A recent article in the American Chemical Society journal examined the potential for generating electricity from the difference in saltiness between freshwater and seawater.

In the process, called "pressure retarded osmosis", freshwater flows naturally by osmosis through a special membrane to dilute seawater on the other side. The pressure from the flow spins a turbine and produces electricity using no fuel and with no greenhouse gas emissions.

The study concluded that using this process on just one-tenth of the global river water flow into the oceans could generate enough power to meet the electricity needs of 520 million people.

Now scientists at Penn State have extended the potential of the idea to producing electricity from any water which contains food waste, domestic waste or animal waste.

The process first uses a naturally occurring exoelectrogenic bacteria, that consumes organic material and produces an electric current, to remove the organic matter from the waste water. They then add ammonium bicarbonate. The ammonium bicarbonate solution works similarly to seawater for producing electricity through osmosis.

After it has been used in the process, the ammonium bicarbonate is easily removed from the water by heating it to 43°C.  The ammonia and carbon dioxide that make up the salt boil out and are recaptured and recombined for reuse. The heat for this would come from waste heat from the process which produced the waste water.

Project leader, Professor Bruce E Logan, said that "Waste heat makes up 7 to 17 percent of energy consumed in industrial processes, There is always a source of waste heat near where this process could take place and it usually goes unused."