de4.pdf

SOUND SLUDGE

Ultrasound treatment of return activated sludge: environmental and economical impacts pa1

SOUND SLUDGE

Ultrasound treatment of return activated sludge:

environmental and economical impacts

Deliverable DE4

SOUND SLUDGE


Preface The present report was established in the framework of the LIFE Environment project ‘Upgrading of wastewater treatment plants with ultrasound treatment for reducing the production of sludge) (SOUND SLUDGE project, LIFE05 ENV/F/000067). The started in October 2005 and is executed by Angers Loire Métropole (Fr), IRH IC (Fr), Groupe IRH Environnement (Fr), IPL Santé Environnement Durable Est (Fr), Fraunhofer Gesellschaft (Al) represented by Fraunhofer Institute for Ceramic Technologies and Systems (IKTS) and TME (Nl). Besides the subsidy of the LIFE Environment program, the project was partly subsidized by the local water agency ‘Agence de l’Eau Loire Bretagne).

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Table of contents 1. Introduction ..................................................................................................................................... 5

2. Project outline and application........................................................................................................ 5

2.1. The water treatment plant................................................................................................. 5

2.2. The US unit: outline and operation ................................................................................... 6

2.3. Sampling and follow-up .................................................................................................... 8

2.3.1. Sludge properties............................................................................................................8

2.3.2. Sludge production ...........................................................................................................8

2.3.3. Sludge quality (environmental and agricultural) ...........................................................10

2.3.4. Greenhouse gas emissions ..........................................................................................10

2.3.5. Water treatment performance.......................................................................................11

2.3.6. Economic related parameters.......................................................................................11

3. Results .......................................................................................................................................... 11

3.1. Stability of WWTP input conditions................................................................................. 11

3.2. Impact of US on return activated sludge properties ....................................................... 13

3.2.1. Direct impact – disintegration performance ..................................................................13

3.2.2. Long term impact ..........................................................................................................13

3.3. Sludge production........................................................................................................... 15

3.4. Sludge quality (environmental and agricultural) ............................................................. 17

3.5. Greenhouse Gas emissions ........................................................................................... 18

3.6. Water treatment performance and emission to the surface water.................................. 18

3.7. Economical performance under the conditions of the WWTP of Saint Sylvain d’Anjou. 19

3.7.1. Introduction ...................................................................................................................19

3.7.2. Capital costs .................................................................................................................20

3.7.3. Operational costs ..........................................................................................................20

3.7.4. Operational savings ......................................................................................................20

3.7.5. Summary economic performance – base situation with land spreading ......................21

3.7.6. Economic performance in case of incineration.............................................................21

4. Extrapolation of results ................................................................................................................. 23

4.1. Power requirement ......................................................................................................... 23

4.2. Economics ...................................................................................................................... 25

4.2.1. WWTP of 100,000 p.e. and land spreading..................................................................25

4.2.2. WWTP of 100,000 p.e. and incineration .......................................................................26

4.2.3. Break even analysis: plant size and disposal costs......................................................27

5. Conclusions .................................................................................................................................. 28

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Abstract Sludge reduction is a current topic. The study presented here evaluates sludge reduction by means of a treatment by ultrasounds applied to recycling sludge in a waste water treatment plant of 6 300 population equivalents. Sludge reduction has been evaluated by comparing the measured amount of sludge produced to the theoretical amount (according to the CIRSEE equation) for periods without and periods with treatment. A sludge reduction of about 30 % has thus been demonstrated. This 30 % value has to be considered with care because of the high level of incertitude of the measured amount of sludge. The treatment by ultrasounds seems to have no impact on the water treatment performance, on the quality of thickened sludge, neither on the direct emission of greenhouse gases. The only exception is a decrease of the treatment performance of phosphorus. The economical evaluation has shown that the treatment is not valuable from an economical point for this small waste water treatment plant with low sludge handling costs (10 €/ton). Extrapolation of the results show that after some process modifications (treatment of thickened sludge and return of the treated sludge in the aeration basins) the process could be economically feasible for waste water treatment plants with a size ≥ 100 000 population equivalent and with final sludge elimination costs > 11 € / ton (DM 20%). If a digestor is present, it is preferable to apply ultrasounds to the sludge file in order to increase biogas production. This final application has proved itself all over the world.

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1. Introduction The overall aim of the project is to show the technical and economical feasibility of a method to reduce the quantity of sludge (= waste activated sludge) produced in a biological water treatment plant without a transfer of the environmental pressure, to promote the use of this method and to inform and train its potential end-users. In the light of this objective, three main working objectives have been defined. The first one consists of a demonstration at full scale of the technical possibilities of a reduction of sludge mass production by means of an ultrasound treatment of the return sludge stream in a water treatment plant. Besides, the evaluation of the overall economical-environmental impact of the method and of the global technical-economical feasibility is a project aim. The final working objective is to integrate data obtained during the project into an expert system allowing for the site specific comparison of different sludge reduction, treatment and disposal scenarios (from a technical, economical and environmental point of view). The present report deals with the following topics: sludge reduction, transfer of environmental pressure and economical feasibility.

2. Project outline and application

2.1. The water treatment plant The wastewater treatment plant (WWTP) of St. Sylvain d’Anjou has a capacity of 6300 population equivalents (p.e.). The treatment principals are as follows:

- nutrient removal activated sludge process, with nitrogen removal realised by a primary anoxic reactor,

- dephosphation through the injection of FeCl3 into the aerobic reactor, - dehydration of the surplus sludge by mixing the sludge with a polyelectrolyte (the dehydrated

sludge is pumped up into a stocking tank before being spread out on agricultural land).

A scheme of the WWTP is given in Figure 1, the design rules are given in Erreur ! Source du renvoi introuvable. . Actually, the WWTP turns at about 50 % of its nominal capacity.

Table1 : Design rules for the influent of the WWTP.

Parameters Values Units

Daily hydraulic flow 1150 m3.d-1 Maximum hydraulic hourly flow 100 m3.h-1 BOD5 378 kg.d-1 COD 630 kg.d-1 SS 560 kg.d-1 TKN 105 kg.d-1 Phosphorus 28 kg.d-1

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Figure 1 : Scheme of the WWTP of Saint Sylvain d’Anjou

2.2. The US unit: outline and operation The design of the US unit has been based on field trials performed at the WWTP of St. Sylvain d’Anjou. In these field trials, the primary disintegration effects for different ultrasound powers applied had been evaluated with the following parameters:

- increase in soluble chemical oxygen demand CODsoluble (the carbon source, which is available to the micro-organisms in the aeration tank) ,

- the reduction of particle size distribution (increase of A0; improvement of mass transfer towards the micro-organisms) ,

- increase in enzyme activity (EA) , - reduction of sludge volume (SV; improvement of settling) , - increase in turbidity.

