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See our publications catalogue at:http://europa.eu.int/comm/environment/pubs/home.htm

OFFICE FOR OFFICIAL PUBLICATIONSOF THE EUROPEAN COMMUNITIES

L-2985 Luxembourg

European Commission

Disposal and recycling routes for sewage sludgePart 3 – Scientific and technical report

14K

H-17-01-003-E

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ISBN 92-894-1800-1

A great deal of additional information on the European Union is available on the Internet.It can be accessed through the Europa server (http://europa.eu.int).

Luxembourg: Office for Official Publications of the European Communities, 2001

ISBN 92-894-1800-1

© European Communities, 2001Reproduction is authorised provided the source is acknowledged.

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Scientific and technical sub-component report

23 October, 2001

European Commission

DG Environment – B/2

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This report aims to describe sewage sludge production, composition, treatment and disposal orrecycling, as well as to review scientific evidence regarding the migration and accumulation ofsubstances and elements contained in sludge into the environment and the food chain, and toidentify the associated risks. It focuses in particular on the recycling routes.

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Sludge� is composed of by-products collected at different stages of the wastewater treatmentprocess. It contains both compounds of agricultural value (including organic matter, nitrogen,phosphorus and potassium, and to a lesser extent, calcium, sulphur and magnesium), and pollutantswhich usually consist of heavy metals, organic pollutants and pathogens. The characteristics ofsludge depend on the original pollution load of the treated water, and also on the technicalcharacteristics of the waste water and sludge treatments carried out.

Sludge is usually treated before disposal or recycling in order to reduce its water content, itsfermentation propensity or the presence of pathogens. Several treatment processes exist, such asthickening, dewatering, stabilisation and disinfection, and thermal drying. The sludge may undergoone or several treatments.

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Once treated, sludge can be recycled or disposed of using three main routes: recycling toagriculture (landspreading), incineration or landfilling. Other, less developped outlets exist, such assilviculture, land reclamation, and other developing combustion technologies including wetoxidation, pyrolysis and gasification. Each recycling or disposal route has specific inputs, outputsand impacts.

Landspreading

Landspreading of sludge or sludge-derived material partially replaces the use of conventionalfertilisers, since it contains compounds of agricultural value. It also contains organic matter,although under a form and at a level below that which would have a significant positive impact onsoil physical properties. Composted sludge however presents a more stable organic matter due tothe addition of a vegetal co-product during the process.

However, landspreading also involves the application of the pollutants contained in sludge to thesoil. These pollutants undergo different transformations or transfer processes. These processesinclude leaching to groundwater, runoff, microbial transformation, plant uptake and volatilisationand enable transfer of the compounds into the air and water, and their subsequent introduction intothe food chain.

Therefore outputs of sludge recycling consist of yield improvement, but also of emissions ofpollution into the soil, and indirect emissions into air and water. Other emissions into the airinclude exhaust gases from transportation and application vehicles.

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Incineration

Incineration is a combustion reaction. Different techniques are currently performed, classifiedbetween mono-incineration when sludge is incinerated in dedicated incineration plants, incinerationwith other wastes, or co-incineration when sludge is used as fuel in energy or material production.Other technologies are also being developed such as wet oxidation or pyrolysis.

Outputs are flue gases, ashes, and wastewater, as well as the production of energy. Thereforeincineration generates emissions into the air (particles, acid gases, greenhouse gases, heavy metals,volatile organic compounds, etc.), soil (disposal of ashes and flue gas treatment residues to landfill,atmospheric deposition of air emissions) and water (flue gas treatment wet processes). Emissionsinto the air may be reduced thanks to flue gas treatment. Emissions depend on the process, but arealso influenced by the sludge type. Energy production generally counterbalances the energy needsfor sludge drying.

Operation of an incineration plant may also produce noise, dust, odour and visual pollution.

Landfilling

There are two possibilities in terms of sludge� landfilling: mono-deposits, where only sludge isdisposed of, and mixed-deposits (most commonly observed), when the landfill is also used formunicipal wastes.

The inputs of landfilling are the waste and additional resources required for the operation of thelandfill site, such as fuel for vehicles, electricity, and additional materials when leachate is treatedon-site. Outputs consist of leachate, landfill gas and energy production when the gas is recovered.

Landfill operation therefore generates emissions into the air (mainly greenhouse gases like methaneand carbon dioxide, reduced when biogases are collected and burnt), and into the soil and water atdumpsites (various compounds such as ions, heavy metals, organic compounds and micro-organisms in leachate). The operation of a landfill also generates other impacts in terms of noiseand dust from the delivery vehicles, as well as odours, land use, disturbance of vegetation and thelandscape.

Other routes

Other sewage sludge recycling routes presently used in Europe include the use of sludge in forestryand silviculture or in land reclamation.

)RUHVWU\�DQG�VLOYLFXOWXUH refer to different kinds of tree plantation and use. The term forestry ismainly used when considering amenity forests, or mature forest exploitation. On the contrary,silviculture is more specifically used when referring to intensive production. From the agriculturaland environmental point of view, differences exist in terms of the impact of landspreading ascompared to the use of sludge in forestry, relating to such factors as the plant species grown, thefauna and flora involved, and the soil types.

Agronomic benefits are increased tree growth and the provision of nutrients to the soil. However,competition with weeds, especially in young plantations may be observed. Excessive rates ofsludge application may also lead to degradation of the upper layer of the soil and the humus, aswell as nitrogen leaching to groundwater. The use of sludge in a forest environment may cause analteration in the characteristics of the ecosystem and, in the case of a mature forest where there isno need to have an additional input of nutrients, may disturb the natural biotopes. More research ishowever needed on this issue.

When considering the risks to humans associated with the presence of heavy metals in sludge, it isassumed that these are lower than those associated with spreading on agricultural land, as forestproducts represent only a very small part of the human diet. However, some risks may still existdue to the transfer of heavy metals to game or edible mushroom species, and in a general manner towild fauna and flora.

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After identifying gaps in knowledge, some recommendations are given in this report concerningsludge application in forest or tree plantations.

Use of sewage sludge in ODQG� UHFODPDWLRQ� DQG� UHYHJHWDWLRQ aims to restore derelict land orprotect soil from erosion through soil provision and increased vegetal covering. In the case ofindustrial sites, topsoil may often be absent or if present, damaged by storage or handling. Soil orsoil forming materials on site may be deficient in nutrients and organic matter. Other problems mayexist, such as toxicity, or adverse pH levels. All these problems create a hostile environment for thedevelopment of vegetation.

Possible solutions include the use of inorganic fertilisers or imported topsoil, which can be veryexpensive depending on location and availability. An alternative solution is the use of organicwastes such as sewage sludge, which is already performed in Sweden, Finland, Germany and theUnited Kingdom.

Sludge application takes place using the same machinery as in recycling to agriculture. Somespecific machinery for sludge projection may be needed when applying sludge in areas whereaccess is difficult.

It was assumed that risks are lower than in the case of spreading on agricultural land, when its useis not related to food production. However, no data is available concerning the potential impacts onwild fauna and flora. Moreover, the amount of sludge applied as well as the application of sludge tosloping land to reduce erosion go against current regulatory prescriptions for the use of sludge inagriculture, inducing risks in terms of pollutants application.

Developing technologies

Several technologies presenting an alternative to conventional combustion processes are currentlybeing developed or introduced onto the market. These technologies mainly include by the wetoxidation process, pyrolysis, and the gasification process. Other technologies may be found, whichare most often combinations of these three main processes.

These technologies present advantages in terms of flue gas and ash treatment. Moreover, they alsoseem to have reduced impacts on the environment compared to conventional combustion processes.

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A review of current scientific knowledge concerning pollutants transfer mechanisms in thedifferent environment media and the food chain has been carried out in order to assess the possibleimpacts on the environment and human health.

Each route has specific transfer processes, but transfers relating to landspreading covers most of thesignificant transfers relating to the other routes, with the exception of air emissions.

Heavy metals

The presence of numerous metals in soil and sludge has been reported in the literature. Onceapplied to the VRLO�they�are distributed between the different soil media. Scientific evidence showsthat they accumulate in the upper layers of the soil, due to binding to the different existing organicor mineral particles. Their mobility and biovailability to plants and micro-organisms may beinfluenced by several factors of which the pH level of the soil is the most important. Heavy metalsare naturally present in soil at varying levels, and may originate from several anthropogenicsources such as fertilisers, animal manure, sludge, or atmospheric deposition. However, variety inthe metal levels in European soils may also be due to the diversity of the extracting methods usedrather than differences in the field. In order to ensure the quality of the comparisons, aharmonisation of the sampling and measurement methods would be required.

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Micro-organisms species present in the soil are numerous. Some of them are important for soilfertility and therefore for agricultural production. Concern has been expressed about theconsequences of metal provision to the soil on the micro-organisms population and biodiversity.Available scientific literature shows contradictory results, depending on the species taken intoconsideration, the local conditions of the experiments, and the confusion of short-term laboratoryexperiments with long-term field trials. Some authors mentioned the ability of microbialpopulations to adapt to changing conditions, which may be considered a result of negative pressureon the population. On the basis of long-term field trials, some studies concluded that soil micro-organisms’ diversity and population could be negatively affected by sludge-borne metals in thelong-term, and by metal levels in soil which were in some cases below current regulatoryprescriptions. It must also be stressed that microbial activity indicators must not be used as the onlyindicators of microbial reaction to metal application, as they do not reflect changes in populationstructure.

/HDFKLQJ�WR�JURXQGZDWHU�appears to be a negligible phenomenon. On the contrary, UXQRII, whenit occurs, may play a significant role in metal transfer. Its importance depends greatly on the localsituation, and the fate of metals needs to be further documented.

3ODQW�XSWDNH�occurs for all heavy metals and is described by transfer factors. Some metals (e.g.copper and zinc) are of biological importance for the plant.

It has been observed that heavy metals are concentrated in the roots and vegetative parts of plantsand are less present in the generative parts such as wheat grain.

Uptake will increase with increasing metal levels in soil, but only applies to the bioavailable part ofthe metals present in soil. However there may be no direct relation between total metalconcentration and bioavailable metals in soil. pH is the most important factor influencing metaluptake. In particular, a decrease in the pH value in soil in the range of pH 7 to pH 4 causes anincrease in the uptake of Cd, Ni and Zn. The same effect is observed for Cu, but is less marked.Lastly, when considering usual acidity levels in agricultural soils, a pH decrease had no observedeffect on Pb and Cr uptake. This information supports the setting of different limit values for Cd,Ni and Zn, and possibly for Cu, for soil with pH values of between 5 and 7 as well as for soil withpH values of higher than 7. Sludge spreading should also be avoided on soil with a pH value below5 and limit values should refer to the bioavailable part of metals in soil rather than to the totalconcentration, although it is not possible at the moment to define for all heavy metals what is thebioavailable fraction.

8SWDNH of metals by animals occurs through contaminated plant consumption or soil ingestion.However little information is available concerning metal quantities ingested and absorbed and theirsubsequent toxicity levels to animals. Metals do not seem to accumulate in meat. More focus isneeded concerning possible Pb and Cd transfer to offal, as in some cases this could lead to levelsnearing acceptable limits in foodstuffs. Transfer of Pb and Cd across the placenta and into the milkwas observed during indoor feeding trials, but there are likely to be few practical consequences forfinished animals. Concentration of Cu in the milk was not influenced by the ingestion of sludge-amended soil. A quantitative assessment of this contamination pathway is not available at thepresent time.

In a general manner, KXPDQ�H[SRVXUH�WR�KHDY\�PHWDOV�may be attributed to several sources anddepends on many factors such as diet, actual absorption, and food processing. Consumption ofcontaminated crops appears to be the main means of exposure to sludge-borne metals. It is assumedthat the specific contribution of sludge-borne metals to the human diet is very low, when takinginto account the observed level of metals present in soil, and considering the surface area overwhich sludge spreading takes place.

Organic pollutants

Numerous organic compounds are present in sludge. Once applied to the land, they are distributedthroughout all soil media and undergo several retention and transport processes. They are

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physically, chemically and biologically transformed in other intermediary compounds during theirmineralisation, for which no data is presently available. The degradation pathway of the organiccompounds and thus the duration before reaching negligible concentration in soils may greatlydepend on the aerobic or anaerobic degradation conditions.

/HDFKLQJ� RI� RUJDQLF� SROOXWDQWV to ground water appears to be insignificant but, unlike metals,cannot be neglected in some cases. The importance of this mechanism depends on the properties ofthe compounds and the soil. It appears on the one hand that many compounds present short half-lifevalues, reducing the risk of leaching to groundwater. On the other hand, persistent compounds suchas PCDD/Fs or PCBs show an affinity with soil particles and will therefore bind to soil rather thanleach to ground water. 5XQRII, when it occurs, may play an important role in the transfer oforganic compounds.

Even if definitive evidence is lacking, it appears that soil PLFUR�RUJDQLVPV are not affected bysludge-borne organic pollutants in most cases and that they are able to adapt to changingconditions.

Most organic pollutants are QRW�WDNHQ�XS�E\ SODQWV. However, a risk of contamination of the foodchain exists when spreading sludge directly onto crops, especially on plants which are to beconsumed raw or semi-cooked.

Soil and sludge ingestion on land used for grazing is the main route for DQLPDO contamination.Accumulation of bioaccumulative compounds such as PCDD/Fs, PCBs or PAHs may occur in meatand milk. However, it is presently not possible to assess the quantities and fates of organiccompounds ingested by animals.

It appears that the consumption of animal products is the major source of KXPDQ exposure tosludge-borne organic pollutants, due to the ingestion of soil by livestock. As in the case of heavymetals, it is assumed that the specific contribution of sludge-borne organic pollutants to the humandiet is very low, when considering the reduced proportion of the utilised agricultural area ontowhich sludge spreading takes place.

Lastly, it should be noted that at the present time no universally accepted and validated analyticalmethod exists for analysing most organic compounds. There is also a lack of data concerning levelsof organic pollutants in European sewage sludge as no regular survey has been performed in thepast.

Therefore, considering presently available knowledge on organic compounds, it appears at thepresent time, that:

- transfer to water is low, micro-organisms adapt to changing conditions in soil, and numerousorganic compounds are rapidly degraded in soil. Attention should therefore mainly be given tocompounds with higher half-life time values,

- from the point of view of crop protection, no limit value seems to be necessary as transfers toplant do not occur for most organic compounds,

- restrictions should focus on bio-accumulative compounds spread on grazing land such as PCBsand PCDD/Fs. In this case deep injection of sludge could reduce the risk of livestockcontamination by organic pollutants,

- a survey of organic pollutant levels in sludge should be performed by sludge producers,focusing on the specific organic pollutants identified within the waste water catchment area ofthe WWTP.

Pathogens

There are five main types of pathogens observed in sludge: bacteria, viruses, fungi and yeast,parasitic worms, and protozoa. Humans and animals are sensitive to some of these organisms,which may cause numerous pathologies ranging from simple digestion troubles to lethal infections.

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Sludge-borne pathogens are mainly present on the VRLO surface or at shallow depths where sludgehas been ploughed into the soil. Pathogen penetration depends on the effective depth of the soil, itstexture (particularly its clay content), its organic matter content and also on possible cracks,prolonged drought, faults or absence of vegetation.

Survival of pathogens in soil depends on numerous direct or indirect factors. Indirect factors areclimatic factors such as sunlight, temperature, desiccation or pH, characteristics of the soil (texture,moisture etc.), quantity of sludge spread, the pathogen content of the sludge, its organic content andthe eventual presence of competing organisms. Direct factors are related to the biologicalcharacteristics of the pathogen, and especially to the form under which it may survive. Parasites’eggs or cysts are the longest survivors – one to two years in certain favourable circumstances.Depending on the conditions and the organisms themselves, survival periods may vary from a fewdays to several years. The pathogenic agent population decreases faster when the sludge is spreadon the soil surface rather than when it is ploughed into it.

Transfer to JURXQGZDWHU is only assumed to occur in some particular cases, while VXUIDFH�ZDWHUcontamination is more likely to occur when runoff water transports pathogens which are bound tosoil particles.

Survival on SODQWV is shorter than in soil, due to the effects of desiccation and sunlight.

Transmission to grazing GRPHVWLF� DQG� IDUP� DQLPDOV takes place via ingestion of contaminatedfeed and soil.

+XPDQV can mainly be affected by consuming raw or semi-cooked contaminated vegetables ormeat.

