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Energy self-sufficient sewage wastewater treatment plants: is optimized anaerobic sludge digestion the key? P. Jenicek, J. Kutil, O. Benes, V. Todt, J. Zabranska and M. Dohanyos
ABSTRACT The anaerobic digestion of primary and waste activated sludge generates biogas that can be converted into energy to power the operation of a sewage wastewater treatment plant (WWTP). But can the biogas generated by anaerobic sludge digestion ever completely satisfy the electricity requirements of a WWTP with ‘standard’ energy consumption (i.e. industrial pollution not treated, no external organic substrate added)? With this question in mind, we optimized biogas production at Prague’s Central Wastewater Treatment Plant in the following ways: enhanced primary sludge separation; thickened waste activated sludge; implemented a lysate centrifuge; increased operational temperature; improved digester mixing. With these optimizations, biogas production 3
increased significantly to 12.5 m per population equivalent per year. In turn, this led to an equally significant increase in specific energy production from approximately 15 to 23.5 kWh per population
P. Jenicek (corresponding author) J. Kutil J. Zabranska M. Dohanyos Department of Water Technology and Environmental Engineering, Faculty of Environmental Protection, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague, Czech Republic E-mail: [email protected] O. Benes V. Todt Veolia Water, Czech Republic
equivalent per year. We compared these full-scale results with those obtained from WWTPs that are already energy self-sufficient, but have exceptionally low energy consumption. Both our results and our analysis suggest that, with the correct optimization of anaerobic digestion technology, even WWTPs with ‘standard’ energy consumption can either attain or come close to attaining energy selfsufficiency. Key words
| anaerobic digestion, digestion efficiency improvement, energy consumption, energy production, energy self-sufficiency, sewage sludge, wastewater treatment
INTRODUCTION In the last decades, the goals of wastewater treatment have become ever more ambitious and, thus, the technologies used are ever more sophisticated. The result is that both the energy required for and consumed by the treatment processes has risen dramatically. Simultaneously, of course, the minimization of energy consumption has become a major goal for wastewater treatment plant (WWTP) operators. This has led many researchers to investigate various aspects of energy selfsufficiency in WWTPs (Chudoba et al. ; Svardal & Kroiss ; Balmer & Hellström ; Jenicek et al. ). They all seem to agree that two preconditions are necessary for improving the energy balance in municipal WWTPs:
• •
the optimization of the total energy consumption during wastewater treatment, and the application of anaerobic digestion technology.
doi: 10.2166/wst.2013.423
Generally, the easiest way to increase biogas production and, thus, improve the energy balance of a WWTP is to supply an external organic substrate (Schwarzenbeck et al. ; Balmer & Hellström ). Recently, however, Nowak et al. () reported examples of energy self-sufficient municipal WWTPs whose self-sufficiency was achieved without the need for such a substrate. This suggests that it will be beneficial to focus the further development of wastewater treatment technologies on increasing the amount of pollution removed by anaerobic digestion and on low energy consuming processes, such as the autotrophic removal of nitrogen from wastewater (Seghezzo et al. ; Mulder ). Focusing on these areas of development will raise the possibility not only of energy self-sufficient or energy neutral WWTPs, but even of energy positive WWTPs whose surplus power and heat can be supplied to other consumers.
P. Jenicek et al.
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Energy self-sufficient sewage wastewater treatment plants
Most of the technological proposals made in relation to energy self-sufficient, or even energy positive, WWTPs are based on the application of anaerobic digestion, which can be used for direct wastewater treatment (especially in developing countries and in industrial wastewater treatment) and for sludge digestion (Seghezzo et al. ). The advanced activated sludge process commonly used in municipal WWTPs convert about half of the wastewater pollution into sludge (Sanin et al. ). Although sludge is widely considered to be an unfavorable byproduct of wastewater treatment, it is, in fact, a useful raw material for the production of energy. In this paper, we estimate the amount of wastewater energy and sludge energy that can be used for wastewater treatment, and compare our estimates with full-scale energy consumption and production data from the Central WWTP in Prague, Czech Republic.
ENERGY IN WASTEWATER The concept of a self-sufficient WWTP is usually based on exploiting the chemical energy of wastewater pollutants. This energy may be defined in different ways, the most frequent being a simplification that expresses the energy available by means of chemical oxygen demand (COD). This simplification assumes a population equivalent (PE) of 120 g COD (per person) per day and a calorific value for organic pollution of 14 kJ/g of COD (Svardal & Kroiss ). On this basis, the energy bound in wastewater can be estimated as 170 kWh/(PE.year). The maximum original energy is subsequently lost, step by step, during treatment processing. Figure 1 shows that over 40% of the original energy is lost by the oxidation of both organic pollution compounds and nitrogen compounds during the activated sludge process and by effluent discharge
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of the rest of the pollution. Another loss is associated with the residual organic matter in the digested sludge. And the low electrical efficiency of combined heat and power (CHP) generation represents a further energy loss of between 60 and 70%.
