Twinning Tech to make Electricity from Waste Water

04 March 2012
Posted by Ben Valsler.

By combining two existing but limited technologies, researchers in America have devised a novel way to generate electricity from waste water - and all it took was a pinch of the right salt.

The two technologies are Microbial Fuel Cells (MFC) and Reverse Electrodialysis (RED).  MFCs use naturally occurring microbe species known as Exo-electrogenic bacteria, which break down organic matter and release electrons, creating a voltage.

Microbial Reverse-Electrodialysis CellRED relies on a salinity gradient, using seawater and freshwater separated by a stack of selectively permeable membranes that only allow either positive or negative ions through.  These membranes are connected to electrodes, and together contribute to an electric current.  Each membrane only provides a small amount to the current, so a working RED cell requires many layers of membrane.  To keep the supply of sea- and fresh-water, RED systems presently need to be built by the coast, and when using natural water sources, suffer from fouling unless the water is extensively filtered first.

By combining MFCs with RED and feeding in waste freshwater, Professor Bruce E. Logan at colleagues at Penn State University were able to make a "Microbial Reverse-electrodialysis Cell" or MRC, that not only generated significantly more energy than a fuel cell alone, but also needed fewer membranes than a traditional RED.

By using ammonium bicarbonate as their salt source instead of seawater, they were able to increase the efficiency and sever the tie to coastal regions.  Ammonium bicarbonate solution is easily produced using waste heat, and can be reclaimed and recycled within the system.

Logan and colleagues argue that this technology could tip the energy balance of water purification, as their system helped to purify waste water while generating 0.94 kilowatt-hours of electricity per kilogram.  Traditional treatment of waste water with activated sludge consumes 1.2 kilowatt-hours per kilogram of organic matter.  This would provide a strong incentive to build MRC treatment plants in energy poor areas - providing both power and sanitised water.

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