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e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 687–694

avai lab le at www.sc iencedi rec t .com

journa l homepage: www.e lsev ier .com/ locate /eco leng

mergy required for the complete treatment of municipalastewater

aolo Vassallo ∗, Chiara Paoli, Mauro FabianoIPTERIS, Department for the Study of Territory and its Resources, University of Genova, Corso Europa 26, 16132 Genoa, Italy

r t i c l e i n f o

rticle history:

eceived 1 August 2008

eceived in revised form

3 October 2008

ccepted 16 November 2008

eywords:

astewater

nvironmental services

mergy analysis

a b s t r a c t

The continuous increase of human pressure on the environment and the concomitant

pollution threat call for more complete and efficient environmental protection systems.

Wastewater treatment plants are a technological response to the accumulation of pollution

that occurs during the human-dominated phases of water cycle. In recent years, thanks to

significant improvements in sewage treatment methodology, a number of upgrades have

been assessed to improve the efficiency and functionality of treatment systems. Nonethe-

less, this activity requires large material and energy consumptions that have to be carefully

accounted for when evaluating the efficiency of the process. In this work we present an

emergy approach to the evaluation of a wastewater treatment plant located along the Lig-

urian coast (NW Mediterranean Sea). Besides the evaluation of the water treatment plant

system, a preliminary assessment of the environmental costs in terms of natural fluxes

ewage

igurian Sea

erobic treatment

required for the treatment process was performed. In fact, at the end of the treatment dis-

charged water is still loaded with substances that have to be adsorbed by the receiving

natural system. The work done by nature assimilating this load is generally considered as

free while it is counted as a further cost in the total emergy budget of the water purification

process.

assessment. In contrast, emergy analysis (Odum, 1996) is a

. Introduction

astewater treatment is a requisite process needed for thebatement of polluted water, which is mainly released byuman activities. Pollution risk is particularly increased byhe tendency for human society to concentrate its activitiesn large population centers. If on one hand wastewater treat-

ent is a key activity preventing the onset of environmentalroblems, on the other it implies resources consumptionnd external inputs (e.g. emissions from the use of fossiluels and local environmental impacts elsewhere from the

xtraction of raw materials used to make the machinery anduildings of the wastewater treatment plant) that have toe taken into account when considering the environmental

∗ Corresponding author. Tel.: +39 0103538069; fax: +39 0103538066.E-mail address: paolo.vassallo@unige.it (P. Vassallo).

925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.ecoleng.2008.11.002

© 2008 Elsevier B.V. All rights reserved.

performances of a treatment plant (Björklund et al., 2001).Therefore, the efficiency of wastewater treatment processeshas to be evaluated using a systemic perspective, which aimsat accounting for all the direct and indirect costs related toboth the human and natural contributions to water treatment.In this sense the economic way of thinking fails to evaluatethe role played by the biosphere through its environmentalservices (i.e., dilution, microbial decomposition, etc.) contri-butions to wastewater treatment that are generally regardedas free contributions and thus ignored during the economic

methodology able to incorporate environmental services intoa whole-system analysis. This is because emergy considersall the direct and indirect fluxes required to reach a certain

r i n g

688 e c o l o g i c a l e n g i n e e

level of production or maintain a service by translating all theinputs to a system in a single metric, i.e., measuring an equiv-alent ability to do work (Odum and Odum, 2003). In emergyaccounting all available energy inputs to the system are mul-tiplied by a factor, called the transformity, which measuresthe energy of one kind needed to support 1 J of work doneby that type of energy in the system. When the flux is moreeasily evaluated by means of mass (or even currency), suit-able conversion factors or emergy per unit values are appliedto transform mass or money into emergy. Both transformitiesand other conversion factors provide the means to evaluate allthe fluxes supporting the system in a common unit of measurecalled the solar emjoule (sej).

