What ever goes down the drains in your sinks and showers or when you flush your toilet travels through sewers to a wastewater treatment plant. The output of the wastewater treatment plant is safe to discharge into a river or lake. In fact, some wastewater treatment plants boast that their effluent is safe for drinking. The process includes putting raw sewage into large settling tanks. Processing the resulting unsanitary sludge is energy intensive and expensive. What if it's possible to treat wastewater without accumulating sludge? What if the output of sewage can be not only potable water, but also electricity?
According to a study released in December 2010 by the American Chemical Society, such a process is not a pipe dream, but a potential reality. Only one previous study, in 2004, measured the internal chemical energy in municipal waste water. That one dried wastewater in an oven, analyzed the dried sample in a bomb calorimeter, and determined the chemical oxygen demand (COD). From the COD, it inferred that wastewater contains a significant amount of energy.
The 2010 study points out that oven drying destroys some of the potent volatile organic compounds in the sample. Its authors therefore freeze dried wastewater samples, only a few milliliters per day. It takes a month or two to obtain enough solids to analyze using this method, but it avoids burning off important chemicals. The resulting analysis estimates 20% more energy per liter than the earlier study.
How much energy are we talking about? The 2004 study estimated that, world wide, municipal wastewater contains a continuous supply of somewhere between 70 and 140 gigawatts of energy annually. To produce that much energy in petroleum-fired electric generating plants would require burning 52-104 million tons of oil.The world's largest wind turbines could generate 70-140 gigawatts per year--if there were 12 to 24 thousand of them working continuously! A large nuclear plant produces about 1 gigawatt of electricity annually, so it would take about 70-140 of them.
And that's just the 2004 study, which underestimated the available energy and didn't even consider the energy potential from industrial or agricultural wastewater. Some large hog farms, for example, simply push manure into large lagoons, which smell foul and cause significant health hazards. As it turns out, both of these other two waste streams potentially contain more energy than municipal wastewater.
How do we convert this potential into electricity we can actually use? Formerly, it required using an anaerobic digester to extract methane from wastewater (or garbage). That methane can be used in place of natural gas in an electric power plant. The digester works only when kept artificially warm, which requires electricity. Perhaps the main reason no one did than on an industrial scale is that no one thought to measure energy potential in wastewater until 2004. Newer technology has rendered methane extraction unnecessary. What's more, it operates at room temperature or lower.
Microbes in waste water (and garbage, for that matter) consume the organic compounds, converting them into carbon dioxide, water, and energy. A microbial fuel cell converts a portion of this chemical energy directly into electricity. By the time the water and electricity leave the fuel cell, the organic compounds have been consumed. Whatever is left contains nothing to spread, compost or treat by any of the other expensive processes now needed to get rid of sludge.
Neither microbial fuel cells nor any other means of generating electricity are 100% efficient. Nevertheless, they seem to offer a faster way than either wind or solar power to generate electricity with renewable resources and stop burning fossil fuels and accumulating nuclear waste. The science and technology are still in infancy. More research is needed before we can stop relying on coal for half of our electricity. But the fact is that we flush energy down the toilet. The sooner we can tap into it and use it, the better.