Working with energy and mass balances: a conceptual

framework to understand the limits of municipal

wastewater treatment

J. M. Garrido, M. Fdz-Polanco and F. Fdz-Polanco

ABSTRACT

At present all municipal waste water treatment plants (WWTPs) are energy consumers. Electrical

energy requirements for oxygen transfer are large in secondary biological systems. Nevertheless,

from a thermodynamic point of view chemical oxygen demand (COD) is an energy source.

Combustion of every kilogram of COD releases 3.86 kWh of energy. In this manuscript some

measures are presented, from a conceptual point of view, in order to convert the actual concept of

wastewater treatment as an ‘energy sink’ to an ‘energy source’ concept. In this sense, electrical self-

sufficiency in carbon removal WWTPs could be obtained by increasing the sludge load to the

anaerobic sludge digester. Nitrogen removal increases the energy requirements of WWTPs. The use

of a combined two-stage biological treatment, using a high loaded first stage for carbon removal and

a second stage combined nitrification–anammox process for nitrogen removal in the water line,

offers a way to recover self-sufficiency. This is not a proven technology at ambient temperature, but

its development offers an opportunity to reduce the energy demand of WWTPs.

J. M. Garrido (corresponding author)Chemical Engineering Department,School of Engineering,University of Santiago de Compostela,Campus Sur,E-15782,Santiago de Compostela,SpainE-mail: juanmanuel.garrido@usc.es

M. Fdz-PolancoF. Fdz-PolancoDepartment of Chemical Engineering and

Environmental Technology,University of Valladolid,47011 Valladolid,Spain

Key words | COD balance, energy balance, nitrogen balance, wastewater treatment sustainability

INTRODUCTION

Wastewater is a mixture rich in water (>99%), with a small

amount of pollutants (<1%) that in wastewater treatmentplants (WWTPs) are transformed into by-products (carbondioxide and sludge). Regardless of the technology used

and the size of the facilities, at present almost all municipalWWTPs using aerobic biological processes for removingorganic matter are net electrical energy consumers. Aerobicprocesses used are net energy consumers due to the fact that

the oxidation of organic matter requires oxygen transfer, andaeration systems demand high amounts of electrical energy.Additionally, nitrogen treatment processes consume more

electrical energy than processes simply removing chemicaloxygen demand (COD) due to the additional oxygen andpumping requirements for the nitrification–denitrification

process (Jonasson ).Accepting as specific energy parameter the ‘source elec-

trical energy use intensity’ (EUI), defined as annual energyused on the facility divided by the average influent flow

(kWh/m3), the literature overview shows big differencesamong different facilities. EUI average values of0.78 kWh/m3 treated wastewater were reported in the

USA (US EPA & US ED ). Lidkea () analyzed

three Canadian WWTPs with average flow rate of56,000 m3/d, and found an average EUI value of0.35 kWh/m3. EUI in WWTPs of Flanders is on average

0.30 kWh/m3 (Fenu et al. ). Jonasson (), comparingseveral European facilities, obtains an EUI average value of0.30 kWh/m3 for Austria and 0.47 kWh/m3 for Sweden.EUI due to secondary treatment accounts for 0.2 kWh/m3

treated water in these two countries. Differences betweenthese two countries are related to energy consumption forpumping. The use of membrane technologies increases

energy consumption. EUI associated with the use of mem-brane bioreactors is between 0.8 and 1.2 kWh/m3. The useof conventional activated sludge (CAS) equipped with tertiary

membrane filtration and ultraviolet disinfection increasesEUI up to 0.593 kWh/m3 (Fenu et al. ; Maere et al. ).