A number of previous approaches to the environmentalevaluation of treatment plant systems have been recentlydeveloped also by means of emergy analysis (e.g. Björklundet al., 2001; Geber and Björklund, 2002; Nelson et al., 2001;Siracusa and La Rosa, 2006; Zhou et al., 2007; Chen et al., 2007).In this study we present (1) an emergy approach to a typicalthree phases wastewater treatment plant located along theLigurian coast (the human-dominated phases of wastewatertreatment) followed by (2) an emergy-based evaluation of theoutflows released in the coastal zone at the end of the treat-ment process (the natural phases of wastewater treatment).The treatment process is able to strongly reduce the organiccontent of municipal wastewater, but the residual composi-tion of discharged water is still loaded with materials thatnature has to adsorb. The work that nature has to do to pro-cess these concentrated materials will be evaluated taking intoconsideration the requirements for the complete assimilationof the discharged organic matter.

Main objectives of this study were:

(1) to evaluate the efficiency of the wastewater purificationprocess in terms of emergy required per served person andper unit of treated organic matter;

(2) to assess the natural work required for complete treatmentof the waste after the effluent is released from the plant.

This paper represents a further step on the path to realizea bottom-up design for the evaluation of the fluxes exploitedon a typical Mediterranean coastal zone. We have performedemergy analysis of a fish farm (Vassallo et al., 2007) and twosmall marinas along the Ligurian coast (Paoli et al., 2008a,b);our overall aim is to analyze the activities related to coastalzone with an integrated approach in order to lead differentstakeholders (fishermen, marinas managers, beach managers,policy makers, etc.) to an environmentally sound manage-ment of their activities.

2. Materials and methods

2.1. Study area

The study was performed at the Savona wastewater treat-

ment plant, which is located in the suburbs of Savona, oneof the main cities in the Ligurian region (NW Italy, 44◦20′02′′N,8◦30′25′′E). The treatment plant occupies an area of 45,000 m2.It was built in 1999 and serves 10 coastal municipalities that

3 5 ( 2 0 0 9 ) 687–694

total of 250 km2 spread along a 34 km coastline. The local pop-ulation is slightly lower than 130,000 inhabitants. The plantwas designed to serve a maximum population of 166,000 per-son equivalents (p.e.). During 2005, the wastewater channeledto the plant for purification reached 11,900,000 m3. The sewagetreatment efficiency of a municipal wastewater plant is gener-ally measured by its ability to decrease the organic matter loadin wastewater. Two parameters are generally considered forthis evaluation: chemical oxygen demand (COD) and biologicaloxygen demand (BOD). They are both measures of the oxygenrequired in a body of water, either the BOD or COD decompo-sition of the organic matter content. The oxygen demand isinterpreted as an indirect measure of the organic matter loadbecause the treatment process is supposed to be completelydependent on aerobic oxidation. That is, the organic matteris decomposed (by the microbial community or by chemicalreactions) requiring oxygen which produces water, CO2 andenergy as in the simplified reaction reported in Eq. (1):

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (1)

We assumed that this reaction happens on the bottom whereorganic matter carried by sewage is expected to fall due to itshigher density.

Both COD and BOD are generally expressed in milligramsper litre (mg/l), which indicates the mass of oxygen consumedper litre of solution.

During 2005, the treated wastewater in the Savona plantcorresponded to 2,980,000 kg BOD and 5,640,000 kg COD.Abatement performances must comply with the current Ital-ian legislation in terms of organic matter load dischargedto the sea (D.Lgs. 3 April 2006, no. 152:25 mg/l BOD, 125 mg/lCOD). Abatement percentages for the Savona plant during2005 reached 96% for BOD and 87% for COD.

Treatment is achieved by means of a typical three phasesprocess (Fig. 1). The wastewater is first treated mechanically bycleaning filters and sand traps and in a pre-sedimentation pro-cess. Secondly, an aerobic microbial decomposition process isperformed in aeration basins, followed by end-sedimentationbasins. Finally a disinfection process is performed by means ofperfusion with ozone. After treatment, the water is dischargedinto the sea from a pipe (900 mm diameter) releasing treatedwater at 100 m depth and 1500 m from the coastline. The aer-obic digestion process during the second phase of treatmentgenerates sludge that settles out and is collected. Before dis-posal the sludge is digested anaerobically (Fig. 1).

