|Fig. 1: An example of a sludge digestion tank. (Source: Wikimedia Commons)|
The accumulation of large amounts of waste in countries with high population is a growing problem. Dirtying the water and polluting the air, waste or sludge requires treatment and in some cases can be converted to be used as fuel.
Human waste is typically dealt with at sewage plants or water treatment plants. The waste is dried and converted into a bio-solid which is either left in a landfill or used as fertilizer. This method is wasteful and harmful to the environment.  Instead, some plants put the sludge into digester tanks (like that in Fig. 1) where the sludge undergoes anaerobic digestion. In this process, bacteria convert organic matter to methane and produce a bio-gas (a mixture of methane and carbon dioxide). The bio-gas is harnessed to generate heat or electricity to power the plant. 
The energy produced by this method of waste to energy can be used in various ways such as: production of cement or building materials, pyrolysis, gasification, supercritical (wet) oxidation, hydrolysis at high temperature, production of hydrogen, acetone, butanol, or ethanol, and direct generation of electrical energy by means of specific micro-organisms.  As stated before, the direct generation of electrical energy can be used to power the plant or power the anaerobic digestion process to produce more methane and thus more energy.
In a recent study, Elizabeth Heidrich attempted to verify the results of a previous study and compared samples between domestic wastewater and wastewater from industrial facilities. The industrial sample contained about 16.8 kilojoules per liter of internal chemical energy, which is 7.6 to 20 percent more than the study done in 2004. 
We can make an estimate for the total energy available in all sewage waste in the world by finding the total dry mass of all sewage and multiplying it by the energy content of dry sewage. We estimate the total dry mass by the amount of feces produced by an average person (128 g/day of wet mass plus 29 g/day of dry mass to get 157 g/day) multiplied by the number of people on earth (7.6 billion) to get 1.2 billion kg/day.  To get the yearly amount, we multiply by 365 and get 438 billion kg of sewage waste per year. We multiply this number by the energy content of sewage (12.51 MJ/kg) to get 5.48 × 1012 MJ/year = 5.48 × 1018 J/year.  This is a similar process used for the estimated upper bound of 5.98 × 1018 J/year of energy given by Cash. 
So, is converting waste to energy a feasible energy source? In the case of wastewater plants using fuel cells to produce electricity to help run the plant and help run the waste conversion, it may be feasible. The costs required to produce the electricity is at most equivalent to the benefits of recycling energy and disposing of waste that would otherwise pollute the water and air or take up space.
As far as becoming a cost efficient and high energy outputting energy source, waste to energy is not feasible. We have a generous estimate of 8.68 × 1018 J/year, which is only a fraction of the total primary energy supply (around 13,000 Mtoe = 5.44 × 1020 J/year). 
© Franklin Huang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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