ways to optimize the energy balance of municipal wastewater(energía)

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Journal of Cleaner Production xxx (2014) 1e7

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Journal of Cleaner Production

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Ways to optimize the energy balance of municipal wastewatersystems: lessons learned from Austrian applications

Otto Nowak a, *, Peter Enderle b, Petar Varbanov c

a Nowak Waste Water Consult (NW2C), Colmarplatz 1, 7000 Eisenstadt, Austriab iwConsulting Engineers e.U. (iwConsult), Nikolaiplatz 4/II, 8020 Graz, Austriac Centre for Process Integration and Intensification e CPI2, Research Institute of Chemical and Process Engineering e M}UKKI, Faculty of InformationTechnology, University of Pannonia, Egyetem u. 10, 8200 Veszpr�em, Hungary

a r t i c l e i n f o

Article history:Received 2 December 2013Received in revised form19 August 2014Accepted 21 August 2014Available online xxx

Keywords:Municipal wastewaterWastewater treatmentEnergy optimizationWastewater heatAnaerobic treatment

* Corresponding author. Tel.: þ43 676 337 09 56.E-mail addresses: [email protected]

iwconsult.at (P. Enderle), [email protected]

http://dx.doi.org/10.1016/j.jclepro.2014.08.0680959-6526/© 2014 Published by Elsevier Ltd.

Please cite this article in press as: Nowak, O.,Austrian applications, Journal of Cleaner Pro

a b s t r a c t

This paper discusses some of the major ways in which the energy balance of municipal wastewatersystems can be optimized. In Austria, two advanced municipal wastewater treatment plants withnutrient removal are energy self-sufficient. At these plants the total consumption of electric energy issmaller than the energy production by means of Combined Heat and Power (CHP) generation usingbiogas from anaerobic sludge digestion. By additional measures like the addition of organic waste to thedigesters (“co-digestion”), the use of the thermal energy of the wastewater for space heating andalternative wastewater and waste options using alternative processes, municipal wastewater systems caneven become “energy-positive”. The studies have shown that wastewater treatment plants are capable ofreaching up to 180% energy generation compared to the energy needs, while switching from wastewaterto cooling water regeneration as the heat source of heat pumps for district heating can offer electricitysavings of up to 45%. However, negative effects on the environment like insufficient wastewater treat-ment or the release of methane gas to the atmosphere have to be avoided.

© 2014 Published by Elsevier Ltd.

1. Introduction

Considering the energy content of wastewater, two forms ofenergy have to be taken into account: thermal energy andchemically-bound energy of the organics contained in the stream.The latter is most commonly expressed as COD (Chemical OxygenDemand). Thermal energy is the larger part, but it has to be reusedas close to the source as possible, whereas the chemically-boundenergy can be transported via the sewer system with only littlelosses.

The concept of using municipal wastewater for residentialheating bymeans of heat pumps exists sincemany years (Funamizuet al., 2001). In Europe, more than 100 wastewater heat recoverysystems are meanwhile in operation mainly in Switzerland and inScandinavia e e.g. for Oslo (Venkatesh and Brattebø, 2011) and forStockholm (Pandis Iveroth et al., 2013).

(O. Nowak), peter.enderle@u (P. Varbanov).

et al., Ways to optimize the enduction (2014), http://dx.doi

In Austria, the theoretical potential of thermal energy recoveredfromwastewater via heat pumps amounts up to 450 GWh/y (Bucarand Schinnerl, 2007). In Germany, the potential of thermal heatbound inmunicipal wastewater is expected to be as high as to coverapproximately 5% of the space heating demand by using heatpumps (Müller and Butz, 2010). Some heat recovery systems havebeen launched recently (Butz and Müller, 2010). However, in uti-lizing the thermal energy of wastewater in an economic andecological way, the sewer system as well as the potential systemuser have to fulfill some basic requirements: minimal wastewaterflow of 15 L/s, heat demand of at least 100 kW, short distance be-tween heat source and heat sink, high operation performance ofheat pumps, etc. (Müller and Butz, 2010). The integration of heatexchanger systems at existing sewer systems is basically possible,but cost-intensive. Necessary rehabilitation measures at existingsewer networks, however, provide a good opportunity to integrateheat recovery devices. Although there are a number of applicationswhere the thermal energy from municipal wastewater is used, it isin general more efficient to reuse the heat from industrial waste-water or from cooling water.