Nevertheless, from a thermodynamic point of view,organic matter in wastewater can be considered not as an

‘energy sink’ but an ‘energy source’. All the organic com-pounds included in the wastewater contain energy storedwithin their chemical bonds. However, recovery of most of

2294 © IWA Publishing 2013 Water Science & Technology | 67.10 | 2013

doi: 10.2166/wst.2013.124

this energy, in terms of electricity, is a difficult task in

WWTPs. A large fraction of this energy is dissipated asresidual heat when secondary aerobic biological reactorsare used. At commercial level, only anaerobic digestion of

either wastewater or sludge may recover a fraction of thisenergy in the biogas. However, only sludge methanizationmay be used in cold or temperate regions of the world.Digester gas-fueled electric power generators are used to

recuperate energy from biogas. Cogeneration units usinginternal combustion engines are the most widely usedenergy recovery systems in WWTPs. They convert 30–42%

of the energy into electricity. Fuel cells also may be usedfor recovering energy from methane. Nevertheless, electricalefficiency of fuel cells is not much larger, between 36 and

45%. Moreover, cogeneration using fuel cells is moreexpensive than internal combustion engines (Brown &Caldwell ). Energy recovery using microbial fuel cellsor microbial electrolysis cells is still a challenge, due to

some concerns related to energy losses, high costs, and feasi-bility of using such technologies in full-scale facilities (Loganet al. , ; McCarty et al. ).

To perform energy balances, it is necessary to calculatethe ‘energy content’ (EC) of the wastewater. The paper ofShizas & Bagley () seems to be the first experimental

approach to determine the energy stored in domestic waste-water. Using an oven to dry the samples and a calorimeterbomb, they found that the heat of combustion (ΔUC) is lar-

gely dependent on the fraction considered: wastewater(�ΔUC¼ 3.2 kJ/g dry); primary sludge (�ΔUC¼ 15.9 kJ/gdry); secondary sludge (�ΔUC¼ 12.4 kJ/g dry); anaerobi-cally digested sludge (�ΔUC¼ 12.7 kJ/g dry). Converting

these experimental values based on dry matter into valuesbased on COD the new calculated values are very similarfor all fractions: raw wastewater (�ΔUC¼ 14.70 kJ/g

COD); primary sludge (�ΔUC¼ 11.12 kJ/g COD); second-ary sludge (�ΔUC¼ 12.05 kJ/g COD); an aerobicallydigested sludge (�ΔUC¼ 11.68 kJ/g COD). Taking COD as

reference, the experimental values of the energy stored inthe different streams of the WWTP did not vary so much.

In a more recent paper, Heidrich et al. () present

experimental results for two samples of wastewater fromdifferent facilities; both samples were dried in an oven orby freezing in order to minimize loss of volatiles. Foroven-dried samples the EC values were 22.5 and 17.7 kJ/

kg COD while for freeze-dried samples the values reportedare 28.7 and 17.8 kJ/kg COD. Heidrich et al. () obtainedlarger values than those of Shizas & Bagley (), as the

former authors corrected the combustion heat obtainedtaking into account a possible formation of nitric acid

from nitrogen gas during the experiments. Differences in

the experimental values obtained between the experimentaldata of Shizas & Bagley () and Heidrich et al. () aresuggesting that another methodology should be followed in

order to set the value of the heat of combustion of organicmatter.

COD is a conservative parameter easy to measure andfollow during wastewater treatment. COD may be converted

to methane in anaerobic digesters. Considering stoichi-ometry, it is easy to set that 1 kg CH4 is equivalent to 4 kgCOD. By applying Hess’s law and stoichiometry of the reac-

tions, and adopting from Perry () that the heat ofcombustion of methane is 55.53 kJ/g CH4, the heat of com-bustion of COD is calculated as:

�ΔUC ¼ 55:53kJ=g CH4ð Þ= 4 g COD=gCH4ð Þ¼ 13:88kJ=gCOD

This theoretical value of 13.88 kJ/g COD is in goodagreement with the experimental value 14.70 kJ/g COD pro-posed by Shizas & Bagley () for raw wastewater. Thus,

this energy potential, equivalent to 3.856 kWh per everykilogram of COD oxidized, will be considered throughoutthe calculations.

