Monday, May 2, 2011
Grand Ballroom C-D, 2nd fl (Sheraton Seattle)
Anaerobic digestion of microalgae can help make algal-based bioenergy sustainable by recovering biogas fuel and nutrients. However, the high cellular protein content of certain algal species results in high ammonia concentrations in the digesters, limiting their treatment capacities. This study was carried out to develop an anaerobic digestion system to convert microalgae into biogas fuel at high organic loading rates (OLR). Anaerobic digestion was investigated in laboratory-scale batch and continuous digesters using Spirulina (Arthrospira platensis) as an algal feedstock. Experiments included testing at different OLR and hydraulic retention time (HRT). Biogas and methane production and solids reduction were measured to determine the digester performance. The pH and concentrations of ammonia and volatile fatty acids were determined as indicators of process stability. The methane yield after 29 days of batch digestion, per mass of volatile solids (VS) loaded was 0.394 L/gVS, which is comparable to that of food wastes. This suggests the feasibility of using microalgae as a feedstock for anaerobic digestion. A continuous digester at OLR of 2.0 gVS/L d and HRT of 25 d yielded methane at 0.300 L/gVS (76% of the 29-day batch yield). The volumetric methane production rate was 0.608 L/L d at 64% methane content. Ammonia concentration during the continuous digestion averaged 4600 mg/L. Methods are being investigated to reduce ammonia concentration while maximizing the OLR.