From Shite To Light

The recent survey of readers of Consumer magazine which voted the Christchurch City Council New Zealand's best local authority cited our Wastewater Treatment Plant at Bromley one of the city's successes. This success is being continued with the purchase of a new gas engine generator which will ensure more economic power generation from the biogas produced as a byproduct from the plant's treatment of the city's sewage.

The engine runs on the biogas produced by the anaerobic digestion of the wastewater solids treatment process. The wastewater solids are collected in the primary and secondary sedimentation stages of the treatment process. The biogas contains two-thirds methane and one-third carbon dioxide and is produced from the breakdown of the organic matter in the wastewater solids by the action of bacteria.

Large, enclosed tanks of 5,000 cubic metres in volume, called digesters, are heated to 37oC, and a floating cover traps the gas produced. The bacteria producing the methane thrive under these conditions, provided they are kept up to temperature and are well mixed with their continuous wastewater solids smorgasbord.

In the past the amount of gas produced was more than the previous generators could process so the excess was flared-off. The new generator has the capacity to generate power from all the gas which makes the plant a large net energy exporter to the Southpower electricity network.

The US-made generator, purchased at a cost of $1.6 million was bought with the assistance of a loan of $500,000. This money was made available through the Crown Energy Efficiency Loan Scheme administered on behalf of the government by the Energy Efficiency and Conservation Authority (EECA). The Loan Scheme was established in 1989 to offer loan finance to government departments and other publicly funded bodies for energy efficiency investments.

The benefits to the environment from the reuse of the waste are also translated into benefits for city ratepayers. A $300,000 per year credit for the energy sold to Southpower will reduce the operating costs of wastewater treatment. On congratulating the City Council for the initiative it had shown in making use of a waste resource, EECA's chief executive, Martin Cummer, said, "Today the savings the Council would make as a result of the upgraded technology mean the investment will pay for itself in five years".

Mike Bourke
Liquid Waste Manger
Ph: 371-1364