The technical equipment for the ultrasound system consists of the following main components:

- suction tube with flushing valve (tube from sludge pit to feeding pump) - feeding pump (with dry run protection and overpressure sensor) - connection tube between feeding pump and ultrasound unit - ultrasound unit (noise protection box with three flow-through vessel and ultrasound processor

with generator - flow meter - return tube with flushing valve from ultrasound unit to anoxic reactor - switch cabinet for controlling and monitoring the ultrasound system.

The US unit contains three sonotrodes with a power of 2 kWh each. The sonotrodes are installed in a three-door stainless steel cabinet (Figure 2). Only 6% of the return sludge is being treated by the US unit, thus allowing to treat the whole amount of sludge present within a period equivalent to the mean sludge age (24 days). This part stream return sludge is taken from the return sludge pit after the clarifier. The feeding pump for the ultrasound unit is placed besides this return pit. Ultrasound unit and switch cabinet will be placed besides the anoxic tank. The connection between the feeding pump and the ultrasound unit is made by an underground tube. After sonication the return sludge will be directly returned into the anoxic reactor (Figure 3).

influent

1+1 pumps 100 m3/h

fine screeningaquaqard:6mm

grit

screw concoyer

grease chamber

(7m3)

combined treatment grit and grease removal

aerobic reactor

with 2 surface aerators

anoxic reactor

1-1 pumps (155 m3/h)

stripping tank

clarifier

outlet

brook La Veillière

extracted sludge

sludge return1-1 pumps (100 m3/h)

influent

1+1 pumps 100 m3/h


grit

screw concoyer

grease chamber

(7m3)


aerobic reactor


anoxic reactor

1-1 pumps (155 m3/h)

stripping tank

clarifier

outlet

brook La Veillière

extracted sludge


influent

1+1 pumps 100 m3/h


grit

screw concoyer

grease chamber

(7m3)


aerobic reactor


anoxic reactor

1-1 pumps (155 m3/h)

stripping tank

clarifier

outlet

brook La Veillière

extracted sludge


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Figure 2 : Sonotrodes in the stainless steel cabinet

Aerobic partAnoxic part

Final sedimentation tank

Disintegrated

partial flow

Recycle flowInflow

effluent

Return-sludge

Surplus sludge

RLS-Teilstrom

ultrasound

plant (6kW)

agitator 2

agitator 1

SP 1

SP 2

SP 3

SP 4-2

SP 5

SP 4-1

Figure 3 : Implementation of the Ultrasound unit in the WWTP and sampling points (SP) for analysing the sludge properties.

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2.3. Sampling and follow-up

2.3.1. Sludge properties The sampling points for follow up of sludge properties are also shown in Figure 3. The sampling at the points SP1, SP2, SP3 and SP 4 (SP4-1 was the sampling point for return sludge before installation of ultrasound unit, SP4-2 after installation) and the analysis of the accordant suspensions have been carried out:

• during initial analysis before implementation of US (in addition to WWTP routine analysis), • during field test with pilot US device and • each time when disintegration performance of ultrasound unit was tested (SP4-2 before, SP5

after US). In total 6 on-site measurements were done during the project period (duration of each 2-3 days, with at least one sampling of all SP per day on site). The parameters that were measures at the different sampling points are given in Table 2.

Table 2 : measured parameters for follow up of sludge properties

SP Sample Parameters 1 Inlet Conductivity, SS, CODhom, CODsoluble, NH4

+-N

2 Effluent Conductivity, SS, CODhom., CODsoluble, turbidity, NH4

+-N, NO3-N Ptotal (hom.)

3 Mixed liquor SS, VS, SVI, A0*, EA**, CODhom., CODsoluble, NH4

+-N, Ptotal (Filtr), Ptotal (sludge homogenized)

4 return activated sludge (RAS) SS, VS, SVI, A0, EA, CODhom., CODsoluble, NH4

+-N, Ptotal (Filtr), Ptotal (sludge homogenized)

5 return activated sludge (RAS) after US A0, EA, CODsoluble * A0... volume specific surface (integral parameter of particle size distribution) ** EA... enzyme activity There were three main targets aimed to achieve by means of the onsite measurements:

• additional control of WWTP condition’s stability while duration of project (representativeness of results),

• control stability of disintegration performance, • recording WWTP-process data in order to determine possible impact of using ultrasound in a

configuration like shown in Figure 3.

2.3.2. Sludge production

The evaluation of the sludge reduction by the US treatment was performed by comparing the measured production with the calculated production according to a theoretical formula adjusted to the period without treatment. Measured production The measured sludge production has been calculated by means of the parameters given in Table 3. Sludge production in week X has then been calculated by using equation 1 : Sludge produced week X = MS after extraction week X+1 + MB extracted week X+1 - MB after extraction week X (kg MS) equation 1

With : MS after extraction = mass of sludge in the aeration tanks after extraction

= V tanks (m3) x SS a.t. (g.l) / 1000

MS extracted = mass of sludge extracted = Vsludge-ex (m3) x SSsludge (g/l)

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Table 3 : Parameters followed-up and method of measurement

Parameter Symbol Method of measurement Daily volume at the entrance and the exit of the WWTP

Vtreated Venturi canal installed at the exit of the WWTP with an ultrasound probe to measure water height, daily inegration of all data

Volume of extracted sludge Vsludge-ex Electromagnetic flow rate meter installed downstream of the thickening table, integration of data

COD and SS content of the water at the entrance of the WWTP

CODentrance and SSentrance

Mean daily sample taken by automated sampler at the entrance of the WWTP, reconstitution of two mean samples per week and analysis of COD (NF T 90-101) and SS (NF T90-105-2)

SS content of thickened sludge SSsludge Mean sample of the thickened sludge by means of an electrovanne installed at the bottom of a homogeinised tank (agitators) of 1 m3 by which the thickened sludge flows before being rejected in the stocking tank, analyses of SS (NF T90-105-2)

SS content of the aeration tanks

SSa.t. Sampling of 4 samples a week (2 just before juste and 2 just after the extraction of sludge) of the sludge in the aeration tank, analysis of SS (NF T90-105-2)

The amount of sludge produced calculated according to equation 1 has been compared to the amount extracted during the same period. A minor difference of 2.1 % has been observed. Equation 1 will be used, given the fact that it takes into account an eventual change in the stock of sludge in the aeration tanks. The hypotheses made are: (i) the stock of sludge in the final sedimentation tank does not change over the whole experimental period and (ii) the two aeration tanks are perfectly homogenized.