DESCRIPTION AND ENERGY BALANCE OF PRAGUE’S WWTP Prague’s Central WWTP treats pollution of 1.6 million PE by a combination of mechanical, chemical and biological processes. The biological step can be characterized as an RDN (regeneration–denitrification–nitrification) activated sludge system (Kos et al. ), with primary sedimentation and anaerobic sludge digestion being the crucial elements of sludge treatment. Originally opened in 1965, the plant has been upgraded several times. The most recent reconstruction in 1997 was driven by the need to increase the wastewater treatment capacity and to upgrade the activated sludge process technology for more efficient nutrient removal. At the same time, the primary sedimentation process was chemically enhanced by the addition of Fe3þ and Al3þ salts. The addition of Fe3þ (2–4 g/L) before primary clarifiers enables an efficiency of COD removal of about 50%. Together, these measures influenced both the amount and composition of the sludge produced and, thus, led to the need to intensify sludge treatment. The following four intensifying measures were applied to anaerobic sludge digestion at Prague’s Central WWTP: (i) Thickening of activated sludge: centrifuges were introduced to increase the TSS concentration of WAS. (ii) Upgrade of thickening centrifuges to ‘lysatethickening centrifuges’ (Zabranska et al. ): by implementation of this sludge disintegrating device, the centrifugal forces created in the centrifuge were deliberately applied to efficient sludge disintegration. (iii) Increase in the operating temperature to a thermophilic temperature (from 40 to 55 C). (iv) Enhancement of digester mixing: continuous mixing was applied to both the first and second stage digesters. W
Figure 1
|
Typical transformation of chemical energy of organic pollution (COD) during conventional wastewater treatment with anaerobic digestion of sludge (adapted from Cornel et al. (2011)).
Today, the plant’s sludge treatment facilities consist of 12 digesters, each with an operational volume of 4,800 m3, operated at a thermophilic temperature (55 C). The digesters, configured as a series of first and second stage digesters, are W
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operated in a semi-continuous mode with about 24 feedings daily. The first-stage digesters are mixed and heated; the second-stage digesters (of the same volume, but provided with a gas holder) are mixed, but not heated. The total solid retention time of the two stages is around 25 days. The waste activated sludge (WAS) is pre-thickened in a gravity thickener and then thickened from ca. 7 to 70 g/l and disintegrated in lysate-thickening centrifuges. The disintegrated sludge is mixed with primary sludge and the mixture then fed into the first-stage digesters. The digested sludge is dewatered by dewatering centrifuges and, after composting, applied to land as fertiliser and soil conditioner.
Figure 2
|
BIOGAS PRODUCTION AND ENERGY BALANCE OF PRAGUE’S WWTP The combination of technological enhancements described above resulted in a significant improvement in biogas production at the plant and, consequently, in its energy balance. In batch laboratory tests, Dohanyos et al. (b) reported improvements in the methane yield from activated sludge thickened by a lysate centrifuge of between 11.5 and 31.3%, depending on sludge quality. However, because many measures were implemented simultaneously, it is not possible at this stage to determine which strategy makes the most significant contribution to the improvement of a plant’s digestion efficiency. The enhanced anaerobic digestion of sludge increased the specific biogas production (SBP) to 600–700 m3/kg added volatile suspended solids (VSS) (Jenicek et al. ) so that the average biogas production is now above 50, 000 m3/d, with a methane content of about 63%. This is relatively high volumetric biogas production, about 1 m3 per m3 reactor volume per day, compared to typical values in similar WWTPs 0.5–1.0 m3/(m3 d) (Chudoba et al. ). In 2011 the SBP per population equivalent was 12.5 m3/ (PE.year) which equates to a specific energy value of 71 kWh/(PE.year). This means that 41.7% of the initial COD, at an average COD concentration of 670 mg/l, was transformed into biogas, an exceptionally high conversion rate. Figure 2 shows how the COD balance evaluation proposed by Nowak () was applied to the evaluation of the data from Prague’s Central WWTP. Using this scheme, we identified two main reasons for the high specific biogas production. The first is the high efficiency of primary sedimentation (47.5%); the second is the high efficiency of the upgraded anaerobic digestion process (65%).
Estimation of average COD balance for Prague’s Central WWTP in 2011, expressed per population equivalent 120 g COD.