This work will show how the concept of a WWTP as‘energy sink processes’ may be converted to ‘energy sourcesystems’ in the near future, at least from a theoretical

point of view. For doing so, mass and energy balances willbe analyzed, considering a 400,000 population equivalent(p.e.) municipal WWTP. For this purpose, different scen-

arios to treat wastewater and meet nutrient and CODdischarge limits will be considered.

MATERIALS AND METHODS

Wastewater treatment plant

Energy and mass benchmarking will be considered to ana-

lyze a municipal WWTP of 400,000 p.e., treating74,500 m3/d wastewater and an organic load of 50,000 kgCOD/d. The population equivalent of this plant was fixedat 125 g COD/(p.e.·d) and 12 g total nitrogen (TN)/(p.e.·d),

which are equivalent to the 60 g biochemical oxygendemand/(p.e.·d) considered by the European urban waste-water treatment directive 91/271/EEC (EC ). The plant

layout of the facility included in the water line at least a pri-mary treatment, secondary CAS treatment and anaerobic

2295 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

sludge digester, as common elements for benchmarking

(Figure 1). Modification of this basic facility included twoscenarios that would be used for analyzing aWWTPdesignedfor COD removal and three additional scenarios for analyz-

ing WWTPs designed for nutrients removal.

Scenarios for COD WWTPs

Scenario 1. Conventional facility for carbon removal; Appar-ent biomass yield in the CAS was fixed at 0.5 g COD/g CODand efficiency of primary sedimentation was fixed at 30%.

Scenario 2. Improving primary sedimentation. Similar toscenario 1 but efficiency of primary sedimentation wasincreased to 40%, by using lower overflow rates and

adding flocculants in the primary sedimentation tanks assuggested by Siegrist et al. ().

Scenarios considered for nutrient removal WWTPs

Scenario 3. Conventional nutrient removal plant. A predeni-trification process was considered as the nutrients removal

system.

Scenario 4. Similar to scenario 3, but autotrophic nitrogen

removal of the centrate is considered by using a system com-bining both partial ammonia oxidation and anammoxprocesses.

Scenario 5. Autotrophic nitrogen removal in the water line isconsidered as tertiary treatment. High-rate CAS operatedwith very low solids retention time (SRT) was considered

as secondary treatment in order to increase secondarysludge production.

COD balances

COD balances were calculated using a spreadsheet (Excel).The information required to perform COD balances in the

secondary treatment, apparent biomass yield and oxygenrequirements, was determined with the assistance ofBiowin software.

Assumptions considered during the study

Energy requirements of the physical stages (e.g. pumping,degritting, operation of primary and secondary settlers,sludge dewatering units) and the sludge treatment are fixedat 20 Wh/(p.e.·d). This value is similar to that of only

19.2 Wh/(p.e.·d) reported for Austria by Jonasson ().EUI for the physical stages is only 0.135 kWh/m3, and8,000 kWh/d are required for treating the 74,500 m3/d

above indicated. By doing that it is assumed that these phys-ical stages are optimized and that energy consumption dueto these operations cannot be reduced to a perceptible

extent. Specific energy consumption associated withoxygen transfer in the biological stages was fixed at 1.0 kgO2/kWh, which is in between the typical oxygen transfercapabilities associated with various types of aerator (Metcalf

& Eddy ). Thus, the oxidation of 1 kg COD in the CASrequires 1 kWh electrical consumption.

Mass and energy balances are carried out following

COD evolution along the WWTP, while energy balancesare performed using the EC or combustion enthalpy(�ΔUC), so the EC of a stream in the flow diagram can be

calculated as a function of the COD mass flux (FCOD), andassuming an EC of 3.856 kWh for each kilogram COD(Equation (1)):

EC kWh=dð Þ ¼ FCOD kg COD=dð Þ×ΔUC kWh=kg CODð Þ(1)

Total COD of raw wastewater (50,000 kg/d) was dividedinto four COD fractions as suggested in Metcalf & Eddy(), and considering the typical characteristics of raw

sewage (Rieger et al. ): soluble inert COD (SI), 4% totalCOD; soluble biodegradable COD (SS), 10% of total COD;particulate inert COD (XI), 20% of total COD; and particu-late biodegradable COD (XS), 66% of total COD. SI was

neither oxidized nor separated in the sedimentation units ofthe WWTP. A fraction of XI is separated as primary sludge,and the remaining fraction present in the primary treated

wastewater is wasted in the secondary sludge. This COD frac-tion cannot be methanized. XS may be partly oxidized in the

Figure 1 | Mass (% COD, referred to the influent) and energy (MWh/d) balances for

scenario 1, a conventional WWTP for 400,000 p.e.

