Floating geomembrane covers for odour control and biogas collection and utilization in municipal lagoons

The use of anaerobic lagoons as the first pond in waste stabilization pond systems in tropical and warm-temperature climates is considered a highly co st-effective and practical way to treat municipal wastewater. These anaerob ic ponds, designed with hydraulic residence times of up to six days, can ef fect BOD5 removals of 60 to 80%. The subsequent aerobic stabilization pond surface area is greatly reduced over systems designed without anaerobic tre atment up front due to the organic load reduction occurring anaerobically. in lagoon systems with mechanical aeration, operation costs can be greatly reduced. While odour is a concern with anaerobic treatment, anaerobic ponds treating municipal wastewater can be designed to be relatively odour-free given suf ficiently low wastewater sulfate concentrations. However, when sulfate conc entrations are high, or when odour control or greenhouse gas emissions are significant issues, or when the wastewater is relatively high in organic st rength resulting in commercial production of methane gas, anaerobic lagoons can be covered, and the biogas collected and burned both to produce energy and reduce emissions and odour. The City of Melbourne treats approximately 50% of its municipal wastewater at the Western Treatment Plant in waste stabilization ponds designed with a naerobic ponds as the first pond in the system. Each of three pond systems at the Western Treatment Plant receives an average dry weather flow of 120, 000 m(3)/d with an average strength of 400 mg/L BOD5. This paper describes the design, installation and commissioning of two 3.9 hectare floating, sel f draining, geomembrane covers on the anaerobic section of two of these lag oon systems. Biogas collection and utilization were an important part of th e installation. A description of how the biogas is collected, the quantitie s generated and an overview of the control system used to operate the bioga s handling facility is also included. Particular emphasis was placed on max imizing biogas utilization in design of the biogas control system.