Such an elevated level of biogas production is sufficient to cover a large part of the total energy demand of a WWTP (Figure 3). In the specific case of Prague’s Central WWTP, the self-sufficiency rate could be even higher, but, due to the low capacity of the CHP units, only 92% of the produced biogas is actually used to generate electricity. Although some full-scale data suggest that an electricity production efficiency from biogas in the range of 35–40% is realistic (Braun et al. ; Weiland ), the Prague plant currently achieves around 31%. Improving this efficiency is another potential way of increasing the plant’s energy selfsufficiency.
DISCUSSION Recently, similar data regarding full energy self-sufficiency were presented for Austrian WWTPs in Strass and Wolfgangsee-Ischl (Nowak et al. ). Table 1 compares the
Figure 3
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Energy self-sufficiency of Prague’s Central WWTP in 2011.
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Comparison of COD balance of several WWTP types
Typical WWTP (Cornel et al.
Typical WWTP (Lazarova et al.
WolfgangseeIschl (Nowak
2011)
2012)
et al. 2011)
Prague
Oxidation N, C þ effluent
43%
53%
38%
36%
Digested sludge
31%
21%
24%
23%
Biogas
26%
26%
38%
42%
Austrian and Prague COD energy balance data with data presented by Cornel et al. () and Lazarova et al. () for a ‘typical’ WWTP. The data in Table 1 confirm that for energy self-sufficiency it is necessary to significantly increase the part of COD that is converted into biogas. However, there are various ways of achieving this goal. At Wolfgangsee-Ischl, the most important factor is apparently the extremely long solid retention time (80 days) in the mesophilic digesters (Nowak et al. ). In Prague, the solid retention time is much shorter (25 days), but anaerobic digestion is operated at thermophilic temperatures (Zabranska et al. ) and lysate-thickening centrifuges are used to improve WAS degradability (Dohanyos et al. a). Another important factor determining the part of COD converted to biogas is the efficiency of the primary clarifier and, consequently, both the amount of primary sludge produced and the ratio between primary sludge and WAS. In both of the self-sufficient Austrian WWTPs (WolfgangseeIschl, Strass) the efficiency of primary sedimentation is high. According to Nowak et al. (), the efficiency of COD removal in the primary clarifier in Wolfgangsee-Ischl is 37%. Using the data presented in the scheme in Figure 2, we calculate that this efficiency reaches even 47.5% in Prague due to the chemical enhancement of the primary sedimentation process. The importance of the primary sludge/WAS ratio can be seen in Figure 4, which shows the relationship between SBP and the relative amount of primary sludge in the treated raw sludge. An individual SBP of 150–400 m3/t is assumed for WAS and of 400–850 m3/t for primary sludge (both values are for added VSS). It is obvious that the maximization of biogas and electricity production is a major task on the road to energy selfsufficient, or even energy positive, WWTPs. However, equally important is the strict optimization of the total energy consumption of the plant. In Prague, in 1997, the specific electricity production was around 15 kWh/(PE.year). The data in Table 2 show
Figure 4
|
Influence of the relative amount of primary sludge on specific biogas production from the mixture of primary sludge and WAS, based on the long-term monitoring of Prague’s sludge degradability by laboratory batch tests.
that the optimization of both sludge management and anaerobic sludge digestion can increase biogas production to such an extent that it is realistic to achieve a specific electricity production of about 25 kWh/(PE.year). Such electricity production is high enough to cover the energy requirements of advanced wastewater treatment technology and, hypothetically, to eventually create an energy surplus of up to 25% (based on a comparison of Prague’s electricity production and Strass’s electricity consumption). The actual data do not enable us yet to label Prague’s Central WWTP as self-sufficient because its aeration energy consumption is relatively high. This is primarily due to the fact that the plant still uses the older-style aeration tanks with shallow depths; the water level above the fine bubble aerators is just about 3 m, which, together with potential diffuser fouling, causes low oxygen transfer efficiency compared with the significantly deeper modern aeration tanks. Conversely, the aeration energy consumption at Strass is exceptionally low due to both the highloaded first stage of the activated sludge system and the deammonification of the reject water (Wett ). The total energy consumption often considerably increases sludge disintegration, but this is not the case with a lysate-thickening centrifuge, which, compared with a common thickening centrifuge, only results in an increase in specific energy consumption of about 0.06 kWh/kg TSS (Fabregat et al. ). It is quite difficult to compare WWTPs with regard to energy consumption because, even without the influence of industrial wastewater, municipal WWTPs differ in various aspects. The characteristics of the raw wastewater and the quality of effluent required can both significantly affect the specific energy consumption. Recently, more stringent
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Table 2
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Comparison of electricity consumption with electricity production for selected WWTPs – all data expressed in kWh/(PE.year) Nordic (Balmer 2000)
Strass (Nowak et al. 2011)
Wolfgangsee-Ischl (Nowak et al. 2011)
Prague
Aeration and mixing
–
9.1
11.5
18.9
Other consumption
–
10.8
7.7
11.3
Specific electricity consumption
31–47
19.9
19.2
30.2
Specific electricity production
18.6–19.8
21.4
20.6
23.5/25.4a
Total balance
16.1 to 27.8
þ 1.6
þ 1.4
6.7/4.6a
a
Theoretical data for the case that all of the produced biogas would be used for electricity production.
