Floating Membrane Cover System Solves Odor Problem At Australian Treatment Plant

The Melbourne Water Corporation in Australia had a serious wastewater treatment odor problem which was solved through the design and construction of the world’s largest floating membrane cover system, built by Geomembrane Technologies Inc. (GTI) of Fredericton, New Brunswick, Canada.

With a serviced population of 2.3 million people, the Melbourne Water Corporation’s Western Treatment Plant has been in service since 1897, covers an area of 27,000 acres, and combines lagoon and land treatment to process an average flow of 124 million gallons of sewage daily — 52 percent of Melbourne’s sewage.

The anaerobic treatment process generates biogas that was the subject of odor complaints and greenhouse gas concerns. Biogas contains mostly methane, with smaller amounts of carbon dioxide, nitrogen, hydrogen sulphide and water vapor. A cover system was required to capture biogas and send it through a pipeline for odor treatment and future utilization as a fuel.

Melbourne Water wanted a cover design that could be installed without taking the lagoons out of service. They also wanted a system that was low maintenance and could facilitate maintenance activities, when required. The covers had to be self-draining, have structural integrity to withstand pressures caused by scum development, and facilitate biogas migration / collection. All of these requests were met by the designers and fabricators.

The project included an ISO 9001- compliant quality assurance plan and an environmental management plan. The cover design and pre-fabrication was completed in North America, with the individual cover sections then shipped to Australia for installation. Geomembrane Technologies Inc. also provided training of operators and maintenance personnel, performance testing, and aftercare assistance.

The project involved covering the inlet ends of two lagoons (25 West and 55 East) using a three-layer floating insulated cover made of a high performance reinforced geomembrane (Seaman XR5), a middle layer of polyfoam insulation, and a bottom layer of high density polyethylene. The underskin provides protection of the cover system from scum development and scum movement under the cover. The XR5 material provides minimal expansion / contraction due to changes in ambient temperature, is ultra-violet light resistant, and has high tensile strength. The three-layer nature of the cover system, with top layer floating free and bottom layer welded to the foam insulation, provides a lightweight but durable structural integrity to the cover system which allows buoyancy and access for maintenance and inspection.

The action of accumulated rainwater and wind can play havoc on the longevity of cover systems and the degree of maintenance required. The cover installed for Melbourne Water incorporates a network of weights, floats and self-regulating gravity drains that allow passage of rainwater into the liquid below. There are no folds in the design for collection of rainwater, and since there are no folds, there are fewer wear areas in the cover that require periodic maintenance.

The foldless cover design allows the designer to use the entire walled perimeter of the lagoon as a transmission zone for biogas since the cover sweeps from the top of the wall, where there is a gas-tight seal, down to the liquid surface. The sweep area, along with a system of floats and weights, allows unhindered conveyance of biogas from the central areas of the cover to the perimeter where it is conveyed through wall penetrations and piping to the biogas blowers. (GTI’s cover systems are protected by patents and patents pending.)

The covers dimensions are 650 ft. by 560 ft. for 25 West lagoon and 700 ft. by 650 ft. for 55 East lagoon. Biogas is captured under the covers and conveyed for either treatment in a flare or utilization as a fuel for electrical generation. Construction of an electrical generation system is currently underway (by others). The covers are specially designed to store biogas in low electrical demand periods so that it can be used in periods of greater demand when electricity sells for a higher price.

Biogas flows from each of the lagoons is expected to be between 195,000 f3 / d and 635,000 f3/d. It is expected that electricity will be generated between 8 and 16 hours per day, only during peak demand period. Given an average biogas production of 1,342,000 f3 /d at a methane content of 65 percent, the expected generator set sizing is 6000 kW.  With an average electricity cost of $0.07/kWh, the value of the biogas is estimated to be $1.8 million annually.

The biogas collection and treatment system is controlled by 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, and changes to the software can be made through the Internet.

The floating cover system at Melbourne Water’s Western Treatment Plant has been in operation for over three years now and continues to operate successfully.