The Role of Anaerobic Digestion in Wastewater Management by Vincent Vutai et al. Anaerobic Digestion. in Wastewater Management

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1 The Role of Anaerobic Digestion in Wastewater Management This article considers the role anaerobic digestion can play in energy recovery from wastewater.

2 Wastewater systems potentially contribute significant negative impacts not only on regional water bodies, but also on global energy, climate, and sustainability. Energy recovery from wastewater is one way to reduce the negative impacts and achieve greater resource recovery. 1,2 The most common form of energy recovery is anaerobic digestion. Anaerobic digestion is the biological degradation of organic matters in the absence of oxygen and converts the chemical energy in organic carbon to biogas. 3 Typically, anaerobic digestion has been used for wastewater sludge treatment and reduction, agricultural manure management, and food waste management. 3 To accomplish more sustainable resource recovery and reduce overall energy footprint, wastewater can be regarded as renewable resource to convert embedded chemical energy to biogas. 4 And as a result, anaerobic digestion can play a significant role in energy recovery from wastewater. In the United States, currently there are 16,000 publicly owned treatment works. Of those, only 544 use anaerobic digestion. 3 That means currently there are at least 15,456 facilities that send the sludge to landfills or incinerators that attributes to global warming and air pollutions. The importance of anaerobic digestion arises with the ability to convert the organic compounds in waste to biogas. Biogas is comprised of 60 70% methane and 30 40% carbon dioxide, as well as other trace gases. A combined heat and power (CHP) engine can then use biogas to create electricity and heat. Another option is to use the compressed biogas as fuel for fleet vehicles. For fleet vehicles, biogas is compressed in order to power alternative fuel vehicles. Compressed methane or natural gas (CNG) is often viewed as a cleaner alternative to diesel fuel. With this ability to create electricity and fuel, wastewater treatment plants (WWTPs) can not only power their entire plant, but also receive revenue by sending excess electricity to the grid. Currently, few WWTPs are sending electricity back to the grid. Most WWTPs that use anaerobic digestion as one of the treatment process either use biogas to heat the buildings, heating influents or flare the excess biogas or profit from tipping fees paid by other local companies. A tipping fee is a fee charged for the amount of waste disposed of by customers to a landfill. 5,6 Not only can anaerobic digestion create energy from waste, but also through the anaerobic digestion process, digestate that is the material remaining after the methanogenesis stage is called biosolids. The biosolids can be further treated to produce higher quality biosolids either grade A or B, which can be sent to local farms or nursery stores as a fertilizing compound or as soil conditioner. The Co-Eat Model The Co-Eat model that was developed by U.S. Environmental Protection Agency (EPA) researchers as a co-digestion economic analysis tool allows the user to input current operating parameters that tailor the model to plant specific situation. 7 This model intends to see the impacts of co-digestion on an existing anaerobic digestion system. The model was developed in order to see the impacts of what co-digestion can do have on an existing anaerobic digestion system. The Co-Eat model can predict biogas production based on volatile solids (VS) destroyed per day or per year. 7 It can also estimate other economic parameters such as tipping fees, value of the biogas, and disposal cost. There is a scenario Figure 1: Anaerobic digestion process. Source: JFS & Associates