2296 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

CAS or methanized in the anaerobic digester. It was assumed

that secondary sludge was composed of a mixture of XS andXI. Futhermore, microorganisms generated during secondarytreatment were included in the XS fraction. SS is partly oxi-

dized and partly converted by microorganisms in the CAS.TN mass flux in the influent was 4,800 kg/d. For nutrientremoval scenarios 4 and 5, TN removal was fixed at 80% inaccordance with the 92/271/EEC European urban waste-

water directive (EC ).Biomethanization of the biodegradable COD fractions

of primary and secondary sludge is supposed to be similar.

The differences in the methane capacity obtained for thesesludges are related to the presence of a large fraction of XI

in the secondary sludge. Sixty-nine percent of COD destruc-

tion of XS was fixed in the anaerobic digester (Kabouris et al.) in order to calculate COD transformation in methane.Thus EC of biogas was calculated (Equation (2)):

EC kWh=dð Þ ¼ 0:69 FCOD-XS kg COD=dð Þ× ΔUC kWh=kg CODð Þ (2)

where FCOD-XS(kg COD/d) represented the COD mass flux

associated with biodegradable particulate COD for primaryand secondary sludges. Electrical efficiency of fueled elec-tric power generators was fixed at 35% (Brown &Caldwell ).

RESULTS AND DISCUSSION

Analyzing a conventional WWTP there is a first question toelucidate: is it possible to obtain a positive energy balance?The answer is clear: if the EC of the wastewater is higherthan the energy requirements of the WWTP, the plantcould be an energy source. Electrical energy requirements

of the conventional WWTP are fixed at 63 Wh/(p.e.·d)(Jonasson ). This amount is equivalent to 0.34 kWh/m3

treated wastewater in the considered WWTP. In our scen-

ario of 400,000 p.e. and 50,000 kg COD/d the electricalenergy requirements per day amount to 25,200 kWh/d.Nevertheless, the gross energy associated with wastewaterCOD is much larger, 203,500 kWh/d. If the whole COD

amount could be converted into methane, 71,225 kWh/delectricity would be generated. This was calculated byfixing efficiency of the electricity generator at 35%. This

first result is conclusive. The wastewater holds much moreenergy than that necessary to operate WWTPs. Nevertheless

from a practical point of view this limit will be lower. First,

nonbiodegradable COD, accounting for 24% of total CODin this study, cannot be transformed to methane. Second, afraction of biodegradable COD will be lost in the water

line due to its oxidation in the CAS. Thus, the goal is todemonstrate the technical feasibility of the energy recupera-tion processes.

COD removal scenarios

Scenario 1

Figure 1 depicts the mass and energy balances of the con-

ventional WWTP designed for carbon removal. Massbalances were performed using a basis of 100% for the50,000 kg/d COD mass load to the WWTP and assuming

that primary clarification efficiency is 30% (COD). Energybalances were calculated by using Equation (2), thus are pro-portional to the CODmass flux. Apparent biomass yield wasestimated to be 0.5 kg COD/kg COD, using the Biowin®

simulation software, corresponding to a CAS operatedwith a SRT of around 7 d.