total nitrogen discharge limits have forced Prague’s WWTP to implement upgraded technology, including enlarged aeration tanks. Thus, it will be very interesting to follow further developments in its specific energy consumption. The energy balances presented in this paper are based on the direct energy consumption and production in a WWTP. However it is important to emphasize that the total energy balance should include all factors indirectly contributing to energy consumption, such as the chemicals and other products used, the influence of transportation, and so forth. Conversely, several WWTP processes bring energy savings. One such process is the substitution of fossil fuels by sludge. Another example of an energy-saving process is the substitution of mineral fertilizers by recycling the nutrients contained in the sewage sludge (N, P, K, S, etc.) for use in agriculture. Thus, when all forms of energy in a WWTP are considered, a comprehensive energy balance emerges that could be a more objective indicator of the plant’s potential to become energy self-sufficient (Remy ).
CONCLUSION The energy content of sewage is several times higher than the energy required for its efficient treatment. Our results show that, due to the anaerobic digestion of the sludge produced during wastewater treatment, the goal of energy selfsufficient sewage WWTPs is a realistic one. Moreover, we have shown that energy self-efficiency could be achieved without the addition of either external organic substrates or industrial wastewater. Even a sewage WWTP using energy-intensive secondary biological treatment can produce sufficient biogas to come close to energy self-sufficiency. The full-scale data from Prague’s WWTP suggest that, together with the skilful management of energy consumption, the optimization of anaerobic sludge digestion can be the key to energy self-sufficiency.
ACKNOWLEDGEMENTS This study was supported by the Central Wastewater Treatment Plant of Prague and by research project MSM 6046137308.
REFERENCES Balmer, P. Operation costs and consumption of resources at Nordic nutrient removal plants. Water Science and Technology 41, 273–279. Balmer, P. & Hellström, D. Performance indicators for wastewater treatment plants. Water Science and Technology 65 (7), 1304–1310. Braun, R., Weiland, P. & Wellinger, A. Biogas from Crop Digestion. IEA Bioenergy, Task 37 Brochure, International Energy Agency, Paris, France. Chudoba, P., Sardet, C., Palko, G. & Guibelin, E. Main factors influencing anaerobic digestion of sludge and energy efficiency at several large WWTP in central Europe. Journal of Residuals Science and Technology 8, 89–96. Cornel, P., Meda, A. & Bieker, S. Wastewater as a Source of Energy, Nutrients and Service Water. Treatise in Water Science. Elsevier, Oxford. Dohanyos, M., Zabranska, J. & Jenicek, P. a Innovative technology for the improvement of the anaerobic methane fermentation. Water Science and Technology 36 (6–7), 333–340. Dohanyos, M., Zabranska, J. & Jenicek, P. b Enhancement of sludge anaerobic digestion by use of a special thickening centrifuge. Water Science and Technology 36 (11), 145–153. Fabregat, A., Bengoa, C., Font, J. & Stueber, F. REMOVALS, Reduction, Modification and Valorisation of Sludge: A Particular Approach. IWA Publishing, London. Jenicek, P., Bartacek, J., Kutil, J., Zabranska, J. & Dohanyos, M. Potentials and Limits of Anaerobic Digestion of Sewage Sludge: Energy Self-Sufficient Municipal Wastewater Treatment Plant? Water Science and Technology 66 (6), 1277–1281. Kos, M., Wanner, J., Sorm, I. & Grau, P. R-D-N Activated sludge system. Water Science and Technology 25 (4/5), 151–160. Lazarova, V., Choo, K. H. & Cornel, P. Water-Energy Interactions in Water Reuse. IWA Publishing, London.
