Getting Electricity from Sewage

Treating human waste is a very expensive problem for most countries. Without treatment, raw sewage can spread disease and kill off the living organisms in our streams and rivers. Contamination of drinking water with sewage is common in countries that do not have the resources to treat their sewage. Sewage treatment is expensive but it does protect our waterways and prevents a lot of human illness. Unfortunately, the need to treat human sewage will never go away as long as people continue to gather together in the cities and towns of our world. About 60 percent of the human waste generated in the U.S. goes to waste treatment plants. Fourteen billion gallons of water are needed each day to carry that waste to the treatment plants. Even so about 10 percent of the waste mentioned above is not treated and goes in the form of raw sewage into our streams and rivers.

There are sewage treatment plants all over the industrialized world that utilize two relatively simple means to treat the sewage. The primary means used to treat sewage is to allow the solid material to settle to the bottom of a sedimentation tank. The sludge that settles to the bottom is pumped into an oxygen-free digester (anaerobic; secondary treatment). There, anaerobic bacteria help to breakdown many of the organic (carbon-containing) compounds in the sludge. The sludge is then allowed to dry and can oftentimes be used a fertilizer.

The liquid on top of the sludge in the sedimentation tank is then pumped to an oxygen (aerobic) containing digester (secondary treatment). The bacteria in the digester then breakdown the organic (carbon-containing) material in the sewage. The biological oxygen demand (BOD) is determined when assessing whether a digester has done its job or not. BOD is the amount of oxygen needed to breakdown the organic material in the sewage. If the sewage were released without this treatment the organisms in the waterways would use up valuable oxygen to breakdown the organic materials killing the fish and other animals dependant on oxygen in the water. By the time treated water is released around 90 percent of the BOD is removed. The water is chlorinated to eliminate most of the bacteria and then the water is released.

All of this takes time and large amounts of electricity to pump the liquids and to aerate (put oxygen in) the sewage. Some researchers from Pennsylvania State University (PSU) have found a very simple way to generate electricity from this sewage and at the same time reduce the BOD by 80 percent. In the Environmental Science and Technology (volume 38, number 7, pages 2281-2285), Hong