One of the most popular topics in today's world is finding ways to reduce waste. There is considerable drive to increase recycling, decrease energy consumption, and reduce emissions. However, one form of waste we often ignore is our own human excrement. It's a topic that's often ignored because of taboos, but dealing with human waste is a problem all societies have faced. Failing to remove it properly can often have deadly consequences, as is currently illustrated by the cholera outbreak in Haiti.  Thus, significant resources in the forms of money, energy, and manpower are spent on our excrement. Without drastically altering our diets, it's unlikely we'll be alter our biological functions to reduce waste, but there is plenty of unharnessed energy left in our excrement. A significant portion of the energy content of the food we ingest remains after it leaves our bodies, but it is largely ignored in modern waste treatment. This report will explore the possibilities for this untapped energy source.
A simple theoretical analysis can yield an upper-bound on the amount of energy that can be extracted from human feces. The mass of waste produced depends heavily on the amount of dietary fiber consumed. [2,3] High-fiber diets can produce upwards of four times the wet-stool mass of one deficient in fiber.  Age and gender are two other important factors. Adults with a high-fiber diet produce an average of 349 g/day of wet-stool.  Thus, a generous estimate of the mass of feces produced by the world's 6.8 billion people is 866 billion kilograms per year.  However, only 260 billion kilograms of viable fuel are produced, because water makes up approximately 70% of the stool weight. [5,6] The energy content of dry stool is about 2.3 x 107 J/kg . Thus, a high-fiber population produces 5.98 x 1018 J/year of energy, well short of the 1020 J/year of energy consumed in the world. 
Although, we'll never be able to power the world on our own biological waste there is still a significant amount of untapped energy available. In addition, a portion of the world's 1020 J/year is used to treat this waste, but it's hard to estimate how much because the methods of treating the waste vary significantly. In developed nations, centralized plumping transports the waste to large waste-water treatment facilities, but 2.6 billion people still have no form of toilet.  About 1% of England's total electricity is spent on wastewater treatment, but in some places excrement can simply be left exposed or even dumped into water sources.  Thus, developing methods to harness the energy in feces can benefit both the richest and poorest nations, but the techniques will likely be drastically different.
Excrement isn't a fuel you can simply just burn, because it's mostly water. Drying and burning it is possible, but the varied composition of feces will cause poor combustion and numerous emissions. Fortunately, generating methane gas from excrement is a very well understood process. It's used everyday at many wastewater treatment plants and on farms and dairies with animal waste. The waste is broken down by anaerobic bacteria in digester devoid of oxygen. The bacteria feed on the excrement and produce methane gas. This methane is traditionally considered a useless by-product of the digestion process and it is often just burned off. Some wastewater plants use the combustion to heat their facilities and the digester, but the vast bulk of this energy is wasted. This process is very attractive because it can be scaled from individual residences to city-wide networks.
The applications for the generated methane are as varied as the waste treatment procedures around the world. In areas with no centralized plumbing, individual or community digesters are ideal. Not having an existing infrastructure means the systems can be designed to optimize digestion efficiency. The small amounts of methane produced can be used for cooking and heating, as is already being done with some community kitchens in Africa. 
Not everyone has been quite as wasteful with the methane produced. Farms around the world are using their animal waste to power their facilities and even sell electricity back to the grid.  One might be tempted to combine human waste with food and animal waste treatment digesters, but this should only be done if proper precautions for handling human waste are applied.
Developed nations with centralized plumbing will have to work within the constraints of these systems. A large portion of the clean water consumed (e.g. 30% in the United States) is used for toilets. Unfortunately, a large amount of energy is consumed cleaning and transporting this water, and it's entirely unfeasible to overhaul these infrastructures. Thus, in developed nations the best place to extract some energy is at existing waste treatment facilities.
The quantity of methane produced at these plants is significant enough to be used to generate electricity on a large-scale and cities are beginning to see the value in this. The city of Cleveland is currently constructing a steam-powered electricity plant at it's wastewater treatment facility. The plant will be able to produce 25% of its total electricity demand and expects to pay off the investment in 11 years.  San Antonio and San Diego have plans to pipe the collected methane and sell it to utility companies. [12,14] New York City is currently seeking proposals for how to efficiently dispose of its waste for the next twenty years. 
Burning methane isn't the only option. The methane can be used to generate hydrogen for fuel cells. A pilot program in California is currently creating enough to fuel 50 cars per day.  Disposing of human waste in space has always been a problem. It's currently hauled back to earth at an enormous cost. The first United Nations' satellite is going to test the possibility of generating hydrogen from human waste in space when it launches in 2011. 
The waste our bodies produce doesn't have to be a significant strain on our already limited resources. Harnessing it as a renewable energy source can also improve sanitation and reduce water pollution throughout the world. It's not a solution to the world energy problem, but the technology already exists and it is being shown to be economically feasible in a variety of situations.
© William Cash. 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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