The Anaerobic Digester to Renewable Natural Gas (RNG) projects make use of a variety of organic feedstocks, including dairy manure, food waste, source-separated organics (SSO), institutional, commercial, and industrial (IC&I) waste, yard waste, agricultural waste, vegetable biomass, and fats, oils, and grease (FOG), among others. The total solids content of the feedstocks we utilize ranges from 5% to 15%. For feedstocks with higher total solids contents, up to 35%, we implement additional feedstock processing, sand and grit removal, hydrolysis, and alternate digester technologies. Pre-digester separation technologies are also deployed to manage feedstocks with lower total solid contents while minimizing volatile solids losses on a case-by-case basis.
Typically, the hydraulic retention times for standard heated mesophilic digesters range from 21 to 25 days, while for higher temperature thermophilic digesters, they are around 12 to 14 days. The biogas yields from these anaerobic digesters vary depending on the organic content of the feedstock, such as carbohydrates, proteins, lipids, vegetable oils, and lactose, ranging from 3 cubic feet of biogas per pound of feedstock to 12 cubic feet per pound of feedstock. The raw biogas usually contains approximately 50%-70% methane prior to further processing and conditioning to around 99% methane to meet the pipeline quality RNG specifications for injection into the natural gas transmission or distribution system.
To meet RNG specifications, we employ various technologies for processing and conditioning biogas, including hydrogen sulfide (H2S) scrubbing, pretreatment and dehydration, volatile organic compound (VOC) and contaminants removal, and carbon dioxide (CO2) removal or recovery, as well as methane (CH4) upgrading systems.
Additionally, these projects utilize spent digestate from anaerobic digesters to produce Class A or B biosolids and fertilizers with optimal nitrogen, phosphorus (P205), potassium (K20), sulfur, carbon, and carbon to nitrogen (C:N) ratio. The sale of fertilizers creates additional value streams alongside valuable RNG sales.