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Mulder, A. The quest for sustainable nitrogen removal technologies. Water Science and Technology 48, 67–75. Nowak, O. Benchmarks for the energy demand of nutrient removal plants. Water Science and Technology 47 (12), 125–132. Nowak, O., Keil, S. & Fimml, C. Examples of energy selfsufficient municipal nutrient removal plants. Water Science and Technology 64 (1), 1–6. Remy, C. Life Cycle Assessment of Conventional and SourceSeparation Systems for Urban Wastewater Management. Dissertation, Department of Water Quality Control (http:// opus.kobv.de/tuberlin/volltexte/2010/2543/), Technical University, Berlin. Sanin, F. D., Clarkson, W. W. & Vesilind, P. A. Sludge Engineering: The Treatment and Disposal of Wastewater Sludges. DEStech Publications, Inc., Lancaster, PA. Schwarzenbeck, N., Bomball, E. & Pfeiffer, W. Can a wastewater treatment plant be a powerplant? A case study. Water Science and Technology 57 (10), 1555–1561.
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Seghezzo, L., Zeeman, G., van Lier, J. B., Hamelers, H. V. M. & Lettinga, G. A review: the anaerobic treatment of sewage in UASB and EGSB reactors. Bioresource Technology 65 (3), 175–190. Svardal, K. & Kroiss, H. Energy requirements for waste water treatment. Water Science and Technology 64 (6), 1355–1361. Weiland, P. Biogas production: current state and perspectives. Applied Microbiol. Biotechnol. 88, 849–860. Wett, R. Development and implementation of a robust deammonification process. Water Science and Technology 56 (7), 81–88. Zabranska, J., Dohanyos, M., Jenicek, P. & Kutil, J. Disintegration of excess activated sludge – evaluation and experience of full-scale applications. Water Science and Technology 53 (12), 229–236. Zabranska, J., Dohanyos, M., Jenicek, P., Zaplatilkova, P. & Kutil, J. The contribution of thermophilic anaerobic digestion to the stable operation of wastewater sludge treatment. Water Science and Technology 46 (4–5), 447–453.
First received 16 January 2013; accepted in revised form 10 June 2013
© IWA Publishing 2013 Water Science & Technology
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68.8
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2013
Energy self-sufficient sewage wastewater treatment plants: is optimized anaerobic sludge digestion the key? P. Jenicek, J. Kutil, O. Benes, V. Todt, J. Zabranska and M. Dohanyos
ABSTRACT The anaerobic digestion of primary and waste activated sludge generates biogas that can be converted into energy to power the operation of a sewage wastewater treatment plant (WWTP). But can the biogas generated by anaerobic sludge digestion ever completely satisfy the electricity requirements of a WWTP with ‘standard’ energy consumption (i.e. industrial pollution not treated, no external organic substrate added)? With this question in mind, we optimized biogas production at Prague’s Central Wastewater Treatment Plant in the following ways: enhanced primary sludge separation; thickened waste activated sludge; implemented a lysate centrifuge; increased operational temperature; improved digester mixing. With these optimizations, biogas production 3
increased significantly to 12.5 m per population equivalent per year. In turn, this led to an equally significant increase in specific energy production from approximately 15 to 23.5 kWh per population
P. Jenicek (corresponding author) J. Kutil J. Zabranska M. Dohanyos Department of Water Technology and Environmental Engineering, Faculty of Environmental Protection, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague, Czech Republic E-mail: [email protected] O. Benes V. Todt Veolia Water, Czech Republic
equivalent per year. We compared these full-scale results with those obtained from WWTPs that are already energy self-sufficient, but have exceptionally low energy consumption. Both our results and our analysis suggest that, with the correct optimization of anaerobic digestion technology, even WWTPs with ‘standard’ energy consumption can either attain or come close to attaining energy selfsufficiency. Key words
| anaerobic digestion, digestion efficiency improvement, energy consumption, energy production, energy self-sufficiency, sewage sludge, wastewater treatment
INTRODUCTION In the last decades, the goals of wastewater treatment have become ever more ambitious and, thus, the technologies used are ever more sophisticated. The result is that both the energy required for and consumed by the treatment processes has risen dramatically. Simultaneously, of course, the minimization of energy consumption has become a major goal for wastewater treatment plant (WWTP) operators. This has led many researchers to investigate various aspects of energy selfsufficiency in WWTPs (Chudoba et al. ; Svardal & Kroiss ; Balmer & Hellström ; Jenicek et al. ). They all seem to agree that two preconditions are necessary for improving the energy balance in municipal WWTPs:
• •
the optimization of the total energy consumption during wastewater treatment, and the application of anaerobic digestion technology.
doi: 10.2166/wst.2013.423
Generally, the easiest way to increase biogas production and, thus, improve the energy balance of a WWTP is to supply an external organic substrate (Schwarzenbeck et al. ; Balmer & Hellström ). Recently, however, Nowak et al. () reported examples of energy self-sufficient municipal WWTPs whose self-sufficiency was achieved without the need for such a substrate. This suggests that it will be beneficial to focus the further development of wastewater treatment technologies on increasing the amount of pollution removed by anaerobic digestion and on low energy consuming processes, such as the autotrophic removal of nitrogen from wastewater (Seghezzo et al. ; Mulder ). Focusing on these areas of development will raise the possibility not only of energy self-sufficient or energy neutral WWTPs, but even of energy positive WWTPs whose surplus power and heat can be supplied to other consumers.