Two observations allow to reinforce these first hypothesis. The application of US did not have an effect on:

- the height of the sludge vowel in the final sedimentation tank - the concentration at the bottom of the final sedimentation tank was as high as 17,0 ± 0.7 g/l

and 14.8 ± 2.9 g/l during periods with and without US treatment respectively. Theoretical evolution of sludge production The theoretical sludge production was calculated based on the entrance parameters of the WWTP according to equation 2 1. Sludge production (kg/kg BOD5) = (SSentrance (1 – VSentrance) + SSentranceVSentranceVSrefractory + (0.82 + 0.21 log (MC)BOD5, entranceBOD5,treatment performance + 0.17 % TNKnitrifiedTNKentrance / BOD5

equation 2

With : Parameter Value SSentrance Flux SS at the entrance of the WWTP Variable BOD5,entrance Flux BOD5 at the entrance of the WWTP Variable VSentrance Volatile solids at the entrance of the WWTP 70 % VSrefractory Refractory part of the volatile solids 30 % MC Massif charge Variable BOD5 treatment performance Treatment performance of BOD5 99 % TNKnitrified Percentage of total N that is nitrified 70 % TNKentrance Total nitrogen at the entrance of the WWTP 37 mg/l

Adjustment of the theoretical evolution of sludge p roduction for the period without US The theoretical sludge production calculated according to equation 2 underestimates the weekly measured sludge production by a mean of 37 % for the period without US treatment. In order to compare the effect of the US treatment, it is necessary that the theoretical evolution for the period without treatment equals the measures production. An adjustment of the parameters has thus been necessary. A multiplication parameter has been added to equation 2, resulting in equation 3. Sludge production (kg/kg BOD5) = ((SSentrance (1 – VSentrance) + SSentranceVSentranceVSrefractory + (0.82 + 0.21 log (MC)BOD5, entranceBOD5,treatment performance + 0.17 % TNKnitrifiedTNKentrance / BOD5) * k

equation 3

1 Cornier J.C., Fayoux C., Lesouef A., Villesot D. (1994). “Les nouvelles contraintes d’exploitation des usines d’épuration.” Techniques Sciences et Méthodes, n° 7-8, pp 392-406.

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A value of k of 1.37 allows to minimize the moderated error over three weeks (period close to the mean sludge age) between the measured production and the theoretical evolution. The necessity to apply an adjustment parameter can be partly explained by the presence of a physico-chemical treatment of phosphorus by FeCl3. On the WWTP, the amount of FeCl3 injected in the reactor is however very small (molar ratio Fe/P = 0.24) which results in a production of physico-chemical sludge in an amount representing only 4% of the produced sludge. In order to explain the lasting 33 %, the hypothesis was made that the amount of sludge measured overestimates the amount produced. This hypothesis is reinforced by the fact that the mean sludge production measured during the period without US treatment equals 1.6 kg DM / kg BOD5, a value that is unlikely for a mean massif charge of 0.04 an a mean ratio MES / DBO5 of the water to be treated of 1.2. However, the data and information obtained during the study have not allowed to determine other sources of error that could explain the overestimation of the amount of sludge produced. In the following, the theoretical sludge production is calculated according equation 3.

2.3.3. Sludge quality (environmental and agricultur al) Thickened sludge is being sampled and analysed for organic and inorganic pollutants. These analyses are done on sludge samples taken with and without the ultrasound treatment.

2.3.4. Greenhouse gas emissions The emission of CO2, CH4 and N2O to the air form the anoxic tank, the aeration tank and the sludge pit have been measured. The method is given in the schematised in Figure 4. A floating basin open at its base allows to cover a part of the surface of the basin from which greenhouse gases are emitted. The air within the basin is continuously renewed with fresh air by means of a pump. The air from the basin is extracted by a second pump. This air is analysed directly for CH4, CO2 and N2O by means of gas chromatography and a catarometric detector. Besides, water samples are taken during the air measurements in order to be able to express the emission as a function of the water quality. These measuring campaigns are being preformed during periods with and periods without the ultrasound treatment.

Air200 bars

Inflatable basin posed on the surface of the water

Slacker

Low pressure

Flow rate meter

Air Injection

Computer data

treatmentanalyser

Gas chroma-tographie

Catarometricdetector

Air200 bars

Inflatable basin posed on the surface of the water

Slacker

Low pressure

Flow rate meter

Air Injection

Computer data

treatmentanalyser

Gas chroma-tographie

Catarometricdetector

Figure 4 : Method used for measuring the emissions to the air.

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2.3.5. Water treatment performance The inlet and outlet of the WWTP are analysed for organic and inorganic parameters. Measuring campaigns last for one week. Measuring campaigns take place during periods with and periods without the ultrasound unit being operated.

2.3.6. Economic related parameters Economic related parameters (energy consumption, polymer consumption, FeCl3 consumption) are being followed-up.

3. Results

3.1. Stability of WWTP input conditions Very important for sludge production – its reduction was the main target parameter of the project – is WWTP input of suspended solids and COD. Both parameters correlate, as shown in Figure 5. Red points are data of the continuous WWTP analysis in the period with ultrasound, green data points are from the period without ultrasound treatment and mauve points are data from the IKTS on site measurements (grab samples). Yellow marks stand for average values in each period. Two conclusions can be drawn from Figure 5:

1. there were usual conditions during the onsite measurements of disintegration performance, so obtained data can be seen as reliable,

2. there were relatively stable input conditions regarding suspended solids and COD in the complete trial period, no extreme variations which would imply serious doubts about the findings.

The values of ratios that constitute a reference for sludge production are given in table 4. Also the other parameters like conductivity, pH, P-total and ammonia did not vary very much in the trial period (Figure 6), only WWTP inlet-concentration of phosphorus seems to increase (additional P- and ammonia measurements were started when US started also). It can be claimed that there were stable inlet conditions within the test period, significant impacts from external sources, which might have influenced the results in a negative way or which could have led to inconclusiveness, were not observed. On average the load of suspended solids and COD in the period without ultrasound was slightly lower than in the period with ultrasound treatment.

Table 4: Value of different ratio at the entrance of the WWTP

Ratio Period without US treatment Period with US tr eatment

Mean error Max Min Mean Error Max Min

COD/SS 2.5 0.5 4.0 1.7 2.5 0.5 3.9 2.0

COD/BOD5 2.6 0.3 3.1 2.2 3.2 0.7 4.3 2.3

COD/TNK 9.7 1.8 12.9 7.2 9.4 1.3 11.2 7.5

COD/Ptot 57.6 10.0 74.6 40.7 66.8 9.0 86.2 56.8

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Figure 5 : WWTP input data (Suspended Solids and COD in kg/d) and IKTS measurements

0

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8

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600

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1400

pH, P

-tot

al [

mg/

L], 0

,1*N

H4+

-N [

mg/

L]

Con

duct

ivity

[µm

S/c

m]

Inlet WWTP - Snap shot measurements

conductivity pH P-total NH4-N

US on

US off

Figure 6 : WWTP input data (IKTS grab sample measurements) conductivity, pH, Ammonia and Phosphorus

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3.2. Impact of US on return activated sludge proper ties

3.2.1. Direct impact – disintegration performance

The immediate effect of introducing ultrasound to sewage sludge on its properties (return activated sludge (RAS) in this case) is named “primary disintegration effect”. Effects like sludge reduction, which follow subsequently in the process are called “secondary disintegration effects”. The quantity and stability of primary disintegration effects of the US unit in this trial was proven by analysis before and after US-treatment (SP4-2 and SP5) of

• turbidity in supernatant, • soluble COD, • enzyme activity, • particle size distribution (integrated parameter volume specific surface).