Regarding the chemically-bound energy, in principle the organiccompounds can be degraded biologically under aerobic or

ergy balance of municipal wastewater systems: lessons learned from.org/10.1016/j.jclepro.2014.08.068

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O. Nowak et al. / Journal of Cleaner Production xxx (2014) 1e72

anaerobic conditions. By the latter process, energy can be gained inthe form of methane (CH4). However, some of the COD of waste-water is needed for nitrogen removal by denitrification (Nowaket al., 2011). Furthermore, aerobic treatment with a long solidsretention time of the biomass is necessary also for far-reachingdegradation of organic micropollutants. Under optimal condi-tions, about half of the biodegradable COD of municipal wastewatercan be converted anaerobically to CH4; the other half has to bedegraded aerobically which causes additional energy demand foraeration (Nowak et al., 2011).

Many authors doubt that it is possible to operate a conventionalmunicipal wastewater treatment plant (WWTP) designed as low-loaded single-stage activated sludge plant with nitrification andnutrient removal, so that it would be energy self-sufficient (e.g.Meda et al., 2010). However, some best practice examples of suchWWTPs have shown that aerobic and anaerobic conversion pro-cesses can be managed so that, based on the annual average, moreelectric energy is produced from the biogas (methane) from sludgedigestion by means of CHP units than what is needed for theoperation of the whole plant (aeration, pumping, sludge treatmentetc.). It has to be pointed out that no other organic substrate thanthe sewage sludge of the respective plant has been processed in theanaerobic digesters. Hence, it could be shown that energy self-sufficiency is achievable at conventional WWTPs with nutrientremoval treating ordinary sewage (Nowak et al., 2011).

It is possible to further optimize the overall energy balance ofthe “wastewater system”

e by the addition of “bio-waste” (kitchen leftovers and greenclippings) and of organic waste from agro-industries to thesludge digesters at WWTPs or/and

e by separate collection of the so-called “blackwater” (toiletwastewater) in order to reduce the organic loads discharged intothe sewerage. The “blackwater” can then be processed togetherwith organic waste in separate digesters (“biogas plants”).

In many regions the potential for utilizing the chemical energyof organic waste is by far not exhausted. In the case of biodegrad-able substances the best way to utilize this energy is generallyanaerobic digestion. Until now, anaerobic treatment of waste istypically applied to sludge treatment at larger municipal WWTPs,to the treatment of organically high-concentrated industrialwastewater and occasionally to the treatment of manure, foodleftovers from restaurants and so-called “bio-waste” from house-holds, if it is collected separately. In Austria, like in most countriesof Central Europe, municipal WWTPs with a design capacity ofmore than 30,000 PE (Population Equivalent) are mostly equippedwith mesophilic anaerobic sludge digestion. By using the digestergas as energy source, at least a part of the chemical energy from thewastewater is exploited. Overcapacities of anaerobic digesters atWWTPs are occasionally used for the treatment of organic waste inorder to increase the biogas production. However, not only the gainof biogas should be of interest when using these free capacities byfeeding co-substrates into the digester of WWTPs, but also impactson the operation of the plant.

“Co-digestion” of organic waste may lead to additional pollutionloads, mainly of nitrogen (ammonia), in the reject water from thedewatering of the residues of the fermentation process. Hence, thedenitrification capacity of the plant has to be examined in detail tofind out to what extent organic waste can be applied to the di-gesters without the installation of an additional treatment step fornitrogen removal. Otherwise, an upgrading of the treatment plantto improve nitrogen removal might be necessary. The additionalloads of COD and of phosphorus, however, are comparably low inthe reject water. Also the quality of the residues is important, if the

Please cite this article in press as: Nowak, O., et al., Ways to optimize the enAustrian applications, Journal of Cleaner Production (2014), http://dx.doi

digested sludge is used in gardening or agriculture. At theWWTP ofLeoben, situated in Central Austria, with a design capacity of 90,000PE and equipped with two anaerobic digesters, the free digestercapacities are used since 2004 for the fermentation of organicwaste, mainly from agro-industries (slaughterhouses, dairies andleather production). As the main loading of the plant is only abouthalf of the design capacity, one of the two digesters is enough forsludge treatment. Concerning additional nitrogen loads caused byco-digestion, investigations by Nowak et al. (2007) have shown,that the comparatively lowest release of nitrogen e related to theCH4 produced e occurred from the flotation residues due to thehigher fat content. The relatively highest nitrogen release comefrom the residues from leather industry. However, with all thesesubstrates the ratio of nitrogen in the reject water to methaneproduced has been lower than with waste sludge.

In sparsely populated rural areas, however, the energy of theorganic compounds in domestic wastewater is not used at all. Atsmall advanced biological WWTPs, all steps for wastewater andsludge treatment are aerobic. Consequently, energy cannot begained at all from organic substances. On the contrary, the specificenergy demand at these plants is considerably high, as a lot ofenergy is needed for far-reaching aerobic degradation of theorganic matter. Anaerobic treatment of residues from urbanwastewater and waste management could be a suitable possibilityin order to improve the energy efficiency and to gain energy fromunexploited organic substances. Moreover, for sanitation alterna-tive concepts should be considered, as presented e.g. by Bieker et al.(2010).

