Wastewater treatment is the method by which sewage of both residential and industrial sources is processed to promote public health and reduce environmental impacts on receiving waters. This physical and biological process generates sludge, which after being treated to reduce pathogens, is referred to as biosolids. In the US there are over 16,000 wastewater treatment plants (WWTP), and every year they produce approximately 7 million tons of biosolids according to the EPA.1 These biosolids are handled differently depending upon local conditions, but most are either buried in landfills, land applied for agriculture or incinerated. Reducing the volume of biosolids produced by each facility is desirable for improving operational efficiency since lower volumes are easier to manage and cheaper to handle and dispose. Most facilities utilize either aerobic digestion to process sludge into biosolids, but larger facilities use anaerobic digestion because this process reduces the overall volume of solids left for management. Anaerobic digestion is more complex and capital intensive, so typically only those facilities treating flows higher than 5 million gallons per day (MGD) use anaerobic digestion. Given current economic conditions and rising energy costs, however, anaerobic digestion is becoming more attractive to utility managers as they attempt to offset energy costs. The anaerobic process produces methane gas. Also called biogas, methane can be utilized not only to fire boilers for heating digesters and nearby buildings, but also to fuel internal combustion engines, microturbines or fuel cells to generate power for plant processes such as blowers in the aeration system. There is also the potential for WWTPs to obtain carbon credits for utilizing renewable energy, especially in those states with renewable portfolio standards. Because anaerobic digestion has limited application in the US, this study evaluated economic viability at plants with design flows less than 5 MGD by incorporating codigestion of food waste to improve the production of biogas for use as energy to reduce operational costs and recover capital costs.

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