Floating Geomembrane Covers for Wastewater Treatment Plant in Melbourne

ADI Limited

Category: Water Resources and Treatment

Serving 2.3 million people, the Melbourne Water Corporation’s Western Treatment Plant covers an area of 10,850 hectares and processes an average of 470 million litres of sewage daily — almost half of the city’s discharge. The plant uses an anaerobic treatment process that generates biogas. The gas was causing complaints about odour and concerns with greenhouse gases. Biogas contains mostly methane, with smaller amounts of carbon dioxide, nitrogen, hydrogen sulphide and water vapour.

In 1998, two companies in the ADI Group of New Brunswick designed and built the world’s largest odour control cover system at the Melbourne plant. ADI Limited designed the system, and its affiliate, Geomembrane Technologies, built it.

The $8 million project involved covering the inlet ends of two lagoons using a three-layer floating insulated cover system. The total area covered is 77,000 square metres.

Biogas is captured under the covers and conveyed either for treatment in a flare, or to be used to generate electricity. Construction of an electrical generator is under way by others.

The geomembrane covers are specially designed to store biogas in low electrical demand periods so that the power can be used at periods when electricity sells for a higher price. Given an average biogas production of 38,000 cubic metres a day at a methane content of 65 per cent, the generator is expected to be sized at 6,000 kW. The value of the biogas is estimated at $1.8 million per year.

Three layers

The project was awarded in December 1997 and completed one year later. ADI and GTI proposed and installed a three-layer composite cover system which includes a 30 mil high density polyethylene (HDPE) underskin, a high performance reinforced geomembrane (Seaman XR5) top layer, and 12 mm of polyfoam insulation sandwiched in between.

The three-layer nature of the system, with the top layer floating free and the bottom layer spot welded to the foam insulation, provides a lightweight but durable structural integrity. The underskin provides protection from scum development, while the XR5 top layer gives minimal expansion and contraction in changing temperatures, is resistant to ultra-violet light damage and has high tensile strength.

Biogas is allowed to migrate between the HDPE underskin and XR5 cover through orifices located along the ridge floats and along the perimeter, where it is removed by blowers. When biogas needs to be stored, the pressure beneath the cover is allowed to go positive, forcing the gas up between the cover layers. The cover incorporates a network of weights, floats and self-regulating gravity drains that allow rainwater to pass into the liquid below and prevent pooling. The gas control system consists of an Allen Bradley PLC5 in communication with ADI’s APM Windows based control software using a PC operator interface. The system can be monitored remotely.

The cover was prefabricated into 13 x 70 metre panels at four locations in Canada and the U.S., then shipped to Australia. Individual panels were welded together into 200 metre strips and pulled onto the active lagoon surface.

The reinforced concrete perimeter wall was cast in place around three sides of the existing lagoons, which were maintained in service. The fourth side of the cover is open ended to the remainder of the lagoon system and held by a cabling system designed to withstand waves caused by 150 kilometre per hour winds.

Staff can access all areas of the cover system using small all-terrain vehicles and maintenance can be done while the cover is in service.

The system is performing well with few operating problems. A localized problem at the open end of the cover system was occurring when storm winds pushed floating material onto the edge of the cover, blocking the self-draining devices. However installing an alternative drain design in these locations solved the problem.CCE

Project name: Floating Geomembrane Cover for Odour Control and Biogas Collection

Award winner: ADI Limited (designer) Project team leaders: Hollis Cole, P.Eng., Claude DeGarie, P.Eng., Brent Howe, P.Eng., Paul Henheffer, P.Eng, Victor Cormier, P.Eng., Darin Evans, P.Eng., Jim Piercy, P.Eng.,Troy Gordon

Client/Owner: Melbourne Water Corporation

Other key players: Geomembrane Technologies Inc. (contractor), Seaman Corporation (membrane material supplier), Solmax International, Pactiv Corporation (material suppliers)