There have been recent investigations onwaste and wastewatertreatment touching the plant energy balances. One example is thework by Righi et al. (2013) on the sewage and food waste man-agement and another representative article is on the environ-mental assessment of an urban water system (Lemos et al., 2013).While they correctly put emphasis on Life Cycle Assessment, theconsideration of the plant energy balance and energy self-sufficiency seems to be under-researched and in need of system-atic exploration. The current article aims at filling this gap byconsidering several ways of ensuring positive energy balance ofwastewater treatment and the utilization of the low-grade wasteheat associated with wastewater and industrial cooling watercycles.

2. Heating with sewage or with cooling water

There are potentially two ways of improving the energy balanceof wastewater treatment plants and industrial sites. They arerelated to the thermal and chemical energy carried with thesestreams. Utilization of residual or waste heat carried with waterstreams is the first option and it is discussed in this section.

2.1. Potential heat sources

In regions with moderate climate where space heating isnecessary during the cold season also the sewage temperatures arelow during this season. While the temperature of sewage is around10 �C during winter season, the temperature of cooling water canbe assumed to be around 30 �C.

The temperature of municipal wastewater can be reduced inwinter by means of heat pumps for heating purposes by about 2 �C(from approx. 10 �C to 8 �C). Presuming a temperature of theheating circuit (heat pump output) of 40 �C, a COP (Coefficient OfPerformance) of about 4 is required in this case. This means that thethermal output would be four times higher than the electric energyto be spent.


Fig. 1. Energy balance of Wolfgangsee-Ischl TP for a period of 12 months (September2009 to August 2010) (Nowak et al., 2011).

O. Nowak et al. / Journal of Cleaner Production xxx (2014) 1e7 3

When reducing the temperature of cooling water from 30 �C to24 �C, again with a flow temperature in the heating circuit of 40 �C,the COP is around 8. Therefore, eight times more thermal energycan be gained than electric energy has to be spent. In other words,with the same expenditure of electric energy about two times morethermal energy can be harvested when cooling water is used fordistrict heating with heat pumps as compared to the application ofmunicipal wastewater for the same purpose.

An interesting variation of the low-grade wastewater heat uti-lization is the concept of “low-temperature district heating” (or“cold district heating”) (e.g. Olsen et al., 2008). In this concept theheat of industrial wastewater or cooling water with temperaturesof 30 �Ce40 �C is fed via a heat exchanger into a pipe system to theconsumers. Each single consumer has his own heat pump to pro-duce hot water (ca. 45 �C) by adding a small amount of electricalenergy (ca. 10%).

2.2. Illustrative example 1

For Austria, it can be estimated that about 15,500 m3/h ofmunicipal wastewater could be used as a maximum for districtheating by means of heat pumps. This equals to about 40% of themunicipal wastewater of WWTPs with an average loading of morethan 10,000 PE providing approximately 36 MWof thermal energy.With a COP of 4, an electric energy of about 9 MW has to be appliedto obtain this thermal energy. Hence, in this scenario in total45 MW could be gained by the use of municipal wastewater forspace heating. To get the same energy in total (45 MW) in the caseof the application of cooling water e reducing its temperature from30 �C to 24 �C by heat pumps for district heating e only about5700 m3/h of cooling water have to be used. This is equal to about6% of the estimated total flow of cooling water in Austria (800million m3 per year). With this, 40 MW of thermal energy could beproduced, while 5 MWof electric energy would have to be applied.Thus more than 40% of electric energy could be saved, if coolingwater from the industries could be utilized for district heating bymeans of heat pumps instead of municipal wastewater.

3. Utilization of the chemical energy content of wastewater

The second energy source to utilize is the chemical energybound in waste and wastewater streams in the form of organiccompounds. This can be utilized at two levels. The simplestapproach is to process the wastewater sludge in anaerobic di-gesters. This usually provides sufficient energy to run the plants.The infrastructure and the processing unites, however, can bemuchbetter utilized as investment and loaded with additional substratesfor co-digestion. This also provides synergy effects for enhancedwaste treatment and extra energy generation.

The following two case studies illustrate the application of theseprinciples. The common indicator for energy consumption ofmunicipal wastewater treatment plants kWh/(PE$y) was used toillustrate the energy consumption of the wastewater treatmentplants. “Energy self-sufficiency” is defined as the energy con-sumption in kWh/(PE$y) equal or lower than the energy productionfrom the generated digester gas by CHP.

