The use of anaerobic lagoons as the first pond in waste stabilization pond systems in tropical and warm-temperature climates is considered a highly cost-effective and practical way to treat municipal wastewater. These anaerobic ponds, designed with hydraulic residence times of up to six days, can effect BOD5 removals of 60 to 80%. The subsequent aerobic stabilization pond surface area is greatly reduced over systems designed without anaerobic treatment 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 sufficiently low wastewater sulfate concentrations. However, when sulfate concentrations are high, or when odour control or greenhouse gas emissions are significant issues, or when the wastewater is relatively high in organic strength 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 anaerobic 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 m3/d with an average strength of 400 mg/L BOD5. This paper describes the design, installation and commissioning of two 3.9 hectare floating, self draining, geomembrane covers on the anaerobic section of two of these lagoon systems. Biogas collection and utilization were an important part of the installation. A description of how the biogas is collected, the quantities generated and an overview of the control system used to operate the biogas handling facility is also included. Particular emphasis was placed on maximizing biogas utilization in design of the biogas control system.