Under the AB 32 Scoping Plan, the California Air Resources Board (CARB) identified the Low Carbon Fuel Standard (LCFS) as one of the nine discrete early action measures to reduce California's greenhouse gas (GHG) emissions that cause climate change. In this second installment of Innovations in Energy Solutions, we shed light on the anaerobic digestor technology, which one of many innovations in the development of renewable energy spurred on by the LCFS regulation.
The production of useful energy from waste products, whether it be food scraps, dairy or swine manure or algae involves the process of anaerobically digesting the feedstock (waste) to produce biogas. The anaerobic digestion process itself takes place in an anaerobic digester. This article highlights a few common questions regarding this innovative process.
Q: What does the anaerobic digestion process entail? Anaerobic digestion describes the process whereby microorganism break down the organic material in the waste in the absence of oxygen to generate biogas, which is comprised mostly of methane and carbon dioxide. Methane is the primary component of natural gas hence the production of usable renewable natural gas from the process of anaerobically digesting waste product. Therefore, the goal of any renewable natural gas production from waste product LCFS pathway application is to maximize the methane gas production rate, thus, the importance of the anaerobic digestion process.
Q: Are there anaerobic digester technologies that would enhance the production rate of biogas from the process? While the design specifications of a digestor have to be tailored to the feedstock, there are some common observations with regards to improving the efficiency of the process. Mainly, the operating temperatures of the digester – is it a mesophilic (35 – 38 deg. C) or thermophilic (50 – 65 deg. C) digester and the mean cell residence time (MCRT) – 5 days, 10 days, 15 days or longer? Studies have demonstrated that a longer residence time (i.e., a longer digestion period) results in a higher methane production rate – up to 40% higher with a 15-day MCRT compared to a 10-day MCRT.
Q: Does temperature have an equally dramatic impact? As it turns out, not as much – methane content in the biogas from a thermophilic digester is only about 4 to 5% higher compared to a mesophilic digester. The bottom line is that while a lot of attention is focused on feedstock properties, it is a smart strategy for project operators to also consider maximizing anaerobic digester system variables to increase the efficiency of methane production and content in the biogas.
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Data Source: U.S. Environmental Protection Agency Region 9. Anaerobic Digestion of Food Waste, Funding Opportunity No. EPA-R9-WST-06-004, Final Report, March 2008, Prepared by East Bay Municipal Utility District.