While much of water R&D has been focused on the production of potable water, there has been a growing interest in the generation of energy from wastewater. Scientists believe that wastewater contains up to 9 times the energy needed to treat it. This means that wastewater treatment plants could potentially be energy self-sufficient, while producing excess clean energy for other uses.
Tapping energy from wastewater is not a new concept. Many modern wastewater treatment plants use the biogas released by the decomposition of organic material to supplement their energy needs. Energy is also produced in the incineration of the sludge produced at the end of the decomposition process. But the total energy produced through the current processes is less than 5% of what is estimated to be in there.
Since the energy is stored in the organic material, R&D has unsurprisingly been centred about improving the efficiency of decomposition (and energy recovery in the process), and microbial fuel cells appear to be leading the charge. Microbial fuel cells (MFCs) are based on the anaerobic decomposition of organic substances in wastewater, which produces electrons that when sent through a conductor, results in the flow of electricity. A slight modification of the MFC produces the microbial electrolysis cell (MEC), which produces hydrogen gas. The hydrogen gas can be used as an energy source in place of natural gas.
Both MFCs and MECs are still some way from large scale implementation, but some innovative wastewater treatment plants have already managed to achieve energy self-sufficiency! Check out the San Diego Point Loma wastewater treatment plant, which not only produces enough energy for its own needs but also supplies energy to the power grid. Its energy sources?
Biogas from the digesters that breakdown the organic compounds is used to power generators
Waste heat from the generators keeps the digesters at an optimum temperature
Hydroelectricity -- the plant happens to be located on a cliff, 90ft (27m) above its ocean outfall, and a hydroelectric plant captures the energy when the treated effluent flows down to the sea
Above: A microbial desalination cell developed by Tsinghua University and Penn State University -- a slight modification on the basic MFC
Source: Bruce E Logan Laboratory, Penn State University
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