Therefore the risks of sewage sludge application onto the land – that may be addressed by goodpractices – have to be taken into account as pathogens are present in sludge and may havesignificant impacts on humans and animals. In general, deep injection or ploughing down may berecommended during or after sludge application. Although those practices reduce the deleteriouseffect of weather on micro-organisms, contact with animals, wildlife and humans as well asdissemination into the environment will be reduced.

Sewage sludge may also contain plant pathogens, as well as weed seeds. They mainly originatefrom washing of vegetable and fruit, or from road or roof runoff after aerial deposition. Plantpathogens have in general low optimum growth temperature, so that disinfection will be achievedat a lower temperature than for mammalian pathogens.

3ROOXWDQWV�WUDQVIHU�PRGHOOLQJ

Based on the description of the transfer mechanisms of different sludge-borne pollutants in theenvironment, a PRGHO was developed in order to assess:- the transfer of pollutants in soil (in particular due to runoff and leaching),

- the transfer of pollutants to plants in order to make a comparison with limit values infoodstuffs,

- the accumulation of pollutants in the soil,

- the time before reaching a given limit value of pollutants in soil.

2QO\�KHDY\�PHWDOV�DUH�WDNHQ�LQWR�FRQVLGHUDWLRQ. Knowledge concerning organic pollutants doesnot enable accurate calculations to be made as very little is known about their behaviour anddegradation pathways in soils (moreover, it appears that organic pollutant transfer to plants isnegligible and that this particular route should not involve a significant human health risk). It is notrelevant to apply such calculations to pathogens.

7ZR� VFHQDULRV� DUH� H[DPLQHG�� ZKLFK� UHSUHVHQW� WZR� H[WUHPH� VLWXDWLRQV� RI� ORZ� DQG� KLJKDFFXPXODWLRQ. Several assumptions were necessary in order to perform the calculation. 7KHUHIRUH

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The main results can be summarised as follows:- on a one-year basis, it must be observed that pollutants brought to soil by sludge application

represent a very low proportion of the amount of metals present in soil before sludgeapplication;

- plant uptake of sludge-borne metals may vary, but always represents a minor part of the amountof sludge-borne metals contained in soil; in the long-term, plant uptake will increase withincreasing soil concentration ;

- runoff is the main parameter in the model influencing the heavy metal accumulation in soil ;

- global plant uptake of metals present in soil always remains below the limit values forfoodstuffs. However, in the worst case, it may reach a significant proportion of these limitvalues;

- on the contrary, uptake of metals originating only from sewage sludge application is very low,and reaches, in the worse case of our modelling, 1 % of the limit value for foodstuffs ;

- an equilibrium may be reached after several years between plant uptake and sludge application,indicating that, in some cases, a limit value for metal levels in soil would never be reached ;

- the number of years required before a limit value is reached for metal accumulation in soilwould vary greatly between the two extreme cases considered herein: figures range from around4,500 years to over 34,000 years in the case of low accumulation, and from 20 years to around140 years in the high accumulation scenario.

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Today, many uncertainties remain concerning the transfer of pollutants (especially organicpollutants) to the environmental media and the food chain. Several issues would need to be moreaccurately documented. Amongst these issues, the following may be mentioned:

- The importance of the runoff process in the pollutants’ transfer should be assessed. Mechanismsneed to be understood, as well as quantities of pollutants concerned, and their fate.

- An issue of concern is the degradation pathway of the organic compounds in soil. Compoundsmay be degraded into intermediary chemicals before total mineralisation. The toxicity andleaching potential of these metabolites is not well known. Lysimeter and field studies should becarried out.

- Long-term impacts of heavy metals and organic pollutants, in particular on soil micro-organisms and fertility, are not well documented.

- More data is needed concerning the ingestion and absorption levels of organic compounds and,to some extent, heavy metals by animals.

- There is also a lack of knowledge concerning the specific contribution of sewage sludge topollutants’ transfers.

- A survey of the organic pollutants’ levels in sewage sludge should be performed in the MemberStates in order to gain an accurate appreciation of their occurrence. This may only be possible ifstandard analytical methods are set and broadly accepted.

- Available literature does not always enable a comparison between the different countries, as nocommon research protocol and no trans-national study has been carried out.

- More information is also needed concerning other routes for sludge recycling, such as landreclamation or use in forestry and silviculture. Research should be carried out to preciselyidentify the agricultural benefits of sewage sludge spreading and its environmental and sanitary

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impacts (especially concerning organic pollutants for which no data is currently available).Moreover, currently available information does not enable an assessment and comparison of thebenefits and risk as regard the diversity of European forests.

- Lastly, some interesting new technologies such as wet oxidation, pyrolysis or gasification havebeen developed. More information concerning their environmental impact and their applicationis needed. Tests have not always been carried out on sludge, and this issue requires furtherdocumentation.

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The objective of this intermediary report is to review scientific evidence on the migration andaccumulation of substances and elements contained in sludge into the environment and the foodchain, and identify the associated risks.

The present report aims to:

- present the main waste water treatment technologies and their impact on sludge composition;

- present the sewage sludge treatment processes and the different substances and elementspresent in sewage sludge;

- identify the main routes for sludge disposal and recycling and describe their technologicalcharacteristics;

- review current scientific knowledge on the main biophysical processes involved, flows ofdifferent elements of the sludge through the environmental media and the associated risks;

- develop a simple model describing the flows and accumulations of pollutants in theenvironment.

$V� WKH� WHFKQLFDO� GHVFULSWLRQ� RI� LQFLQHUDWLRQ� DQG� ODQGILOOLQJ� RI� ZDVWHV� DV� ZHOO� DV� WKHLUHQYLURQPHQWDO� DQG� VDQLWDU\� LPSDFWV� KDYH� DOUHDG\� EHHQ� H[WHQVLYHO\� GRFXPHQWHG� HOVHZKHUH�WKLV�UHSRUW�ZLOO�PDLQO\�VXPPDULVH�WKH�DYDLODEOH�FRQFOXVLRQV�IRU�WKRVH�URXWHV�DQG�DGGUHVV�WKHVSHFLILFLWLHV�IRU�VOXGJH� LQFLQHUDWLRQ�DQG�ODQGILOOLQJ�� LI�DYDLODEOH��2Q�WKH�FRQWUDU\��SDUWLFXODUDWWHQWLRQ� KDV� EHHQ� JLYHQ� WR� WKH� GHVFULSWLRQ� RI� WKH� EHQHILWV� DQG� LPSDFWV� RI� VOXGJH� XVH� LQDJULFXOWXUH�DQG�WKLV�UHSRUW�IRFXVHV�HVSHFLDOO\�RQ�WKHVH�LVVXHV�

The different steps covered in this report are presented below:

Wastewater collection

Water treatment

Sludge treatment

Sludge recycling

and disposalImpacts

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Sludge is a by-product of the water clean up process. There are three main categories of sludge:

- sludge originating from the treatment of urban wastewater, consisting in domestic wastewater or in the mixture of domestic waste water with industrial waste water and/or run-offrain water.

- sludge originating from the treatment of industrial wastewater, i.e. water used in industrialprocesses.

- sludge from drinking water treatment. Water has to be treated before its consumption. Theamount of sludge generated from drinking water treatment is significantly lower than thatgenerated from wastewater treatment.

The characteristics of sludge depend on the original pollution load of the treated water, and also onthe technical characteristics of the treatment carried out. Water treatment concentrates the pollutionpresent in water and therefore sludge contains a wide variety of matter, suspended or dissolved.Some compounds may be usefully reused (organic matter, nitrogen, phosphorus, potassium,calcium, etc.) whereas other compounds are pollutants (such as heavy metals, organic pollutants,and pathogens).

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Wastewater collection

Water treatment

Sludge treatment

Sludge recycling

and disposal

Impacts

Sludge from conventional wastewater treatment plants (WWTP) is derived from primary,secondary and tertiary treatment processes. Most often, the sludge produced has a concentration ofa few grams per litre, and is highly biodegradable. Each process has a different impact on the waterpollution load. These are presented below.

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)LJXUH���� wastewater treatment and sludge generation

More information on the European wastewater treatment system is provided in box 1 at the end ofthis chapter.

3.2.1 Pre-treatment

Pre-treatment consists of various physical and mechanical operations, such as screening, sieving,blast cleaning, oil separation and fat extraction.

Pre-treatment allows the removal of voluminous items, sands and grease. The residues from pre-treatments are not considered to be sludge. They are disposed of in landfills.

3.2.2 Primary sludge

Primary sludge is produced following primary treatment. This step consists of physical or chemicaltreatments to remove matter in suspension (e.g. solids, grease and scum).

The most common physical treatment is sedimentation. Sedimentation is the removal of suspendedsolids from liquids by gravitational settling. Sedimentation is usually considered first because it is asimple and cost-effective method. Another physical treatment is flotation. Air is introduced into thewastewater in the form of fine bubbles, which attach themselves to the particles to be removed. Theparticles then rise to the surface and are removed by skimming.

In the mechanical stage, 50 to 70 % of the suspended solids and 25 to 40 % of the BOD5 can beremoved [Werther and Ogada 1999].

Chemical treatments are coagulation and flocculation. Coagulation and flocculation are used toseparate suspended solids when their normal sedimentation rates are too slow to provide effectiveclarification.

Coagulation is the addition and rapid mixing of a coagulant to neutralise charges and collapse thecolloidal particles so they can agglomerate and settle. The flocculation is the agglomeration of thecolloidal particles that have been subjected to coagulation treatment.

3.2.3 Secondary sludge

Secondary sludge is generated from the use of specially provided decomposers to break downremaining organic materials in wastewater after primary treatment. The active agents in thesesystems are micro-organisms, mostly bacteria, which need the available organic matter to grow.There are various techniques such as lagooning, bacterial beds, activated sludge as well as filtrationor biofiltration processes.

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The lagooning technique uses the development of a bacterial population in a lagoon, whichconverts organic matter into CO2 and biomass. Oxygen is fed into the system via the photosyntheticactivity of microphytes (unicellular algae) or macrophytes (plants), although an alternativetechnique consists of artificial aeration of the lagoon. Practically water is passed through severallagoons, each reaching a higher level of de-pollution. This technique is suitable for WWTPs withlarge site areas.

In bacterial beds, the effluent is in contact with bacteria, which are attached to a support.

In activated sludge, bacteria are kept in suspension in the vessel in aerobic conditions. At the end ofthe process, the treated water has to be decanted off in order to separate the cleaner water from theactivated sludge. This treatment generates another type of sludge, called surplus activated sludge.

3.2.4 Mixed sludge

The primary and secondary sludge described above can be mixed together generating a type ofsludge referred to as mixed sludge.

3.2.5 Tertiary sludge

Tertiary sludge is generated when carrying out tertiary treatment. It is an additional process tosecondary treatment and is designed to remove remaining unwanted nutrients (mainly nitrogen andphosphorus) through high performance bacterial or chemical processes.

These treatments are necessary when a high level of depollution is required, for example insensitive areas identified in the Member States.

Nitrogen consumes oxygen when a nitrification reaction takes place in the natural environment. Itis toxic under its ammoniac or nitrate phase, and is responsible of eutrophication. The removal ofnitrogen is a biological process leading to the production of N2.

Each step is carried out by specific bacteria, which need different conditions to grow.

The removal of phosphorus may be performed using chemical processes or biological treatments.Chemical processes consist of chemical precipitation using additives followed by sedimentation.Physical-chemical removal of phosphorus increases the quantity of sludge produced by an activatedsludge plant by about 30 %. Biological treatments employ specific micro-organisms, which areable to store phosphorus. It accumulates within the bacteria enabling its removal with the rest of thesludge.

3.2.6 Digested sludge

After water treatment, additional treatments need to be performed RQ�VOXGJH, in order to:

- reduce its water content,

- stabilise its organic matter and reduce the generation of odours

- reduce its pathogen load,

- reduce its volume and global mass.

Several treatments can be applied to sludge to achieve this. These are described in a following partof this report. One of those transforms the sludge in a way that it is considered as a new type ofsludge usually referred to as “digested sludge”. This so-called digestion process is described furtherin the following part.

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Each kind of treatment has a specific impact on the composition of sewage sludge. We can definefour types of sludge:

- A : primary sludge, primary sludge with physical/chemical treatment or high pollution load1

- B1 : biological sludge (low load)- B2 : biological sludge from clarified water (low and middle load)- C : mixed sludge (mix of A and B2 types)- D : digested sludge

Composition of each kind of sludge is provided below:

$ %� %� & ''U\�PDWWHU��'0� J�/ 12 9 7 10 309RODWLOH�PDWWHU��90� �'0 65 67 77 72 50S+ 6 7 7 6,5 7& ��90 51,5 52,5 53 51 49+ ��90 7 6 6,7 7,4 7,72 ��90 35,5 33 33 33 351 ��90 4,5 7,5 6,3 7,1 6,26 ��90 1,5 1 1 1,5 2,1&�1 � 11,4 7 8,7 7,2 7,93 ��'0 2 2 2 2 2&O ��'0 0,8 0,8 0,8 0,8 0,8. ��'0 0,3 0,3 0,3 0,3 0,3$O ��'0 0,2 0,2 0,2 0,2 0,2&D ��'0 10 10 10 10 10)H ��'0 2 2 2 2 20J ��'0 0,6 0,6 0,6 0,6 0,6)DW ��'0 18 8 10 14 103URWHLQ ��'0 24 36 34 30 18)LEUHV ��'0 16 7 10 13 10&DORULILF�YDOXH N:K�W�'0 4 200 4 100 4 800 4 600 3 000

7DEOH���� impact of treatments on the sewage sludge composition and properties [OTV 1997]

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Sewage sludge contains both compounds of agricultural value and pollutants. Compounds ofagricultural value include organic matter, nitrogen, phosphorus and potassium, and to a lesserextent, calcium, sulphur and magnesium. Pollutants are usually divided between heavy metals,organic pollutants and pathogens.

A table summarising the average composition of sewage sludge in the Member States is providedin the appendix. However, this data has to be taken carefully, as years and time series are different.The figures provided are mean values, and do not take into account differences between small andlarger WWTP. In addition, we are not always confident that it accurately represents the situation ofeach country, especially in the Accession Countries and in some EU countries where nocomprehensive survey has been performed.

1 The load (Cm) is defined as the ratio between the daily mass of pollution to be removed and the mass of

bacteria used for depollution. Usually the following levels are defined:

- high load: Cm>0,5 kg BOD5/kg sludge/day

- middle load: 0,2<Cm<0,5

- low load: 0,07<Cm<0,2

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In order to perform comparisons, typical composition of animal manure and slurry is also providedin appendix.

3.4.1 Organic matter

Organic matter is mainly used for soil improvement. Known benefits of organic matter applicationto soil are the improvement of the physical properties of soil such as structure or improvement ofthe retention capacity of minerals and water. Other benefits of sludge application may be theimprovement of the soil bearing strength, or the reduction of the potential for surface runoff andwater erosion [ADAS 2000].

Degradation of the organic matter can also increase the soil content in compounds of agriculturalvalue (such as N, S, Mg etc.), which are slower released than in the case of mineral fertilisers andtherefore available for a longer period to crop [ADAS 2000]. Organic matter is lastly an energysource for micro-organisms living in soil. Therefore sludge spreading may induce an increase ofthe soil population and activity, and of its mineralisation capacity.

Sludge organic matter is mostly constituted of soluble matter, such as hydrocarbons, amino-acids,small proteins or lipids. Its content in urban sewage sludge is high (usually more than 50 % of thedry matter) but varies according to the treatment and conditioning2 carried out on sludge. Contentlevel may be reduced due to dilution after incorporation of lime or salts for instance. The tablebelow compares the content of organic matter of urban sewage sludge against other urban wastesand animal manure.

2UJDQLF�0DWWHU�FRQWHQW��RI�'0

8UEDQ�VOXGJHAerobic digestion 60 - 70Anaerobic digestion 40 - 50Thermal treatment < 40Lime treatment < 40Composting 50 – 85

8UEDQ�FRPSRVW*UHHQ�ZDVWHV�FRPSRVWLQJ

40 – 6030 – 60

$QLPDO�PDQXUH 45 – 857DEOH���� content of organic matter in sludge after different treatments and in other urban waste

and animal manure [Lineres 2000]

However, a recent UKWIR funded literature review indicated that the minimum threshold level fordetectable effects of sludge additions on soil physical properties was c. 5 tonnes organic matter/ha,i.e. about 10 t dry solids/ha. Considering current quantity limitations for agricultural use of sludgeimplemented in the Member States, those benefits do not occur [Lineres 2000]. Improvement ofsoil physical properties may however be observed when using sludge for land reclamation(provided that limit values for pollutants applied are respected), as amounts of sludge used may bevery important.