P. Jenicek et al.
1740
|
Energy self-sufficient sewage wastewater treatment plants
Most of the technological proposals made in relation to energy self-sufficient, or even energy positive, WWTPs are based on the application of anaerobic digestion, which can be used for direct wastewater treatment (especially in developing countries and in industrial wastewater treatment) and for sludge digestion (Seghezzo et al. ). The advanced activated sludge process commonly used in municipal WWTPs convert about half of the wastewater pollution into sludge (Sanin et al. ). Although sludge is widely considered to be an unfavorable byproduct of wastewater treatment, it is, in fact, a useful raw material for the production of energy. In this paper, we estimate the amount of wastewater energy and sludge energy that can be used for wastewater treatment, and compare our estimates with full-scale energy consumption and production data from the Central WWTP in Prague, Czech Republic.
ENERGY IN WASTEWATER The concept of a self-sufficient WWTP is usually based on exploiting the chemical energy of wastewater pollutants. This energy may be defined in different ways, the most frequent being a simplification that expresses the energy available by means of chemical oxygen demand (COD). This simplification assumes a population equivalent (PE) of 120 g COD (per person) per day and a calorific value for organic pollution of 14 kJ/g of COD (Svardal & Kroiss ). On this basis, the energy bound in wastewater can be estimated as 170 kWh/(PE.year). The maximum original energy is subsequently lost, step by step, during treatment processing. Figure 1 shows that over 40% of the original energy is lost by the oxidation of both organic pollution compounds and nitrogen compounds during the activated sludge process and by effluent discharge
Water Science & Technology
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2013
of the rest of the pollution. Another loss is associated with the residual organic matter in the digested sludge. And the low electrical efficiency of combined heat and power (CHP) generation represents a further energy loss of between 60 and 70%.
DESCRIPTION AND ENERGY BALANCE OF PRAGUE’S WWTP Prague’s Central WWTP treats pollution of 1.6 million PE by a combination of mechanical, chemical and biological processes. The biological step can be characterized as an RDN (regeneration–denitrification–nitrification) activated sludge system (Kos et al. ), with primary sedimentation and anaerobic sludge digestion being the crucial elements of sludge treatment. Originally opened in 1965, the plant has been upgraded several times. The most recent reconstruction in 1997 was driven by the need to increase the wastewater treatment capacity and to upgrade the activated sludge process technology for more efficient nutrient removal. At the same time, the primary sedimentation process was chemically enhanced by the addition of Fe3þ and Al3þ salts. The addition of Fe3þ (2–4 g/L) before primary clarifiers enables an efficiency of COD removal of about 50%. Together, these measures influenced both the amount and composition of the sludge produced and, thus, led to the need to intensify sludge treatment. The following four intensifying measures were applied to anaerobic sludge digestion at Prague’s Central WWTP: (i) Thickening of activated sludge: centrifuges were introduced to increase the TSS concentration of WAS. (ii) Upgrade of thickening centrifuges to ‘lysatethickening centrifuges’ (Zabranska et al. ): by implementation of this sludge disintegrating device, the centrifugal forces created in the centrifuge were deliberately applied to efficient sludge disintegration. (iii) Increase in the operating temperature to a thermophilic temperature (from 40 to 55 C). (iv) Enhancement of digester mixing: continuous mixing was applied to both the first and second stage digesters. W
Figure 1
|
Typical transformation of chemical energy of organic pollution (COD) during conventional wastewater treatment with anaerobic digestion of sludge (adapted from Cornel et al. (2011)).
Today, the plant’s sludge treatment facilities consist of 12 digesters, each with an operational volume of 4,800 m3, operated at a thermophilic temperature (55 C). The digesters, configured as a series of first and second stage digesters, are W
1741
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Energy self-sufficient sewage wastewater treatment plants
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operated in a semi-continuous mode with about 24 feedings daily. The first-stage digesters are mixed and heated; the second-stage digesters (of the same volume, but provided with a gas holder) are mixed, but not heated. The total solid retention time of the two stages is around 25 days. The waste activated sludge (WAS) is pre-thickened in a gravity thickener and then thickened from ca. 7 to 70 g/l and disintegrated in lysate-thickening centrifuges. The disintegrated sludge is mixed with primary sludge and the mixture then fed into the first-stage digesters. The digested sludge is dewatered by dewatering centrifuges and, after composting, applied to land as fertiliser and soil conditioner.