2.2. Emergy concept and accounting

Emergy accounting quantifies the available energy or exergy(expressed in terms of solar energy) used up directly and indi-rectly to make a product or service. In emergy accounting, bothenvironmental and economic inputs, including energy, mate-rials, labor and currency, are all converted into solar emergyequivalents. The renewable energy and resources, like sun-

light, wind, rain, and tides, which are usually regarded asfree externalities of the production process in economics, areincluded in emergy accounting as significant environmentalsupport.

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e c o l o g i c a l e n g i n e e r

The algorithm for emergy accounting has been given bydum (1996). The total annual emergy inflow to the bio-phere from the sun, moon, and deep heat sources fromhe earth makes up the baseline. The baseline emergy ishe driving force for everything physically happening in theiosphere, together with the storages (e.g. fossil fuels, ores,orest biomass, soil organic content) previously built up by thennual emergy inflow and used at rates far exceeding theireplacement rate. In recent years, a number of modificationso the baseline have been introduced (Campbell, 2000; Odumnd Odum, 2000). In this evaluation we used the 9.26E+24 sejaseline (Campbell, 2000); aiming at consistent comparisonsith other studies (i.e., Gronlund et al., 2004; Nelson et al.,

001) we converted their results to the baseline here adoptedultiplying the older values by a 0.981 coefficient.The guidelines for the emergy calculation sketched by

dum (1996) require system diagrams to organize evaluationsnd account for all inputs to, and outflows from, processesUlgiati and Brown, 2002).

.2.1. Human-dominated phases of wastewater treatmentvaluationn Fig. 1, an energy systems diagram of the treatment plants presented and an evaluation table for the assessment ofhe represented flows was constructed from the diagram. All

aterials, energy, labor and environmental services were eval-ated in their common units (kg, J, D , m3, etc.) and thenonverted into solar emergy (sej) through the application ofuitable transformities or other conversion factors. Amongnvironmental services we have included in the calculationhe oxygen consumption used for bacterial respiration dur-ng the aerobic decomposition phase. Previous studies on the

mergy evaluation of treatment plants (Björklund et al., 2001;eber and Björklund, 2002; Nelson et al., 2001; Siracusa anda Rosa, 2006; Zhou et al., 2007; Chen et al., 2007) did not con-ider this item in the emergy budget of the treatment plants,

Fig. 1 – Schematic diagram of a three ph

3 5 ( 2 0 0 9 ) 687–694 689

but this is an effective consumption during the degradationof organic matter that in our opinion has to be counted. Theemergy contributions of oxygen will be considered when eval-uating the performances of waste treatment in this study, butit will be taken out of consideration during comparisons withprevious studies for the sake of homogeneity.

The resources exploited by the system are conventionallygrouped in two types, depending on their origin and replace-ment rate: group F comprises resources imported from outsidethe system. The remainder can be further split into subgroupsR, for renewable local resources, and N, for non-renewablelocal resources (Ridolfi et al., 2005). The greater the F sources,the more the system proves itself not to be self-sufficient,while a huge expense in terms of N inputs mirrors a strongdependency upon resources that cannot be replaced at thecurrent exploitation rate (Ulgiati et al., 1995).

Results from the table were employed for the calculationof a few indices aiming at the evaluation of the efficiency ofthe process. We determined efficiency of the treatment pro-cess in two different ways related to the purchased and thetotal emergy spent to purify the wastewater. Specifically effi-ciency was evaluated as (1) the purchased emergy requiredper capita (F/total served population) and (2) the transformityof treatment emergy used per joule of organic matter treated(U/sewage Joule).

2.2.2. Natural phases of wastewater treatment evaluationTreated water, released as output from the plant, has beenstrongly reduced in organic matter content but is far from purewater. The organic matter load still in the discharged water isreleased to the coastal zone and it has to be assimilated by theenvironment. We developed a conceptual model of the coastal

system partially following the approach suggested by Ulgiatiand Brown (2002) to perform an emergy evaluation of the workdone by the coastal system in assimilating the remainingorganic matter load. Ulgiati and Brown (2002) evaluated the

ases wastewater treatment plant.