Assuming these parameters the electrical energy require-

ments of the plant were estimated as 24.5 MWh/d;8 MWh/d (33%) are associated with the energy require-ments of the physical stages and 16.5 (67%) are required

for the oxygen transfer in CAS. The energy potential of themethane oxidation is much larger, 56.3 MWh/d. Neverthe-less, the electricity produced by the biogas internalcombustion engine was 19.7 MWh/d, only covering 80.5%

of the electricity requirement of the WWTP. The balancesclearly demonstrate that it is not possible to reach an ener-getically self-sufficient WWTP maintaining this

conventional operation strategy. EUI was 0.33 kWh/m3

treated. Electricity consumption in this kind of scenariocould be reduced considering the use of high efficient aera-

tion systems, with oxygen transfer capabilities of 1.7–2.2 kgO2/kWh (Svardal & Kroiss ). The assumption of 1 kgO2/kWh considered in the present study represents the typi-

cal capability of the aerators (Metcalf & Eddy ).Biogas was the only source for energy production and

aeration the main energy consumer. A first energy bottleneckmay be identified in Figure 1: the low efficiency of the pri-

mary system allows that 70% COD goes to the aerobicbiological process, consuming oxygen to ‘burn’ the organicmatter to produce carbon dioxide and residual heat. Another

limitation was related to the low efficiency of anaerobic diges-tion, which only transforms into biogas 29.2% of the influent

2297 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

COD. Thus, self-sufficiency of the plant could be improved by

increasing primary sludge production (scenario 2).

Scenario 2

A route to modify the mass and energy balances is to changethe efficiency of primary sedimentation from 30 to 40%

(Figure 2). This may be done by using lower overflow ratesand adding flocculants to the influent (Siegrist et al. ).The higher COD efficiency of primary sedimentation

increases the mass/energy load to the anaerobic digester,decreasing the amount of COD that will be oxidized andthe energy requirements of the CAS. The electrical energyrequirements of the plant were estimated as 22.0 MWh/d.

Energy requirements of CAS diminished from 16.5 to14 MWh/d. The electricity produced in the internal combus-tion engine was also 22.0 MWh/d, and the facility achieved

self-sufficiency. Other possible strategies to improve orincrease biogas production in the WWTP could be: (i) toincrease COD mass flux of primary sludge by using primary

filtration units; (ii) to improve COD methanization in theanaerobic digester using, for example, thermal hydrolysis

processes; and (iii) to use the anaerobic digestion process.

From these three alternatives, only the second seems to bean option as the use of primary membrane technologies isnot a proven technology and the third is recommended for

sub(tropical) climate conditions (van Lier ), limitingits use in cold and temperate climate regions.

Nutrient removal scenarios

Other scenarios that should be considered include thoseWWTPs that should meet nutrients limits. Phosphorusremoval itself, using either physical or biological processes,does not exert a strong effect on internal energy balances.

However, nitrogen removal exerts a strong influence on theenergy requirements of the WWTP. Oxygen requirementsassociated with nitrogen removal are very large, and compar-

able to those associated with organic carbon removal.For a conventional nutrient removal system, using

nitrification–denitrification processes, ammonia oxidation

requires a large amount of oxygen, around 4.57 kg O2/kgoxidized-N. Thus, energy requirements of nitrification arearound 4.57 kWh/kg oxidized-N. Moreover, 4.57 kWh elec-

tricity is lost with every kilogram of nitrate discharged in theeffluent. The denitrification reaction offers not only anopportunity of removing TN or recovering alkalinity in thewastewater. It is also a way of recovering energy; 2.86 kg

COD are oxidized per kg NO3�-N denitrified. Thus,

2.86 kWh electricity is saved per kg NO3�-N denitrified.

Scenario 3

To quantify the standard conditions a predenitrificationCAS system was considered, assuming an apparent biomassyield of 0.5 kg COD/kg-COD and a SRT of around 7 d.

Nitrogen content, associated with the particulate CODfractions, was 0.06 kg N/kg COD. Figure 3 shows the

Figure 2 | Mass (% COD, in relation to the influent) and energy (MWh/d) balances for

scenario 2; efficiency of primary sedimentation was 40%.

Figure 3 | COD percentage and balance (in relation to 100% in the influent) and total nitrogen balance (9.6 units in the influent, associated with 100 COD units) for scenario 3 (left) and 4 (right).

