Conventional wastewater treatment process designs continue to incorporate expensive, energy-intensive aerobic treatment methods first developed over a century ago. Although industrialized nations can afford to operate these facilities, more than a billion people in the world lack adequate sanitation and cannot afford to build or operate these types of wastewater treatment plants. In order to make wastewater treatment more affordable for all nations, new approaches to treatment must be discovered and developed. One approach is to find new ways of producing useful products from products of treatment.

Dr. Bruce Logan and his research group have discovered that electricity can be produced from wastewater using bacteria in a microbial fuel cell, while at the same time accomplishing wastewater treatment. Bacteria degrade organic matter by oxidizing it or through the removal of electrons from the organic substrate. This oxidation process by bacteria must be linked to reduction of another compound, typically oxygen or nitrate, to which the bacteria add electrons. Some bacteria, such as those that reduce iron minerals, have the ability to transfer these electrons to a surface such as a carbon electrode. By providing a conductive growth surface for bacteria (anode) that is separated from a counter electrode exposed to oxygen (cathode), it is possible to use bacteria to create electricity.

In construction, Logan’s microbial fuel cell is deceptively simple. It is comprised of a plastic tube, measuring just 6 inches long and 2.5 inches in diameter, through which wastewater is circulated. A bacterial biofilm grows on the surface of graphite rods housed inside the tube. These rods function as the anodes. As the bacteria oxidize the organic matter in the wastewater, electrons are released and move as electrical current through the rods into wires outside the liquid container to a cathode constructed of plastic, carbon, and platinum. Wastewater treatment occurs as hydrogen ions are released by the feeding bacteria into the circulating wastewater, reducing its biological oxygen demand.

Biological generation of electricity in a wastewater treatment process represents a new approach to wastewater treatment and much needs to be done to develop this technology. The two specific objectives of Logan’s work are 1) to design and develop new reactors that reduce reactor cost and improve electricity recovery and 2) to demonstrate continuous power generation using domestic wastewater in larger reactors.

There are potential economic benefits of a process that generates electricity from wastewater. For example, complete recovery of electricity from domestic wastewater produced by a community of 100,000 people could conceivably produce 2.3 MW of power. Even if only 0.5 MW of electricity cold be recovered using such a process at a rate of ~103 mW/m2, this would be enough electricity to power 330 homes. If sold, the electricity would be worth as much as $1.7 million based on electricity costs in California ($0.15/kWh). There are also numerous industries where electricity could be harvested from high-strength wastewaters. For example, electricity generation from 20,000 food processing industries that generate one million gallons of wastewater per day could be used to produce electricity worth $1 million per plant, or $20 billion annually.

Through this research it will be possible to develop new, more cost-effective reactors that can be successfully scaled to larger size and tested. This and other work will lead to wastewater treatment systems in the future that will accomplish both wastewater treatment and affordable and renewable electricity generation.