Moreover sludge organic matter is mostly constituted of soluble matter, such as hydrocarbons,amino-acids, small proteins or lipids. There is only a small amount of lignin or cellulose entering inits composition. Therefore, sewage sludge’s organic matter mineralises fast, and its rapiddegradation could generate a peak in the nitrate and pollutant levels in soil

It may be observed that the specific case of composting induces the addition of stable organicmatter to the sludge, originating from the co-product. In this case, organic matter mineralisesslower, and nutrients are slower released, reducing the potential risk of nitrogen leaching to

2 for a description of the different types of treatments, see section 4

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groundwater. ADEME [2001] observes that the rapidity of the compost mineralisation depends onthe type of compost, as well as from its maturity, and that compost decomposition may continueseveral years following its application on soil. It is also assumed that composted sludge could havea more significant impact on soil structure than non-composted sludge. However no information isavailable in the literature concerning a lower threshold level for detectable effects of sludgeadditions on soil physical properties.

3.4.2 Nitrogen and phosphorus content

The table provided in appendix shows the content of nitrogen and phosphorus in sludge in theMember States. The ranges are between 20 and 80 000 mg/kg DM for Nitrogen and 10 and 90 000mg/kg DM for Phosphorus.

The proportion of phosphorus and nitrogen in sewage sludge is comparable to the one of animalmanure.

7RWDO�1(% of DM)

1�±�1+�(% of N total)

3(% of DM)

8UEDQ�VOXGJH 0,9 – 5,2Liquid 1 – 7 2 - 70Semi-solid 2 – 5 < 10Solid 1 – 3,5 < 10Composted 1,5 – 3 10 – 20 0,2 – 1,5

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0,961,0 – 2,4

0,390,04 – 0,44

/LWWHU0DQXUH

2,2 – 4,44 – 7

1050 – 70

0,61 – 1, 610,91 – 3,3

7DEOH���� content of nitrogen and phosphorus in sludge after different treatments and in otherurban waste and animal manure [Lineres 2000]

1LWURJHQ

Nitrogen is mostly found under organic form in sludge, and to a lesser extent under ammoniacform. Other mineral forms of nitrogen are only found as traces.

Treatments carried out on sludge can greatly influence their content of nitrogen and phosphorus, asshown in the table below. For instance, as most of the ammoniac is located in the liquid phase ofsludge, an important part of it will be removed during the thickening and dewatering steps. Thenitrogen content is also influenced by the operation of the WWTP, and the sludge storageconditions: in some cases a reduction of the nitrogen content in stored liquid sludge by 30 % after 4months has been reported [ADEME 1996].

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7\SH�RI�WUHDWPHQW 7RWDO�1(% of DM)

1�1+�

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(% of total N)

/LTXLG�VOXGJHaerobic digestion, gravity thickeningaerobic digestion, mechanical thickeninganaerobic digestionlagooning

5 – 74 – 71 – 71 – 2

5 – 102 – 8

20 – 70N/A

6HPL�VROLG�VOXGJHaerobic digestion, mechanical dewateringanaerobic digestion, mechanical dewateringlime treatment

3 – 5.51.5 – 33.4 – 5

< 5< 5< 10

6ROLG�VOXGJHaerobic digestion, lime treatment (press filter)compostedaerobic, dewatered on drying bedsanaerobic, dewatered on drying beds

2.51.5 – 32 – 3.5

1.5 – 2.5

< 1010 – 20

< 10< 10

'ULHG�VOXGJH 3.5 – 6 10 – 15

7DEOH���� influence of treatment on the nitrogen content of some sewage sludge [ADEME 1996]

As plants can assimilate only mineral nitrogen, the agricultural value of the sludge is alsodetermined by the aptitude of its organic N to be mineralised. The nitrogen availability depends onthe type of sludge. It varies between 4 and 60 %, but within one type of sludge, great variationshave been reported (see table below). The nitrogen availability may be classified as follows:composted sludge < anaerobic digested sludge < aerobic digested sludge. The different treatmentscarried out on sludge may also greatly influence the availability of the nitrogen in sludge, withoutknowing the influence of each one of them [ADEME 1996]. Other factors influencing theavailability of the nitrogen are extrinsic factors: temperature, humidity, pH and texture of the soil,and condition of landspreading. Loss of nitrogen can also occur if volatilisation of the ammoniactakes place, or if nitrates are leached. This may represent a possible risk of groundwater pollution.It may happen if the amount of sludge applied does not correspond to plant needs in nutrients orbecause of the fast degradation of sludge-borne organic matter which could give rise to a peak ofnutrient in soil.

6OXGJH�W\SH $YDLODELOLW\����

Aerobic digested sludge 24-61 %

Anaerobic digested sludge 4-48 %

Digested composted sludge 7 %

Composted raw sludge 4 %

Thermally dried sludge 7-34 %

7DEOH���� nitrogen availability according to laboratory results [ADEME 1996]

3KRVSKRUXV

Phosphorus is used by the plant for its growth, the rigidity of its cell walls, and for the developmentof its root system. Sludge-borne phosphorus is of particular interest as phosphorus is a limitednatural resource.

Phosphorus in sludge is mostly present under mineral form: mineral phosphorus can representbetween 30 and 98 % of the total phosphorus, according to the type of sludge. As in the case of

��

nitrogen, the amount of phosphorus available in sludge depends on the treatments carried out, andis not proportional to the amount of total phosphorus. Variations in composition are illustrated inthe table below. Of course, the amount of phosphorus in sludge is much higher when a specifictertiary waste water treatment for phosphorus removal is carried out. It has also been observed thatcomposted sludge has a lower phosphorus content than non-composted sludge, due to the lowphosphorus content of the co-products used during the composting process. Contrary to nitrogen,phosphorus content in sewage sludge is not significantly reduced after storage.

7\SH�RI�WUHDWPHQW 3�2�

(% of DM)3

(% of DM)

Liquid sludge ;�anaerobic digestionAerobic digestionPrimary sludge, lime treated

4.9 – 6.92.5 – 12.65

2.5 – 12

2.1 – 31.1 – 5.51.1 – 5.2

7DEOH���� influence of treatment on the phosphorus content of some sewage sludge [ADEME1996]

3.4.3 Calcium enrichment

Lime addition to sewage sludge is performed in order to stabilise the sludge (see chapter 4). Toreach a good level of stabilisation, it is recommended to add about 30 % of lime to the dry matter.Lime treatment of sludge therefore generates a product with a useful content in CaO that could beof interest on certain soils. However, as the calcium content may be highly variable in lime treatedsludge, it is generally necessary to analyse sludge before use.

Field studies have shown that lime treated sewage sludge has positive impacts on the pH, structureand permeability of the soil [Lineres 2000]. Calcium is also a useful element for the plant as itstrengthens its cell walls. Calcium supply to the soil may in some cases correct a deficiency of thiselement.

3.4.4 Other compounds of agricultural value

Other compounds present in sludge such as potassium, sulphur, magnesium, sodium and oligo-elements (e.g. boron, cobalt, selenium, iodine) may be of interest in crop production, each of thembeing useful for the plant development and growth. However, they may appear in sludge undervarious forms (for instance magnesium sulphate or magnesium oxide), and their efficiency willdepend on their availability.

It has been mentioned in the UK literature that, as atmospheric sulphur deposition continues to fall,sulphur fertiliser additions are increasingly generating yield responses. Biosolids applicationstypically supply between 140 and 200 kg/ha total SO3, making a valuable contribution to croprequirements [Chambers HW�DO. 2000].

However, the agricultural value of those compounds related to their level in sludge is notextensively documented in literature.

3.4.5 Heavy metals

Numerous heavy metals are present in sludge. Heavy metals may affect plant health and growth,soil properties and micro-organisms, livestock and human health, and accumulate in theenvironment. Their impacts are more accurately described in chapter 6. On certain soils however,for example on copper deficient soils, the heavy metals content of sewage sludge can correct traceelement deficiency.

��

The average content of 7 heavy metals in the Member States is presented below. Data refers to theinformation collected in the Member States for this study, which are summarised in appendix.

'LUHFWLYH�������((&mg/kg DM

5DQJH�LQ�WKH0HPEHU�6WDWHV

mg/kg DM

&G 20 – 40 0.4 – 3.8

&U 1000 – 1750 16 - 275

&X 1000 – 1750 39 - 641

+J 16 – 25 0.3 - 3

1L 300 – 400 9 - 90

3E 750 – 1200 13 - 221

=Q 2500 – 4000 142 - 2000

7DEOH���� average content in sewage sludge of 7 heavy metals in the Member States

In all countries, the average values of composition are clearly under the limits of the 86/278/EECdirective. In most cases, the values given in this table are also below the national limit values set inthe regulations of each country.

For some compounds, such as cadmium and mercury, the values are quite homogeneous: between0.5 and 3.8 mg/kg DM. For other compounds however, there are great differences among Europeancountries. This may be due to the industrial context of each country.

Concerning the Accession Countries, we have collected data from Cyprus, the Czech Republic,Estonia, Latvia, Lithuania, Slovakia and Slovenia. The situation is heterogeneous. Compared withthe values in the Member States, Latvia has sludge of worst quality. The sludge of other countriespresents levels of contaminants, which are comparable or slightly higher than in the Member States(Cd, Pb, Cr in Estonia, for instance) or much higher (Hg, Ni). However, those values are all belowthe actual limits of the 86/278/EEC directive. In Slovakia, only one WWTP, representing 3,3% ofthe total sludge production, produces a sludge, which can not be applied on land (higher Cr contentdue to tannery effluent treatment).

There are three main origins for heavy metals in sewage sludge: domestic effluents, road runoff,and industry. For each metal, the proportion of each origin may be very different, and theimportance of heavy metals originating from the industry depends greatly from the industrialsituation of each country.

As a comparison, typical heavy metal levels in animal manure and slurry is provided in appendix.

3.4.6 Organic pollutants

A wide variety of organic chemicals with diverse physical and chemical properties may be found insludge. They also may affect soils, plant, animals and human health, and have impacts on theenvironment, which are described in chapter 5 and 6 of this report.

In this study we especially take into consideration the compounds to which it is most often referred,but many others are present as traces. The considered compounds are:

- PAH : Polynuclear aromatic hydrocarbons

- PCB : Polychlorinated biphenyls

- PCDD/F : Polychlorodibenzodioxins/furans

��

- AOX : Sum of organohalogenous compounds

- LAS : Linear alkylbenzenesulphonates

- NPE : Nonylphenol and Nonylphenolethoxylates

- DEHP : Di(2-ethylexyl)phtalate

A description of each of them is provided in appendix.

So far, we have not received satisfactory data from the Member States concerning thosecompounds. As they are often not mentioned in the national regulations, no survey has beenregularly performed describing the organic pollutant content in sewage sludge. To compensate themissing information, the concentrations given have been collected from various documentarysources. Therefore, those figures must be considered only as indicative values.

Concerning PCDD/F, data have been taken from the &RPSLODWLRQ� RI� (8� 'LR[LQ� H[SRVXUH� DQGKHDOWK� GDWD [AEA Technology 1999]. It shows that average concentrations of dioxins are quitesimilar among Member States, between 15 and 40 ng I-TEQ/kg DM. According to the study, thiswould indicate that the sources of contamination in the Member States are similar. Industrial inputscan also cause important contamination in sludge. In some cases, more than 1 000 I-TEQ/kg DMhave been reported.

Data available concerning other organic pollutant levels in sludge are not consistent and reliableenough to draw any conclusion.

3.4.7 Pathogens

Sewage sludge contains various micro-organisms, especially when biological treatments are carriedout. Only some of them have health-related impacts. Sludge may also contain plant pathogens. Asfor organic compounds, no satisfactory data could be found for the Member States and theAccession Countries concerning their content in sludge.

Presence of pathogens in sludge is related to the sanitary level of the population, and the type ofindustry in the region. The types of pathogens usually considered are viruses, bacteria, protozoa,and helminths. Their load in sludge varies along time. More information concerning treatments thatmay be performed in order to reduce or destroy pathogens present in sludge is given in part 4.5, andthe fate of pathogens in the environment and their sanitary impact is summarised in part 6.3.

���� ,QGXVWULDO�VOXGJH

Data provided above refers mainly to urban sewage sludge. As a comparison, table 8 describes thecomposition of some types of industrial sludge, namely pulp and paper, and tannery sludge. Moreinformation is available in the WRc report, “Survey of wastes spread on land” [2001].

3.5.1 Pulp and paper industry

Composition of pulp and paper industry sludge depends on the paper production process. Usingvirgin wood fibre generates a liquid effluent mainly loaded with lignin and cellulose, thereforecontaining a higher level of stable organic matter. On the contrary, recycling of waste paperinduces additional steps such as de-inking and bleaching, and therefore generates a so-called de-inking sludge, containing colouring agents and chemicals. Reusing waste paper usually generates agreater amount of sludge than when using virgin wood fibres.

Pulp and paper sludge is therefore a mixture of cellulose fibres, ink and mineral components. Inksused to be produced by using heavy metals. Their usage has however been greatly reduced in thelast 20 years, therefore reducing their level in sludge. The higher content of cellulose fibres makesthe nitrogen availability lower than in the case of urban sludge. As a consequence, nitrogen isreleased more slowly into the soil after application, reducing the risk of leaching to groundwater.

��

3.5.2 Tannery Sludge

Leather manufacturing generates liquid and solid wastes originating from the different steps in thetransformation of the mammalian skin into leather, performed by using several reactive products.Liquid effluents contain collagen fixed to tanning agents and heavy metals originating from thereactive products used during the tanning process. Sludge composition varies according to thespecific process performed on site.

As tannery wastewater is rich in proteins, nitrogen content in the sludge is higher than in the caseof urban sludge, and therefore of interest for landspreading. However, heavy metal (especiallychromium) content may prevent their use in agriculture.

3XOS�DQG�SDSHU�LQGXVWU\�VOXGJH 7DQQHU\�VOXGJH ���'LUHFWLYH

(/(0(176 0LQ 0D[ 0HDQ 0LQ 0D[ 0HDQ

'U\�VROLGV���� 1,7 65 31,60 4,10 13,21 7,38

&�1�5DWLR 12,5 200 77,80

ZDWHU�S+ 4,5 9,4 7,30 6,7 7,20 6,86

$JULFXOWXUDO�YDOXH����'0�

2UJDQLF�PDWWHU 19,1 90,4 63,90 47,61 68,87 54,15

1�7RWDO 0,4 4,9 1,31 3,59 5,60 5,05

1�1+� 0 0,3 0,02

&D2 0,52 19,9 12,50 13,35 21,41 16,02

0J2 0,02 6,5 0,86 0,30 0,51 0,38

3�2� 0,19 8 0,68 0,40 0,88 0,62

.�2 0,06 0,79 0,180 0,12 0,92 0,65

62� 1,270

+HDY\�PHWDOV��SSP� �PJ�NJ�'0�

&DGPLXP�±�&G 0,2 4,4 0,98 0,15 0,07 0,17 20 – 40

&KURPLXP�±&U < 1 44,5 34,10 92,00 162,50 127,60 -

&RSSHU�±�&X 2 349 61,20 8,50 12,80 9,90 1000 – 1750

0HUFXU\�±�+J < 0,01 1,4 0,240 0,03 0,04 0,03 16 – 25

1LFNHO�±�1L < 1 32 12,40 1,10 2,07 1,53 300 – 400

/HDG�±�3E < 1 83 13,10 2,25 5,15 3,67 750 – 1 200

=LQF�±�=Q 1,3 330 135,10 20,40 30,60 26,80 2500 – 4000

$UVHQLF�±�$V <8

2UJDQLF�FRPSRXQGV��SSP� �PJ�NJ�'0�

)OXRUDQWKHQH� 0,01 <0,1 <0,055

%HQ]R��E��IOXRUDQWKHQH� <0,005 0,04 <0,022

%HQ]R��D��S\UHQH� <0,005 0,03 <0,017

6XP�RI���3&% 0,002 <1 <0,5

7DEOH���� composition of some industrial sludge types [WRc 2001] (*: PAH compounds)

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%R[� ��� :DVWHZDWHU� WUHDWPHQW� V\VWHP� LQ� WKH� (XURSHDQ� 8QLRQDQG�WKH�$FFHVVLRQ�&RXQWULHVTables below summarise the level of the wastewater treatment in the Member States and theAccession Countries. In some countries however, such as Finland, access to sewerage may bereplaced by the use of septic tanks.