690 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 687–694

Fig. 2 – Three basic scenarios for increasing organic matter discharge to the coastal zone environment. (A) No organicmatter discharge and background concentration maintained at natural local level; (B) increasing organic matter load that isadsorbed by coastal zone environment through the stimulated microbial activity staying within the carrying capacity of the

cap

environment; (C) organic matter load exceeding the carryingmatter load in the coastal zone environment.

environmental services for the abatement of the emissionsfrom a power plant by means of the dilution process inthe air. Here we used a similar approach for the evaluationof the output dilution in the coastal system together withaccounting for the organic matter load degraded by microbesin the marine environment. This latter calculation examinesthree basic scenarios depending on the mutual interactionof the two main factors affecting the ability of the coastal

zone to assimilate organic pollution: (1) the intensity of theorganic matter discharge and (2) the carrying capacity of theenvironment defined as the maximum ability of the localmicrobial community to mineralize incoming organic matter:

acity and leading to an accumulation of exogenous organic

• scenario (A) no discharge at all: both the organic mattercontent and the microbial community activity on the seabottom are maintained in the natural unperturbed condi-tions (Fig. 2A);

• scenario (B) a discharge that we can consider as “tolerable”for the coastal area, i.e., an organic matter load release thatdoes not exceed the carrying capacity of the sea bottom.The representation of this condition in a system diagram

leads to the creation of a new object (a storage tank) herereferred as “tolerable organic matter load”. It is processedby the microbial activity of the area, which is stimulated bythe discharge itself (Fig. 2B);

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scenario (C) an even more intense discharge that goesbeyond the assimilation capacity of the area: this leadsto the establishment of a further tank in the system herereferred as “un-tolerable organic matter load” that remainsin the environment without being channeled to the highertrophic levels or re-mineralized by bacterial activity (Fig. 2C).

At increasing organic matter load (moving from the firsto the second scenario), higher natural fluxes are requiredy the coastal area for the complete assimilation of dis-harged material. To determine these fluxes, we made twoasic assumptions: (1) the sea bottom degradation activityay increase up to a maximum rate and (2) higher quantities

f released organic matter ask for more oxygen to be decom-osed (Eq. (1)). This implies that with increasing dischargerom the plant (1) the organic matter will have to be spreadver a wider area to be completely processed and/or (2) moreurrent is needed to carry the dissolved oxygen needed forrocessing organic loads into the processing area. The bot-om area required to decompose the organic matter load ishe rate of organic matter discharge from the plant dividedy the maximum rate of degradation per unit area of the seaottom (Eq. (2)). The maximum degradation rate of micro-ial community has been assessed in a previous study andalued at ∼80 gC/(m2 year) (University of Genoa, unpublishedeport). The organic carbon discharge rate from the plant is.12E+08 gC/year:

rea (m2) = released carbon (gC/year)degradation rate (gC/(m2 year))

(2)

he necessary current flushing volume per time is the oxy-en required per time divided by the oxygen concentration inater. The oxygen required per time is given by the quantity ofOD discharged. Considering oxygen content in water equal

o 7.3 mgO2/l, the required water volume is thus calculated asn Eq. (3):

ater volume (l) = COD (mg)O2 conc (mg/l)

(3)

nce the water volume has been calculated the current mov-ng this volume is obtained considering an average currentelocity of 0.06 m/s as in Eq. (4):

inetic energy (J) = 12 × water volume (m3)

×water density (kg/m3) ×velocity2 (m/s)

(4)

he empower ascribable to the coastal area needed to assimi-ate the load is thus the sum of (1) the largest one of the emergynputs due to sun, rain, wind and tide calculated over thenlarged area, (2) the emergy related to oxygen consumptiony microbial community and (3) the emergy of the currentsinetic energy needed to carry the water volume supplying

he increased request for oxygen.