Accordingly Figure 7 shows 4 diagrams with these parameters depending on volume specific energy input. Energy input was changed due to varying flow rate of US feeding pump. Measurements have been carried out in March 2006 (pilot unit), January 2007, July 2007 and February 2008. The values of the parameters with rising energy input varies in a small range because sludge’s disintegratabiliy depends not only on equipment but also on external factors like temperature, sludge age and others which cannot be taken into account in detail. In February 2008 disintegration performance (regarding slope of the fit equations) was comparable to the results achieved with the pilot plant in March 2006 and slightly better than in January and July 2007. It can be concluded that there was no measurable loss of performance from the equipment-side caused by wearing or so (ultrasound equipment was operating 24/h at 7d/week from 18.07.07 until 7.4.2008). Ao- and EA diagrams in Figure 7 also show the higher starting level of the parameters in February 2008 caused by the previous US-operation period (see § 3.1.3). 3.2.2. Long term impact A very clear effect of US was observed on return activated sludge’s enzyme activity and volume specific surface (integral parameter of particle size distribution). Although only a certain amount of RAS was treated a significant increase of RAS enzyme activity and volume specific surface was found in the period with US-operation. Both parameters become directly affected by ultrasound (see 3.1.2). RAS is part of complete mass of activated sludge in the WWTP – so it can be concluded, that ultrasound split stream treatment took effect on the whole activated sludge in the system: fineness of sludge increased slightly but enzyme activity at least doubled (activity of hydrolytic enzymes was measured here). When ultrasound device was turned off parameters decreased again on former level without ultrasound (Figure 8).

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Ultrasound treatment of return activated sludge: environmental and economical impacts page 14

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U]

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EA

[µm

ol/L

*min

]

Espez [kWs/L]


Launch of pilot plant (Januar 2007)

February 2008

start of continuous operation July 2007

Figure 7 : Disintegration performance (turbidity, enzyme activity, soluble COD, volume specific surface) depending on volume specific energy input

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enzyme activity EA [µmol/L*min]

volume specific surface Ao[m²/cm³]

Return activated sludge

Ao EA

US on

US off

Figure 8 : Volume specific surface and enzyme activity of return activated sludge RAS with and without US

3.3. Sludge production Figure 9 shows a good correlation between the weekly sludge production calculated according to equation 3 and the measured production for the period without US treatment. The quality of the adjustment validates the possibility to use equation 3 in order to evaluate the effect of the US treatment on the sludge production. It appears that for periods with US treatment, the measures sludge production is lower that the theoretical one (Figure 10). At the scale of the period of US treatment a mean reduction of the sludge production in the order of 30 % has been observed. The incertitude of the measurement of the sludge production has been evaluated by considering the four possible error sources of equation 1: volume of the sludge extracted (Vse), concentration of SS of the thickekend extracted sludge (SSse), volume of the aeration tanks (Vat) and concentration of SS in the aeration tanks after extraction in week X (SSafter) and after extraction of the precedent week (SSbefore). The values of these error sources and the method of obtaining these values are given in Table 5. By developing on these terms equation 1, we obtain equation 4. The incertitude for a period can then be calculated based on equation 5. Sludge produced (SP) (kg) = Vse (m3) * SSse (kg/m3) + Vat* (SSafter –SSbefore)

equation 4

)(** 111

SSbeforeSSafterVatSSseVseSP d

ns

snn

dn

nweekly −+=∑∑

=

=

=

=

equation 5

The incertitude (on a 95% confidence interval) of the weekly measured sludge production during a given period (period with or period without US) calculated on the basis of equation 5 equals 11%. Because of this high incertitude, the sludge reduction of 30 % should be considered like a trend. However, the coherent data (figure 10) show that sludge reduction is real on the studied WWTP.

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y = 1.04xR2 = 0.994

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calculated production (kg)

mea

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d pr

oduc

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(kg)

Figure 9 : Correlation between the measured and the calculated sludge production during the period without US treatment (cumulated production over 22 weeks).

y = 0.75 xR2 = 0.994

y = 0.74 xR2 = 0.957

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mea

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(kg)

period 1 period 2

Figure 10 : Correlation between the measured and the calculated sludge production during the two periods with US treatment (cumulated production over 24 weeks for period 1 and over 12 weeks for period 2).

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Table 5 : Sources of error for the measurement of sludge production, their values and method of calculation.

Source n° description Title Relative

incertitude u

Method of calculation / source

1 Volume of thickened sludge Vse 0.5 % Technical documentation of the flow rate meter

2 Concentration of SS in the thickened sludge

SSse 8.9 % A mean hourly sample was taken during 24 hours. The samples have been grouped to obtain 4 mean samples for each period of 6 hours. The samples have been analyzed for their SS concentration. To the error thus obtained has been added the error made by the analysis on one and unique sample (determined in error source n° 4).

3 Volume of the aeration tanks

Vat 0 % The volume of the tanks is constant

4 Concentration of SS in the aeration tanks (week before and week of extraction)

SSbefore and SSafter

5.5 % Sampling of 4 individual samples and analysis of the SS concentration on 4 fractions of each sample.

3.4. Sludge quality (environmental and agricultural ) In order to determine an eventual concentration of pollutants in the sludge as a reason of the reduction of the amount of dry matter produced, the concentration of organic pollutants and of metals has been determined. The concentration of DEPH, NPE and PCB in the sludge produced without US treatment was below the detection limit. The concentration of PAH and PCDD/F was very low, 25 times lower to the maximum concentration proposed (for sludge spread out on agricultural land) in the ‘Working Document on Sludge’. Based on these facts, the analyses of organic pollutants has been excluded from the following measuring campaigns. Concerning the metals, the analyses of the sludge sampled during the US treatment is compared to the sludge sampled during the period without US treatment (Table 6). The results show no evidence of a significant impact of the US treatment on the sludge quality. However, this observation has to be taken with care because eventual variations of the metal concentration at the inlet of the WWTP have not been measured. Besides, we obtained only one analysis for the period with US treatment. The comparison of the values of table 6 with the actual legislation and with the concentrations proposed in the ‘Working Document on Sludge’ show that the sludge of the WWTP of Saint Sylvain d’Anjou can be valorised in agriculture.