2&'(�'DWD (XURVWDW�'DWD$FFHVV�WR�SXEOLF�VHZHUDJH1R�DFFHVV

WRVHZHUDJH

1RWUHDWPHQW

3 3�6 3�6�7 7RWDO$FFHVV�WRVHZHUDJH&RXQWU\

% of population

<HDU% of

population

<HDU

Austria �� 1 1 39 35 �� 1997 �� 1995Belgium �� 48 0 27 0 �� 1997 N/ADenmark �� 0 2 14 72 �� 1997 �� 1985Finland �� 0 0 0 77 �� 1997 �� 1995France �� 2 0 0 77 �� 1997 �� 1990Germany �� 1 4 12 72 �� 1997 �� 1990Greece �� 18 26 19 5 �� 1997 N/AIreland �� 7 35 26 0 �� 1997 �� 1990Italy* �� 14 3.1 36 23.6 �� 1995 N/ALuxembourg �� 0 19 57 11 �� 1997 �� 1995Netherlands � 1 0 42 55 �� 1997 �� 1995Portugal �� 34 4 16 1 �� 1997 �� 1990Spain 1�$ N/A 11 34 3 1�$ 1997 �� 1990Sweden � 0 0 6 87 �� 1997 ��� 1990UK � 8 8 61 18 �� 1996 �� 1995

7DEOH���� sewerage access and wastewater treatment in the Member States [OECD 1999 andEurostat 1998-2000; Italy : Cecchi HW�DO. 1996] ; P: primary treatment ; S: Secondarytreatment; T: Tertiary treatment

(($����� ��HDUO\���¶V

$FFHVV�WR�VHZHUDJH1R�DFFHVVWR

VHZHUDJH1R

WUHDWPHQW3 3�6 3�6�7 7RWDO&RXQWU\

% of populationBulgaria �� 35 0 34 0 ��Czech Rep. �� 17 4 52 0 ��Estonia �� 9 27 15 22 ��Hungary �� 12 5 22 6 ��Latvia �� 12 10 51 0 ��Lithuania �� 12 32 29 0 ��Poland �� 23 10 28 4 ��Romania �� 12 6 29 0 ��Slovenia �� 8 5 45 0 ��Slovakia �� 13 16 15 0 ��Total AC �� 18 8 31 2 ��

7DEOH����� sewerage access and wastewater treatment in the Accession countries [EEA 1999] ; P:primary treatment ; S: Secondary treatment; T: Tertiary treatment

��

%R[����6OXGJH�TXDOLW\�LPSURYHPHQWDuring the last 20 years, sludge quality has considerably improved. Some examples are provided infigures 2 and 3 and table 11 for heavy metals and organic pollutants. Other examples may be foundin WRc [1993] and ADEME [1999] reports. The data provided in figure 2 stems from the state ofUpper Austria and concerns 80 up to 140 rural, urban and industrial plants, of which capacitiesvary between <1000 and 500 0000 Inhabitants equivalent.

0

500

1000

1500

2000

2500

3000

3500

1980 1985 1990 1995 2000

PJ�NJ�'0

Zn

Cr

Ni (x10)

Pb (x10)

Cu (x10)

Cd (x100)

)LJXUH���� decrease of heavy metals content in sewage sludge in Upper Austria, 1980 – 2000[Aichberger, 2000]

0

500

1000

1500

2000

2500

3000

3500

1977 1982 1987 1992

PJ�NJ�'0

Zn

Cr

Ni (x10)

Pb (x10)

Cu

Cd (x100)

Hg (x100)

)LJXUH���� decrease of heavy metals content in sewage sludge in Germany, 1977 – 1992/93 [ATV1996]

Eljarrat HW� DO� (1999) also analysed the dioxin content of sewage sludge from 19 WWTP inCatalonia (Spain), and compared the results with figures from archived sewage sludge originatingfrom 15 WWTP, stored between 1979 end 1987. The most recent samples indicated values rangingfrom 7 to 160 pg/g, with a mean value of 55 pg/g and a median value of 42 pg/g, whereas archivedsamples indicated levels between 29 and 8300 pg/g, with a mean value of 620 pg/g and a medianvalue of 110 pg/g. This study showed a reduction in the dioxin amount in sewage sludge.According to the authors, there could also have been a change in the sources to the environmentover time.

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������� �������$2; mg/kg DM 250 – 350 140 – 2803&% mg/kg DM < 0,1 0,01 – 0,043$+ mg/kg DM 0,25 – 0,75 0,1 – 0,6'(+3 mg/kg DM 50 - 130 20 - 601RQ\OSKHQRO mg/kg DM 60 – 120 -3&''�) ng TE / kg DM < 50 15 - 45

7DEOH����� average contents of organic micro-pollutants in sewage sludge in 1988/89 comparedwith data from German publications till 1996 [Leschber 2000]

Those figures show that after a significant improvement, the level of pollutants is nearing a baselevel. These improvements result from reducing the sources of the pollution, mostly point sourcessuch as industrial discharge.

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%R[� ��� 6OXGJH� SURGXFWLRQ� LQ� 0HPEHU� 6WDWHV� DQG� $FFHVVLRQ&RXQWULHV

6OXGJH�SURGXFWLRQ�LQ�0HPEHU�6WDWHV

The amount of sludge produced in each Member State is presented in figure 4.

0

500 000

1 000 000

1 500 000

2 000 000

2 500 000

Germ

any

UK

Franc

eIta

ly

Spain

Portu

gal

Sweden

Nethe

rland

s

Austri

a

Denm

ark

Finlan

d

Belgium

Greec

e

Irelan

d

Luxe

mbu

rg

W�'0

)LJXUH���� sludge production in Member States (t DM)3

The total amount of sludge produced in the 15 European Union countries is about 7 million tons ofdry matter (t DM). As shown in the figure 4, Germany is the first sludge producer, followed by theUnited Kingdom, France, Italy and Spain, all producing more than 500 000 t DM in a year. These 5countries generate altogether nearly 75 % of the European sewage sludge. All other countriesproduce less than 250 000 t DM each. This situation roughly reflects the demography of eachcountry.

The amount of sludge is usually presented in tons of dry matter that should be multiplied tenfold4

to obtain the amount of raw sludge produced. The total amount of raw sludge produced in the EUshould be around 70 million tons. However, it is only a theoretical figure, as the original watercontent of the sludge depends on its type and the treatment applied.

Figure 5 presents the sludge production in the European Union per inhabitant and per day.According to this data, Greece produces the lowest amount of sludge per inhabitant (15,4 gDM/inhabitant/day), whereas Denmark is the most important producer with 78 g.

0,0010,0020,0030,0040,0050,0060,0070,0080,0090,00

Denm

ark

Germ

any

Finlan

d

Sweden

Portu

gal

Austri

a UK

Luxe

mbu

rg

Spain

Italy

Franc

e

Nethe

rland

s

Irelan

d

Belgium

Greec

e

J�'0�LQK��GD\

)LJXUH���� sludge production in Member States (g DM/inh./day)

3 Source: report of the European Commission on the implementation of the European legislation concerning

wastes for the period 1995-1997. Missing data completed from the study on the situation of the agricultural

use of sewage sludge in the European Union [ADEME, 1999].4 10 % corresponding to an average dry matter content of sewage sludge.

��

These differences reflect the diversity of national wastewater treatment systems, including theconnection rate of each country. It is also directly linked to the quality, availability, and type ofsludge taken into consideration in the statistical data.

6OXGJH�SURGXFWLRQ�LQ�$FFHVVLRQ�&RXQWULHV

In the Accession Countries, the production level is more difficult to assess because ofheterogeneous statistical systems, and the low reliability of the data. However, the production isdirectly linked to the national equipment level. Figure 6 shows the amount of sludge produced inthe Accession Countries, when data is available. The values are between 400 tons in Malta and330 000 tons in Poland (assuming a dry matter level of 10 %), which is the country with the largestpopulation.

050 000

100 000150 000200 000250 000300 000350 000

Poland

Czech

Rep

ublic

Sloven

ia

Slovak

ia

Estonia

Litua

nia

Hunga

ry

Latvi

a

Cypru

sM

alta

W�'0

)LJXUH���� sludge produced in the Accession Countries (t DM)5

The sludge production per inhabitant is presented in Figure 7, showing great differences betweencountries. Indeed, there is no correlation between the amount of sludge produced and thepopulation. An explanation of such diversity is to be found in the differences in equipment leveland connection rate to water treatment, but also to the relatively low reliability of the statisticaldata. For instance, there is presently no common classification of waste in the Accession Countries.

0,0020,0040,0060,0080,00

100,00120,00140,00160,00

Sloven

ia

Estonia

Cypru

s

Czech

Rep

ublic

Slovak

ia

Litua

nia

Latvi

a

Poland

Hunga

ryM

alta

J�'0�LQK�GD\

sludge production in the Accession Countries (g DM/inh./day)

5 Data source: screening realised by the European Commission or data collected for this study. No data were

available from Bulgaria and Romania.

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%R[����&RPSDULVRQ�RI�VOXGJH�VSUHDGLQJ�ZLWK�RWKHU�IHUWLOLVDWLRQSURFHVVHVVarious fertilisation media in the Member States have been compared and are presented below.

The amount of nitrogen and phosphorus brought by the animal manure has been calculated using theEurostat figures for animal manure production in the Member States, and applying a coefficient toassess N and P content. This coefficient can differ among countries in the European Union, and a studyis currently being carried out by Eurostat to compare those figures. However, the results of this studywere not available when this report has been written. Therefore, we used coefficients from the FrenchCORPEN (Committee for the reduction of the water pollution by nitrates and phosphorus).

We have established an extreme scenario by assuming that all sludge produced in a country would beused in agriculture, although the disposal and recycling routes can greatly differ among the MemberStates. It must also be reminded that the amount of mineral fertiliser used may sometimes be higher thanwhat it is needed. If only “best practices” would have been considered, this amount would certainly bereduced.

0%

20%

40%

60%

80%

100%

Fin

land

Gre

ece

Fra

nce

Sw

eden

Ger

man

y

Italy

Den

mar

k

Spa

in

UK

Net

herla

nds

Irel

and

Por

tuga

l

Aus

tria

Bel

gium

&

N Animal manure

N Sludge

N Mineral Fertilizer

)LJXUH���� sources of nitrogen fertilisation in the Member States

In all Member States, sludge fertilisation is the least used media.

In the case of nitrogen fertilisation, the amount due to sludge use in agriculture is between 0,1 % inGreece and 2,4 % in Germany. However, considering the low figures, there is no strong differenceacross Member States.

Regarding phosphorus, the part of fertilisation from sewage sludge is between 0,2 % in Greece and 10%in Germany. Unlike nitrogen, there is an important heterogeneity between Member States, as theamount of phosphorus brought by mineral fertilisation is between 25 and 65 %. Phosphorus is a criticalfactor in fertilisation, and sludge utilisation in agriculture occurs on the basis of their phosphoruscontent in several countries.

0%

20%

40%

60%

80%

100%

Greec

e

Finlan

dIta

ly

Franc

eSpa

in

Portu

gal

Sweden UK

Irelan

d

Austri

a

Germ

any

Belgium

& L

uxem

bour

g

Nethe

rland

s

Denm

ark

P Animal Manure

P Sludge

P Mineral Fertilizer

)LJXUH���� sources of phosphorus fertilisation in the Member States

��

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Wastewater collection

Water treatment

Sludge treatment

Sludge recycling

and disposalImpacts

Sludge produced by wastewater treatment plants is usually processed to reduce the water content ofthe sludge, its fermentation propensity and pathogens content. The different steps of the sludgetreatment are described in the table 12. The different treatments, which will be performed on sludgewill depend on its further disposal or recycling.

6WHSV 7\SHV�RI�SURFHVVHV 2EMHFWLYHV

&RQGLWLRQLQJ Chemical conditioningThermal conditioning

- Sludge structure modification

- Improvement of further treatment

7KLFNHQLQJ Gravity thickeningGravity belt thickenerDissolved air flotation

- Obtain sufficient density, strength andsolids content to permit hauling forfurther disposal process

- Reduce the water content of the sludge

'HZDWHULQJ Drying bedsCentrifugingFilter beltFilter press

- Reduce the water content of the sludge

6WDELOLVDWLRQDQG�RUGLVLQIHFWLRQ

Biological processes:$QDHURELF�GLJHVWLRQ$HURELF�GLJHVWLRQ/RQJ�WHUP�OLTXLG�VWRUDJH&RPSRVWLQJ

Chemical processes:/LPH�WUHDWPHQW1LWULWH�WUHDWPHQW

Physical processes:7KHUPDO�GU\LQJ3DVWHXULVDWLRQ

- Reduce the odour generation

- Reduce the pathogen content of thesludge

7KHUPDO�GU\LQJ DirectIndirect

- Highly reduce the water content

7DEOH����� the different steps of sludge treatment

A short description of existing treatment processes usually performed in Member States andAccession Countries is provided below. For each of them, a classification is possible between batchand continuous processes. Batch processes necessitate reaching a given quantity of sludge beforeperforming the treatment. On the contrary, continuous processes allow uninterrupted operation.

��

���� &RQGLWLRQLQJ

A preliminary phase of chemical or thermal conditioning may be conducted to improve furthersludge thickening or dewatering.

Chemical conditioning is realised by using mineral agents such as salts or lime, or organiccompounds (polymers).

Thermal conditioning consists of heating sludge to 150-200 °C for 30 to 60 minutes. Heat changesthe physical structure of the sludge, helping further dewatering. However, as part of the organicmatter may be hydrolysed during the process, it could trigger offensive smells and high pollutedfiltration or centrifugation water during the dewatering steps. It is possible to perform partialthermal conditioning by heating at a temperature of 40 to 50 °C. This solution reduces thecontamination of centrifugation and filtration water. The advantages and disadvantages of each ofthose possibilities are summarised in the table below.

&RQGLWLRQLQJ $GYDQWDJHV 'LVDGYDQWDJHV

&KHPLFDO�PLQHUDO�DJHQWV�

- Improvement of the cohesionand the density of the sludge

- Increase in sludge amount- Reduction of the organic matter

content- Slow reaction

&KHPLFDO�RUJDQLF�DJHQWV�

- Reduction of the mass of sludge,- No modification of the

agricultural value- Lower quantities to be used- Easy to handle and transport

- Costs of the products

7KHUPDO - May be applied to all sludge- Efficient and stable process- Stabilisation and disinfection- Lower sludge amount

- Energy consumption- Odours- Increase in the pollution load of

the filtrate7DEOH����� comparison of the different conditioning processes

���� 7KLFNHQLQJ

Thickening is a first step to reduce sludge water content. Sludge reaches 10 to 30 % dryness, andcan still be pumped. Various existing techniques are presented below.

*UDYLW\�WKLFNHQLQJ

Gravity thickening is a widespread technique, performed in tanks usually fitted with a rotatingploughing system. The gravitational forces bring the thickened sludge at the base of the tank fromwhere it is extracted. Water is collected at the top. The process is capable of thickening the sludgeby 2 to 8 times, bringing it from a few grams/litre to a few tens of grams/litre.

Performing costs are relatively low, as only an electricity supply is needed to operate the harrowand the pumps. The energy consumed is about 5 kWh/t DM.

*UDYLW\�EHOW�WKLFNHQLQJ

The gravity belt thickener consists of an endless filter belt on which thickening takes place in threephases: conditioning, gravity drainage and compression. Flocculated sludge is fed onto the belt and,as it moves along, water passes through the weave of the belt. At the discharge end of the machine,the sludge is further thickened by the compression caused by it being turned over onto itself. Ahigh-pressure wash station continuously washes the belt.

Sludge thickening with a gravity belt thickener is made possible by the addition of polyelectrolyteto the sludge.

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Gravity belt thickeners are used for all types of sewage sludge, although they are at their mosteconomical when handling sludge of less than 1 % DM feed and thickening to 6 % DM.

Primary sludge can be thickened to 10 % DM at which point it is difficult to process furtherwithout expensive pumping systems. Activated sludge is normally thickened to 5 % DM.

Performing a gravity belt thickening requires about 50 kWh/t DM and water.

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The technique of air flotation can be used when the solid particles have a low rate of settlement,and in sewage sludge treatment the process is used to thicken surplus activated sludge.

