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Domestic Wastewater Treatment as a Net Energy Producer Can This be Achieved? Perry L. McCarty,*,†,‡ Jaeho Bae,‡ and Jeonghwan Kim‡ † ‡
Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega MC 4020, Stanford, California 94305, United States Department of Environmental Engineering, INHA University, Namgu, Yonghyun dong 253, Incheon, Republic of Korea
’ INTRODUCTION Water, food, and energy are three of the major resource issues facing the world today. In order to help address these issues, domestic wastewater is now being looked at more as a resource than as a waste, a resource for water, for energy, and for the plant fertilizing nutrients, nitrogen (N) and phosphorus (P).1 Use of reclaimed wastewater for landscape and crop irrigation and indeed for domestic consumption is a widely accepted and growing practice to save water and to make use of the fertilizing elements it contains. Similarly, use of domestic wastewater as a source of energy has a long history, especially through the anaerobic conversion of wastewater’s organic content into methane (CH4) gas, a useful biofuel.2 However, through the conventional practice of aerobic wastewater treatment combined with anaerobic sludge digestion, only a portion of the energy potential of wastewater is captured.3 That contained in the dissolved organic fraction is not recovered, but is removed instead by aerobic processes that require much energy. As a result with traditional approaches, more energy is consumed in wastewater treatment than is gained through digestion. What might we do better toward more complete recovery of the three important resource potentials of domestic wastewaters? Water reuse is already widely practiced where water is in limited supply, but this often increases the energy needed for treatment because of increased water quality requirements for reuse.1 Reducing treatment energy requirements can help offset this need, particularly through more efficient capturing of the biofuel potential in wastewater itself. Reducing net energy requirements for wastewater treatment is a complementary, not an alternative goal to water reuse. The same can be said with respect to nutrient recovery. Additionally, climate change concerns associated with fossil fuel consumption, as well as increasing energy costs, necessitate that greater efforts be made toward better efficiency and more sustainable use of wastewater’s energy potential. While r 2011 American Chemical Society
more efficient water and nutrient recovery from wastewater are important goals in themselves, the focus of this article is how we can more completely recover wastewater’s energy content. Wastewater treatment accounts for about 3% of the U.S. electrical energy load,4 similar to that in other developed countries.5 The energy needs for a typical domestic wastewater treatment plant employing aerobic activated sludge treatment and anaerobic sludge digestion is 0.6 kWh/m3 of wastewater treated, about half of which is for electrical energy to supply air for the aeration basins.3,5 With conventional approaches involving aerobic treatment a quarter to half of a plants energy needs might be satisfied by using the CH4 biogas produced during anaerobic digestion, and other plant modifications might further reduce energy needs considerably.4 However, if more of the energy potential in wastewater were captured for use and even less were used for wastewater treatment, then wastewater treatment might become a net energy producer rather than a consumer. Energy Potential in Domestic Wastewater. Table 1 contains a summary of three energy-related characteristics of domestic wastewater: the energy resource contained in wastewater organics, the external fossil-fuel energy requirements for the production of equivalent amounts of the fertilizing elements N and P, and the energy that might be gained from wastewater’s thermal content. Concerning energy associated with N and P, ∼7% of the world’s natural gas production was used in 1990 to fix atmospheric nitrogen through the Haber-Bosch Process to satisfy the demand for N.6 8 Somewhat less is associated with P production. From a broad environmental perspective world fossil fuel consumption could be reduced through the direct use of wastewater N and P for fertilizer instead of using manufactured fertilizers. Why do we spend energy to rid wastewaters of fertilizing elements, rather than saving energy by using them for plant fertilization? There is also potential energy to be gained from the thermal heat contained in wastewater, energy that may be captured through use of heat pumps for low-energy use such as in the heating of buildings, a practice sometimes used in areas with cold winter climates such as Sweden.9 Heat pumps represent an efficient way to use electrical energy for heating, and operate akin to a refrigeration unit. Electrical energy is used to extract heat from a source—air, ground, or wastewater—and transfer the heat to an area of need such as a building. The source becomes colder and the building warmer. A measure of energy effectiveness is the coefficient of performance (COP), which is Published: July 12, 2011 7100
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Table 1. Energy Characteristics of a Typical Domestic Wastewater energy (kWh/m3) constituent
typical concentrationsa
maximum potential from
required to produce
thermal heat available for
(mg/L)
organic oxidationb
fertilizing elementsc
heat-pump extractiond
organics (COD) total
500
refractory
180
suspended
80
0.31
dissolved
100
0.39
biodegradable
320
suspended dissolved
175 145
0.67 0.56
nitrogen organic
15
ammonia
25
0.29 0.48
phosphorus
8
0.02
water totals
7.0 1.93
0.79
7.0
a
After Tchobanoglous and Burton.42 b Based upon a theoretical 3.86 kWh energy production/kg COD oxidized to CO2 and H2O.3 c Based upon production energy of 19.3 kWh/kg N by Haber-Bosch Process and 2.11 kWh/kg P after Gellings and Parmenter.6 d Energy associated with a 6 °C change in water temperature through heat extraction.