3.1. Case study 1: energy self-sufficient municipal wastewatertreatment plant for nutrient removal

The WWTP of Wolfgangsee-Ischl is a single-stage activatedsludge system with primary sedimentation and anaerobic sludgedigestion. The aeration tanks (5100 m3) are equipped with fine-bubble aeration and stirring devices. The treatment plant was put


into operation in the mid-1980s and afterwards only upgraded bychanging and optimizing mechanical devices.

In 2009, the mean influent load was about 40,000 PE related to120 g COD/(PE$d). Due to summer tourism, in the months of July toSeptember, the influent load equals to around 50,000 PE, whereasduring the rest of the year the influent load is in the range of 33,000to 40,000 PE. Originally, the plant was designed for 100,000 PE, butonly for carbon removale and for phosphorus removal, because thereceiving river flows later into a lake. However, because it was clearduring plant design that at least in the beginning of the plantoperation, the influent load will be much lower than 100,000 PE,the aeration tanks were designed in a way that makes nitrificationand denitrification practicable. Hence, the aeration tanks areoperated with a combination of pre-denitrification and intermit-tent nitrification-denitrification. The effluent temperature variesbetween 7 �C and 17 �C. Now, the effluent standards are 0.5 mg/Lfor total phosphorus, 5 mg/L for ammonia nitrogen at effluenttemperatures above 8 �C and 70% of nitrogen removal on theaverage of all days with more than 12 �C in the effluent.

The N:COD ratio of the influent is in the range of 0.09 and0.10 g N/g COD on the average. COD removal by primary sedi-mentation was found to be about 37% (Nowak, 2003). In the aera-tion tank, the solid retention time (SRT) is about 8 days in summerand about 12 days in winter. The extent of nitrogen removal of theplant is around 76% on the annual average, and about 80% on theaverage of all days withmore than 12 �C in the effluent. This plant isequipped with two large digesters operated in series with an SRT ofalmost 80 days in total. The digester gas is mainly used in con-ventional CHPs. After a second, more efficient CHP unit has beeninstalled with an electric efficiency of ca. 34%, all biogas can beutilized for electricity production and, moreover, the energy de-mand of the plant was further reduced by optimization. The rejectwater from sludge dewatering is not treated separately, but onlyequalized by means of a storage tank. The digested sludge isdewatered by means of a chamber filter press and used inagriculture.

Fig. 1 shows the energy balance of Wolfgangsee-Ischl TP for the1-year-period of September 2009 to August 2010. It can be seenthat it has a slightly positive energy balance on average. Over the12-month-period, 20.6 kWh/(PE$y) of electrical energy were pro-duced from the digester gas. Surplus electric energy from the plantis fed to the grid. For the peak energy demand, electricity as well as



natural gas is taken from the grid. Natural gas is used in order toavoid peaks in the electricity demand from the gridwhichwould becostly. 2.2 kWh/(PE$y) of electric power were fed into the grid,while 0.4 kWh/(PE$y) were taken from the grid. With the naturalgas taken from the gas grid another 0.4 kWh/(PE$y) of electricalenergy were produced by means of the CHP units. In total,19.2 kWh/(PE$y) of electricity were consumed at this WWTP ofwhich 11.5 kWh/(PE$y) were used for aeration and the stirring ofthe aeration tank, and 7.7 kWh/(PE$y) of electric energy were usedfor all the other treatment steps and devices.

Over this 1-year-period (09/2009 to 08/2010), the overall sur-plus of electricity production was 7%. Between the years 2010 and2012, the overall-surplus of electricity production varied between 6and 10%.

It has to be pointed out that the Wolfgangsee-Ischl TP is aconventional single-stage activated sludge plant like thousandsothers worldwide and that there is no additional energy input eneither by “co-substrates” fed into the anaerobic sludge digestersnor by separate devices for electricity production, like photovoltaicor wind power.

The main reason for the neutral to positive energy balance ofthis wastewater treatment plant is the longstanding and on-goingoptimization of all mechanical equipment and an optimal aera-tion control.

3.2. Case study 2: from energy self-sufficiency to energy productionin excess at a municipal wastewater treatment plant

The Strass WWTP is designed as two-stage activated sludgeplant, with a very high-loaded first stage with solids retention time(SRT) below 0.5 days. COD removal in this high-loaded stage isaround 50%. The SRT or “sludge age” in the second, low-loadedstage (aeration tank volume of 10,740 m3) is about 12e14 daysand the temperature varies between 9 �C and 18 �C. Due to wintertourism, the influent load equals to about 220,000 PE during winterseason, whereas during the rest of the year, the influent load issometimes less than 90,000 PE. The N:COD ratio of the influent isaround 0.07 on the average. The extent of nitrogen removal of theplant is around 85% on the annual average. The solids retentiontime in the digesters is around 36 days. Due to the two-stage bio-logical process, a lot of biomass with a lot of nitrogen is transferredto the digesters, and therefore the nitrogen load as ammonia in thereject water from sludge dewatering is very high. Nitrogen from thereject water is removed by deammonification (“anammox”) in aseparate treatment stage to a high extent. For deammonification, atthis plant the DEMON process has been developed (Wett, 2007).The digester gas is utilized in the conventional CHP units.

