Tacoma and Sacramento: Pumped about biogas

In 2012, the City of Tacoma began evaluating different options for collection and processing source separated organics (SSO), in addition to the yard waste collection and composting program that it has provided to citizens for years. The evaluation began with characterization of the feedstocks that could be collected in the city. The city decided to focus on commercial sources (stores, food processors, restaurants, etc) rather than a separate program for residential SSO, in addition to the yard waste program, as other cities have done. The results of several surveys of businesses in the city and a pilot collection program indicated that approximately 3,300 tons per year of SSO suitable for producing biogas could be collected. This thorough bottom-up study identified what could actually be collected, rather than what might be generated based on business types and other information.

The city then proceeded to evaluate the best alternatives for processing the SSO, which included several methods of anaerobic digestion and composting by combining with yard waste. Concept designs, life-cycle costs and revenues were prepared for all the alternatives, including operating costs over the project lifetime. Revenues included fuel cost displacement for the city’s refuse trucks, which, appropriately, would be fueled by the resulting CNG. Costs included all capital costs, including not only the digesters and biogas-to-CNG conversion, but also the costs for 50 new CNG-fueled trucks.

Environmental and social factors were also considered in a “triple-bottom line” analysis. The conclusion was that co-digestion with the City’s sewage sludge, using excess digester capacity available in the city’s wastewater treatment plant, would be the most beneficial method for processing the commercial organics, and would result in a net-zero economic result for the City. Similar co-digestion systems have been implemented in several cities across North America, using excess sewage sludge digester capacity. This approach requires preprocessing the SSO materials so they can be mixed with the sewage sludge without creating upsets in the digester. The next step in this program is a co-digestion pilot study, currently under way, in which pre-processed SSO is added to the digesters.

Meanwhile, in Sacramento, food waste is being converted to CNG with the help of a high-solids anaerobic digestion system from Clean World Partners, Gold River, California, and an alternative vehicle fuel conditioning system from Madison, Wisconsin-based BioCNG. The system initially used about 100 cubic feet per minute of biogas from the food waste digester to produce about 450 diesel gallon equivalents of CNG fuel per day.

The Organic Waste Recycling Facility at the South Area Transfer Station (SATS) in Sacramento, Calif., began accepting 25 tons of food waste per day in December 2012, collected by Atlas Disposal Industries from area food processing companies, restaurants and supermarkets. Through anaerobic digestion (AD), the food waste is converted into renewable natural gas, electricity and heat, with material remaining from the process being turned into fertilizer and soil amendments. Clean World Partners began expanding the facility in 2014 to allow the facility to accept 100 tons per day of food waste. At this level, the system is expected to be capable of producing around 1,500 diesel gallon equivalents per day from the digester biogas, according to BioCNG.

Atlas Disposal also has opened California’s first AD-based renewable natural gas fueling station, which uses natural gas produced at the recycling facility to fuel the company’s fleet as well as other area vehicles running on compressed natural gas (CNG). Currently CNG powers about one-fourth of Atlas Disposal’s vehicles, including waste recycling trucks. According to Atlas, the fuel costs about $2.25 per gallon.

While BioCNG’s units have been employed at several landfills in the Midwest, the Sacramento biodigester is the first project for the company using an organic food waste digester.