Two of the key challenges facing us as a global community in the 21st century are climate
change and access to clean water. Population growth and increasing usage of water has
resulted in ever larger volumes of wastewater. Heightened public awareness and concerns
over long term environmental and health effects have resulted in increasingly tight discharge
standards. These twin drivers fuel the rise in energy required for wastewater treatment.
One of the most energy intensive aspects of wastewater treatment is aeration for the aerobic
oxidation of organic compounds. Typically half the energy used in wastewater treatment is
used for aeration and a key by-product is a large amount of excess biomass (sludge).
Treatment and disposal of this sludge is both costly and generates further environmental
concerns. The treatment of water and wastewater is currently the 4th largest sector for energy
usage in the UK and global warming concerns mean that the industry is under pressure to
achieve higher water quality whilst reducing its carbon footprint.
The organics within the wastewater have a significant calorific value from 7.6 kJ/l (domestic
wastewater) to 16.8 kJ/l (mixed industrial/domestic waste) (Heidrich et al 2011). This energy
concentration is too low for economic recovery using existing technologies. However
significant research is being undertaken into exoelectrogenic bacteria that are able to directly
transfer electrons outside of the cell by the anaerobic oxidation of organic compounds in the
wastewater. Devices that convert these organic compounds direct to electricity are known as
microbial fuel cells (MFCs) and combine biological and electrochemical processes. Whilst
significant research has been undertaken into the potential of MFCs as a potential source of
renewable energy, no large scale plants have yet been installed and the research is still limited
to the laboratory.