3 Table 1. Biogas production and values using Co-Eat model: Quasar Operation. Quasar Biogas Production Using Co-Eat Combined Heat and Power Efficiency (CHP) = 28% Biogas to Natural Gas Conversion Factor = 60% Total Solids % for Food Waste = 8% Total Solids % for Fats, Oil, Grease = 8% Total Solids % for 1 MGD = 1% Natural Gas ($/unit) = $2.44 Electricity ($/unit) = $0.05 Tipping Fees ($/unit) = $0.07 Quasar 4/20/16 Biogas Production (ft 3 /day) 227,091 Annual Biogas Production (ft 3 /yr) 101,138,396 Electrical Energy Generated via CHP (KWh/yr) 4,780,458 Biogas Value (Boiler + Flare) ($/yr) 531,242 Value of Electrical Energy via CHP ($/yr) 239,023 Value of Compressed Natural Gas (Bioler + Vehicle Fuel) ($/yr) 1,126,515 generator built into the model that generates theoretical co-digestion scenarios in order to compare the economics and physical characteristics to that current process. This model was used in the following two case studies. Case Study: Quasar Wooster, OH Quasar is a company that uses anaerobic digestion technology that works with a municipal WWTP to produce biogas from sludge. 8 Quasar uses the biogas to generate electricity and thermal heat, natural gas, or compressed natural gas. 8 The company s mission is to reduce greenhouse gas emissions, divert waste from landfills, and contribute to a cleaner environment while receive economic benefits. 8 At the Quasar operation, digestion feed was coming from sludge from WWTPs, wastes from poultry, grain, food, and other local manufacturing companies such as Smuckers. These multiple feed inputs are called co-digestion because of the mixture of different types of organic wastes. 9 Quasar s facility was operating at around 1 million gallons per day (MGD) and receiving around 60,000 gallons per day of combined food waste, fats, oils, and greases. Typically, food waste is around 5% total solids. 10 With the biogas generated, the CHPs were able to heat the inflow, power the entire plant, and send excess electricity back to the grid. The biosolids that were generated as byproducts of anaerobic digestion were sent to local farms as fertilizer. However, most of the profit Editor's Note: Anaerobic Digestion in the News Scientists around the world are turning wastewater sewage into renewable energy. In Bristol England, the GEN-eco Bio- Bus is fueled by biogas collected via an anaerobic digester. South Korea has a wastewater treatment plant that uses human wastes to power itself. And a wastewater treatment plant in California converts biogas into electricity that is pumped back to the grid. Read more in Powered by poop and pee? by Alison Pearce Stevens.

4 received was through tipping fees of these local food/agricultural companies who sent the waste to Quasar instead of landfill or incineration. Table 1 shows the results of biogas generation, the energy recovered and the revenues from different biogas applications. Case Study: Dayton Wastewater Treatment Plant Dayton, OH The Dayton Water Reclamation plant is a much larger facility compared to Quasar facility. The Dayton plant uses primary sludge and secondary activated sludge from the municipal wastewater. It operates around 38 MGD. With that large input, the facility is able to create a lot of biogas, averaging around 600, ,000 ft 3 per day. Table 2 presents results from the Co-Eat model that show the potential of biogas for Dayton Water Reclamation Plant. 7 Some operating parameters were made identical to compare the two facilities such as CHP efficiency, biogas to natural gas conversion factor, total solids percentage for 1 MGD, natural gas ($/unit), electricity ($/unit), and tipping fees ($/unit). However, operating parameters that were assumed for the base case of Quasar was the total solids percentage of food waste, fats, oils, and greases. It can be seen that using biogas for electrical energy via CHP and CNG, biogas could be a very profitable operation for a wastewater facility. If sending the sludge to a landfill or an incinerator, a plant that is in comparable size as Dayton s plant can lose potential revenue of anywhere between $2 and $3 million per year. Comparing the Quasar operation with the Dayton Water Reclamation Plant, one can see the potential benefits of codigesting different local food wastes. Dayton primarily uses primary sludge and waste activated sludge compared to Quasar s operation of food and agricultural waste. But compared to Quasar, Dayton Water Reclamation Plant is roughly 38 times larger than Quasar s operation with an influent rate of 38 MGD compared to Quasar s 1 MGD. In terms of biogas production, Quasar is not far from Dayton s biogas production. Roughly, the Dayton plant is creating only 62% more biogas production than Quasar. This is significant because Quasar is much smaller operation than Dayton Water Reclamation Plant. This shows the benefit that local food waste and agricultural waste with higher organic contents can have on the biogas production. Conclusion Traditionally, anaerobic digestion is seen as a waste management resource. Many facilities and plants that we have spoken to say they are unable to implement anaerobic digestion technology because of the high capital costs to either build new digesters or upgrade their existing digester technology for the introduction of other organic wastes such as food waste. Table 2. Biogas production and values using Co-Eat model: Dayton WWTP. Dayton WWT Biogas Production Using Co-Eat Combined Heat and Power Efficiency (CHP) = 28% Biogas to Natural Gas Conversion Factor = 60% Total Solids % for 1 MGD = 1% Natural Gas ($/unit) = $2.44 Electricity ($/unit) = $0.05 Tipping Fees ($/unit) = $0.07 Dayton Water Reclamation Plant 4/26/16 Biogas Production (ft 3 /day) 684,295 Annual Biogas Production (ft 3 /yr) 249,767,753 Electrical Energy Generated via CHP (KWh/yr) 11,805,649 Biogas Value (Boiler + Flare) ($/yr) 1,311, Value of Electrical Energy via CHP ($/yr) 590, Value of Compressed Natural Gas (Bioler + Vehicle Fuel) ($/yr) 2,782,002.00