In 2004, after upgrading the CHP with a new unit (electricalefficiency: 39.5%), the WWTP became energy self-sufficient. On theaverage of the years of 2005e2007, 21.4 kWh/(PE$y) of electricenergy were produced from the gas from sludge digestion. Thepeak energy demand has still to be taken from the grid, surpluselectrical energy from the plant, however, is fed into the grid.Approximately 3.2 kWh/(PE$y) could be fed into the grid on theaverage of this period, and 1.7 kWh/(PE$y) were provided from thegrid. In total, 19.9 kWh/(PE$y) of electricity were consumed at theWWTP of which 9.1 kWh/(PE$y) were used for aerating and stirringthe aeration tank, the “rest” (10.8 kWh/(PE$y)) was used for all theother treatment steps and devices including the influent pumpswhich consumed 1.9 kWh/(PE$y). Accordingly, over the wholeperiod of these three years (2005e2007), 6.3% more electricity wasproduced by means of CHP units from the biogas through theanaerobic digestion of the excess sludge from both biologicalwastewater treatment stages than what was needed for the oper-ation of the plant. Hence, like the Wolfgangsee-Ischl TP, the Strass


WWTP was energy self-sufficient during this period on the yearlyaverage without additional energy supply (Nowak et al., 2011).

In 2008, “co-digestion” started with the addition of conditionedkitchen waste (food leftovers) from restaurants to the sludge di-gesters. The organic substrate is directly fed into the digestertogether with excess sludge from biological wastewater treatmentto increase the electricity production from the biogas by the CHP.

Fig. 2 shows the fluctuation of electricity production and elec-tricity consumption in the period from August 2003 until April2010. It can be seen that during the years of 2005e2007, whenenergy self-sufficiency has been achieved for the first time, theenergy production as electricity was most of the time a bit higherthan the consumption of electric energy. By the addition of “co-substrate” the production of electric energy by the CHP has grad-ually increased to around 180% of the demand for electric power(Fig. 2).

At this wastewater treatment plant, no measurable increase ofthe ammonia load in the reject waters from sludge dewatering wasobserved, after co-digestion with kitchen waste as substrate wasstarted. And the amount of “sludge” residues increased onlyslightly, by about 5e10%.

4. Optimization of the energy balance by enhanced anaerobictreatment

4.1. Analysis of the options

Theoretically, from the energy point of view, the best way tofurther optimize the overall energy balance of the “wastewater sys-tem” would be anaerobic treatment of the municipal wastewater inorder to maximize the energy production in the form of biogas(methane). Studies on anaerobic treatment of municipal wastewateras single step, however, have typically revealed that the eliminationof COD (Chemical Oxygen Demand), especially in the case of waste-water with low COD concentration and low temperatures, is ratherunfavorable. This is in particular attributable to an inadequateemission of methane gas from the liquid phase to the air, whereby arelevantpart of the producedmethane gas leaves the anaerobic stagedissolved in theeffluent. Thismeansnotonlya loss of energy, but alsonegative impacts on the greenhouse gas balance due to the higherglobal warming potential of methane compared to carbon dioxide.

A study on anaerobic treatment of municipal wastewater hasrevealed that anaerobic treatment could be an alternative to aerobictreatment in general (Urban et al., 2007). Within this study, testshavebeen carried outwithmunicipalwastewater (gCOD¼ 573mg/L)based on an UASB pilot plant (Upflow Anaerobic Sludge Blanket),whereby operational values between 20 and 25mg/L CH4 have beenfound dissolved in the effluent. To improve the balance of green-house gases of the treatment system, the dissolved methane couldeither be reused by stripping the methane from the effluent (Urbanet al., 2007) or new ways to gain a higher concentration of theinfluent to the anaerobic stage should be considered.

In the following, one possible approach to gain energy frommunicipal wastewater is discussed. Usually, the fresh water de-mand of common flushing toilets is comparably high. In Austria,approximately 130 L of fresh water are used per capita (C) and day,whereby about 30% of the fresh water is used for toilet flushing(OVGW, 2013). To avoid a high dilution of the blackwater theinstallation of vacuum toilets is therefore a promising alternative togenerate a highly concentrated “blackwater” stream. By means ofvacuum toilets, the necessary amount of flushing water could bereduced to 0.8e2 L of water per flush or approximately 8 L of freshwater per capita and day. Thus, in the case of Austria, the waterconsumption could be reduced from 130 to about 100 L per capitaand day.