Table 6: Concentration of pollutants in the thickened sludge.

parameter without US with US

Cu mg/kg DM 241.67 ± 8.33 230.00

Cd mg/kg DM 0.81 ± 0.04 0.62

Zn mg/kg DM 341.67 ± 41.67 220.00

Cr mg/kg DM 21.83 ± 1.17 19.00

Hg mg/kg DM 5.47 ± 3.67 3.30

Ni mg/kg DM 19.67 ± 3.33 17.00

Pb mg/kg DM 19.67 ± 0.33 21.00

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3.5. Greenhouse Gas emissions The emissions of greenhouse gases are given in Table 7. From this table it can be concluded that greenhouse gas emissions vary over time. The most stringent difference between the emission with and the emissions without US is the increase in CO2 emission with the US treatment. This increase would be mainly due to a higher CO2 emission form the aeration basin. In order to verify if this increase in CO2 emission could be due to a higher COD load at the inlet of the WWTP, the CO2 emission has been calculated as a function of COD removed. The results for the aeration basin are as follows:

- 06/07 (no US treatment) : 4.82 *10-2 kg CO2/kg CODremoved - 09/07 (US treatment) : 6.04 *10-2 kg CO2/kg CODremoved - 06/08 (no US treatment) : 4.64 *10-2 kg CO2/kg CODremoved

Besides, the COD flux at the inlet of the WWTP has been quite equivalent for the three periods (mean value of 306, 297 and 291 kg COD/day respectively). The increased CO2-emmission is thus not due to a higher DCO load of the WWTP. If these results would be confirmed by further measuring campaigns, the increase in CO2-emmission could be explained by an accelerated catabolism and respiration ratio of active sludge microorganisms. This complies with a decrease of total sludge production, because more CO2-production means less sludge. Two hypotheses have been emitted to explain this observation:

- formerly inert particular organics have been transferred into a biodegradable status by the ultrasounds

- microorganisms themselves get more degraded because of the more permanent presence of active hydrolytic enzymes.

However, it has to be kept in mind that even if the CO2-emmission might be increased, the US treatment does not induce any additional pressure on the environment as the total greenhouse gas emission is not changed (Table 7).

Table 7: Emission in equivalent CO2 per m3 treated water (g/m3).

CH4 C02 N2O TOTAL US no yes no no yes no no yes no no yes no date 06-07 09-07 06-08 06-07 09-07 06-08 06-07 09-07 06-08 06-07 09-07 06-08 anoxic basin 0.3 0.3 0.2 2.0 1.8 1.6 < dl 0.1 2.1 2.3 2.2 3.9

aeration basin 2.9 4.4 0.5 30.4 41.4 17.6 16.3 1.8 1.1 49.5 47.5 19.3

sludge pit 13.3 11.7 23.3 1.0 0.8 1.0 1.5 0.2 0.1 15.8 12.7 24.5

total 16.5 16.4 24.0 33.4 44.0 20.2 17.8 2.1 3.3 67.6 62.4 47.7

3.6. Water treatment performance and emission to th e surface water The results obtained at the direct exit of the US unit show that that the US increased the concentration of fine particles. This could harm the effluent quality. The treatment performance of the WWTP without the US treatment (mean value for 4 to 5 days, error for 3 campaigns) and with the US treatment (mean value for 5 days, 1 campaign so no error) are given in Table 8. Concerning MES, DCO and hard DCO, the US treatment has no impact on the treatment performance.

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Table 8: Treatment performance (%) of the WWTP of Saint Sylvain d’Anjou.

Parameter without US treatment with US treatment MES 98.04 ± 0.44 98.03 DCO 92.91 ± 1.98 92.92 DCOhard 31.49 ± 4.22 35.71 DBO5 96.86 ± 0.61 98.11* NTK 94.67 ± 1.22 96.49** PO4 93.02 ± 3.73 88.32 Ptotal 91.59 ± 3.87 86.38 * the treatment performance has been calculated on the basis of 2 values, the concentration of DBO5 in the outlet being < 3 mg.O2/l in 3 out of the 5 samples ** the treatment performance performance has been calculated on the basis of 2 values, the concentration of NTK being < 2 mg/l in 3 out of the 5 samples. The treatment performance of phosphorus seems to decrease when the US unit is working. Table 9 shows that concentrations of the outlet of the WWTP are indeed increased during the US treatment. It was verified that the injection of FeCL3 and the molar ratio Fe/P have not changed over the different follow-up periods. A precise calculation of this ratio for the period ‘without US 3’ and for the period ‘with US’ has been possible. In both cases, the value equals 0.2 and can thus not explain the difference observed. Even if the higher phosphorus concentration at the outlet has to be confirmed, the result seems coherent. In effect, at an unchanged concentration in the sludge, a lower sludge production decreases the phosphorus flux that leaves the WWTP by the sludge file and increases the phosphorus flux at the outlet of the WWTP. This phenomena has been observed in other studies concerning at source sludge reduction, for example in the study on ‘Biolysis O’2. Table 9 shows that sludge concentrations are indeed increased during the US treatment. However, for the WWTP of Saint Sylvain d’Anjou, the discharged water still replies with the French legislation.

Table 9 : Concentration of Ptotal (mean values and error for 5 mean daily samples) at the entrance and the outlet of the WWTP and the treatment performance (%).

Campaign Entrance (mg/l)

Outlet (mg/l)

Treatment performance %

without US 1 0.24 ± 0.10 0.72 ± 0.30 95.51 without US 2 0.18 ± 0.04 0.50 ± 0.00 95.65 without US 3 0.30 ± 0.20 0.46 ± 0.12 95.49 with US 0.82 ± 0.21 1.15 ± 0.13 86/38

3.7. Economical performance under the conditions of the WWTP of Saint Sylvain d’Anjou

3.7.1. Introduction The economic performance of US treatment to reduce sludge at a WWTP can be analysed in an integral way or in a partial/marginal way. Whereas an integral assessment would take on board all relevant parameters of the performance of a WWTP, including the “normal” operation of the WWTP, the partial/marginal analyses focuses on the differences between the initial state (without the application of US) and the performance of a WWTP that uses US for sludge treatment.

2 Rewcastle M., Taylor T., Churchley J., Lebrun T., Perrin. (2004). “Full-scale trial of Degremont’s Biolysis ‘O’ sludge minimisation technology on an activated sludge plant in the UK.” The 9th European Biosolids and Biowastes Conference, November 14-17, Wakefield, UK. Session 06, paper 16, PP 1-14.

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In this chapter, a partial/marginal economic performance analysis is performed, highlighting the additional investments, costs and savings due to the application of US. The economic performance of an US-unit therefore depends on:

- Additional capital costs of US: depreciation costs and interest; - Additional operational costs of US: electricity, maintenance, replacement of sonotrodes; - Additional operational savings (due to sludge reduction): savings on chemicals use, internal

sludge handling (electricity) and disposal costs (land spreading, land filling or incineration). The calculations are based on a 30 % reduction in sludge production by the US treatment.

3.7.2. Capital costs Investment costs for the non-commercial ultrasound system used in Saint Sylvain were about € 60,000, which is € 10,000 per kW installed power. ‘Non-commercial’ means that it was designed and installed to demonstrate ‘proof of principle’ with regard to sludge reduction by means of RAS-treatment in a small WWTP under well controlled conditions. The US-device was designed to operate with various settings for research purposes, thus it was not consequently constructed under commercial aspects. Commercial systems normally require a lower level of adjustment opportunities. Also a pre-evaluation of economical feasibility is usual before setting up an ultrasound disintegration system. Annualised for 10 years (lifetime) with an interest rate of 5%, the investment of - in the particular case of the WWTP of Saint Sylvain - € 60,000 leads to annual capital costs of € 7,770.