5 Time and funding constrains are other obstacles many facilities and plants face when trying to adopt anaerobic digestion. Many process engineers do not have the time or resources to further investigate new technologies such as anaerobic digestion due to their existing workloads and the complexity of retrofitting the existing plants. Many plants and facilities either keep status quo or hire outside consultants if budget allows. generation. Sludge from wastewater treatment plants is being sent to landfills or burnt in incinerators, creating greenhouse gases that affect the environment while the utilities do not tap into the revenue potential. If the biogas is harnessed from the sludge and wastes from local food and agricultural industries, it can achieve overall system efficiency with economic and environmental benefits. Implementing this technology would allow reductions in greenhouse gas emissions. Another obstacle facing anaerobic digestion application is the stigma that biosolids as composting waste produces a foul odor. Aside from all the obstacles, anaerobic digestion can be a great technology on energy recovery and potential revenue For our future work, we want to explore the return on investment when implementing a new anaerobic digestion system for a facility. The system analysis such as Life Cycle Assessment (LCA) should be done not just for anaerobic digestion unit process, but the system of wastewater treatment train as a whole to evaluate trade-offs of this new technology. em Acknowledgment: The U.S. Environmental Protection Agency and the University of Cincinnati Research Training Program supported this project. The authors would like to acknowledge Phil Bennington from Dayton Reclamation Plant for providing the data required for this project along with the continued support and encouragement, the staff from Quasar for providing a tour of their facility, as well as the necessary information regarding their operation, and U.S. EPA researchers Dr. Steve Rock and Jonathan Ricketts for the discussions of the Co-Eat model. Disclaimer: The views expressed in this article are those of the authors and do not necessarily reflect the view or policies of the U.S. Environmental Protection Agency. Any mention of specific products or processes does not represent endorsement by the U.S Environmental Protection Agency. Vincent Vutai and Mingming Lu are both with the Department of Biomedical, Chemical, and Environmental Engineering at the University of Cincinnati. Xin (Cissy) Ma is with the U.S. Environmental Protection Agency s National Risk Management Research Laboratory, Sustainable Technology Division, Cincinnati, OH. vutaivl@uc.edu. References 1. McCarty, P.L.; Bae, J.; Kim, J. Domestic Wastewater Treatment as a Net Energy Producer Can This be Achieved?; Environ. Sci. Technol. 2011, 45, (17), Guest, J.; Skerlos, S.; Barnard, J.; Beck, M.; Daigger, G.; Hilger, H.; Jackson, S.; Karvazy, K.; Kelly, L.; Macpherson, L.; Mihelcic, J.; Pramanik, A.; Raskin, L.; Van Loosdrecht, M.; Yeh, D.; Love, N. A New Planning and Design Paradigm to Achieve Sustainable Resource Recovery from Wastewater; Environ. Sci. Technol. 2009, 43, Ma, X.; Xue, X.; González-Mejía, A.; Garland, J.; Cashdollar, J., Sustainable Water Systems for the City of Tomorrow A Conceptual Framework; Sustainability 2015, 7, (9), Grant, S.B.; Saphores, J.-D.; Feldman, D.L.; Hamilton, A.J.; Fletcher, T.D.; Cook, P.L.M.; Stewardson, M.; Sanders, B.F.; Levin, L.A.; Ambrose, R.F.; Deletic, A.; Brown, R.; Jiang, S. C.; Rosso, D.; Cooper, W.J.; Marusic, I. Taking the Waste Out of Wastewater for Human Water Security and Ecosystem Sustainability; Science 2012, 337, (6095), California Energy Commission. See (accessed June 18, 2016), 6. Waste Management. See (accessed June 18, 2016), 7. U.S. Environmental Protection Agency (EPA) Organics: Co-Digestion Economic Analysis Tool (CoEAT). See index-2.html (accessed June 18, 2016), 8. Quasar Energy Group. See (accessed June 18, 2016). 9. JFS & Associates. See Moriarty, K. Feasibility Study of Anaerobic Digestion of Food Waste in St. Bernard, Louisiana: A Study Prepared in Partnership with the Environmental Protection Agency for the RE-Powering America's Land Initiative, Siting Renewable Energy on Potentially Contaminated Land and Mine Sites; NREL/TP-7A , 2013.