If the third scenario is reached, the natural fluxes will note sufficient for the complete assimilation of the organic mat-er input. This would lead to the accumulation of organic

3 5 ( 2 0 0 9 ) 687–694 691

matter on the sea bottom surrounding the discharge pointand the depletion of oxygen content in water (i.e., hypoxia,anoxia events, hypertrophic conditions). As a consequencethe removal of this organic matter load could be achievedonly by spending more time in processing or by invest-ing further external fluxes, most probably as a result ofhuman intervention, focused on the recovery of the impactedarea.

3. Results

3.1. Human-dominated phases of wastewatertreatment

We performed the analysis of Savona treatment plant activityduring 2005. Results are reported in Table 1.

The highest emergy contribution to the total emergy bud-get is due to the sewage reaching the plant. As already detectedby other authors, a very large amount of emergy is due tothe wastewater flow (Björklund et al., 2001). In fact, sewageis generated by a large number of different sources spread allover the served area; the produced wastewater is then gath-ered, transported and concentrated through the collecting webbefore being processed by the plant. These processes, togetherwith the high transformity of the sewage itself (due to its mate-rials and energy content), make this item have a large effecton plant performances.

Oxygen consumption is the largest renewable contributionto the total emergy budget while building materials and elec-tricity play the dominant role among non-renewable externalresources. The overall treatment cost is driven by the externalpurchased resources consumed, which are 97% of the totalplant’s emergy budget for treatment.

3.2. Natural phases of wastewater treatment

When water exits the plant and it is released into the sea,it is far from pure since, even with the current Italian legis-lation limits on BOD and COD, wastewater is still carrying anorganic matter load. As a consequence the purification processis not terminated at the plant and if we enlarge the windowof interest considering the whole process required (completeorganic matter mineralization due to the activity of coastalenvironment) a number of further environmental costs haveto be taken into account. As described above, we identifiedthree scenarios that allowed us to make an emergy assess-ment of the natural contributions. It is necessary to place thedischarge from Savona plant in one of them.

Samples collected both at the sea surface and near the bot-tom in the proximity of the discharge point of the Savona plant(impact in Table 2) and in a control area 2 miles North West ofthe discharge point (control in Table 2) showed that the plantoutlet does not significantly affect the receiving environment.In fact, the dissolved oxygen concentration is not differentbetween the two sites neither is it different with sea depth.If the organic load exceeded the carrying capacity of the area,

anoxic conditions would develop and oxygen concentrationwould be expected to drop.

As a consequence we can assume the carrying capacityof the area has not been exceeded. This is evidence that

692 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 687–694

Table 1 – Emergy table for the wastewater treatment process (resources and material consumption exploited duringpurification in plant; T1 = max of items 1–3 + sum of items 4–12) and total emergy (emergy of treatment raised by emergycarried by sewage; T2 = T1 + item 13).

Item Input U.M./year sej/unit Ref. Emergy (sej/year) Type

1. Sun 1.85E+14 J 1 Odum (1996) 1.85E+14 R2. Wind (kin) 3.97E+10 J 1.47E+03 Odum (1996) 5.96E+13 R3. Rain (chem) 1.58E+11 J 1.79E+04 Odum (1996) 2.88E+15 R4. Geot. heat 1.41E+11 J 3.37E+04 Odum (1996) 4.85E+15 R5. Oxygen 2.85E+09 g 5.06E+07 Ulgiati and Brown (2002) 1.47E+17 R6. Human lab. 9.53E+10 J 7.24E+06 Odum (1996) 7.03E+17 10% R, F7. Water 9.35E+09 g 7.16E+06 Vassallo et al. (2006) 6.83E+16 64% R, F8. Electricity 3.96E+13 J 1.71E+05 Odum (1996) 6.89E+18 F9. Fuel 9.52E+11 J 6.47E+04 Odum (1996) 6.28E+16 F10. Building 4.66E+09 g 6.94E+18 F10a. Steel 7.91E+07 g 4.07E+09 Buranakarn (1998) 3.28E+17 F10b. Concrete 4.58E+09 g 1.41E+09 Buranakarn (1998) 6.60E+18 F10c. Bricks 1.45E+06 g 2.18E+09 Buranakarn (1998) 3.22E+15 F10d. Aluminium 6.57E+05 g 1.25E+10 Buranakarn (1998) 8.34E+15 F10e. HDPE 1.89E+05 g 5.17E+09 Buranakarn (1998) 9.96E+14 F10f. PVC 5.98E+04 g 5.76E+09 Buranakarn (1998) 3.51E+14 F11. Chemicals 8.15E+08 g 3.73E+08 Odum and Odum (1983) 3.10E+17 F12. Maintenance 1.91E+04 D 1.56E+12 Marchettini et al. (2007) 3.04E+16 FT1. Treatment process emergy sej 1.52E+19 U13. Sewage 6.66E+14 J 3.73E+06 Björklund et al. (2001) 2.53E+21 FT2. Total emergy sej 2.55E+21 U