the ratio of the amount of heat energy transferred per unit of electrical energy used to drive the heat pump compressor. Typical values are in the range of 2 5, with 3 4 being common. In mild climates, air is often the source from which heat is extracted, but in cold climates winter air temperatures may be too cold for such use. In such conditions, ground or groundwaters, or indeed sufficiently warm wastewater provide other options. With wastewater, the heat energy associated with a 6 10 °C drop in water temperature is what might be available, providing the final temperature is sufficiently above the freezing point. The potential of a heat pump to be economical is in large measure dependent upon the relative unit energy cost of alterative heating fuels such as natural gas or fuel oil. Where alternative available fuels are much less expensive than electricity, the potential for wastewater to serve as an energy source for heat pumps diminishes. The most direct and commonly exploited and useful energy source in wastewater is the organic fraction as measured by the chemical oxygen demand (COD), which indicates the amount of oxygen (O2) required to oxidize the organic material to carbon dioxide (CO2) and water (H2O). In Table 1 the organic fraction is divided between dissolved and suspended, and between biodegradable and refractory. Suspended solids may be concentrated in a settling tank, with the resulting primary sludge anaerobically digested for CH4 production, but CH4 results only from the biodegradable fraction. Through thermal, chemical, or electrical processes, some of the refractory portion may be conditioned to increase biodegradability and CH4 production,2 but the energy cost for this may offset the gains. Thermal processes such as incineration have the potential to extract energy from both the biodegradable and refractory fractions of the sludge. However, unless water content can be reduced below ∼30%, more energy is required for incineration than is produced through combustion.10 Thus, thermal processes are generally not energy producers. The soluble organic fraction cannot be concentrated easily and so is subjected to treatment processes that can treat dilute
streams with short detention times. Here, aerobic treatment processes have been found to be very effective, albeit with a relatively high cost in energy. An energy-savings goal would be to use a process that both captures the energy potential in the dissolved organics and meets effluent standards effectively. Anaerobic versus Microbial Fuel Cells for Treating Dissolved Organics. A major challenge is to capture the energy potential of the dissolved organic component in domestic wastewater, and to do so with little offsetting energy expenditure and costs. One possibility is to replace secondary aerobic treatment with secondary anaerobic treatment. Another is an evolving novel method, microbial fuel cells (MFCs), which accomplish direct biological conversion of organic energy into electricity, an approach that is hoped may achieve more efficient conversion than is currently possible with anaerobic treatment.11 With the anaerobic approach CH4-driven engines are used to turn generators to produce electricity. Here only about 30 40% of the CH4 energy is converted into electricity,12 the remainder is given off as heat, which may or may not be useful. Chemical fuel cells offer another approach to produce electricity from CH4, perhaps increasing the efficiency of conversion to 50%.12 An important question is whether MFCs, which are enjoying much current research,13 are likely to meet or exceed such transfer efficiencies and to do so at comparable or lower cost? A brief review of each option is in order. Some energy is always lost in a conversion process. In anaerobic treatment of domestic wastewater about 8% of the potential energy is lost in the conversion of higher energy organics such as carbohydrates into CH4, a lower energy organic. Another 7% is lost from the conversion of a portion of the organics into the cells of microorganisms necessary to carry out the reactions. Wastewater treatment itself is not 100% efficient, and so additional losses result here, perhaps 5%. These combined losses total about 19%, meaning that the CH4 produced would contain only about 81% of the original biodegradable organic energy potential. Through combustion only about 35% of the 7101