Figure 2
|
BIOGAS PRODUCTION AND ENERGY BALANCE OF PRAGUE’S WWTP The combination of technological enhancements described above resulted in a significant improvement in biogas production at the plant and, consequently, in its energy balance. In batch laboratory tests, Dohanyos et al. (b) reported improvements in the methane yield from activated sludge thickened by a lysate centrifuge of between 11.5 and 31.3%, depending on sludge quality. However, because many measures were implemented simultaneously, it is not possible at this stage to determine which strategy makes the most significant contribution to the improvement of a plant’s digestion efficiency. The enhanced anaerobic digestion of sludge increased the specific biogas production (SBP) to 600–700 m3/kg added volatile suspended solids (VSS) (Jenicek et al. ) so that the average biogas production is now above 50, 000 m3/d, with a methane content of about 63%. This is relatively high volumetric biogas production, about 1 m3 per m3 reactor volume per day, compared to typical values in similar WWTPs 0.5–1.0 m3/(m3 d) (Chudoba et al. ). In 2011 the SBP per population equivalent was 12.5 m3/ (PE.year) which equates to a specific energy value of 71 kWh/(PE.year). This means that 41.7% of the initial COD, at an average COD concentration of 670 mg/l, was transformed into biogas, an exceptionally high conversion rate. Figure 2 shows how the COD balance evaluation proposed by Nowak () was applied to the evaluation of the data from Prague’s Central WWTP. Using this scheme, we identified two main reasons for the high specific biogas production. The first is the high efficiency of primary sedimentation (47.5%); the second is the high efficiency of the upgraded anaerobic digestion process (65%).
Estimation of average COD balance for Prague’s Central WWTP in 2011, expressed per population equivalent 120 g COD.
Such an elevated level of biogas production is sufficient to cover a large part of the total energy demand of a WWTP (Figure 3). In the specific case of Prague’s Central WWTP, the self-sufficiency rate could be even higher, but, due to the low capacity of the CHP units, only 92% of the produced biogas is actually used to generate electricity. Although some full-scale data suggest that an electricity production efficiency from biogas in the range of 35–40% is realistic (Braun et al. ; Weiland ), the Prague plant currently achieves around 31%. Improving this efficiency is another potential way of increasing the plant’s energy selfsufficiency.
DISCUSSION Recently, similar data regarding full energy self-sufficiency were presented for Austrian WWTPs in Strass and Wolfgangsee-Ischl (Nowak et al. ). Table 1 compares the
Figure 3
|
Energy self-sufficiency of Prague’s Central WWTP in 2011.
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Table 1
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Energy self-sufficient sewage wastewater treatment plants
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2013
Comparison of COD balance of several WWTP types
Typical WWTP (Cornel et al.
Typical WWTP (Lazarova et al.
WolfgangseeIschl (Nowak
2011)
2012)
et al. 2011)
Prague
Oxidation N, C þ effluent
43%
53%
38%
36%
Digested sludge
31%
21%
24%
23%
Biogas
26%
26%
38%
42%
Austrian and Prague COD energy balance data with data presented by Cornel et al. () and Lazarova et al. () for a ‘typical’ WWTP. The data in Table 1 confirm that for energy self-sufficiency it is necessary to significantly increase the part of COD that is converted into biogas. However, there are various ways of achieving this goal. At Wolfgangsee-Ischl, the most important factor is apparently the extremely long solid retention time (80 days) in the mesophilic digesters (Nowak et al. ). In Prague, the solid retention time is much shorter (25 days), but anaerobic digestion is operated at thermophilic temperatures (Zabranska et al. ) and lysate-thickening centrifuges are used to improve WAS degradability (Dohanyos et al. a). Another important factor determining the part of COD converted to biogas is the efficiency of the primary clarifier and, consequently, both the amount of primary sludge produced and the ratio between primary sludge and WAS. In both of the self-sufficient Austrian WWTPs (WolfgangseeIschl, Strass) the efficiency of primary sedimentation is high. According to Nowak et al. (), the efficiency of COD removal in the primary clarifier in Wolfgangsee-Ischl is 37%. Using the data presented in the scheme in Figure 2, we calculate that this efficiency reaches even 47.5% in Prague due to the chemical enhancement of the primary sedimentation process. The importance of the primary sludge/WAS ratio can be seen in Figure 4, which shows the relationship between SBP and the relative amount of primary sludge in the treated raw sludge. An individual SBP of 150–400 m3/t is assumed for WAS and of 400–850 m3/t for primary sludge (both values are for added VSS). It is obvious that the maximization of biogas and electricity production is a major task on the road to energy selfsufficient, or even energy positive, WWTPs. However, equally important is the strict optimization of the total energy consumption of the plant. In Prague, in 1997, the specific electricity production was around 15 kWh/(PE.year). The data in Table 2 show
Figure 4
|
Influence of the relative amount of primary sludge on specific biogas production from the mixture of primary sludge and WAS, based on the long-term monitoring of Prague’s sludge degradability by laboratory batch tests.
