Aconsiderable 30 MW a year could poten- tially be saved at South African waste- water treatment plants should the methane-rich biogas emitted as a by-product be exploited for its energy-generation potential.

Jason Gifford, spokesperson for the Energy Division of WEC Projects, a firm which is implementing South Africa’s first biogas-to-power plant at a municipal wastewater treatment works, explains that biogas produced during the sewage treatment process has the potential to reduce the plant’s dependence on the national electricity grid.

“Biogas can be used to fuel gas engine generators at treatment works and provide a percentage of the electricity these plants use for operation,” he explains.

Considering that there are around 50 major municipalities in South Africa with a wastewater treatment plant large enough to operate an effective biogas plant, 30 MW is a conserva- tive estimate of the possible electricity saving.

Treatment plants traditionally use a natural process, known as anaerobic digestion, which involves the decomposition of organic matter in the absence of oxygen, to convert a large proportion of the solid sewage sludge, produced in the mainstream treatment processes, into biogas.

Following anaerobic digestion, the biogas produced is termed raw fuel, as it contains water, hydrogen sulphide as well as a variety of volatile organic and chemical compounds that can damage the moving parts in engines.

“For biogas to be an effective fuel, it therefore needs to be conditioned and have these contami- nants removed to ensure long-term sustainability of the plant engine and a lower overall running cost,” says Gifford.

A biogas-to-power plant may provide wastewater treatment works with an on-site energy source that could enable it to offset a portion of its running costs.

Critically, Gifford emphasises that, as the mandate of municipal water works is not to generate electricity, but to clean wastewater, it is important to outsource the running of biogas- to-power plants to specialised contractors.

“In certain international cases, the function of biogas-to-energy operations at water works has exceeded the initial mandate of cleaning effluent and has become the primary focus of the entire plant, resulting in a deterioration of the quality of the treated effluent,” he notes.

Gifford adds that the true financial benefit of municipal biogas plants is evident when savings are not lost to other divisions within the municipality, but reinvested into the effluent treatment works for maintenance and upgrade.

In addition, he says, the cost of a fully commissioned biogas-to-power operation, including full automation and remote monitoring, can be recouped after four to seven years, making it a financially viable long-term energy option.

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