Fig. 2. Energy production and energy consumption, both as electricity, and the ratio of energy production to energy consumption at Strass TP.


Almost 60% of the COD load in domestic wastewater results fromhuman excreta, asmentioned e.g. by Otterpohl et al. (1999) or Peter-Fr€ohlich et al. (2007). Therefore, on the basis of a per capita load of120 g COD/d in the total wastewater, the per capita COD in theblackwater is about 70gCOD/d.Hence, a highly concentratedorganicwastewater stream can be generated by using vacuum toilets with aminimum concentration of about 9 g COD/L. Thiswastewater streamcan be collected via vacuum sewerage in small communities anddwellings and treated anaerobically in order to gain energy.

In general, the necessary infrastructure (i.e. vacuum seweragesystems) to collect highly concentrated blackwater for up to 100households has already been realized in some cases in Germanyand in Austria within the last decade. The latest example for such asystem is the “Jenfelder Au” project, the revitalization of the formerLettow-Vorbeck military barracks in a new urban district in the cityof Hamburg. In the course of this project, approximately 630households are planned to be connected to a separating sanitarysystem based on vacuum technology with the aim to generateenergy from blackwater and to integrate a novel wastewaterinfrastructure (Skambraks, 2011).

4.2. Illustrative example 2

A significant amount of waste available in all households is “bio-waste”, mainly kitchen leftovers and, particularly in the country-side, also green clippings. By adding kitchen leftovers to theanaerobic system e preferentially via vacuum system crushed inadvance by waste disposers e the energy benefit can be increased

Table 1Specific energy content (COD, methane, thermal and electrical energy) of different sourc

ParameterC … capita

Domestic wastewater Black-water Sludg

CODtotal [kg/(C$y)] 44 ca. 26 ca. 26CODconverted [kg/(C$y)] 27e30 20e22 13e1Methane [m3/(C$y)] 9.5e10.5 7e7.7 4.5e5Energyth [kWhth/(C$y)] 52e58 38e42 25e3Energyel [kWhel/(C$y)] 33e37 24e27 16e2

“converted” means “anaerobically convertible to CH4”.values for “Blackwater” derived from (Peter-Fr€ohlich et al., 2007).values for “Sludge from municipal WWTP” derived from (Nowak, 2003).values for “Bio-waste collected” derived from (UBA, 2009).


considerably. In Table 1, the specific COD loads, as a measure fororganic compounds and energy content, of these different wastestreams are compared. “COD”, as kg/(C$y), and “methane”, as m3

N/(C$y), are used to express the energy content and the content oforganic matter. These parameters are more conservative than“organic matter” or “biogas”, because COD can be directly con-verted to methane (0.35 m3

N CH4/kg COD) and methane directlyinto thermal energy (10 kWh/m3 CH4).

For the calculation of the example shown in Table 1, an electricalefficiency of 35% and a total efficiency of 90% of the CHP unit havebeen estimated. It can be seen fromTable 1 that the specific COD loadper capita (C) or per Population Equivalent (PE) is about the same in“toilet wastewater” (blackwater) as the specific COD load in thesludge from conventional municipalWWTPs. However, significantlymore energy canbegained fromblackwater as sludge frombiologicalWWTPs is already partly stabilized. No data is available for the totalmass of “bio-waste” including green clippings. Thus, these valueshave to be estimated from data of currently collected “bio-waste” inAustria without green clippings (in Table 1: “Bio-waste” collected).

The data in Table 1 show that from blackwater and from “bio-waste” (collected) about 60 kWhth of thermal energy and about40 kWhel of electrical energy could be produced per capita and year.

5. Discussion

This paper discusses some of the major ways to optimize theenergy balance of municipal wastewater systems. It has beenshown in Case Study 1 that conventional wastewater treatment

es of organic substances from households.

e from WWTP “Bio-waste” collected “Bio-waste” total (estimated)

ca. 14 ca. 306 10e10.5 20e24.6 ca. 3.5 ca. 7.71 ca. 19 ca. 420 ca. 12 ca. 27



plants can be adjusted to provide small annual average energyexcess between 6 and 10%, although on a daily operation basis therecan be periods of energy deficit. Through adding “biowaste” to thewastewater sludge digester for the purpose of co-processing (co-digesting), the on-site electricity production can even increase toaround 180% of the own energy demand.