that the optimization of both sludge management and anaerobic sludge digestion can increase biogas production to such an extent that it is realistic to achieve a specific electricity production of about 25 kWh/(PE.year). Such electricity production is high enough to cover the energy requirements of advanced wastewater treatment technology and, hypothetically, to eventually create an energy surplus of up to 25% (based on a comparison of Prague’s electricity production and Strass’s electricity consumption). The actual data do not enable us yet to label Prague’s Central WWTP as self-sufficient because its aeration energy consumption is relatively high. This is primarily due to the fact that the plant still uses the older-style aeration tanks with shallow depths; the water level above the fine bubble aerators is just about 3 m, which, together with potential diffuser fouling, causes low oxygen transfer efficiency compared with the significantly deeper modern aeration tanks. Conversely, the aeration energy consumption at Strass is exceptionally low due to both the highloaded first stage of the activated sludge system and the deammonification of the reject water (Wett ). The total energy consumption often considerably increases sludge disintegration, but this is not the case with a lysate-thickening centrifuge, which, compared with a common thickening centrifuge, only results in an increase in specific energy consumption of about 0.06 kWh/kg TSS (Fabregat et al. ). It is quite difficult to compare WWTPs with regard to energy consumption because, even without the influence of industrial wastewater, municipal WWTPs differ in various aspects. The characteristics of the raw wastewater and the quality of effluent required can both significantly affect the specific energy consumption. Recently, more stringent
P. Jenicek et al.
1743
Table 2
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|
Energy self-sufficient sewage wastewater treatment plants
Water Science & Technology
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68.8
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2013
Comparison of electricity consumption with electricity production for selected WWTPs – all data expressed in kWh/(PE.year) Nordic (Balmer 2000)
Strass (Nowak et al. 2011)
Wolfgangsee-Ischl (Nowak et al. 2011)
Prague
Aeration and mixing
–
9.1
11.5
18.9
Other consumption
–
10.8
7.7
11.3
Specific electricity consumption
31–47
19.9
19.2
30.2
Specific electricity production
18.6–19.8
21.4
20.6
23.5/25.4a
Total balance
16.1 to 27.8
þ 1.6
þ 1.4
6.7/4.6a
a
Theoretical data for the case that all of the produced biogas would be used for electricity production.
total nitrogen discharge limits have forced Prague’s WWTP to implement upgraded technology, including enlarged aeration tanks. Thus, it will be very interesting to follow further developments in its specific energy consumption. The energy balances presented in this paper are based on the direct energy consumption and production in a WWTP. However it is important to emphasize that the total energy balance should include all factors indirectly contributing to energy consumption, such as the chemicals and other products used, the influence of transportation, and so forth. Conversely, several WWTP processes bring energy savings. One such process is the substitution of fossil fuels by sludge. Another example of an energy-saving process is the substitution of mineral fertilizers by recycling the nutrients contained in the sewage sludge (N, P, K, S, etc.) for use in agriculture. Thus, when all forms of energy in a WWTP are considered, a comprehensive energy balance emerges that could be a more objective indicator of the plant’s potential to become energy self-sufficient (Remy ).
CONCLUSION The energy content of sewage is several times higher than the energy required for its efficient treatment. Our results show that, due to the anaerobic digestion of the sludge produced during wastewater treatment, the goal of energy selfsufficient sewage WWTPs is a realistic one. Moreover, we have shown that energy self-efficiency could be achieved without the addition of either external organic substrates or industrial wastewater. Even a sewage WWTP using energy-intensive secondary biological treatment can produce sufficient biogas to come close to energy self-sufficiency. The full-scale data from Prague’s WWTP suggest that, together with the skilful management of energy consumption, the optimization of anaerobic sludge digestion can be the key to energy self-sufficiency.
ACKNOWLEDGEMENTS This study was supported by the Central Wastewater Treatment Plant of Prague and by research project MSM 6046137308.
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First received 16 January 2013; accepted in revised form 10 June 2013