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Environmental Science & Technology CH4 energy might be converted into electricity, the remaining 65% is given off as heat.12 Overall then, the electricity so produced would contain only about 28% of the original energy potential in the biodegradable wastewater organics. Perhaps this could be increased to 40% with more efficient electrical generation or through the use of chemical fuel cells.12 However, the heat produced from CH4 combustion need not be lost, but can be used for heating buildings or other purposes. Energy losses do result with MFCs as well, and they can be substantial.11,14,15 Power production is the product of current and cell voltage. First there is the Coulombic loss,11 the portion of wastewater organics that are not converted into current. This loss may be similar to an anaerobic system with a 7% loss to microbial growth and 5% loss due to treatment inefficiency, or about 12% combined. Then there is the loss in electrochemical potential or voltage, which translates as a decrease below the theoretical value of about 1.1 V for wastewater organics.11 For example, if the effective MFC voltage were half of that or 0.55 V, then 50% of the potential energy would be lost. Combined with the Coulombic loss, transfer of energy from the soluble organics to electricity would be 44%, still perhaps higher than with anaerobic treatment, but not much. However, voltage losses in MFCs currently tend to be much greater than 50%. Typical losses are 0.1 V at the anode and 0.5 V at the cathode for a combined loss of 0.6 V or over half of the theoretical value.14 Further substantial voltage loss results from the associated movement of electrons through electrical wires and especially from ion transport between electrodes, the latter is a function of distance between electrodes, equaling about 1 V/cm of distance with typical wastewater. The most optimistic projections for MFCs result from studies with high organic concentrations and simple substrates.13,14 With low reactor organic concentrations associated with efficient wastewater treatment more voltage loss is expected.15 Thus achieving the electrical generation efficiency that is already practical with anaerobic systems presents a great challenge for MFCs. Also, a MFC system has been estimated to cost 800 times that of an anaerobic system based upon available technologies,14 thus presenting another major challenge. These and other challenges11,13,14,16 suggest several major breakthroughs are needed for MFCs to become competitive with electricity generation through anaerobic wastewater treatment. Anaerobic Wastewater Treatment of Domestic Wastewater. Complete anaerobic treatment of domestic wastewater has the potential to achieve net energy production while meeting stringent effluent standards. Anaerobic wastewater treatment is well over a century old, starting with the relatively inefficient septic and Imhoff tank processes.17 However, over the past 50 years more efficient anaerobic processes have been developed leading to suggestions in the 1980s that they be applied to more fully treat domestic wastewaters.18,19 Since then there have been a number of applications of full-scale direct anaerobic treatment of domestic wastewater, particularly in developing countries such as Brazil, Colombia, Mexico, Egypt, and India, where anaerobic treatment is considered to be a low-cost wastewater treatment alternative.20 22 Low temperature and low organic concentrations are often cited as barriers to direct anaerobic treatment of domestic wastewaters. However, many laboratory studies have shown good performance at temperatures as low as 5 °C and with hydraulic retention times (HRT) of only a few hours.23 25 Biochemical oxygen demand (BOD) removals expected with present anaerobic reactors range from 70 to 80%, not quite sufficient to meet stringent regulatory