Thus, it could be shown that the most important measure forreducing the demand for external electric power at a wastewatertreatment plant is the anaerobic treatment of the sludge. By thisprocess “biogas” (methane)and thereforeenergycanbegained. In thisregard, it has to bementioned that methane production from sewagesludge atwastewater treatment plants usually remains constant afterthe implementation of energy optimizationmeasures as the chemicalenergy content of the wastewater (¼ influent) keeps about the same.Therefore,methane emissions are about the samebefore and after theimplementation of energy optimization measures. Anaerobic treat-ment of raw wastewater with low COD concentration instead ofanaerobic treatment of sewage sludge, however, can cause a signifi-cant increase of methane emissions as a certain amount of methaneremains dissolved in the effluent. Thismeans not only a loss of energybut also negative impacts on the greenhouse gas balance.

Considering the aspect of primary energy reduction per personand year, however, only a certain amount of primary energy couldbe saved through anaerobic sludge digestion. Taking into account,that not all people are connected toWWTPswith digesters and thata part of the influent load to municipal WWTPs comes from in-dustries about 50 kWh of primary energy could be saved per personand year. In Austria, the total consumption of primary energy isaround 50000 kWh per person and year. Therefore, even if it ispossible to collect enough organic waste to increase the gain ofdigester gas by the factor of 3e150 kWh per person and year, only0.3% of the consumption on primary energy could be covered by“biogas” from municipal WWTPs. Nevertheless, even this smallfraction could be a valuable contribution to more efficiency withinthe area of municipal wastewater systems and to the integration ofunused renewable energy sources in the network of energy supply.

The utilization of residual low-grade (low-temperature) heat inwastewater or cooling water as the heat source end of a heat pumpcan be also named as one major aspect to optimize municipalwastewater systems In this case the regeneration of cooling waterprovides a thermodynamically more favorable option saving about45% of electricity input. From Illustrative Example 1 it can be seenthat for total 45 MW of space heating only 5 MW of electricitywould be needed if cooling water is used as the heat source vs.9 MW electricity input if municipal wastewater is used as the low-temperature heat source of the heat pump.

Therefore, the utilization of low-grade heat in wastewater orcooling water may enable the ignition of new paths of urban heatsupply and resource efficiency based on decentralized or semi-central head recovery systems.

6. Conclusion

In many communities the water supply and wastewater facil-ities are the largest consumers of electricity in the urban infra-structure and wastewater is commonly regarded as being only anenergy sink. However, there are a number of possibilities to reducethe energy demand of wastewater treatment and disposal andthere are some potential ways of extracting energy from waste-water as discussed within this paper. In general, the implementa-tion of energy optimization measures is accompanied by thereduction of CO2 emissions. The amount of CO2 emissions saved,however, cannot be generalized as the reduction of CO2 emissionsdepends on the particular situation of electricity produced/used inthe country where the wastewater treatment plant is situated.


More important than the discussion on CO2 reduction is theaspect that energy self-sufficiency can be reached even at typicalconventional municipal WWTPs with nitrification (ammoniaremoval) as well as nitrogen and phosphorus removal. This meansnot only a reduction in energy consumption but also a substantialcost reduction in the field of wastewater treatment. This is ofespecial interest to those countries in which the specific costs ofenergy have a major share of the overall costs for wastewatertreatment. Furthermore, a valuable contribution to more efficiencywithin the area of municipal wastewater systems can be made bythe utilization of available and underutilized renewable energysources although the amount of overall energy savings seems to befrom rather insignificant role having a look on the overall energyconsumption of industrialized countries. Nevertheless, integratedsystem solutions based on the principals of Zero Emissions mayoffer more flexibility up to the overall aim of energy self-sufficienturban infrastructure systems in the future.

In general, the discussed ways to optimize the energy balance ofmunicipal wastewater systems are appropriate all over the world.The given and particular frame conditions (e.g. climate, technicalstandard, know-how, experience, population density, etc.), how-ever, determine the implementation of the given potential.

Acknowledgments

We acknowledge the financial support of the Hungarian Stateand the European Union under the TAMOP-4.2.2.A-11/1/KONV-2012-0072.

Nomenclature

CHP combined heat and power (generation)COD chemical oxygen demandCOP coefficient of performancePE population equivalentSRT solids retention timeWWTP wastewater treatment plant

References

Bieker, S., Cornel, P., Wagner, M., 2010. Semicentralised supply and treatment sys-tems: integrated infrastructure solutions for fast growing urban areas. WaterSci. Technol. 61 (11), 2905e2913.

Bucar, G., Schinnerl, D., 2007. Technical and Economic Market Potential in Terms ofWastewater Heat Recovery in Austria. Report, IEE Project WasteWater Heat.Grazer Energieagentur GmbH, Austria (in German).