3.7.3. Operational costs Operational costs consist of costs for energy consumption of the US-unit (20,260 kWh/year, based on 3,355 kWh/month), maintenance costs (1% of investment costs each year is assumed) and costs of replacement of the sonotrodes. The energy consumption of the US-unit at the WWTP of St Sylvain was measured monthly. The consumption of the US treatment equals 3,355 kWh per month. This represents an increase in energy consumption of 17% compared to the total energy consumption of the WWTP (agitators included). At a price per kWh of € 0.05, annual energy costs of one US-unit are calculated at € 2.013 (= 3,355 kWh x 12 x € 0.05/kWh). Maintenance is only required for wearing parts of the feeding pump. Other components are free from maintenance. Costs are about 1% of the initial investment, which is calculated at € 600 each year. The US-unit at Saint Sylvain has three sonotrodes. Minimum operation time of each sonotrode is 3 years, and experiences show that on average 1 sonotrode has to be replaced each year. The replacement costs of one sonotrode is about € 5,000.

3.7.4. Operational savings Operational savings can be achieved if application of US results in less sludge production. At the WWTP of St Sylvain sludge production is reduced by about 30 % due to US treatment (= 420 tonnes of the amount of 1400 tons in 2005). Costs to dispose (wet) sludge (4% DS) are € 10/ton (or € 250 per ton DS) for land spreading. As a result, sludge reduction of 420 tonnes (30% of 1400) results in savings on (external) handling costs of € 4,200 on a yearly basis. The plant in St Sylvain has a sludge press for dewatering, but it was not used during US treatment (no savings on electricity use). Content of dry solids (DS) in the sludge which is disposed is (only) 4%.

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At the WWTP of St Sylvain FeCl3 and polymers are used to treat sludge. For both substances, no significant change in the consumption is observed after the start up of the US treatment. Optimisation and adjustment of chemicals use was not targeted within the trial, so it was only followed up but not varied.

3.7.5. Summary economic performance – base situatio n with land spreading The WWTP of Saint Sylvain is a small plant with a capacity of 6,300 population equivalents. In combination with low disposal costs for land spreading (€ 10 per ton), US treatment is not economic feasible for the WWTP of Saint Sylvain. Additional annual costs are calculated at € 15,383, annual savings at € 4,200. The net additional annual costs are thus € 11,183 (capital costs, operational costs and savings). The annual net operational costs of € 3,413 cannot be paid back in this case (Table 10).

Table 10 : Economic performance of US treatment at the WWTP of Saint Sylvain – base situation: land spreading

WWTP St Sylvain 6,300 p.e. - 100% land spreading 1,400 ton sludge (DS 4%) - 30% reduction sludge 56 ton dry sludge - electricity costs € 0,05 / kWh - disposal costs € 10 / ton (DS 4%) Investment in 1 US-unit € 60,000

Capital costs, annuity 10 years, 5% € 7,770 Electricity (sonotrodes) 40,260 kWh/y € 2,013 Maintenance (1% inv costs) € 600 Replacement of 1 sonotrode per year € 5,000 Total operational costs € 7,613 Total annual costs € 15,383 Savings on sludge handling (land spreading) 420 ton/y € 4,200 Total operational savings € 4,200

Annual net operational savings -€ 3,413 Payback period*) years -17.6

*) Payback period is defined by “investment divided by annual net operational savings” (not taking into account interest).

3.7.6. Economic performance in case of incineration Under current conditions, application of US treatment at the WWTP of Saint Sylvain d’Anjou is not economical feasible (see § 3.7.5). The plant is small and disposal costs (for land spreading of WWTP sludge) are low. Therefore, too little money can be saved to offset the additional costs of US treatment. The question is whether changes in the conditions could lead to an economical feasible application of US in a WWTP (with comparable technical lay-out as the plant in Saint Sylvain d’Anjou)? To find an answer to this question, two different situations have been analysed:

- First, the base situation is differentiated. In stead of land spreading of the sludge, it is assumed that the sludge is incinerated (as is the case in many EU member states);

- Next, the base situation will be extrapolated to a plant of 100,000 p.e. with either land spreading or incineration of the disposed sludge.

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In many European countries, a part of sludge is being incinerated. Incineration is far more expensive than land spreading (about € 450 per ton DS, with at least 20 % DS per ton3). In Table 11, the base situation results are re-calculated in case the wet sludge is dewatered from 4% DS to 20% DS and incinerated. Energy savings of dewatering (because less sludge needs to be dewatered) is calculated as savings per ton sludge (not) dewatered.

Table 11 : Economic performance of US treatment at the WWTP of Saint Sylvain in case of sludge incineration

WWTP St Sylvain 6,300 p.e. - 100% incineration 1,400 ton sludge (DS 4%) - 30% reduction sludge 280 ton sludge (DS 20%) - electricity costs € 0,05 / kWh 56 ton dry sludge - disposal costs € 90 / ton (DS 20%) - dewatering costs € 10 / ton 4 Investment in 1 US-unit € 60,000

Capital costs, annuity 10 years, 5% € 7,770 Electricity (sonotrodes) 40,260 kWh/y € 2,013 Maintenance (1% inv costs) € 600 Replacement of 1 sonotrode per year € 5,000

Total operational costs € 7,613 Total annual costs € 15,383 Savings on electricity (dewatering) 420 ton/y € 4,200 Savings on sludge handling (incineration) 84 ton/y € 7,568

Total operational savings € 11,768

Annual net operational savings € 4,155 Payback period years 14.4

The table shows that if the sludge has to be incinerated (and therefore, be dewatered), US treatment is still not economic feasible for a plant like St Sylvain which is using aerobic sludge treatment. The payback period of the investment is more than 14 years, which is larger than its technical lifetime of 10 years. This can be explained as follows:

- To incinerate sludge, the DS content needs to be about 20%; - The sludge produced at the St Sylvain plant has a DS content of about 4%; - To enable incineration of such sludge, a dewatering step needs to be included, to increase the

DS content from 4% to 20%; - As a consequence of the dewatering, the total amount of sludge to be incinerated is reduced

from 1400 ton/y to 280 ton/y. This additional dewatering step would cost annually € 14,000; - Application of ultrasound would reduce the amount of sludge to be dewatered by 30% = 420

tons (and thus would save dewatering costs of 420 x € 10 = € 4,200); - Application of ultrasound would (also) reduce the amount of sludge to be incinerated by 30% =

84 tons. Because processes on many small and very small WWTP’s are similar to the one in Saint Sylvain d’Anjou and because sludge disposal routes and costs are comparable, it can be summarized that ultrasound disintegration is not economic feasible for small plants with only aerobic sludge treatment, despite a demonstrated significant sludge reduction of almost 30%. However, on the base of that 3 Source: DRSH, 2007, “Jaarverslag 2007”. (annual report of DRSH, a WWTP sludge processing/incineration company in Dordrecht, the Netherlands, with an annual sludge processing of 400,000 ton (21.8% DS)). 4 Based on “BAT document on WWTP sludge” (Huybrechts, VITO, Belgium, 2000).