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standards.22,26 30 Because of this and other experiences, it has been commonly concluded that effluent “polishing” or a posttreatment step is necessary to meet effluent standards.29,31,32 However, recent studies with anaerobic membrane bioreactors indicate that polishing may be accomplished within an anaerobic reactor itself while providing a good quality effluent with low suspended solids and BOD concentrations.33 35 Hypothetical Anaerobic Treatment System for Energy Recovery and Efficient Treatment. What might be the characteristics of a system designed for the efficient anaerobic treatment of domestic wastewater? Good treatment efficiency and low cost relative to that of conventional activated sludge treatment would be necessary. Additionally, CH4 is a powerful greenhouse gas with a global warming potential about 25 times that of CO2,36 and thus must not be allowed to escape to the atmosphere.37 As a useful biofuel, CH4 should instead be captured and used as a renewable source of energy. To meet U.S. effluent standards of 30 mg/L for both BOD and total suspended solids, the system should be designed to achieve an average effluent concentration of 15 mg/L for each. A hypothetical anaerobic treatment system to illustrate the potential outcomes of such treatment is illustrated in Figure 1. This includes a conventional primary settling tank in order to remove settleable suspended materials before secondary treatment, with resulting biosolids sent to a conventional anaerobic digester. The effluent then passes to a secondary anaerobic membrane bioreactor that can prevent loss of biological solids to the effluent and thus maintain a sufficiently high solids retention time (SRT) as required for efficient biodegradation of organics.2 A countercurrent air-stripping unit is the final process shown, the purpose of which is to remove and use the dissolved CH4,18 as well as to add O2 to the effluent stream. Membrane bioreactors are widely used today for aerobic wastewater treatment, as they are capable of producing a high quality effluent with low suspended solids concentration and small footprint relative to traditional aerobic treatment systems, but have a higher energy usage as required to reduce membrane fouling.38 However, a potential significant reduction in the membrane energy cost might be obtained using a new anaerobic reactor design, the anaerobic fluidized membrane bioreactor (AFMBR), which combines a membrane system with an anaerobic fluidized bed reactor (AFBR).35 An AFBR contains particulate media such as granular activated carbon (GAC) that is suspended in the reactor by the upward velocity of the fluid being treated. Wastewater treatment is effected by a biofilm attached to the media. The AFBR is particularly effective for low strength wastewaters as it has good mass transfer characteristics and can retain a high concentration of active microorganisms without organism washout at short detention times of minutes to a few hours,2 a necessity for economical anaerobic treatment of low strength wastewaters. By placing membranes within the reactor itself, the moving action of the suspended media along the membranes reduces fouling, and at low energy expenditure.35 In an initial AFMBR study to treat a dilute wastewater of about 500 mg COD/L at a reactor detention time of 5 h, the total energy expenditure for operating the reactor and fluidizing the GAC media used was 0.058 kWh/m3 of wastewater treated, about one-tenth of the energy requirement for a typical aerobic membrane bioreactor.35 Achieved was an effluent COD of 7 mg/L (99% removal) and less than 1 mg/L of suspended solids. While much yet needs to be done to evaluate effectiveness with domestic wastewater under ambient conditions and to optimize 7102
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Figure 1. A hypothetical system for complete anaerobic treatment of domestic wastewater.
