With anaerobic digestion of wastewater solids and other organic feedstocks, an energy-rich gas is produced which usually contains between 60 and 70 percent methane. This biogas (often called digester gas) is increasingly valuable in today’s energy markets and is a renewable fuel which has important implications for sustainability goals.
Capturing methane and using it wisely minimizes methane emissions to the atmosphere – a critical greenhouse gas concern. Biogas can be used wherever natural gas is used; however, required gas characteristics vary depending on the combustion method or device. Characteristics required for stationary biogas applications at the treatment plant are much less stringent than required to meet pipeline quality natural gas or vehicle fuel needs.
The San Francisco Public Utilities Commission operates all wastewater treatment plants in San Francisco and handles all biogas generated, which is a byproduct of plant-treated sewage. Part of the energy that powers the SFPUC’s Oceanside and Southeast wastewater treatment plant is generated from biogas. This renewable biogas-fueled cogeneration technology provides both heat and electricity for wastewater treatment plant operations. San Francisco can generate up to 3.2 megawatts of renewable energy by capturing and then combusting the methane gas released during the decomposition and “digestion” of our sewer sludge. Options for biogas use, along with technologies for biogas processing and handling, are presented and discussed in the San Francisco Public Utilities Commission’s Long-Term Biosolids Management Plan.