Butz, J., Müller, E.A., 2010. Abwasserw€armenutzung e Erfahrungsbericht über Pla-nung und Bau der Anlage in Bretten (Wastewater heat recovery: experiencereport on design and installation of a facility in Bretten). KA Korresp. AbwasserAbfall 57 (8), 765e770 (in German).

Funamizu, N., Iida, M., Sakakura, Y., Takakuwa, T., 2001. Reuse of heat energy inwastewater: implementation examples in Japan. Water Sci. Technol. 43 (10),277e285.

Lemos, D., Dias, A.C., Gabarrell, X., Arroja, L., 2013. Environmental assessment of anurban water system. J. Clean. Prod. 54, 157e165.

Meda, A., Cornel, P., Henkel, J., 2010. Wastewater as a source of energy e canwastewater treatment plants be operated energetically self-sufficient?. In: 7thIWA Leading-edge Conference on Water and Wastewater Technologies, 2 e 4June 2010, Phoenix (AZ), U.S.A.

Müller, E.A., Butz, J., 2010. Abwasserw€armenutzung in Deutschland: aktueller Standund Ausblick (Wastewater heat recovery in Germany: current status and futureprospects). KA Korresp. Abwasser Abfall 57 (5), 437e442 (in German).

Nowak, O., 2003. Benchmarks for the energy demand of nutrient removal plants.Water Sci. Technol. 47 (12), 125e132.

Nowak, O., Melcher, R., Enderle, P., 2007. Evaluation of the reject waters from co-digestion of solid wastes from agro-industries in a municipal WWTP. WaterSci. Technol. 55 (10), 37e44.

Nowak, O., Keil, S., Fimml, C., 2011. Examples of energy self-sufficient municipalnutrient removal plants. Water Sci. Technol. 64 (1), 1e6.

Olsen, P.K., Lambertsen, H., Hummelshøj, R., Bøhm, B., Christiansen, C.H.,Svendsen, S., Larsen, C.T., Worm, J., 2008. A new low-temperature district


http://refhub.elsevier.com/S0959-6526(14)00891-9/sref1















































heating system for low-energy buildings. In: The 11th International Symposiumon District Heating and Cooling, 31 August e 2 September, Reykjavik, Iceland.

Otterpohl, R., Albold, A., Oldenburg, M., 1999. Source control in urban sanitation andwaste management: ten systems with reuse of resources. Water Sci. Technol. 39(5), 153e160.

OVGW, 2013. FreshWater Usage According to Sectors. OVGW eAustrian Associationfor Gas and Water. http://www.wasserwerk.at/home/alles-ueber-wasser/verbrauch. Pageview: 7th May 2013 [in German].

Pandis Iveroth, S., Vernay, A.L., Mulder, K.F., Brandt, N., 2013. Implications of sys-tems integration at the urban level: the case of Hammarby Sj€ostad, Stockholm.J. Clean. Prod. 48, 220e231.

Peter-Fr€ohlich, A., Pawlowski, L., Bonhomme, A., Oldenburg, M., 2007. EU demon-stration project for separate discharge and treatment of urine, faeces andgreywater e part I: results. Water Sci. Technol. 56 (5), 239e249.

Righi, S., Oliviero, L., Pedrini, M., Buscaroli, A., Casa, C.D., 2013. Life cycle assessmentof management systems for sewage sludge and food waste: centralized anddecentralized approaches. J. Clean. Prod. 44, 8e17.


Skambraks, A.K., 2011. Hamburg water cycle in the settlement jenfelder Au e

integrating the infrastructure services of water and Energy. In: IWA ConferenceCities of the Future 2011, 22.05. e 25.05.2011, Stockholm, Sweden.

UBA e Umweltbundesamt, 2009. Die Bestandsaufnahme der Abfallwirtschaft in€Osterreich e Statusbericht 2009 (Inventory of the Austrian Waste Managemente Status Report 2009). Environment Agency Austria, Department of WasteManagement, Klagenfurt, Austria (in German).

Urban, I., Weichgrebe, D., Rosenwinkel, K.-H., 2007. Anaerobic treatment ofmunicipal wastewater using the UASB technology. Water Sci. Technol. 56 (10),37e44.

Venkatesh, C., Brattebø, H., 2011. Energy consumption, costs and environmentalimpacts for urban water cycle services: case study of Oslo (Norway). Energy 36,792e800.

Wett, B., 2007. Development and implementation of a robust deammonificationprocess. Water Sci. Technol. 56 (7), 81e88.









http://www.wasserwerk.at/home/alles-ueber-wasser/verbrauch

http://www.wasserwerk.at/home/alles-ueber-wasser/verbrauch






































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