performance, the potential for anaerobic domestic wastewater treatment to be energy producing, cost-effective, and to meet environmental discharge requirements has been demonstrated. Comparisons with Conventional Activated Sludge Treatment. An evaluation was made of the potential benefits of anaerobic domestic wastewater treatment compared to a conventional activated sludge system with sludge digestion, assuming wastewater composition listed in Table 1. Figure 2a illustrates that with full anaerobic treatment a doubling of CH4 production over conventional aerobic treatment is obtained, and energy production greatly exceeds the energy needs for plant operation (Figure 2c). Anaerobic domestic wastewater treatment could be a net energy producer. Another significant advantage is that the quantity of digested sludge resulting from anaerobic treatment is much less than with aerobic treatment (Figure 2b), another highly significant cost as well as energy benefit. Issues That Need Addressing. While complete anaerobic domestic wastewater treatment has potential energy and cost savings, there are important issues that need to be addressed. First, for climate change concerns, CH4 must not be allowed to escape to the atmosphere but should be collected and used. Energy for stripping CH4 is anticipated to be less than 0.05 kWh/m3, as much less CH4 would have to be transferred than with O2 in an aerobic treatment system, and both have similarly low solubility. Because of its importance, research on cost and energy-efficient methods for such CH4 capture is needed. An associated problem that also requires more attention is sulfate (SO42-) reduction to sulfide (S2-), which competes with CH4 production and produces a toxic and corrosive gas (H2S).2,37 Another issue is the removal of wastewater nutrients, which is being required more frequently because of the adverse environmental impacts that nutrients can have on receiving waters. There are many approaches here that can be used with anaerobic treatment such as chemical precipitation for P 30,39 or its conversion into struvite (NH4MgPO4 3 6H2O) for recovery as fertilizer.39 For N removal, the traditional approach with nitrification and denitrification is highly energy consuming as well as
wasteful of the fertilizing potential offered. A less energy-wasteful approach is the newer anammox process, which oxidizes ammonia (NH3) with nitrite (NO2 ) to produce harmless N2 gas.40 This is a low-oxygen-consuming process that does not require organics for denitrification, organics that are better converted into CH4 for energy production. Another option aimed at recovering both N and P nutrients and being applied in Europe is source-separation of urine so that it does not become part of the domestic wastewater. Urine contains a majority of the N and P nutrients and might be treated separately and less expensively to recover the nutrients for use in fertilizer.41 In water-poor areas where the treated wastewater might be used for crop or landscape irrigation, both the water and the nutrients can be reused, and energy requirements are significantly less than for potable reuse where reverse osmosis may be required. When coupled with complete anaerobic treatment, reuse for irrigation is perhaps one of the best ways to capture the full resource potential of wastewaters. Anaerobic secondary treatment to reduce energy and operating costs for municipal wastewater treatment has good potential, more pilot as well as fundamental studies to better explore options for effluent CH4 removal and to optimize treatment would appear worthwhile. What Can We Do Now? While complete anaerobic treatment of domestic wastewater has perhaps the best current potential for capturing wastewater’s organic energy content, retrofitting existing conventional aerobic wastewater treatment plants to anaerobic facilities could be costly. The complete anaerobic approach might best be applied with new treatment systems once sufficient experience with them is gained. In the mean time, other practices can help to significantly reduce the overall energy requirements for water supply and treatment, and better capture wastewater’s total resource potential.4 Energy requirements in aerobic wastewater treatment systems can be reduced through upgrading energy-inefficient equipment, better control of aeration systems to deliver only the O2 actually needed, and through the use of more energy-efficient aeration diffusers. Reducing the solids retention 7103
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Figure 2. Comparative estimates of CH4, sludge, and energy production per cubic meter of wastewater treated for full anaerobic treatment versus conventional aerobic treatment with sludge digestion. (a) CH4 production (STP) associated with primary sludge digestion (blue) and secondary treatment (red). (b) Volume of digested sludge resulting from primary treatment (blue) and from secondary treatment (red). (c) Biogas energy produced (blue) and energy used in overall wastewater treatment (red).
times in aeration basins also results in smaller O2 and energy requirements, with more of the wastewater organics converted into biosolids that can be sent to digesters for increased CH4 production. Also many thermal, physical, chemical, and electrical methods are now available that increase the biodegradability of biosolids with potential for reducing overall energy requirements.2 Perhaps the most readily adaptable approach to reduce external energy requirements with existing treatment plants is to make full use of the CH4 produced from conventional anaerobic digesters through use of combined heat and power (CHP) systems (cogeneration). The U.S. Environmental Protection Agency (EPA) estimates that of the 16 000 municipal wastewater treatment facilities operating in the U.S., roughly 1000 operate with a total influent flow rate greater than 19 000 m3/day, a size considered sufficient for CHP.12 However, only 544 of these facilities employ anaerobic digestion, and only 106 of these now utilize the biogas produced to generate electricity and/or thermal energy. EPA estimates that if all of the 544 treatment plants that already have anaerobic digestion adapted CHP, the energy reduction would be equivalent to removing the emissions of approximately 430 000 cars.12 The bioenergy production potential here is significant.