Notes: (1) Sun (J) = area × solar radiation × (1 − albedo) (area = 4500 m2, solar radiation = 5.14E+09 J/(m2 year), albedo = 0.2); (2) wind(J) = area × air density × drag coefficient × velocity3 × s/year (area = 4500 m2, air density = 1.3 kg m−3, drag coefficient = 1E-03, velocity = 2.8 m/s,s/year = 3.14E+07 s/year); (3) rain (J) = area × precipitation × water density × Gibbs energy (area = 4500 m2, precipitation = 0.71 m/year, air den-sity = 1000 kg m−3, Gibbs energy = 4.94E+03 J kg−1); (4) geothermal heat (J) = area × heat flow (area = 4500 m2, heat flow = 3.14E+06 J/(m2 year)); (5)oxygen (g) = (BODin − BODout) × treated sewage (BODin = 0.25 g/l, BODout = 1.08E-02 g/l, treated sewage = 1.37E+10 l year−1); (6) human labor (J) = nemployees × working days × energy consumption (n employees = 41, working days = 222 days year−1, energy consumption = 1.05E+07 J day−1);(7) water (g) = volume consumed × water density (volume consumed = 9352 m3 year−1, water density = 10E+6 g m−3); (8) electricity (J) = kWhconsumed × J/kWh conversion (kWh consumed = 1.10E+07 kWh year−1, J/kWh conversion = 3.6E+06 J/kWh); (9) fuel (J) = volume consumed × J/m3

conversion (volume consumed = 22.27 m3 year−1, J/m3 conversion = 4.27E+10 J/m3); (10) building (g) = building materials amount/plant lifespan(10a steel = 3.81E+09 g, 10b concrete= 2.29E+14 g, 10c bricks = 7.23E+7 g, 10d aluminium = 3.28E+07 g, 10e HDPE = 9.46E+06 g, 10f PVC = 2.99E+06 g,plant lifetime = 50 years); (11) chemicals (g) = yearly chemicals consumption (yearly chemicals consumption = 8.15E+08 g year−1); (12) chemicals(g) = yearly maintenance costs (yearly maintenance costs = 1.19E+04D year−1); (13) sewage (J) = sewage yearly volume × sewage energy content

07 J/m

(sewage yearly volume = 1.19E+07 m3, sewage energy content = 5.58E+

discharge from Savona treatment plant belongs to the secondscenario of Fig. 2.

These appraisals led to the evaluation of the emergy fluxesrequired to process the wastewater after it is discharged(Table 3).

The total emergy required for the complete assimilation ofthe wastewater discharge in the environment is entirely com-posed by renewable fluxes and is 2.6 times greater than the

natural fluxes exploited during the human-dominated treat-ment phases (R in Table 1). Among contributions listed inTable 3, rain plays the dominant role and it is followed byoxygen consumption.

Table 2 – Dissolved oxygen concentration in water at thedischarge point and in a control area 2 miles apart.

O2 concentration (mg/l)

Impact Control

Sea surface 7.32 7.18Sea bottom 7.43 7.38

3).