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positive process result, extrapolation can be made to estimate an economic break-even-point for this technology (see § 4.2.3).

4. Extrapolation of results

4.1. Power requirement Extrapolation of results to other processes and WWTP, especially in terms of exact calculation of sludge reduction, energy use etc. is very difficult, caused by the complex relations and interactions between those process variables which determine sludge production in each case. But by means of the results this report bases on it seems possible to estimate the optimal operational window for the US device in this particular case. Equation 6 was the base for the extrapolation approach. For that purpose factor “a” – it stands for “fraction of inert organic material”- in the equation (equation 6) could become extended to “aUS-Dis.” with an additional term to describe impact of US (equation 7). It is based on the assumption that, due to ultrasound, a certain part of these inert organics becomes biodegradable in the water purification process. Previous theoretical simulations let presume that main effect regarding sludge reduction by means of ultrasound is not mainly generated by changed bio-kinetic parameters like biomass yield or decay rate or others. How much of inert organic fraction is transformed that way must depend on disintegration performance of the disintegration device (quality of primary disintegration effects – expressed as “conversion factor CF”) and the proportion between total treated amount and untreated amount of sludge within one sludge age.

⋅⋅+⋅⋅⋅⋅−−

⋅+= −

−

)15(

)15(

,55 072,11

072,1)1(]/[ T

D

THDaerationinlet

HSRTk

SRTYkb

BOD

SSaYBODkgkgproductionSludge

aerationinlet

Parameters Unit Value

Fraction of inert particular matter (inlet) a kg DS/k g DS 0,6

Heterotrophic Biomass Yield Y H kg/kg BOD 5 0,75

Natural decomposition rate b 1/d 0,2

Decay rate/autolysis parameter k D 1/d 0,17

measured data; BOD 5 ≈≈≈≈ 0,5 COD Source: ATV A131

T=18,8 °C

aerobic SRT=0,27*real SRT

CF is a function mainly of US-unit flow rate if there are no other possibilities to change primary disintegration effects. In that concrete case a CF was figured out that resulted in a sludge reduction of 32.5 %. CF and US-flow rate are indirect proportional. If mathematical description for CF=f(flow rate US unit) is known as function of volume flow (Figure 11) prediction of achievable effects depending on flow rate of ultrasound unit is possible (Figure 12) by means of equation 6 extended by disintegration factor according to equation 7.

⋅⋅−⋅=•

−totalba

USDis.US V

SRTVCFaa

sin,

1 treated amount of sludge mass within 1 SRT

Conversion factor CF =f(disintegration performance)CF = convers proportional to throughput,

equation 7

equation 6

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y = -64,434x + 148,65

R² = 0,7874

y = -0,2757x + 0,5875

R² = 0,8312

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

0,45

0

20

40

60

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120

0 0,5 1 1,5 2 2,5

En

zym

e a

ctiv

ity

EA

[u

nit

s]

Tu

rbid

ity

[N

TU

]

US throughput [m³/h]

Disintegration performance of 6 kW-System @ St. Sylvain

Turbidity NTU EA

working point where CF was 1,7

end point, CF=0

Figure 11 : deriving Conversion factor CF from correlation between throughput of US-system and primary disintegration effects (measured as enzyme activity and turbidity).

0

10

20

30

40

50

60

70

80

0

5

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0 0,5 1 1,5 2 2,5 spe

cifi

c E

ne

rgy

[k

Ws/

L]&

tre

ate

d a

mo

un

t p

er

SRT

[%]

Slu

dg

e r

ed

uct

ion

[%

]

Throughput US-Unit [m³/h]

Sludge reduction specific Energy treated amount

optimum operational window

for installed system

Figure 12 : Estimation of optimum operational window for 6 kW-US-unit in concrete trial configuration

A very rough estimation how to size a US unit for larger plants would be possible on a linear way if is preconditioned that a certain CF is essential to achieve a certain sludge reduction, that more or less exact this CF is generated at the same US-energy impact level with the sludge at the other WWTP. The upper part of Table 12 shows the power needed for US-disintegration for WWTP of different sizes

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if return activated sludge with a dry solids (DS) content of 3 g/l is being treated. Because of the very high power needed (270 kW in case of a 100 000 p.e.) this is a very expensive and not a realistic scenario. In fact low concentrated RAS with about 3 g/L DS would not be treated because it is inefficient. In larger plants normally continuous thickening machines are installed. So a part of thickened surplus activated sludge would have to be taken, disintegrated and recycled to aeration. The required ultrasound unit could become downsized dramatically: if the thickening ends up with approximately with 70 g/L (e.g. with a centrifuge) there is a volume reduction factor of 3/70=0.043. The lower part of Table 12 shows the power requirement in case the US treatment is applied to a thickened sludge with a DS content of 7% (case 1) and 4% (case 2).

Table 12 : Rough linear estimation for sizing up an US-desintegration unit in order to treat retrun activated sludge or thickened return activated sludge.

p.e. (x 1000) 100 80 60 40 10

WITHOUT THICKENING

CSB after dis. 11068 8853 6640 4427 1107 input WWTP in

kg/d

SS after dis. 4500 3600 2700 1800 450

Throughput 54 43 32 22 5,5 m³/h

power 270 215 160 110 27,5 Installed power

[kW]

specific Energy 18,0 18,0 18,0 18,0 18,0 kWs/L

SRT 12,0 12,0 12,0 12,0 12,0 d

basin volume 51333 41067 30800 20533 5133 M³

CF 1,66* 1,66* 1,66* 1,66* 1,66* -

CF*treated

amount 0,50 0,50 0,50 0,51 0,51

a after dis. 0,30 0,30 0,30 0,29 0,29

WITH THICKENING

3 3 3 3 3 DS in g/L before

Thickening

70 70 70 70 70 DS in g/L after

Thickening case 1

40 40 40 40 40 DS in g/L after

Thickening case 2

7% DS 11,6 9,2 6,9 4,7 1,2 Installed power

(kW) case 1

4% DS 20,3 16,1 12,0 8,3 2,1 Installed power

(kW) case 2

*value found in Saint Sylvain trial

4.2. Economics

4.2.1. WWTP of 100,000 p.e. and land spreading The economic assessment is based on the scenario of a 100,000 p.e. with treatement of thickekend sludge with a DS content of 7%. The installed power needed is equal to 11,6 kW (see Table 12), this means that two US-units will be installed. Sludge production of a plant of 100,000 p.e. is calculated at 9,240 ton (33 x 280 tons with content DS 20%). The results are displayed in Table 13.

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Table 13 shows that US treatment for a plant of 100,000 p.e. could be economic feasible under the assumed circumstances (land spreading of dewatered sludge at low costs). The investment of 2 US-units can be paid back within 10 years. However, the calculation also shows that the economic advantage of US treatment would be small (not even enough to finance interest payments over ten years). If external sludge handling costs are higher than € 11.10 per ton (20% DS), or € 55.50 per ton DS, the application of US becomes economically interesting.