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Another change in thinking directed toward more energyefficient systems is the use of distributed, rather than the centralized treatment systems favored in the past due to economies of scale. Centralized plants are generally located down gradient in urban areas, permitting gravity wastewater flow to the treatment plant, while the demand for reclaimed wastewater generally lies up gradient. This means higher energy demands for pumping of the reclaimed wastewater back to areas of need. These energy costs can be reduced through use of smaller distributed treatment plants located directly in water short areas. The Sanitation Districts of Los Angeles County has satellite treatment systems located in up-gradient communities where reclaimed wastewater is applied to percolation beds for mixing with groundwaters used for domestic consumption. The biosolids produced are sent through a trunk sewer to a centralized plant located near the Pacific coast, where sufficient CH4 is produced to satisfy most of the energy needs through a CHP system at the plant. Distributed treatment systems are even used at small scale. The upscale Solaire apartment complex, located adjacent to the Hudson River on Manhattan Island, New York City, has its own membrane biological treatment system in the basement to reclaim 95 m3/day of apartment wastewater for irrigation of its rooftop gardens and for use in toilets and the building’s cooling system. Excess wastewater and biosolids are sent to New York City’s North River Wastewater Treatment Plant for biogas and energy production. Wastewater energy is thus captured efficiently, and the demand on the city’s water system is reduced, as is the load on the North River Plant. The Monterey, CA, Regional Water Pollution Control Plant is located in a prime vegetableproducing but water-short agricultural area, and uses anaerobic treatment coupled with CHP to produce 50% of the plants energy requirements. The 76,000 m3/day of reclaimed water produced is applied to 4900 ha containing vegetable crops to satisfy their need for both irrigation water and plant nutrients, thus all three of wastewater’s important resources are being utilized. These examples well demonstrate how overall energy requirements for treatment can be reduced through more energy-efficient practices in addition to capturing wastewater’s energy potential, while simultaneously capturing its water and fertilizing nutrient resources. Today there is increased understanding of the importance of working toward better sustainability in our water and wastewater treatment systems. Toward this end the further development and wider application of advanced treatment systems, such as the anaerobic membrane bioreactor, that can better capture the full energy and the water and nutrient resource potential contained in wastewater is a highly desirable goal.
’ AUTHOR INFORMATION Corresponding Author
*Phone: 650-723-4131; fax: 650-725-3164; e-mail: pmccarty@ stanford.edu.
’ BIOGRAPHY Dr. McCarty is Emeritus Professor at Stanford University and WCU Professor at Inha University in Korea. He is coauthor of the textbooks, Chemistry for Environmental Engineering and Science and Environmental Biotechnology Principles and Applications. He is recipient of the Tyler Prize for Environmental Achievement and the Stockholm Water Prize. Dr. Bae is Professor in the Department of Environmental Engineering at Inha 7104
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Environmental Science & Technology University with primary interests in biogas recovery from solid wastes and wastewaters. Dr. Kim is an Assistant Professor in the same department at Inha University. The main focus of his research is on the development and use of membrane processes.