The specific gravity of fine suspended solids is lowered by the attachment of micro-bubbles andbrought to the surface, where the thickened sludge is removed by a scraper. Its application insewage sludge treatment involves dissolving air under pressure and subsequently releasing thepressure in the flotation vessel. Adding a polymer is sometimes needed, when it is necessary toreduce the matter in suspension.

The performance of this process is higher than the one of the gravity thickening. However, theenergy costs are also higher: 100 to 130 kWh/t DM.

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The different thickening systems are compared below

$GYDQWDJHV 'LVDGYDQWDJHV

*UDYLW\�WKLFNHQLQJ - Easy to perform- Low energy consumption- Low investment costs

- Needs important room- Low performance on

biological sludge*UDYLW\�EHOW�WKLFNHQLQJ - Easy to perform

- Compact- Work force need- Cleaning water consumption- Polymer use compulsory

'LVVROYHG�DLU�IORWDWLRQ - Easy to perform- Little room needed- Little H2S emission

- Not adapted to variableregimes

- High energy consumption7DEOH����� comparison of the different thickening processes

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Dewatering is the following step after thickening, and allows further reduction of the sludge watercontent. Dewatered sludge has a dry matter content of up to 30 %.

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One of the simplest techniques for dewatering sewage sludge is the open air drying bed. Thistechnique is used mainly on small WWTPs whenever sufficient inexpensive land is available andthe local climate is favourable for year-round operation of the beds. This technique may be lessefficient in cold climates.

It consists of a sand and gravel area about 0.3 m thick on which sludge is spread. The water isdrained and sent to the head of the plant. The sludge is then atmospherically dried.

This process allows a DM content of 40 to 50 % to be reached in some countries, depending on theduration of the drying. This level is reduced to 10% in Nordic countries. It offers the potential oflower operating costs and minimal maintenance requirements, which may offset the disadvantageof high land requirements, weather dependency, and potential odours. However, this technique isrelatively labour intensive.

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Centrifuging is a mechanical process that uses centrifugal forces to separate the thickened sludgefrom the centrifugate. Centrifuges are used in dewatering applications because they are compact,have high throughput capacity, and are simple to operate. Solid-bowl and basket centrifuges are themost commonly types used.

It is possible to use centrifuging either as a thickening process or as a dewatering process.

The process can produce increases in the dry matter of up to 15 to 25 %. It is also possible to use ahigh performance centrifuge, gaining an additional 5 %.

However, the energy needs of this process are significant: from 25 to 80 kWh/t DM, and it is alsonecessary to add a polymer to the sludge.

)LOWHU�EHOW

In the filter belt process, the sludge, mixed with a polymer, is dewatered on the same principle asgravity belt thickening. It is then pressed between two belts. There are different kinds of machinesavailable, depending on the level of pressure applied to the sludge (low, middle or high pressure,respectively about 4, 5 and 7 bars). The process may be combined with a gravity belt thickening.

It is possible to increase the level of dry matter by 10 to 20 %, depending on the type of sludge andthe pressure applied.

To perform this process, costs include polymer, water and energy (about 35 kWh/t DM).

)LOWHU�SUHVV

It is possible by using this technique to reach a high dewatering level, between 30 to 45 %generally. The investment costs however are quite high, especially for high capacities.

Plate and frame filter presses are commonly used to dewater sludge. Conventional filter pressesconsist of rows of vertical plates between which sludge is injected under pressure. The filtrate iscollected before separating the plates. The sludge cakes then fall and are collected. In some cases,membranes are placed between the plates, which can be filled with water in order to improve thedewatering rate. In this case however operating costs are significantly higher.

A preliminary conditioning is usually required either with salts or lime. Electricity needs are about30-40 kWh/t DM. Investment costs are reduced with increasing capacities.

&RPSDULVRQ�RI�WKH�GLIIHUHQW�GHZDWHULQJ�SURFHVVHV

The different dewatering processes are compared below.

��

$GYDQWDJHV 'LVDGYDQWDJHV

'U\LQJ�EHGV - Easy to operate- Adapted to small WWTP- Functions throughout the

year- Low operation costs- High DM content reached

- Land requirement- Weather dependency- Risk of odours- Workforce requirements

&HQWULIXJLQJ - Continuous operation- Compact- Possible automation

- Specialised maintenance- Sludge texture- Noise- High energy consumption- High investment costs

)LOWHU�EHOW - Continuous operation- Easy to perform- Moderate investment costs

- Limited water contentreduction

- Cleaning water consumption- Supervision necessary

)LOWHU�SUHVV - High water content reduction- Structure of the sludge- Possible automation

- Discontinuous operation- Low productivity- Consumption of mineral

conditioner- Supervision necessary- High investment cost

7DEOH����� comparison of the different dewatering processes

���� 'U\LQJ

The drying of sludge allows:

- the elimination of the interstitial water, to reduce the volume of the sludge,

- stabilisation, and disinfection when DM exceeds 90 %,

It is also done in order to:

- increase the calorific value of the sludge, before thermal oxidation,

- allow spreading using techniques similar to those used for mineral fertilisers,

- reduce the transportation costs.

Drying is a thermal treatment. Heat can be transferred either directly or indirectly to the sludge. Inthe first case, it requires an intensive contact between gas and sludge material. The most importanttypes of dryers are the revolving drum dryer and the fluidised bed dryer. In the second case, heat istransferred to the material to be dried via heat conduction through a heat transfer surface. Thus, theheating medium is not in contact with the sludge. The drying takes place at different temperatures.However, at higher temperatures (> 300 °C), it must be carefully controlled to ensure that there isno formation of dioxin and furan compounds.

The level of DM reached is between 35 and 90 %. Partial drying enables a DM content of 30 to 45% to be reached at which percentage it is possible auto-combust the sludge. Those processes inhibitthe re-growth of bacteria, mainly because of the reduced moisture level which may be reached.

Energy requirements for drying are much higher than dewatering when comparing volume ofextracted water (tEW). Therefore in most cases, drying takes place after a dewatering phase.Energy needs for electricity or gas are shown in the table below:

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3DUWLDO�GU\LQJ�������'0� 7RWDO�GU\LQJ�������'0�

)XHO� �/�W�'0� 120 300

(OHFWULFLW\� �N:K�W�'0� 30 50

7DEOH����� energy needs of the drying treatments [OTV 1997]

Energy requirements are important, but may be strongly reduced if an energy source is available onsite (biogas or steam).

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The stabilisation aims at reducing the fermentation of the putrescible matter contained in the sludgeand the emission of odours. Disinfection consists of eliminating pathogens.

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Applied to thickened sludge, anaerobic digestion (also referred to as methanisation) aims atreducing, stabilising, and partially disinfecting the treated volume of sludge. It consists of confiningthe sludge in a vessel at a temperature of about 35 °C.

The anaerobic digestion is divided in three main phases:

- hydrolysis of the macromolecules in smaller components

- production of acidic compounds from those smaller compounds

- gasification, generating carbon dioxide and methane.

The table below shows the input and performance parameters of the anaerobic digestion.

3DUDPHWHU 7\SLFDO�YDOXH

Volumetric (d) 15-18

Volatile solids loading (kg VS/m3.d) 0.8-1.6

Solids loading (kg SS/m3.d) 1.0-2.0

Operating temperature (°C) 30-37

pH 6.6-7.5

Feed sludge concentration (%DS) 3-8

Total solids destruction (% input load) 30-35

Feed sludge volatile compounds (% of solid input) 70-80

Volatile solids destruction (% of VS input) 40-50

Gas production (m3 /kg VS destroyed) 0.8-1.2

7DEOH����� input and performance parameters of the anaerobic digestion [CIWEM 1996]

The biogas produced is often reused in boilers, to maintain a temperature around 35 °C. It may alsobe used to produce electricity on the plant.

It is usually recommended to have the sludge remaining in the digester for more than 20 days, toguarantee a good stabilisation and disinfection.

Other techniques based on the same process exist. Differences consist in the medium or hightemperature used for operation.

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Sludge is placed in a vessel with aerobic micro-organisms. Heat is generated when these bacteriadegrade organic matter. In adequate conditions, the temperature can rise to over 70 °C. Bysubjecting sludge to these high temperatures for a particular period of time, most harmfulorganisms are destroyed. It is usual to subject the sludge to a temperature of 50 to 65 °C. for 5 to 6days. In these conditions, volatile matter is reduced by about 40 %. The process is simple in designbut has a high energy cost: 5 to 10 times more than anaerobic digestion.

The influence of the process on the sludge composition has been given in chapter 3 (table 4).

/RQJ�WHUP�OLTXLG�VWRUDJH

Storage of sludge has two essential purposes: regulating the flows of sludge to agriculture andhomogenising its composition.

Odours may arise, and an increase of the dry matter and a reduction of the organic matter havebeen observed. A reduction of the nitrogen content also takes place: nitrogen is converted intoammonium and to ammoniac in a gaseous form, resulting in decreasing the agricultural value of thesludge.

Long-term storage of sewage sludge has a disinfecting property, reducing the amount of virusesand bacteria in sludge. Its efficiency depends on the duration of the storage. However parasites arethe most resistant pathogens and it has been reported that long term storage would not affect theirinfectious potential. In cold climates, this process does not enable to reach a sufficient level ofdisinfection.

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Composting is an aerobic process consisting of aerating sludge mixed with a co-product such assawdust or animal manure. Composting produces excess heat, which can be used to raise thetemperature of the composting mass. The mix then evolves for several weeks. There are three typesof processes:

- Windrow: the sludge cake is mixed with a bulking agent and set out in piles. The compostingmaterial is turned mechanically to introduce air and prevent excessive temperatures. Thissystem requires a large land area.

- Aerated static piles: the sludge is mixed with a bulking agent and laid over perforated pipes, oron a floor through which air is blown.

- Vessel systems: after mixing with a bulking agent, sludge is injected at the top of a vessel,where a harrow permits an equal repartition. In the lower part, air is injected, and the endproduct is extracted at the base of the vessel.

The composting process is used to several purposes. Composted sludge presents a higheragricultural value (see chapter 3), reaches a good level of disinfection, and is stabilised, reducingtherefore the arising of odours. It also has a humus-like aspect, which, together with the reductionof odours, makes the acceptance of its use easier. Lastly, composting is used to reduce the watercontent of the product, as it may reach over 60 % of dry matter, making also its handling easier.

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Lime treatment consists of the addition of lime to sludge, in order to raise its pH to 12, thusdestroying or inhibiting the biomass responsible for the degradation of the organic compounds. Thetreatment helps also disinfecting sludge, increasing its dry matter content and making handlingeasier.

The dry mass increase depends on the initial dry matter content and the amount of lime supplied. Itis usually recommended to add 30% of lime to the dry mass of sludge, otherwise the treatmentwould not avoid fermentation in the long term.

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Lime treatment is not recommended when sludge is incinerated in a fluidised bed incinerator, as itmay disturb its good operation.

The necessary energy to perform this treatment is about 5 kWh/t DM, used for pumping andmixing.

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Nitrite treatment consists of maintaining sludge in an acid environment (about pH 2 or 3 accordingto the level of treatment expected) for 30 minutes where it undergoes the action of nitrite ion.

This treatment is an efficient stabilisation process. Sludge may be stored for several monthswithout generating odours. Concerning disinfection, two levels may be reached: partial disinfection(pH 3; bacteria are eliminated) or advanced disinfection (pH 2; spores are eliminated as well).

This treatment is performed on thickened sludge. Its impact on the sludge structure facilitatesfurther dewatering.

Nitrite treatment is adapted

- to small WWTP, where lime treatment could be expensive,

- when lime treatment is not possible before land spreading,

- before incineration as it improves its auto-combustibility.

3DVWHXULVDWLRQ

Pasteurisation consists of heating the sludge to a temperature of 70 to 80 °C for a short period(about 30 minutes). This treatment allows reduction of the amount of pathogens in the sludge, butcan not be considered as a stabilisation process in itself.

0DLQ�IDFWRUV�GXULQJ�WKH�GLIIHUHQW�VWDELOLVDWLRQ�DQG�GLVLQIHFWLRQ�WUHDWPHQWV

Generally speaking, the decrease in the micro-organism population follows an exponential kinetic.This kinetic depends on a specific decay rate, which depends on the type of micro-organism, andexogenic factors, including temperature, moisture, duration and pH. The table below (grey areas)indicates the main factors to be controlled when performing the different kinds of stabilisation anddisinfection treatments.

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7DEOH����� deciding factors of the stabilisation and disinfection treatments

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Stabilisation and disinfecting treatments described above do not have similar effects on micro-organisms and pathogens. The table below gives a summary and their best conditions of practice.

7UHDWPHQW &RQGLWLRQV

9HU\�HIIHFWLYH�WUHDWPHQWV

Thermophilic anaerobic digestion 55 °C ; 10 days

Thermophilic aerobic digestion 55 °C ; 10 days

Composting 50 – 60 °C ; 15 – 30 days

Pasteurisation 70 – 80 °C ; 30 min

Lime treatment pH 12, 10 days, (30 % addition to DM)

/HVV�RU�ORZ�HIIHFWLYHQHVV�WUHDWPHQWV

Psychrophilic anaerobic digestion 20 °C ; 30 days

Psychrophilic aerobic digestion 20 °C ; 30 days

Chemical conditioning and mechanical dewatering -

7DEOH����� effectiveness of the stabilisation and disinfection treatments6

When assessing the quality of sludge after treatment, difficulties arise relating to the facts that:

- it is not possible to test all pathogens, as there may be a wide range of micro-organismspresent in sludge,

- tests are in some cases expensive,

- results may not be available before one day, or in some cases several weeks.

Therefore, monitoring indicators are being developed, which have to safely describe the level ofdisinfection reached in the sludge after treatment. Indirect parameters may be used, depending onthe disinfection treatment used. These may be the temperature, pH, reaction time, moisture level,number of turnings etc. This enables a reduction in the pathogen analysis. The literature alsosuggests favoring the analysis of certain micro-organisms, thus determining:

- the level of some micro-organisms which behave in the same way as pathogens, areconsistently present in sludge, and are easy to cultivate and identify

- the level of some particular pathogens, based on the assumption that if the most resistantpathogens are destroyed, the less resistant will be destroyed as well.

WRc [2001] suggested using (��FROL or &ORVWULGLXP�SHUIULQJHQV for this purpose.

6 Source: ADEME 1994

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%R[����6OXGJH�WUHDWPHQWV�LQ�WKH�0HPEHU�6WDWHVAccording to the information provided by the Member States to the Commission7,

• In Germany, many different techniques are used, but no information was given about theprevailing processes.

• In Luxembourg, sludge is digested and then conditioned with lime or iron salts. Mechanicalprocesses are used for dewatering. Polyelectrolytes are added to sludge which are notconditioned in order to facilitate dewatering.

• In the Walloon region of Belgium, sludge is digested, aerobically stabilised, mechanically orthermally dried, or conditioned with lime or polyelectrolytes. No information was availablefrom the Flemish region.

• In the United Kingdom, used methods are mesophilic anaerobic digestion, composting, limestabilisation, liquid storage, dewatering and thermal drying.

• In Denmark, sludge is digested in a heat digestion chamber or in a bioreactor, stabilised byaeration, composted (in controlled conditions during two weeks at a temperature of 55°C),conditioned with lime or pasteurised at a temperature of 70°C during one hour.

• In France, sludge is subject to prolonged aeration, aerobic or anaerobic stabilisation, limeconditioning, thermal drying or composting.

• In Finland, sludge is anaerobically digested, or composted. Other methods such as aerobic orlime stabilisation are of decreasing importance.

• In Sweden, sludge undergoes following techniques: thickening (gravity thickening or flotation)stabilisation (aerobic, anaerobic, lime), conditioning, dewatering (centrifuge, filter belt press, airdrying), thermal drying and composting.

• In Ireland, sludge is dewatered (filter belt or centrifugation) before landfilling, or undergoesanaerobic digestion. Over 60% of the sludge used in agriculture originates from the DublinWWTP, where sludge is dewatered (centrifuge) and thermally dried.

• In Portugal, the technologies employed are drying beds (drainage on sand bed and evaporationof humidity), thickening, mechanical dewatering (band filters, filter presses, vacuum filters orcentrifuge) and various stabilisation processes.

No additional data is currently available about treatments carried out before stabilisation anddisinfection (dewatering, and thickening).