2298 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

evolution of TN and COD mass balances for scenario 3.

Considering wastewater composition, 100 mass units ofCOD are associated with 9.6 mass units of TN in theinfluent. Nitrogen load treated in the WWTP was 4,800 kg

TN/d. TN of the primary influent was the same as in theinfluent. Nitrogen recycled in the centrate counteractednitrogen separated in the primary sludge. From the 9.6 TNunits present in the influent, 20% of TN was washed out

with the effluent (1.9 units), and 59% TN was removed byusing the denitrification reaction (5.7 TN mass units). Elec-trical requirement was 33.7 MWh/d, which is much higher

than the 24.5 MWh/d estimated for scenario 1, with theCOD removal WWTP. Nitrogen removal itself accountedfor 9.2 MWh/d of the electrical requirements. On the

other hand, energy generation was the same as for scenario1, 19.7 MWh/d. EUI was 0.45 kWh/m3 and electricity gen-erated only covered 58.5% of the electricity requirements.Thus, the use of the conventional nutrient removal system,

using nitrification–denitrification processes, moves waste-water treatment away from self-sustainability. This doesnot mean that electrical self-sufficiency cannot be achieved

in nutrient removal WWTPs, but makes it more difficult.In this sense, Nowak et al. () presented two examplesof Austrian WWTPs designed for nitrogen removal, in

which self-sustainability was achieved: the Wolfgangsee-Ischl and the Strass WWTPs. For the first WWTP, the effi-ciency of both primary sedimentation and COD

transformation in methane in the anaerobic digester washigh, 37 and 61.8% respectively, and makes possibleself-sufficiency. The self-sufficiency for the Strass WWTPwas favored, among other factors, by the low N to COD

ratio of the influent, of only 0.07 g N/g COD, and the highelectrical efficiency of the electric power generator used,40% which was 5% higher than that considered for the pre-

sent work.Another possible alternative for reducing the energy

requirements of the WWTPs might be the use of auto-

trophic nitrogen removal processes using both partialammonia oxidation to nitrite and the anammox reaction(Siegrist et al. ). Nevertheless the oxygen requirement

of this autotrophic process is still large, around 2.22 kgO2/kg N removed.

Scenario 4

The WWTP is similar to that of scenario 3 (Figure 3), butnow an autotrophic removal process, e.g. Canon process

(Third et al. ), is considered for treating TN of the cen-trate. Eleven percent of TN treated with this process will be

oxidized to nitrate (0.2 TN units). The secondary CAS is a

conventional denitrification process, which should treat alower TN amount that that estimated for scenario 3 (8.0versus 9.6 TN units, respectively). Energy requirements

and generation of scenario 4 (33.6 and 19.7 MWh/d,respectively) are almost the same as those obtained forscenario 3. Denitrified nitrogen considered for both scen-arios was the same (5.7 units). Whatever the scenario,

ammonia is oxidized to nitrogen gas as the final productusing either conventional nitrogen removal or anammoxreaction. Nitrogen nitrate considered in the effluent of

scenario 4 was the same as that in scenario 3. Thus, elec-tron acceptor and energy requirements associated withoxygen transfer are almost the same in both scenarios.

Nevertheless, the use of the anammox process reducesdenitrification requirements in the CAS, allowing optimiz-ation of primary sedimentation as was analyzed inscenario 2.

Scenario 5

The use of autotrophic nitrogen removal systems, using nitri-fying and anammox biomass, will be considered in the waterline. This process is still under development by several

research groups (Siegrist et al. ). On the other hand,TN removal in scenario 5 was similar to that of 88% referredto by these authors for the anammox reaction. Eleven per-

cent was transformed to nitrate and the remaining 1% wasassimilated by the biomass. Figure 4 depicts the plantlayout of the WWTP with secondary treatment for COD

removal at SRT of 1 d, using, for example, a high-rate aera-tion CAS and autotrophic tertiary treatment for nitrogenremoval. Electrical requirements diminished from

33.7 MWh/d of scenario 3 to 23.9 MWh/d. An EUI ofonly 0.32 kWh/m3 was estimated. This process is self-sufficient as energy generation due to biogas combustion isestimated to be 26.7 MWh/d, covering 111% of the energy

requirements of the plant.