’ ACKNOWLEDGMENT This publication was supported by the WCU (World Class University) program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (grant number R33-10043). ’ REFERENCES (1) Asano, T.; Burton, F. L.; Leverenz, H. L.; Tsuchihashi, R.; Tchobanoglous, G. Water Reuse, Issues, Technologies, and Applications; McGraw-Hill: New York, 2007; p 1570. (2) Speece, R. E., Anaerobic Biotechnology and Odor/Corrosion Control; Archae Press: Nashville, TN, 2008; p 586. (3) Owen, W. F. Energy in Wastewater Treatment; Prentice-Hall, Inc.: Englewood Cliffs, 1982; p 373. (4) EPA Office of Water. Wastewater Management Fact Sheet, Energy Conservation, EPA 832-F-06-024; U.S. Environmental Protection Agency: Washington DC, 2006; p 7. (5) Curtis, T. P., Low-energy wastewater treatment: strategies and technologies. In Environmental Microbiology, 2nd ed.; Mitchell, R., Gu, J. D., Eds.; Wiley-Blackwell: Hoboken, NJ, 2010. (6) Gellings, C. W.; Parmenter, K. E., Energy efficiency in fertilizer production and use. In Knowledge for Sustainable Development—An Insight into the Encyclopedia of Life Support Systems; Gellings, C. W., Blok, K., Eds.; Eolss Publishers: Oxford, 2004; Vol. II, pp 419 450. (7) Galloway, J. N.; Cowling, E. B. Reactive nitrogen and the world: 200 years of change. Ambio 2002, 31 (2), 64–71. (8) Krichene, N. World crude oil and natural gas: A demand and supply model. Energy Econ. 2002, 24, 557–576. (9) Lindtrom, H. O. Experiences with a 3.3 MW heat pump using sewage water as heat source. J. Heat Recovery Syst. 1985, 5 (1), 33–38. (10) WEF-ASCE. Design of Municipal Wastewater Treatment Plants; Water Environment Federation: Alexandria, VA, 1992; Vol. II, p 829. (11) Logan, B. E.; Hamelers, B.; Rozendal, R.; Schroder, U.; Keller, J.; Freguia, S.; Aelterman, P.; Verstraete, W.; Abaey, K. Microbial fuel cells: Methodology and technology. Environ. Sci. Technol. 2006, 40 (17), 5181–5192. (12) Opportunites for and Benefits of Combined Heat and Power at Wastewater Treatment Facilities, EPA-430-R-07-003; U.S. Environmental Protection Agency: Washington DC, 2007; p 42. (13) Pant, D.; Van Bogaert, G.; Diels, L.; Vanbroekhoven, K. A review of the substrates used in microbial fueld cells (MFCs) for sustainable energy production. Bioresour. Technol. 2010, 101 (6), 1533–1543. (14) Rozendal, R. A.; Hamelers, H. V. M.; Rabaey, K.; Keller, J.; Buisman, C. J. N. Towards practical implementation of bioelectrochemical wastewater treatment. Trends Biotechnol. 2008, 26 (8), 450–459. (15) Liu, H.; Logan, B. E. Electricity generation using an air-cathode single chamber microbial fuel cell in the presence and absence of a proton exchange membrane. Environ. Sci. Technol. 2004, 38, 4040–4046. (16) Pant, D.; Singh, A.; Van Bogaert, G.; Gallego, Y. A.; Diels, L.; Vanbroekhoven, K. An introduction to the life cycle assessment (LCA) of bioelectrochemical systems (BES) for sustainable energy and product generation: Relevance and key aspects. Renewable Sustainable Energy Rev. 2011, 15, 1305–1313. (17) McCarty, P. L., One hundred years of anaerobic treatment. In Anaerobic Digestion 1981; Hughes, D. E.; Stafford, D. A.; Wheatley, B. I.; Baader, W.; Lettinga, G.; Nyns, E. J.; Verstraete, W.; Wentworth, R. L., Eds., Elsevier Biomedical Press Inc.: Amsterdam, 1981; pp 3 22. (18) Lettinga, G.; Roersma, R.; Grin, P. Anaerobic treatment of raw domestic sewage at ambient-temperatures using a granular bed UASB reactor. Biotechnol. Bioeng. 1983, 25 (7), 1701–1723.
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Environmental Science & Technology
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(40) Strous, M.; VanGerven, E.; Zheng, P.; Kuenen, J. G.; Jetten, M. S. M. Ammonium removal from concentrated waste streams with the anaerobic ammonium oxidation (anammox) process in different reactor configurations. Water Res. 1997, 31 (8), 1955–1962. (41) Larsen, T. A.; Alder, A. C.; Eggen, R. I. L.; Maurer, M.; Lienert, J. Source separation: Will we see a paradigm shift in wastewater handling? Environ. Sci. Technol. 2009, 43, 6121–6125. (42) Tchobanoglous, G.; Burton, F. L., Wastewater Engineering: Treatment, Disposal, Reuse, 3rd ed.; McGraw-Hill, Inc.: New York, 1991; p 1334.
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dx.doi.org/10.1021/es2014264 |Environ. Sci. Technol. 2011, 45, 7100–7106