4. Discussion

The emergy used in the human-dominated treatment phasesis mainly supplied by the consumption of external non-renewable resources. Still, the emergy used for the treatmentis low in relation to the amount of emergy the wastewateris carrying, with a ratio of emergy in treatment to emergy insewage of about 1:140. This is due both to the great quantity ofsewage reaching the plant and to its high transformity. Hightransformity may, according to Odum (1991, 1994, 1996) indi-cate a high potential impact and this makes the treatmentprocess essential for the maintenance of good environmentalquality.

We compared the efficiency of the treatment process andnatural work performed during wastewater treatment witha number of other studies of three phases treatment plants(WWTP), a plant coupled with constructed wetlands (TP + CW)

and a natural wetland (NW) (Table 4). This comparison focusedon the evaluation of the environmental costs related to theprocesses needed to purify sewage and the assessment ofplant efficiencies.

e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 687–694 693

Table 3 – Emergy table for the assessment of the environmental effort for the assimilation of the discharged organicmatter load. Total emergy is raised by the maximum of items 1–4 + sum of items 5–6.

Item Input U.M./year sej/unit Ref. Emergy (E+16 sej/year) Type

1. Sun 1.60E+16 J 1 Odum (1996) 1.60 R2. Wind 1.60E+13 J 1.47E+03 Odum (1996) 2.40 R3. Rain 1.93E+13 J 1.79E+04 Odum (1996) 35.13 R4. Current 2.44E+08 J 1.08E+07 Odum (2000) 0.27 R5. Tide 5.72E+11 J 2.43E+04 Campbell (2000) 0.96 R6. Oxygen 8.33E+08 g 5.06E+07 Ulgiati and Brown (2002) 4.30 R

Total 40.39 U

Notes: (1) Sun (J) = area × solar radiation × (1 − albedo) (area = 3.90E+06 m2 from Eq. (2) where released carbon = 3.12E+08 g year−1 and degradationrate = 80.02 gC/(m2 year), solar radiation = 5.14E+09 J/(m2 year), albedo = 0.2); (2) wind (J) = area × air density × drag coefficient × velocity3 × s/year(area = 3.90E+06 m2 from Eq. (2) where released carbon = 3.12E+08 g year−1 and degradation rate = 80.02 gC/(m2 year), air density = 1.3 kg m−3, dragcoefficient = 1E-03, velocity = 2.8 m/s, s/year = 3.14E+07 s/year); (3) rain (J) = area × precipitation × water density × Gibbs energy (area = 3.90E+06 m2

from Eq. (2) where released carbon = 3.12E+08 g year−1 and degradation rate = 80.02 gC/(m2 year), precipitation = 0.71 m/year, air den-sity = 1000 kg m−3, Gibbs energy = 4.94E+03 J kg−1); (4) current (J) = kinetic energy of current, kinetic energy of current = 2.44E+08 J from Eq. (4)where water density = 1035 kg m−3 (velocity = 0.06 m/s and water volume calculated as in Eq. (3) with COD = 8.33E+11 mg and O2conc = 7.3 mg/l); (5)tide (J) = area × 0.5 × tides/year × tide range2 × water density × gravity (area = 3.90E+06 m2 from Eq. (2) where released carbon = 3.12E+08 g year−1

and degradation rate = 80.02 gC/(m2 year), tides/year = 730, tide range = 0.2 m, water density = 1025 kg m−3, gravity = 9.8 m/s2); (6) oxygen(g) = CODout × released sewage (CODout = 6.07E-02 g/l, released sewage = 1.37E+10 l year−1).

Table 4 – Purification efficiency and natural effort comparison with previously published studies.