Table 13 : Economic performance of US treatment “example WWTP with 100,000 p.e. and sludge land spreading”

Larger plant 100,000 p.e. - 100% land spreading continuous thickening machines - 30% reduction sludge 9,240 ton sludge (DS 20%) - electricity costs € 0,05 / kWh 1,848 ton dry sludge - disposal costs € 10 / ton (DS 20%) - dewatering costs € 10 / ton Investment in 2 US-units € 120,000

Capital costs, annuity 10 years, 5% € 15,541 Electricity (sonotrodes) 80,520 kWh/y € 4,026 Maintenance (1% inv costs) € 1,200 Replacement of 2 sonotrodes per year € 10,000

Total operational costs € 15,226 Total annual costs € 30,767 Savings on electricity (dewatering) ton/y € 0 Savings on sludge handling (land spreading) 2,772 ton/y € 27,720

Total operational savings € 27,720 Annual net operational savings € 12,494 Payback period years 9.6

4.2.2. WWTP of 100,000 p.e. and incineration If sludge produced by a WWTP of 100,000 p.e. has to be incinerated in stead of applied on agricultural land, savings on sludge handling can make the US treatment economically interesting. In

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Table 14, the economic performance of applying US treatment at a WWTP of 100,00 p.e. and incineration of the disposed sludge is presented.

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Table 14 shows that US treatment for a plant of 100,000 p.e. and sludge incineration will be economically very interesting. The additional costs are more than covered by the savings on (external) sludge handling. The investment of 2 US-units can be earned back in six months. The large difference with the example presented in paragraph 3.3. is the difference in unit costs of sludge handling. If these unit costs are high (as is the case with incineration compared to agricultural use) the costs savings easily cover additional investment and operational costs.

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Table 14 : Economic performance of US treatment “example WWTP with 100,000 p.e. and sludge incineration”

Larger plant 100,000 p.e. - 100% land incineration Continuous thickening machines - 30% reduction sludge 9,240 ton sludge (DS 20%) - electricity costs € 0,05 / kWh 1,848 ton dry sludge - disposal costs € 90 / ton (DS 20%) - dewatering costs € 10 / ton Investment in 2 US-units € 120,000

Capital costs, annuity 10 years, 5% € 15,541 Electricity (sonotrodes) 80,520 kWh/y € 4,026 Maintenance (1% inv costs) € 1,200 Replacement of 2 sonotrodes per year € 10,000

Total operational costs € 15,226 Total annual costs € 30,767 Savings on electricity (dewatering) ton/y € 0 Savings on sludge handling (incineration) 2,772 ton/y € 249,757 Total operational savings € 249,757

Annual net operational savings € 234,531 Payback period years 0.5

4.2.3. Break even analysis: plant size and disposal costs After analysis of the base situation for the WWTP of Saint Sylvain d’Anjou and the different situations (incineration in stead of agricultural use, larger scale) the question rises “at what level of external disposal costs and at what plant size will an investment in US treatment break even with cost savings”? As the analysis in § 4.2.1 shows, the break even point in terms of external disposal costs for a plant of 100,000 p.e. will be somewhat higher than € 10. Calculated exactly, annual net operational savings are nil if sludge disposal costs are € 55,50 per ton DS. At external sludge disposal costs lower than € 55,50 per ton DS for a plant with only aerobic treatment (no sludge digestion) and a capacity of 100,000 p.e., US treatment is not economic feasible under considered conditions described before (and in report DE1)5. Additional effects like prevention of foam and bulking sludge – another effect of ultrasound use - can have positive impact on suitability, not only economically. In Figure 13, a break even analysis is presented, showing for different plant sizes the costs of application of US to reduce one ton of sludge (100% DS). These costs range from over € 1,000 per ton DS for very small plants, to about € 50 per ton DS for large WWTP’s. Combinations in the green area are cost effective; combinations in the orange area are not cost effective. If external sludge disposal is cheap, € 100 per ton DS (€ 20 per ton 20% DS), the application of US is only interesting at larger WWTP’s. This is the case if the sludge is landfilled. But in the case of incineration of the sludge, at costs of € 450 per ton DS, the use of US for sludge reduction is feasible at smaller WWTP’s too!

5 Using ultrasound disintegration prior to digestion (which is the today main type of US-application at WWTP’s) to increase biogas production, as well as to reduce the amount of disposed sludge, is more likely to be cost-effective on a lower level of sludge disposal costs.

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landfill

incineration

base situation St Sylvain

0

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600

700

800

900

1000

0 6 500 10 000 50 000 100 000 150 000 250 000

WWTP size (p.e.)

€ pe

r ton

Dry

Mat

ter d

ispo

sed

Figure 13 : Break even line of combinations WWTP size and disposal costs per ton dry matter

5. Conclusions The main target of this project was to demonstrate the potential for sewage sludge reduction in aerobic waste water treatment by means of ultrasound treatment under well defined conditions in a full scale (size of WWTP: 6000 population equivalents). The results of the full test should allow drawing conclusions regarding technical aspects of ultrasound treatment, about effects on waste water treatment process and about the overall impact on the environment. After a certain time needed for stabilising onsite sludge handling process and a number of pre-tests a 6 kW low amplitude (low wear) ultrasound system was installed to treat return activated sludge (RAS) in a continuous way. The Ultrasound system was in operation for 264 days without any break and showed a very stable disintegration performance. During that time as well as at the periods without ultrasound treatment waste water parameters, sludge properties and effluent quality were observed. It could be shown, that - WWTP-input conditions were continuously analyzed and showed stability in all trial periods - a sludge reduction of about 30 % was achieved due to using the ultrasound device. Proof of that finding was done by sludge mass balance and comparison between theoretical and recorded sludge production, these data fitted very well. In terms of sludge properties a significant increase of enzyme activity in the RAS was found while ultrasound was in operation as well as a slight increase of phosphorus concentration in the effluent. Reason for last mentioned fact is not completely clear, because during operation of ultrasound dosage of FeCl3 for phosphorus precipitation was lower than in times without ultrasound treatment. Dosage adjustment is recommendable to guarantee compliance with legislation rules. No environmental pressures have been observed.

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On base of the findings a method for extrapolation of results was developed which can be used to draw conclusions regarding design of ultrasound devices for other WWTP. From an economical point of view US treatment at the scale of the St Sylvain plant (6,300 p.e.) is not economical feasible. Even if sludge handling costs (costs for dewatering and incineration) would increase from € 10 to about € 30 per ton sludge produced, US treatment would not save costs in that particular case. Nevertheless, using ultrasound in order to reduce aerobic sludge production at larger plants could lead to cost savings which would generate short payback times. For example, the break-even-point of sludge disposal costs for a 100,000 p.e. WWTP with US treatment of thickened return activated sludge can be estimated at about € 11 per ton sludge (sludge at 20% DS). In that case a detailed cost-benefit-analysis is recommendable. Using ultrasound disintegration prior digestion (if existing) is more likely to be cost efficient also with lower sludge disposal costs.

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