Biowaste is a by-product of the water recycling process. It takes the form of a micro-organism-rich sludge that builds up when sieved water is sent to sit in an aeration tank for up to 15 days.

When the Melton Recycled Water Plant, owned by Western Water, was built over 30 years ago, an anaerobic digester was installed to break down this sludge into biogas that, historically, was used to maintain the elevated temperatures of around 35–40° Celsius within the digester by burning the gas within a boiler. Any excess biogas was flared.

In line with a goal to reduce its carbon emissions to zero by 2017, Western Water developed a plan to put this excess gas to a more sustainable use, says Managing Director John Wilkinson.

To this end, Western Water recently commissioned a $2 million cogeneration facility that utilises the excess biogas produced during the sludge break-down process to generate electricity to power the plant.

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The biogas produced by the anaerobic digester is approximately 70 per cent methane. This gas is treated, pressed, compressed and then burnt in a Capstone 200 kilowatt (KW) turbine to produce approximately 1.7 gigawatt hours of electricity per annum.

The power delivered by the gas turbine is fed into the existing site power grid and is used to offset the power consumption onsite by about 60 per cent. The Melton Recycled Water Treatment Plant contains two treatment plants – one that delivers Class C Recycled Water and one that delivers Class A Recycled Water. Typically a Class A Treatment Plant consumes significant amounts of energy. However the power delivered by the gas turbine is sufficient to power the Class A Treatment Plant, and about half of the Class C Treatment Plant.

Engineering estimates predict that the facility, built by Aquatec-Maxcon, will enable Western Water to reduce its greenhouse gas emissions by 1,800 tonnes per year.

The technology

The Capstone turbine has an external heat exchanger module capable of extracting over 276 kW of heat from the turbine’s exhaust. A proportion of this heat will be used to maintain high temperatures in the anaerobic digester, and the excess will be further used onsite to power the Class A treatment plant, and part of the Class C treatment plant.

The Capstone turbine is a technology that has been adapted historically from aeronautical turbines, specifically those found in helicopters, to operate on biogas, natural gas, coal seam gas, landfill gas and many other fuel sources. Typically these engines have been used on coal seam gas in Queensland and in building heating and ventilation services around Australia to provide building power and heat.

The Capstone CR200 micro gas turbine was sourced from the USA and sold by Australian Capstone distributer Aquatec-Maxcon.

The gas turbine is tolerant to fairly high levels of hydrogen sulphide (up to 5000 parts per million) within the biogas but like most gas turbines is not very tolerant of siloxanes. Siloxanes (if present) in the fuel will, when burnt in the combustion chamber, foul the turbine blade with silica by-products and lead to rapid wear and failure of the turbine blades.

Siloxanes are present in biogas produced at any waste water treatment plant (that has a domestic catchment) as they are typically generated from domestic shampoos and cosmetics. The pre-treatment of the biogas to remove these siloxanes (to non-detectable levels) was the key innovation provided by Aquatec-Maxcon.

Leading the way

Melton Recycled Water Treatment Plant is Australia’s first application of the Capstone micro turbine for use with biogas that has been generated at a recycled water plant.

Presently Western Water is looking at other opportunities within its recycled water plants to implement this technology.