F/population (E+14 sej/p.e.) Transformity (E+04 sej/J) F/R Source

Savona WWTP0.88

2.23 55This study

Savona WWTP (no O2 consumption) 2.21 122Florida WWTP 1.00 2.98 220

Nelson et al. (2001)Yucatan WWTP 2.30 13.96 2530Swedish WWTP 1.54 9.37 2998

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wt

Swedish TP + CW 1.80Swedish NW 1.54

Among the cases considered, the Savona treatment plantad the lowest value of external resources purchased perquivalent person served thus showing the best balanceetween plant activity and real needs of population. The effi-iency of the Savona treatment plant is also confirmed byhe transformity of the treatment process. In fact, the treat-

ent process led to an average emergy cost per treated cubiceter of 1.25E+12 sej/m3, corresponding to a transformity of

.23E+04 sej/J. This is the lowest value in comparison with pre-ious studies (Table 4) confirming that water purification inavona treatment plant is a very efficient process.

High waste processing efficiency is also coupled with aood ability to exploit renewable resources, since Savona hashe lowest F/R ratio among the WWTPs considered. Only thereatment process based on the natural wetland system hadn F/R ratio lower than Savona. The low F/R ratio for the nat-ral wetland system was mostly based on the use of naturalmergy in the purification process that required a large areand a long time to remove waste. F/R ratio also shows the rele-ance of oxygen consumption; when its contribution is takennto account the renewable resources used are more than dou-led in the total emergy budget (F/R = 55 if oxygen is accounted

nd F/R = 122 if oxygen is excluded from calculation).

The emergy expense related to the natural phases ofastewater treatment (Table 3) was low compared with the

otal emergy of the purification process (3% of total emergy

Geber and Björklund (2002)10.13 14210.82 9

budget, U in Table 1) but their assessment produces a moreaccurate evaluation of the total emergy required for the pro-cess, i.e., the total treatment costs. In order to perform thisevaluation we assumed that the entire benthic microbial activ-ity is devoted to the mineralization of the organic matter andthat this process occurs with the maximum rate of degrada-tion maintained over the entire time needed to decomposethe waste. These assumptions probably lead to an underesti-mation of the effort that the environment must make to carryout the purification process. Moreover, if the calculated valueaccurately captures the real effort of environment needed toprocess the waste, it would lead to the exclusive use of acoastal zone area equal to ∼4 km2. However, this ignores themultiple other valuable activities taking place in the coastalarea (i.e., recreational bathing tourism, game or industrial fish-ing).

5. Conclusions

The analysis of a three-phases wastewater treatment plant ledto the evaluation of the emergy costs required to both human

and natural systems to purify sewage. The wastewater treat-ment process proved to be an emergy-expensive activ-ity requiring large amounts of non-renewable resources.Nonetheless, this amount is still low, if compared with the

r i n g

r

694 e c o l o g i c a l e n g i n e e

emergy carried by the sewage itself making the treatmentprocess useful, because it avoids the release of concentratedmaterials harmful to environmental quality. The Savona treat-ment plant proved to be efficient in comparison with waste-water treatment plants examined in previous studies both inrelation to the emergy costs per member of the served popu-lation and in terms of the transformity of treated wastewater.

The treatment process continues until the limits estab-lished by the legal framework for the quality of the waterdischarge are reached. This means that part of the organicmatter carried by the sewage is released in the environmentas output from the plant. Here we have presented a prelimi-nary approach to the evaluation of the work that nature mustdo in assimilating this load.

Assuming that nature is able to mineralize all the organicmatter in the sewage discharge, the total emergy involved inthe “natural” treatment process is accounted as for increasedrate of natural fluxes required to support microbial degrada-tion (scenario B in Fig. 2). Nonetheless, if the carrying capacityof nature is exceeded, further fluxes will be required (sce-nario C in Fig. 2). The discharge of an “intolerable” load wouldmost probably lead to the generation of a new storage in theenvironment due to the inability of the ecosystem to mineral-ize the organic matter surplus. The removal of this load andthe restoration of the original environmental conditions, willrequire more time for processing or more human interven-tion to treat the excess waste, thus making the overall processmore and more expensive.

Acknowledgements

Authors warmly thank Matteo Gambino for its help duringthe data gathering process and data analysis and two anony-mous reviewers whose precious suggestions let us improvethe quality of the study.

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