Co-Digestion of Primary Sludge and Waste Activated Sludge for Methane Production

The ratio of primary sludge (PS) and thickened waste activated sludge (TWAS) is a vital factor in the production of digester gas in the wastewater plants. Many wastewater plants do not track sludge ratio, but instead, use a specific monthly or annual sludge ratio that is calculated as an average for their processed PS and TWAS. Lab scale batch biochemical methane potential (BMP) and semi-continuous experiments were used to study different PS and TWAS ratios in a municipal wastewater treatment plant. The batch BMP study showed that the mixing ratio 67% to 33% (PS:TWAS) was the optimum ratio for the highest digester gas yield of 209 mL CH 4g TCOD. The semi-continuous study showed that the ratio 72% to 28% (PS:TWAS) at 15 days hydraulic retention time was better than the ratio 67% to 33% (PS:TWAS) in producing higher yield (188 mL CH /g4TCOD compared to 163 mL CH /g TCOD4 respectively).