High Rate Low Solids Methane Fermentation of Sorghum, Corn, and Cellulose

Abstract

Sorghum, sorghum/alpha-cellulose mixture, and corn were anaerobically digested at 55°C at effluent solids contents of 8-12% total solids (TS), using trace nutrient supplementation. Volatile solids (VS) loading rates at much higher levels than conventional maxima were maintained without volatile fatty acid (VFA) accumulation. Semi-continuously fed digesters with organic loading rates (OLR) up to 12 gVS kg-’ d-’ produced methane at rates up to 3.3 L kg-’ d-‘. Continuous feeding of corn at an OLR of 18 gVS kg-’ d-’ resulted in a methane production rate of 5.4 L kg-’ d-‘. VS removal efficiencies at maximum OLRs were 60% (sorghum) and 67% (corn). At an OLR of 4 gVS kg-’ d-l, sorghum alone as a feedstock led to excess ammonia-N accumulation. Excess ammonia did not accumulate at sorghum loading rates of 8 and 12 gVS kg-’ d-l, nor with a sorghum/alpha-cellulose mix loaded at 8 gVS kg-’ d-l. Instantaneous gas production rates were directly related to feedstock cell soluble content, with peak instantaneous biogas production rates from corn (OLR of 8 gVS kg-’ d-‘) approaching 25 L kg-’ d- ’ following a three-day feeding.