No data is available in the report mentioned above concerning sludge treatments in Greece.However Tsagarakis HW�DO. [1999] documented the management of sewage sludge from WWTP inGreece. It appears that sewage sludge mainly undergoes gravity thickening and dewatering throughdrying beds or mechanical dewatering with bed filters. Digestion is employed mainly in largeconventional activated sludge plants. Composting is only performed in one WWTP in Greece.

7 European Commission; 1999; Report from the Commission, Implementation of Council Directive

91/271/EEC of 21 May 1991 concerning urban wastewater treatment, as amended by Commission

Directive 98/15/EC of February 1998; COM(98)775, http://www.europarl.eu.int/basicdoc/basicdoc-en.htm

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Wastewater collection

Water treatment

Sludge treatment

Sludge recycling

and disposalImpacts

In this part, the operation of the different recycling and disposal routes for sewage sludge isdescribed, before summarising the inputs, outputs and impacts of each of them.

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5.1.1 Technical description

Landspreading is a way for recycling the compounds of agricultural value present in sludge to land.All sludge types (liquid, semi-solid, solid or dried sludge) can be spread on land. However, the useof each of them induces practical constraints on storage, transport and spreading itself.

The sludge production from a given WWTP is more or less constant throughout the year, but theuse on farmland is seasonal. Therefore, storage capacity must be available on the WWTP or on thefarm, either separately or in combination with animal slurry when allowed by the nationalregulations. Average storage duration is about 6 months. Storage on fields may also be practicallyobserved. This however should only be performed shortly before spreading, and with solid andstabilised sludge in order to reduce risks of leaching.

Liquid sludge may be stored in concrete tanks (mostly for small WWTP) or lagoons. It can bepumped to be transported.

Semi–solid sludge may be stored on a platform, which must be waterproof, or in tanks. Sludge pitsmay also be found. As in most cases this type of sludge cannot be pumped, sludge has to beconveyed by using specific hauling equipment such as grabs. Odours may arise when sludge ishandled to be conveyed.

The structure of solid sludge enables storage on piles. Handling implies the use of a crane or atractor.

Dried sludge does not present any specific constraint. If sludge however is pulverulent, storagemust be monitored in order to prevent any explosion and emission of particles to air.

Transportation is the most expensive aspect of this route. It is possible to use tankers for liquidsludge or articulated lorries for other sludge types.

Sludge can be applied to the fields by using trailer tank or umbilical delivery system and may beapplied by surface spreading (it is however of importance to reduce the formation of aerosols toreduce the risk of odour nuisance) or directly injected into the soil. Dried sludge may be suppliedby using the same equipment as for solid mineral fertilisers. The spreading equipment has also tobe adapted to the type of sludge.

Culture types, soil occupation, accessibility of the field, meteorological conditions influencelandspreading. Mostly, the practice can be performed at two times in the year: at the end ofsummer, after harvesting, or in spring, before ploughing and sowing.

As already stated, sewage sludge contains compounds of agricultural significance such as nitrogen,phosphorus, potassium, organic matter or calcium, making its use relevant as an organic fertiliser.Moreover, the cost of this route may be cheaper than other disposal routes. However, the presenceof pollutants in sludge implies that the practice should be carefully done and monitored. To thispurpose, in some countries, codes of practice and spreading schemes have been established,summarising the regulatory obligations. Periods for spreading, types of culture, adequate recordkeeping are described in order to manage the sanitary and environmental risks.

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5.1.2 Impacts and benefits

Inputs of landspreading are sludge and additional resources such as petrol for transportation andapplication, and room for storage.

Outputs consist of yield improvement and fertiliser substitution, but also emission of pollutants tosoil, and indirect emissions to air and water. Other emissions to air are exhaust gases fromtransportation and application vehicles. Dried sludge may also be emitted to the air duringtransportation when trucks are not covered.

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)LJXUH���� representation of the inputs and outputs for landspreading

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As already stated in part 3, sludge contains compounds of agricultural significance such asnitrogen, phosphorus, potassium and eventually calcium. Sludge application therefore replacesconventional fertiliser application. It also contains organic matter, but usual application rate isbelow that which would have a significant positive impact on soil structure.

Landspreading also involves the application to soil of the pollutants contained in sludge. Thosepollutants undergo different transformations and transfer processes, which are more preciselydescribed in chapter 6. Among those processes, leaching, runoff, volatilisation could enable thetransfer of the compounds into the air and water, and their introduction into the food chain.

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Disamenities may take place because of landspreading operation odour. Operation accidents canalso happen, generating an increase in the emissions to soil and a possible reduction of agriculturalyields.

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5.2.1 Technical description

Incineration is a combustion reaction. Different types of incineration may be considered, separatedin the following categories, according to the draft directive on the incineration of wastes:

- mono-incineration when sludge is incinerated in dedicated incineration plants,

- incineration with other wastes, mainly household wastes,

- co-incineration when sludge is used as a fuel in plants whose purpose is the generation ofenergy or production of material products such as coal power plants or cement plants,

Other processes such as wet oxidation or pyrolysis are developing technologies involving thermaloxidation, and are described in a following part of this report. Figure below summarises allpossible thermal oxidation routes for sewage sludge.

Dewatering

Drying

0RQR�LQFLQHUDWLRQ,QFLQHUDWLRQ�DQG�FR�LQFLQHUDWLRQ $OWHUQDWLYH�SURFHVVHV

- Multiple Hearth Furnace- Fluidised Bed- Combined MHF-FBC- Cyclone Furnace- Smelting Furnace- Rotary Furnace

- With municipal solid waste- With coal in power plants- With other fuels- In other processes:

- Wet oxidation- Pyrolysis- Gasification- Pyrolysis-gasification- Pyrolysis- combustion

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- In cement production- In brick making- In asphalt works

)LJXUH����� different routes and related technologies of thermal processing of sludge [afterWerther and Ogada 1999]

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As described in the preceding figure, several types of furnaces may be used for sludge mono-incineration. It is however increasingly performed in fluidised bed incinerators. This is mainly dueto the fact that:

- this process permits virtually complete combustion at relatively low temperatures,

- the amount of inert bed material prevents sudden temperature changes in the furnace,

- intermittent operation is possible,

- maintenance costs are low [Werther and Ogada 1999].

Moreover Mininni [2000] described the partitioning of Cr, , Pb, Sn and Zn during sludgeincineration. Cd, Pb, Sn, and Zn showed enrichment factors from 4 to 26 in the fly ash produced inrotary kiln furnace tests, in contrast with negligible enrichment factors measured in fluidised bedtests. Therefore, since metal emissions into the atmosphere are strictly related to the enrichmentfactors in the fly ash, those test suggest that, from the environmental point of view, sludgeincineration with fluidised bed furnaces is safer than rotary kiln furnaces.

The fluidised bed system consists of a combustion chamber lined with a refractory material at thebase of which a bed of sand is brought to a high temperature and held in suspension by hot air.

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Sludge is introduced inside or above the bed of sand and burnt at a temperature of 900 °C, for a fewseconds. The process is described in the figure below.

1 Sludge feed2 Supplementary combustible3 Combustion gas4 Flue gas5 Fluidised bed6 Combustion chamber7 Air admission chamber8 Window9 Heat exchanger10 Flue gas treatment

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)LJXUH����� operation of a fluidised bed incinerator [source: mg engineering, Lurgi Envirotherm]

In order to reduce the consumption of extra-combustibles, a heat recovery system is generallyinstalled. It warms the air that is injected into the combustion chamber.

For the incinerator to operate adequately, it is necessary to have sludge with a dry matter content of30 to 45 %. These kinds of incinerators can operate 24 hours a day, without specific supervision.

Designated incinerators may be installed on site in a WWTP, when handled capacities justify sucha cost intensive technique. However, designated incinerators may be shared, and therefore burnsludge from different origins. This practice is more or less widespread in Europe and depends onthe country8.

Flue gas has to be treated, to remove acid gases, heavy metals under gaseous and particulate form,and dust. Different treatments are possible for flue gas, such as electrostatic precipitators and bagfilters to remove particulate matter, scrubbers and wet processes for acid gases. They may becombined. It is interesting to observe that sludge quality and the fluidised bed incinerationtechnology often do not imply a specific treatment of nitrous oxides and dioxins [OTV 1997]. Thechoice of system depends on the emission limits, which have to be reached, and the possiblerecycling of the ashes.

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Incineration consists of using already existing installations, usually the ones designed for municipalwaste incineration, limiting additional investment. The technique is especially attractive if theincinerator is settled near the WWTP.

If the calorific value of the sludge is similar to that of municipal wastes (about 60-65 % DM [OTV1997]), sludge can easily be added to the waste. When sludge is dry, it must be carefully mixed to

8 Source : OTV and EEA

��

the waste, to avoid accidents during combustion, such as explosions. It is also possible to introducethickened sludge, reducing the treatment costs (dewatering and/or drying costs). In this casehowever, a reduced calorific value implies to restrain the proportion to waste (about 20 % of thetonnage).

There are different techniques for injecting the sludge: sludge can be mixed before the combustionwith the waste, injected under pressure in the furnace or at the exit of the combustion chamber.

The investment costs are much lower than in the case of mono-incineration, as the process onlynecessitates a modification of an existing installation (sludge injection system, eventually sludgetreatment on site).

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Other possibilities of incineration of sewage sludge are the use as fuel in coal-fired power plantsand cement kilns. The main interest of sludge in cement production is its calorific value.

When using sludge as a fuel in cement production plants, the maximal sewage sludge feed rateshould not be more than 5% of the clinker production capacity. Consequently, for a 2000 t/daycement kiln, a maximum of 100 t/day dry sludge might be used, without worsening the clinkerquality [Werther and Ogada 1999]. Other sources9 however recommend that cement quality shouldbe controlled to ensure that the sludge does not impact the mechanical properties of the product.Gas and effluents of the plants have also to be monitored.

Full-scale tests have been performed in power plants in Germany, Netherlands and Belgium. Thetests showed that co-combustion had little effect on the emission of gases; a slight increase in theheavy metals content of the ash has been reported. Examples of the emissions from the Franken IIpower plant near Erlangen in Germany are shown in the table below.

8QLW 0HDVXUHG�YDOXHV /LPLW�YDOXHV��

7RWDO�GXVW mg/m3 2.0 3012[�DV�12� mg/m3 195.0 30062[�DV�62� mg/m3 64.0 200+&O mg/m3 6.0 -+) mg/m3 1.2 -7RWDO�& mg/m3 < 3.0 -&2 mg/m3 18.0 -7O��&G mg/m3 0.002 0.05+J mg/m3 0.002 0.056E��$V��3E��&U��&2�&X��0Q��1L��9��6Q

mg/m3 < 0.05 0.5

3&''�) ngTE/m3 < 0.0004 0.17DEOH����� emission values of the Franken II power plant near Erlangen (220 MWh), Germany

[VDI 2000]

5.2.2 Impacts and benefits

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In order to operate an incineration plant, necessary inputs are wastes and/or sewage sludge. Thewater content of sewage sludge may be variable. Additional resources are needed, which aremainly:- water, which does not need to be of primary quality,

9 CEN TC 308, Recommendations to preserve and extend sludge utilisation and disposal routes.10 Directive 2000/76/EC of the European Parliament and of the Council of 4 December 2000 on the

incineration of waste.

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- fuels used for starting up the facility- auxiliary materials such as calcium carbonate, especially for flue gas treatment.

Outputs are the possible energy recovery, flue gases, ashes, and wastewater. Therefore incinerationgenerates emissions to air, soil and water, mainly located at the incineration site or at the landfillwhere ashes are disposed of. Emissions depend on the process, but also on the sludge type. In orderto reduce those emissions, treatments have to be performed on flue gas and water (see previouspart).

In most cases, the energy recovery will be counterbalanced by the energy used for reducing thewater content of sludge. Indeed, considering the water content of sludge,

- either dewatered sludge is burnt, and in this case the energy generated during combustion willcontribute to removing the remaining water contained in sludge. In some cases of mono-incineration additional fuel may even be needed,

- or, sludge may be dried before incineration, and in this case the energy recovered willcounterbalance the energy used for the purpose of drying.

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Following pollutants are found in the flue gas, i.e. before any treatment: particulates and fly ash,dioxins, heavy metals bound to particles or under gaseous form (mainly Hg, but also Cd), acidgases (SO2, HCl, HF), nitrogen oxides (NOX), carbon dioxide (CO2) and organic compounds boundto particles and volatile organic compounds (VOC). Flue gas treatment is performed in order toreduce their content in the fumes emitted to the air.

At sludge incineration temperature, dioxins and furans are completely destroyed, so that in theincinerator they are present in negligible concentrations. However, in the flue gas cleaning stages,where the gas temperature is below 450 °C, new formations of dioxins and furans may take place[Werther and Ogada 1999]. It has to be observed that in the case of mono-incineration, the amountof dioxins and NOX present in the raw flue gas is low enough to avoid the implementation of aspecific gas treatment process for these compounds [OTV 1997]. This is confirmed by Werther andOgada [1999], who observed that sewage sludge combustion is characterised by low net emissionsof NOx with the conversion ratio of fuel N to NOx being less than 5%.

Average composition of the flue gas is given in the table below:

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&RPSRVLWLRQ�RIWKH�IOXH�JDV

mg/Nm3

/LPLW�YDOXHV��

mg/Nm3

Daily average values

62� 300 to 3 500 50

+&O 50 to 400 10

&2 5 to 50 50

12; 50 to 200 200/400

'XVW 25 000 to 65 000 10

+) 0 to 6 1

'LR[LQV N/A 0.1 ng/m3

7DEOH����� composition of the flue gas of an incinerator (at the exit of the combustion chamberand before any flue gas treatment) [OTV 1997]

Once emitted into the air, pollutants are dispersed in the atmosphere. Their concentration dependson several factors, depending on the local conditions (climatic conditions, wind direction, windspeed, distance from the incineration plant) or the physical and chemical properties of thecompounds. Atmospheric deposition to the soil can also take place.

Lastly, emissions to air may be due to handling of ashes and combustion residues.

Emissions to air and especially dust, dioxins, heavy metals, VOC, NOx, CO and SO2 may haveadverse health effects. Those pollutants as well as CO2 can also have impacts on ecosystems andclimate change. Damages to buildings may also occur, particularly due to particulate matter, NOX

and SO2.

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Water emissions occur due to flue gas treatment, when a wet process is performed. However, watertreatment reduces the pollutant content of this wastewater. The pollutants present in this wastewaterare mostly the same as those released to the atmosphere with the fumes.

Emissions to water may also be caused by leaching of ashes disposed of to landfills.

Groundwater as well as surface water are concerned by those emissions, which can give rise toadverse health effects and ecotoxicity.

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Emissions to soil are due to the disposal of ashes or the flue gas treatment residues to landfill, orthe use of ashes in road construction. Bottom or grate ash are largely reused, whilst fly ash andresidues from the flue gas cleaning system are generally placed in hazardous waste landfills.

They are also the consequence of the atmospheric deposition of the pollutants emitted to theatmosphere. It must be observed that flue gas treatment residues contain much more pollutants thanash.

The composition of the ashes of a sewage sludge incinerator is given in table 22.

11 Directive 2000/76/EC of the European Parliament and of the Council of 4 December 2000 on the

incineration of waste

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proportion of dry weight %

6L2� 54.9

$O�2� 18.4

3�2� 6.9

)H�2� 5.8

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.�2 1.9

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7DEOH����� composition of ashes of a sewage sludge incinerator [Hudson HW�DO. 1992 in CIWEM1995]

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Disamenities may take place because of the operation of an incineration plant. Among those, noise,dust, odour and visual pollution may be evoked. Operation accidents can also happen, generatingan increase in the emissions to air, reducing the energy recovery, and leading also to health impactson operating personal.

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5.3.1 Technical description

So far, landfilling has been a major route for sludge disposal. However, it should be a limited outletin the future, because of the European legislation on the landfilling of waste which states that“Member States shall set up a national strategy for the implementation of the reduction ofbiodegradable waste going to landfills”12 no later than 16.07.2003. This solution is being chosenwhen no other way exists, that is:

- when concentration of contaminants makes it unsuitable for land spreading or other method ofrecycling,

- when farmland, forestry or land reclamation are not feasible owing to location or topographyor when the total costs would be uneconomic,

- if no incineration capacity is available on or near the site.

There are two possibilities for landfilling sludge: mono-deposits, where the landfill is only used forsludge, and mixed-deposits, when the landfill is used for municipal wastes as well. There is nospecific technical constraint in the conception of the landfill for the disposal of sludge. However,the conditions for disposal of sludge are set out in regulations in each country.

