US sanitary sewer systems collect about 14 billion gallons (~53 billion liters) of sewer water a day. Remarkably, this water contains about 26,000 tons of a potential renewable energy source even after treatment at waste water treatment plants (WWTPs). The end product of those plants is called biosolids, which have a calorific value of low grade coal – about 12 MJ/kg. If all of the biosolids could be converted to power, 9.5 million tons of coal extraction and combustion would be avoided annually. When added to existing energy production at WWTPs (biogas produced in anaerobic digesters), US WWTPs have the potential to produce 18,000 gigawatt hours of clean energy each year.
    Our approach combines all of the energy components from sewage (methane from the anaerobic digester and the energy locked within the biosolids) to produce electricity and heat in two stages. In the first stage, we produce gaseous fuel called synthetic gas (syngas). In the second stage, we convert it into power in contemporary power plant based on an internal combustion engine, a combined cycle gas turbine, or solid oxide fuel cell. Our modeled thermal efficiency of the most efficient configuration will approach 85%. The flow diagram is shown in Fig.1.