The site

Hobart City Council’s cogeneration system is located at the Macquarie Point Wastewater Treatment Plant, adjacent to Macquarie Wharf in Hobart’s central business district. The system treats wastewater from the Hobart CBD and surrounding suburbs.

The plant treats approximately 11 megalitres of wastewater per day to secondary standard. The plant is comprised of two primary sedimentation tanks, three high rate trickling filters and four secondary clarifiers. Sludge from the digesters is treated in two primary and two secondary anaerobic digesters.

Hobart City Council installed a cogeneration system because it provided a sustainable energy source for the wastewater treatment plant and was financially feasible. The system also effectively removes fugitive emissions of methane gas from the anaerobic digesters and provides power during mains power outages.

Technology

Hobart City Council installed a Guascor FG180 gas engine generator of 142 kilowatt electric (kWe) rating was installed, closely matching capacity with the estimated gas supply. The heat from the gas engine’s cooling circuits and exhaust gases are recovered – a total of approximately 255 kilowatt thermal (kWth) – and used to provide the digester sludge heating demand.

The total gas produced by the sludge digestion process is consumed by the gas engine in cogenerating electrical power to supply the wastewater treatment plant loads and simultaneously recover heat, as hot water, for sludge heating.

Energy purchase and supply

The plant is expected to produce 700 megawatt hours of electricity per annum. The electrical power generated by the system is used onsite, with no power exported off-site, to replace approximately 50 per cent of the imported power from Aurora Energy. This provides an income stream that is being used to repay the capital cost of the generating set, heat recovery equipment and associated installation works.

The generator operates continuously, with automatic cut out under low gas conditions and is accredited under the Mandatory Renewable Energy Scheme. The generator is also capable of being operated in ‘island’ mode to provide emergency power during any mains power outage.

Fuel for the plant is supplied by biogas generated from the wastewater treatment plant’s anaerobic digesters.

Heat recovery and use

Prior to the installation of the cogeneration system the digesters were heated using two biogas fired boilers. These boilers have been retained, but are generally not required when the cogeneration system is operational.

Heat from the cogeneration system, estimated at 250 kWth is collected from the gas engine and from the exhaust through a heat exchanger. This waste heat is recovered through an Alfa Laval plate heat exchanger which transfers the heat from the cogeneration loop to the boiler loop. This in turn transfers the heat to the sludge that is circulated from the digesters via the sludge heat exchangers. Any excess heat from the cogeneration loop or the boiler loop is discarded to cold reclaimed water via two additional heat exchangers.

Environmental impact and the project success

The project is expected to save 360 tonnes per annum of CO2 equivalent greenhouse gas emissions. The system has produced an average of 100 kWe. This compares to a design output of 140 kWe and is attributed to lower than anticipated gas production.

Future Outlook

Hobart City Council says that since most modern wastewater treatment plants do not include anaerobic digesters, the potential to provide similar systems in Hobart is limited.

Funding arrangements and acknowledgements

The system was funded by the Hobart City Council. The concept was developed by Scott Morgan of Hobart City Council and Geoff Nash of Bridgewater Engineering. Their input is gratefully acknowledged.