Figure 4 | Nitrogen and COD balances considering 100 g COD as basis of calculus in the

WWTP, considering autotrophic nitrogen removal in the water line.

2299 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

Summary

Table 1 summarizes the main results obtained in the fivedifferent scenarios analyzed. It should be stressed that

most of the results were obtained using a simple thermodyn-amic analysis. The main objective of this study was todetermine the thermodynamic limitations of sewage treat-ment. Of the gross energy associated with wastewater

COD, only a fraction can be recovered as electricity. Fordoing this, WWTP design capacity was fixed at 400,000 p.e.,in which an anaerobic digester was used for sludge stabiliz-

ation. Other authors (Svardal & Kroiss ) presentedenergy balances for WWTPs with different design capacitiesranging from 5,000 to more than 100,000 p.e.

Electrical self-sufficiency is possible in those WWTPsdesigned for carbon removal, e.g. by increasing the amountof COD treated in sludge anaerobic digesters (scenario 2).For those WWTPs in which nitrogen removal is required,

self-sufficiency depends on other variables, e.g. COD/Nratio of the influent, efficiency of anaerobic digesters or theuse of high efficiency electric power generators. Combined

two-stage biological treatment, as proposed in scenario 5,using a high loaded first stage for carbon removal and asecond autotrophic nitrification/denitrification stage for

nitrogen removal in the water line, offer an opportunity torecover the electrical self-sufficiency. This process still isunder development, and represents a challenge to reduce

the energy consumption. Other researchers presented inter-esting proposals for recovering energy, water, and nutrientsfrom WWTPs. Verstraete & Vlaeminck () propose theuse of a new process, ZeroWasteWater, but its future appli-

cation probably might be limited to those WWTPs in whichresource recovery is an issue. ZeroWasteWater implies a

large number of modifications in both the WWTP and the

sewerage system, in order to increase the influent COD con-centration, and also relies on the development of theautotrophic nitrogen removal process at ambient tempera-

ture. Another possible strategy to recover energy is the useof anaerobic technology, but its application is limited to(sub)tropical regions (van Lier ). In this sense, develop-ment of autotrophic nitrification/denitrification processes in

the water line is also an opportunity to reduce nitrogen con-tent of the anaerobic effluents.

CONCLUSIONS

Energy self-sufficiency in COD WWTPs could be obtainedby increasing the sludge load to the anaerobic sludge diges-

ter. This could be accomplished by increasing efficiency ofprimary sedimentation, diminishing sludge age in the CASor improving sludge destruction in the anaerobic digester.

Nitrogen removal increases the energy requirements ofthe WWTPs. More effort should be put into nutrientremoval CAS, by diminishing nitrate (energy) washout

with the effluent. The use of the anammox process for treat-ing the centrate is not a way for directly reducing energyrequirements. However, this process reduces denitrificationrequirements in the CAS, allowing an increase in primary

sludge and methane production in the plant. On the otherhand, the use of the anammox process in the water lineoffers a way to recover self-sufficiency. This could be

obtained by using a secondary high-rate CAS and tertiaryautotrophic nitrogen removal technologies. This is not aproven technology, but its development for treating nitrogen

in the water line offers an opportunity to recover the energysustainability of the WWTPs.

ACKNOWLEDGEMENTS

The authors are grateful to the Spanish Ministry of Scienceand Technology, through the Novedar-Consolider Project,

which funded this study (CSD2007‐00055).

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Table 1 | Comparison of the results obtained in the five different scenarios analyzed

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Electricitygeneration(MWh/d)

Self-sufficiency(%)

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First received 3 October 2012; accepted in revised form 25 January 2013

2301 J. M. Garrido et al. | Working with energy and mass balances Water Science & Technology | 67.10 | 2013

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