Waste deposit in landfill undergoes the following steps:

12 Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste

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- initial aerobic phase: the degradation first occurs under aerobic conditions, during which aerobicmicro-organisms consume the available oxygen in the deposit. This step is rather short (about14 days). The organic content of the leachate increases.

- Acetogenesis: as the level of oxygen decreases, acetogenic and fermentative bacteria decomposethe easily degradable material of the waste. The pH value decreases in the deposit, increasingconsequently the solubility of inorganic substances such as heavy metals. High organicpollution of the leachate is observed.

- Anaerobic methanogenesis: methanogenic bacteria proliferate during this phase, increasing theproduction of methane. The pH value increases, and the organic content of the leachatedecreases. The gas production then reaches a stable composition.

Little is known concerning the evolution of the deposit in the long term, that is more than 30 years.

When processing a mono-deposit, the compacted structure of the deposit in the cell is notfavourable for gas formation. However, should this happen, its composition would not be verydifferent from usual municipal wastes deposit: 50 to 60 % methane, 40 to 50 % carbon dioxide,plus trace elements.

In mixed deposit with municipal solid wastes, sludge is not the principal ingredient: its proportionreaches usually 20 to 25 % of the total deposit. Technically, disposing to landfill in mixed depositdoes not represent the same constraints (especially on water content) as in mono deposit. It is alsoto be observed that thanks to the physical structure of the deposit, the gas formation is enhanced,and the heavy metals content in leachate is reduced. Sludge can usefully be disposed of beforeclosure of the cell, in order to gain space.

5.3.2 Impacts and benefits

The inputs of landfilling are municipal solid wastes and sewage sludge, and additional resourcesneeded for the operation of the landfill site, such as fuel for vehicles, electricity, and additionalmaterials when leachate is treated on site.

Outputs consist of leachate, landfill gas and energy production when the gas is recovered. Landfilloperation generates therefore emissions to the air, soil and water.

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)LJXUH����� representation of inputs and outputs to a landfill site [after COWI 2000].

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Emissions to air are the release of landfill gas, when this is not recovered on site for energygeneration, and dust released during the handling of waste. Other emissions originate from theexhaust gas of the engines used on site.

The generation of landfill gas is of about 10 m3 per ton of deposited waste per annum, but maychange according to the dimension of the landfill, the input rate, the age of waste, and the physicaland chemical characteristics of the deposit. The main components of the landfill gas are methane(between 50 and 60 %) and carbon dioxide (between 40 and 50 %).

Many other VOC13s have been reported as traces, accounting in general for less than 1 % of thevolume of gas generated. The EEA [2000] reported the presence of 12 halogenated hydrocarbonsand about 30 hydrocarbons in landfill gas, with levels ranging from 0,02 mg/m3 to over 600 mg/m3.The amount of VOCs released to the atmosphere is lower when landfill gas is used or flared. In thiscase however, dioxins may be generated. Volatile substances migrate between the landfill and theatmosphere due to diffusion and pressure difference. VOCs originate from the waste, but newsubstances are also generated by the chemical and biochemical transformations occurring in thedeposit.

Carbon dioxide and methane have impacts on the climate, and trace compounds may be toxicand/or carcinogenic, with varying threshold values.

An issue of concern is the impact of landfills on the health of people leaving in the neighbourhoodof such an installation. Several studies have been conducted, not permitting to identify anyexposure pathway or causal link.

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Leachate generated within a landfill is emitted to soil and water, and the amount generated dependson the climatic conditions and landfill cover. The composition of the leachate varies over time. Itcontains several compounds such as ions (Ca2+, K+, Na+, NH4

+, CO32-, SO4

2-, Cl-), heavy metals,organic compounds (chlorinated organics, phenol, benzene, pesticides) and micro-organisms. It canalso contain dissolved methane, which is present in the landfill gas. Emissions may be reducedwhen leachate is collected on site and treated. Otherwise, leachate may leach through the soil to thewater table, or be directly released in surface water, and have impacts on human health andecosystems. Leaching depends on the physical and chemical properties of the compound, but alsoon the soil properties and environmental factors.

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Operation of a landfill generates other impacts in terms of- noise and dust from the delivery vehicles- odours- vermin, rats and birds- land use, disturbance of vegetation and landscape- in some cases, bad operation of a landfill can also cause fires, explosion, and accidental

emission of leachate or landfill gas.

13 Volatile Organic Compounds

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5.4.1 Use in forestry and silviculture

Forestry and silviculture refer to different kinds of tree plantation and use. The term forestry ismainly used when considering amenity forests, or mature forest exploitation. On the contrary,silviculture is more specifically used when referring to intensive production, such as energycoppices or poplar plantation. These definitions are used in this chapter.

Use of sewage sludge in forestry and silviculture appears to be an alternative to the recycling ofsludge to agricultural land. Relatively small areas could permit the spreading of an important partof the sludge production in Europe. For instance, 50% of the French sludge production could bespread on 1% of the forest area when considering a rate of application of 3 tDM/ha/year [INRA1999]. From an economical point of view, this route is of interest if areas are available forspreading in the neighbourhood of the WWTP, in order to reduce transportation costs and relatedpollution.

From the agricultural and environmental point of view, even if similarities between landspreadingand use in forestry may be observed concerning environmental impacts, great differences exist due,amongst other factors, to the specificity of the species grown, the fauna and flora involved, and thesoil types. The issue of sludge recycling to forestry and silviculture has not been addressed to thesame extent as its recycling to agricultural land, and much less information is available in theliterature concerning this outlet. Some conclusions can however be drawn about agriculturaladvantages and constraints as well as about environmental and sanitary impacts. Good practicescorresponding to the state of the actual knowledge may also be found in scientific literature.

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6OXGJH� DSSOLFDWLRQ�PD\�EH�SHUIRUPHG�DW� GLIIHUHQW� WLPHV� GXULQJ� WUHH� JURZWK. Landspreadingmay be done before plantation, when considering re-afforestation or implantation of an intensiveculture. In silviculture, sludge application may also be performed just after sowing or after eachcut. In forests, sludge application could occur practically all year round, in accordance with goodpractices and local conditions.

$�\LHOG�LPSURYHPHQW following sewage sludge application on forest soils has been reported in theliterature. In general, an LQFUHDVH�LQ�KHLJKW��GLDPHWHU�DQG�VXUIDFH�DUHD�FRYHUDJH of the trees havebeen observed. However, results depend upon the species, in some cases even on the individual,and on the local conditions [INRA 1999]. Yields are further improved when considering youngplantations. It must be observed that results refer to tests mainly performed with a single sludgeapplication, at very high rate of application [UBA 1998].

Another benefit which has been reported in the literature is the LPSURYHPHQW� RI� WKH� VRLOFRPSRVLWLRQ�LQ�WHUPV�RI�DJURQRPLF�YDOXH (especially Ca, Mg, S and trace elements), which is ofparticular interest compared to sludge spreading in agriculture, as forest soils are often particularlypoor in such compounds [INRA 1999].

Sewage sludge also contains organic matter, which may act as a soil improver (see part 3.4.1). Thechemical nature of the sludge organic matter (mainly easily degradable organic compounds)induces a fast mineralisation in soil. Therefore, VHZDJH�VOXGJH�SUHVHQWV�D�ORZ�KXPLF�YDOXH [INRA1999]. Moreover, as already mentioned above, it has been reported that the minimum thresholdlevel for the detectable effects of sludge additions on the physical properties of agricultural soil wasc. 5 tonnes of organic matter per ha, i.e. about 10 t dry solids/ha. Below this level of application,those benefits are not expected to occur. However, these statements concern agricultural soils, andmore information is needed concerning the specific impact of sewage sludge organic matter on thephysical properties of forest soil. It may be assumed that the main benefit of sewage sludge organicmatter would be its use by the micro-organisms present in the upper soil layer and humus and therelease of compounds of agricultural value during its degradation.

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Sludge spreading in forestry and silviculture however also presents disadvantages, mainly relatedto the high level of application mentioned in field tests. For instance, excess rates of liquid sludgeapplication could lead to the formation of dense, macropore-free barrier layers creating anaerobicconditions in humus and soil. As a consequence, KXPXV�DQG�XSSHU�VRLO�OD\HU�GHJUDGDWLRQ�PD\EH�REVHUYHG as well as GHVWUXFWLRQ�RI�KDELWDWV for soil biota and reduced microbial activity [UBA1998].

When sludge is spread on young plantations, it has been reported that FRPSHWLWLRQ with othervegetal species may occur, reducing the intake of compounds of agricultural value by young trees.Sludge spreading also induces an LQFUHDVH�LQ�WKH�SUROLIHUDWLRQ�RI�ZHHGV [INRA 1999].

Regarding practical constraints, application of liquid sludge in forested areas may be done by usingthe same spreading machinery as for landspreading (see figure 15). On the contrary, whenspreading other sludge types, either specific spreading machinery is necessary or the machineryused for spreading on agricultural land needs to be adapted, due to the presence of the trees, and inorder to avoid any degradation. Moreover, sludge application in timber or mature forest is onlypossible on forest paths, inducing an uneven repartition of the nutrients.

)LJXUH����� sludge spreading in forest (France)

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In a general manner, emissions to soil, air and water, as well as other environmental impacts aresimilar to those concerning agricultural landspreading. However, some differences have beenreported in the scientific literature.

As for sludge use in agriculture, DFFXPXODWLRQ�RI�KHDY\�PHWDOV�LQ�WKH�XSSHU� OD\HUV�RI�WKH�VRLO(mainly up to 10 cm) may be observed. Forest soils are often acidic and may therefore induce anincreased circulation of metals. The Austrian UBA [1998] reported that no leaching of heavymetals to groundwater has been observed in experimental studies but also that more research isneeded on this issue, especially concerning sandy or macroporous soils.

An issue of concern when spreading sludge in forests is QLWUDWH� OHDFKLQJ: the Austrian UBAreported that nitrogen loads which exceeded the forest stand’s capacity led to strongly increasednitrification and to increased leaching of nitrate together with potassium, magnesium and calcium.This could occur especially when sludge is applied all at once at high levels.

Some LQGLUHFW�LPSDFWV�RQ�ZLOGOLIH�HFRORJ\ have also been reported, since sludge application andrelated yield improvement increased the availability of food resources for numerous animal speciessuch as deer, small mammals and birds. Selective browsing of forest trees has been observed by

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animals trying to reach the nutrient-richer foliage, inducing injuries to mature trees and destructionof young plantations, as well as a related increased susceptibility to pests and pathogenic fungi.Overall, the number of species did not seem to be affected by sludge application [UBA 1998].

Sanitary impacts of sludge application are not well enough documented to draw any conclusions.7KH�ULVN�DQDO\VLV�VKRXOG�� LQ�DQ\�FDVH��GLVWLQJXLVK�IRUHVW�DUHDV�DFFRUGLQJ�WR�WKHLU�XVH: forestsopened to the public present higher risks than those used for silviculture. In order to reduce the riskof the exposition to pathogens, recommendations usually include a restricted access to the area onwhich sludge has been spread during 3 to 12 months following its application [INRA 1999]. Whenconsidering the risk linked to the presence of heavy metals in sludge, LW�KDV�EHHQ�DVVXPHG�WKDWULVNV� DUH� ORZHU� WKDQ� WKRVH� LQGXFHG� E\� VSUHDGLQJ� RQ� DJULFXOWXUDO� ODQG�� DV� IRUHVW� SURGXFWVUHSUHVHQW�RQO\�D�VPDOO�SDUW�RI�WKH�KXPDQ�GLHW [INRA 1999]. However, some risk may exist dueto the transfer of heavy metals to game species, or to some edible mushroom species which areknown to accumulate heavy metals.

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Results mentioned above rely on experimental studies. However, these concern very differentconditions and species, making comparisons and extrapolation (especially extrapolation to thevariety of forests which may be found in the different Member States of the EU) difficult. It mustmoreover be stressed that most of the experiments concern a XQLTXH�DSSOLFDWLRQ�RI�VOXGJH�DW�KLJKUDWH�RI�DSSOLFDWLRQ [UBA 1998]. More information is therefore needed concerning repeated sludgespreading at lower rates, especially in terms of yield response, and environmental and sanitaryimpacts. Moreover, experiments reported in the literature cover a maximum of 13-18 years, to becompared with an average timber rotation of 120 years in forests. There is therefore no dataavailable in the long-term effects, and relevant experiments should be performed.

Most of the experiments have been performed with OLTXLG�VOXGJH, making it necessary to improvethe knowledge concerning this route when using other sludge types.

Lastly, more research should be carried out regarding the environmental and sanitary impacts ofsludge spreading in forests and silviculture, especially concerning the transfer of heavy metals togame species as well to some edible mushrooms which are known to accumulate heavy metals, andconcerning organic pollutants for which no specific data is presently available.

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If sludge spreading is to be performed on forests or in silviculture, some good practices andrecommendations may be found in the literature which are summarised below.

First of all, spreading should be avoided in forests to which the public may have access, in order toavoid any contact with the sludge. Game or mushroom supply areas should also be excluded fromthe spreading perimeter. If spreading is to be performed in forests to which the public could haveaccess,- sludge should be disinfected or access should at least be prohibited to areas on which sludge

would have been spread during 3 to 12 months [INRA 1999],- the public should be informed,- sludge should be stabilised in order to avoid odour problems, and thereby facilitating the

acceptance of sludge spreading.

Sludge spreading could be privileged in silviculture, that is on intensive tree production, such asenergy coppices. Poplar production should however be excluded, as it is most often performed inwet areas, in which water contamination could occur.

In a general manner, sludge spreading should be avoided on sloping land, in areas adjacent topotable water reservoirs, on sandy soils, and in wet areas. An agreement at local level should bereached before sludge spreading is performed. Moreover, the public should be informed of thepractice.

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Lastly, recommendations regarding application rates may be found in the literature, relying on thecomparison of the results of several field tests. The Austrian UBA recommended a rate ofapplication between 100 and 200 m3/ha/year of liquid sludge, i.e. 5 to 10 tDM per hectare,assuming a dry matter content of 5%. CIWEM [1996] also recommends an application of 200m3/ha/year, introducing the following restrictions:

- The maximum application rate of nitrogen during the pole stage should be 1000 kg N/ha,- The application rate of sludge should be reduced to 100 m3/ha on wet soils,- The application rate of liquid sludge should be reduced to 100 m3/ha on slopes of 15-25

degrees, and further reduced to 50 m3/ha on slopes of 15-25 degrees with sparse vegetation,- The application rate should be increased to 100 t DM/ha during pre-planting and early

establishment.

5.4.2 Land reclamation and revegetation

Use of sewage sludge in land reclamation and revegetation aims to restore derelict land or protectthe soil from erosion, depending on the previous use of the site. In the case of industrial sites,topsoil may often be absent or if present, damaged by storage or handling. Soil or soil formingmaterials on site may be deficient in nutrients and organic matter. Other problems may exist, suchas toxicity or adverse pH. All these problems create a hostile environment for the development ofvegetation [WRc 1999].

Possible solutions include the use inorganic fertilisers or imported topsoil, which can be veryexpensive depending on location and availability. An alternative solution is the use of organicwastes such as sewage sludge, which is already used in Sweden, Finland or the United Kingdom.

According to the site use, several situations may arise, for which the provision of sludge could beuseful. They are summarised in the following table. In the case of industrial sites, the benefits ofsludge application also include the improvement of nutrient status, the addition of organic matter,and control of acid generation.

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7DEOH����� objectives of the sludge spreading in land reclamation

Sludge application is performed by using the same machinery as in the case of recycling toagriculture. Some specific machinery for sludge projection may be needed when applying sludgeon areas to which access is difficult.

When aiming to increase soil quantity on the site, two techniques are observed in the field: sludgemay be either directly applied before mixing with the soil present on the site or mixed with soilbefore application. The amount of sludge usually applied is much higher than in the case oflandspreading. As an example, some experiments were made to develop ski runs in La Plagne(France). To reach a soil thickness of 5 cm, it was necessary to use about 100 to 150 tons per

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hectare [ADEME 1999]. WRc [1999] also indicates some typical application rates for differentreclamation sites which are summarised in the table below

7\SLFDO�DSSOLFDWLRQ�UDWHV7\SH�RI�ODQG 5HDVRQ�RI�DSSOLFDWLRQ

Cake (t DM/ha) Liquid (m3/ha)

Low maintenance amenity Nutrient status improvement 50 100

Disturbed agricultural soils Nutrient status improvement 100 -

Sites lacking top-soil Organic matter addition 100-500 -

Acidic colliery spoil Acid generation control > 500 -

7DEOH����� summary of typical sludge application rates for different reclamation sites

Information is needed to assess the risk of this practice as no sufficient data is available concerningenvironmental and sanitary impacts. It was assumed that risks are lower than in the case ofspreading on agricultural land, when considering that its use is not related to food production. Inthe case of ski run development, sludge is usually applied in September, ensuring a sufficient timeperiod before the use of the land for grazing. However, no data is available concerning the potentialimpacts on wild fauna and flora. Moreover, the amount of sludge applied as well as the use ofsludge on sloping land for erosion reduction go against actual regulatory prescriptions for the useof sludge in agriculture. A risk may therefore arise because of the amount of pollutants or nitrogenapplied to land. WRc [1999] recommends that sludge should be sampled and analysed for heavymetals on a regular basis to ensure that, at the application rates required for land-reclamation,heavy metals additions are well within acceptable limits.

In any case, sludge used for land reclamation and revegetation in field experiments is alwaystreated sludge, in order to ensure sufficient disinfection and to reduce the presence of odours.

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Several technologies presenting an alternative to conventional combustion processes are currentlybeing developed or introduced into the market. These technologies are mainly represented by thewet oxidation process, pyrolysis, and the gasification process, which are described below. Othertechnologies may be found, which are most often combinations of these three main processes.

As mentioned by Werther and Ogada [1999], co- and mono-combustion of sewage sludge will stillplay an important role in the future, since alternative technologies are just introduced into themarket and therefore do not benefit from the same experience. However, those technologies presentadvantages in terms of flue gas and ash treatment. Moreover, they also seem to have reducedimpacts on the environment compared to conventional combustion processes.

5.5.1 Wet oxidation

Liquid sludge is set in contact with an oxidative gas such as oxygen in a wet environment, at atemperature of around 250 °C and under high pressure (70 to 150 bars) in a continuous process.Temperature and pressure levels, the use of catalyser, and the gas used (oxygen or air) differentiatethe existing processes.

Sludge is changed in three main products:

- a liquid phase containing easily degradable organic matter, which is easily treated when sentback at the head of the station

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- clean combustion gases, which do not have to be treated, as the relatively low temperature of theprocess avoids generation of compounds such as PCDD/F or NOx. Moreover, as the reactiontakes place in a wet environment, no dust is released to the atmosphere

- mineral residues in a liquid phase which has to be treated.

The process is presented in the figure below:

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Organic pollutants are broken down, and heavy metals concentrate in the solid residue, except formercury which is found in the gas (see table 25).

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7DEOH����� fate of heavy metals in the wet oxidation process [OTV]

No important preliminary treatment is required before performing wet oxidation: in most casesthickening is sufficient. However, the mineral residues and the liquid phase have to be separatedafter oxidation. In some cases, the mineral residuals may be treated in the same way as incinerationashes.

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Data are available concerning the ATHOS process developed by OTV. Its characteristics aredescribed below:

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7DEOH����� characteristics of the ATHOS wet oxidation process [OTV] (*after catalytic treatment)

For the time being, the technique is not widespread enough to assess costs. However it seems to becompetitive in terms of investment and operation costs for WWTP up to 200 000 equivalentinhabitants in comparison with incineration on site [OTV 1997]. Wet oxidation also allowsreducing the dewatering/drying stages.

5.5.2 Pyrolysis

Pyrolysis is a thermal process treatment in absence of oxygen. Waste is not burnt, but brought to atemperature of 300 to 900 °C. The process produces two kinds of residues: solids containingmineral matter and carbon, and hot gases. The process is presented in figure 17, and the matterbalance (in the case of household wastes) is detailed in table 27.

PyrolysisSludge

Hot gases

Solid residue

Temperature(450 to 750 °C)Absence of air

)LJXUH����� presentation of the pyrolysis process

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&DUERQDWHG�VROLGV��� ����NJMineral residues : 95 kgMetals : 50 kgInert, glass, stones : 45 kgCarbonated residue : 210 kg

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7DEOH����� matter balance of household waste pyrolysis [Free University of Brussels]

As the products of the process have a calorific value, pyrolysis is considered as a pre-treatment,requiring further valorisation of the solids and gases.

Analyses of the composition of the gaseous product of the pyrolysis of sludge have shown thatgenerally H2, CO, CO2 and hydrocarbons are the main compounds found in the gas. Theproportions however depend of the sludge type. CO is the dominant compound, with hydrocarbonsrepresenting in some cases an important part of the gas. Composition of the gas also depends on thetemperature of the pyrolysis, as described in the table below [Werther and Ogada, 1999]. Thecomposition of gases implies their treatment and use on site. They can also be cracked, as itfacilitates their further use.

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+���ZW��� 2.5 2.59 3.2 4.62

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+\GURFDUERQV��ZW��� 33.54 36.04 31.12 16.45

7DEOH����� pyrolysis gas composition at different temperatures (in Werther and Ogada [1999])

Solids can be considered as low-quality coal. They have the advantage of easy storage. Whenconsidering units of small capacity (10.000 t/year up to 50.000 t/year), this valorisation may takeplace later, and in another place. The solid residue is then dechlorinated, minerals such as metalsare separated, and it may for instance be considered as a substitution fuel in power or cementplants. On the contrary, for units with a capacity set between 25.000 and 500.000t/year,valorisation is possible on site, and the process is then considered as an integrated process.

During an integrated process, a gasification step of the solid may be added, or gases and solidresidue may be directly burnt, and fumes generated have to be treated to comply with emissionlimits. In some cases, when the pyrolysis and the energetic valorisation of the by-products areseparated, a dechlorination of the solid is possible, and a separation of the minerals (especiallymetals) may take place.

Pyrolysis presents the following advantages:

- a reduced gas emission in comparison with incineration (by about 30%),

- reduced or no emission of PCDD/F, due to the low temperature of the process,

- a possible separation and valorisation of the materials.

Besides, the reduced size of the plant and the lower temperature enable to reduce the investmentcosts. This may offset the technical constraint of building an airtight enclosure.

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There are presently different available techniques, developed by German (PKA, Siemens,Thermoselect) or French (Nexus, Thide, Traidec) firms. About 15 plants in Europe and Japan havebeen built for waste treatment, some of which still being pilot plants, for capacities ranging from0,5 to 25 tons per hour.

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7DEOH����� state of the development of pyrolysis [after Fontana 1999]

On the basis of experiments, the table below establishes the matter balance of processing pyrolysison sludge in comparison with municipal waste:

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7DEOH����� residues of the pyrolysis process [Free University of Brussels]

Municipal waste pyrolysis seems to be worthwhile in terms of costs, in comparison withincineration when considering small capacities (below 200 000 t/year) and scattered habitats. It hasnot been confirmed whether a similar conclusion could be applied to sewage sludge.

5.5.3 Gasification

Gasification is a thermal process during which a combustible material is converted with air oroxygen to an inflammable gas and an inert residue. It has been used for a long time to produce gaswith coal. This kind of process is performed at a high temperature: between 900 °C and 1100 °Cwith air, or between 1000 °C and 1400 °C with oxygen. Gasification with oxygen, which is themost often performed one, generates a gas containing 55 to 60 % N2, with a calorific value of 4 to 7MJ/Nm3.

The gasification process enables the flue gas volume to be drastically reduced since carbon dioxideand water, internally formed, participate in the reaction and the unwanted N2 may be avoided bysupplying pure oxygen. Comparisons given in literature indicate that whereas during mono- and co-combustion of sewage sludge, 24-30 m3 per kg of dry sludge of flue gas are formed, gasificationwith pure oxygen generates only 1.7 m3 [Werther and Ogada 1999].

Pyrolysis can also be considered as a gasification process, but performed in absence of oxygen.Both processes may also be performed together: gasification can be applied to the solid residue ofthe pyrolysis.

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It is a new method when applied to sludge, and therefore not very well documented. The input iseither digested or undigested mechanically dewatered sludge [EEA 1998].

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In order to compare the environmental impacts of the different recycling routes, Life CycleAssessment (LCA) methodology may be applied to sewage sludge. Some studies are available inthe literature describing LCA studies carried out mainly in Germany, France and Switzerland. Mainfindings are presented below whilst more information on the applied methodology, especially onthe impact weighting methods used, is given in each publication.

5.6.1 LCA for the ARA Region Bern, Switzerland14

A LCA has been performed for the Canton of Bern, Switzerland. The study aimed at comparing sixroutes. Three of them addressed the recycling of sludge in agriculture whilst the other three focusedon the disposal of sludge respectively in an incinerator, in a cement plant and in a plant using apyrolysis followed by a combustion process. The impact assessment phase did not take organicpollutants in consideration and was performed by using five weighting methods, which each have adifferent approach for comparing the impacts of the routes considered.

As for any LCA analysis, conclusions are valid within the assumptions made for this study,especially concerning the impacts taken into consideration. It was concluded that there was noglobal difference in the results obtained between the different landspreading disposal routes.Differences appear however when considering each environmental impact independently.Therefore the choice of one specific route to reduce one impact would be made against anotherenvironmental impact. An important conclusion was also the substantial contribution of transport tothe overall pollution.

By comparing the incineration routes, the incineration in cement kilns represented the best option,while the two other options could not be easily differentiated.

The global comparison could not definitely distinguish between the various solutions: theincineration routes have a big advantage on the soil pollution avoidance (this study only consideredheavy metals, and did not take into consideration atmospheric deposition of metals, but impacts ofashes disposed of in landfills), while the landspreading routes present an advantage concerning theeutrophication of waters.

In conclusion, the study suggested a compromise between the various solutions, and the need todefine and differentiate “good” and “bad” sludge.

5.6.2 LCA for the City of Bremen, Germany15

The study carried out in Bremen compared the landspreading route with two other possibilities: co-incineration in a coal-fired power plant, and gasification.

The impact on the environment was assessed using local demographic values, by comparison withGerman mean values. No difference could be found between the different options, as it would havebeen necessary to classify the related environmental impacts.

Each option may have a good impact on one specific indicator and a bad one on another: it istherefore necessary to decide which one has priority. ,Q� DQ\� FDVH�� WKH� QHJDWLYH� LPSDFW� RI� WKH

14 CHASSOT G. M.; CANDIDAS T.; 1997; Ökologische Beurteilung vershiedener Entsorgungsvarianten für

den Klärschlamm der ARA Region Bern, Bericht15 FRANKE B.; HÖPER G.; 1995; Ökobilanz zur Klärschlammentsorgung in Bremen : Landwirtschaft,

Mitverbrennung, Flugstromvergasung; Korrespondenz Abwasser 9/95 pp. 1529-1541

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WUDQVSRUW�KDV�EHHQ�VWUHVVHG. It was therefore decided by the city to give a financial advantage tolandspreading near the city.

5.6.3 LCA for the French Water Agencies, France16

The study aimed at comparing ten routes: six of them related to sludge use in agriculture (includingspreading of composted sludge), two to co-incineration with household waste, one to dedicatedincineration, one to disposal to landfill. Each route was specific to a WWTP size. The weightingmethod used for this study was the Environmental themes method.

Within the assumptions made for this study, there was no global difference in the results obtainedbetween the various landspreading routes, with a small disadvantage however to routes withintensive drying (more energy consumption).

A global comparison could not definitely distinguish between solutions: the landspreading routespresents a disadvantage on soil pollution, while incineration and landfilling routes present adisadvantage on air pollution.

$Q�LPSRUWDQW�FRQFOXVLRQ�ZDV�DJDLQ�WKH�KLJK�LPSDFW�RI�WUDQVSRUW�RQ�WKH�SROOXWLRQ�JHQHUDWLRQ,and also the necessity to take into account benefits of substituting sludge to mineral fertilisers.

16 Agence de l’Eau Rhin Meuse - Arthur Andersen; 1999; Audit environnemental et économique des filières

d'élimination des boues d'épuration, http://www.eaufrance.tm.fr/francais/etudes/modele.asp?fiche_id=54

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Figure 18 summarises the disposal routes for sewage sludge in the Member States. The data refersto the years 1996, 1997 or 1998 depending on the country.

0%

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)LJXUH����� sludge routes in the Member States; time series from 1996 to 1998 according to thecountry [ADEME 1999]

Agricultural use is at the present time the principal outlet for sewage sludge, accounting for about2.7 million t DM representing about 38 % of total sludge production. Landfilling is the secondmajor route, representing 37 % of total sludge production. Incineration accounts for about 9% ofsludge produced in the Member States. Countries present very diverse profiles: agricultural usecounts for about a few percents in Netherlands, up to 70 % in Luxembourg.

In addition, as these data were collected before the ban on the disposal to sea on December 31st,1998, a part of the sludge produced in UK and Ireland has since been redirected to other routes. Inthe case of Ireland, the sludge that was disposed to sea is at present directed to agricultural use.

When trying to obtain a more accurate view of alternative outlets such as land reclamation or use inforestry, we found that relevant data is differently integrated to national statistics. Land reclamationand silviculture are often considered as agricultural use, or integrated to other uses, without beingprecisely documented. Tsagarakis HW�DO� [1999] performed a survey on sewage sludge disposal inEurope. 80 % of the sludge is disposed of via landfill, whereas use in agriculture and use in forestryonly concerns 6 % and 4 %, respectively, of sludge. The remaining 10 % is disposed of within theWWTP.

However, we found that other uses in Member States cover:- for Austria: composting and revegetation- for Spain: disposal to sea when allowed- for the Netherlands: composting- for Portugal: discharge to the surface waters- for Denmark: use in cement kilns

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According to the European Commission, a general increase by 18 % in the sludge quantity isforeseen by 2005. It must be stressed that this figure should be higher, as it does not take intoaccount Italy and Sweden, from which no data were available. This average increase does notreflect the situation in each country. While in Austria, sludge production is expected to stay even, itshould grow by 197 % in Ireland.

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���� ���� (YROXWLRQ����Belgium 85 159 87Denmark 151 160 6Germany 2 227 2 787 25Greece 59 99 68Spain 685 1 088 59France 820 1 172 43Italy 800 N/A

Ireland 38 113 197Luxembourg 8 14 75Netherlands 209 401 92

Austria 200 195 -3Portugal 245 359 47Finland 136 160 18

UK 1 195 1 583 32Sweden 230 N/A7RWDO ����� ����� 18

7DEOH����� forecast of the production of sludge in the Member States by the year 200517

France, UK, Luxembourg, Germany and The Netherlands plan to further develop incineration.Agricultural use of sewage sludge will also increase in Ireland, Finland, UK and Portugal. It shouldconcern about 55 % of the sludge produced in the European Union, whereas landfilling shouldconcern about 19 % and incineration 23 %.

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)LJXUH����� forecasts of the destination of sludge in the Member States by the year 2005

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Figure 20 presents the current situation of sewage sludge disposal and recycling in AccessionCountries. These data are taken from the screening realised by DG Environment or from thequestionnaire sent for the purpose of this study. Some data are still partial (Poland) or not availableat all (Bulgaria, Cyprus and Lithuania).

17 Source: Report from the Commission, Implementation of Council Directive 91/271/EEC of 21 May 1991

concerning urban wastewater treatment, as amended by Commission Directive 98/15/EC of February 1998;

COM(98)775; 1999; Sweden and Italy have not provided any data on the destination of sludge.

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Bulgaria

Cyprus

Czech Republic

Estonia

Hungary

Latvia

Lituania

Malta

Poland

Romania

Slovakia

Slovenia

Other

Green areas

Sylv iculture

Land rec lamation

Disposal to sea

Landf illing

Inc ineration

A gricultural Use

)LJXUH����� sewage sludge routes in the Accession Countries18

Agricultural use and landfilling are the two major outlets for sewage sludge in Accession Countriesaccounting for about 250 000 and 400 000 t DM i.e. respectively 31 and 50 % of total sludgeproduction. Disposal to sea is the only disposal route in Malta, as agricultural landscape and typesof culture (horticulture and fruit production) make the use in agriculture difficult. In Slovakia, otherroutes are the use of sludge in silviculture, green areas and land reclamation. As incineration is arather expensive technology for all kinds of wastes, it is not a widespread technique: it is onlyperformed in the Czech Republic, concerning 1 % of the sludge.

18 Source : questionnaire established for the purpose of the study and European Commission