Enhanced Biogas Production in Pilot Digesters Treating a Mixture of Sewage Sludge, Glycerol, and Food Waste

Anaerobic co-digestion of a ternary mixture of sewage sludge with food waste and residual glycerol was evaluated in pilot scale units, with the aim to increase methane production in sludge anaerobic digesters at sewage treatment plants. Two pilot scale anaerobic digesters (useful volume of 320 L) operated simultaneously for 60 days at room temperature (24 degrees C on average). One digester received only sludge in the feed, and the other received the ternary mixture. The feeding routine of the digesters respected a semi-continuous mode. The units continuously promoted the mixing of the sludge through a bottom-up recirculation system with a hydraulic retention time of 30 d. The digester fed with just sewage sludge had greater instability during the whole period of operation, producing about 3 L d(-1) of biogas with 23% methane, volatile solids (VS) removal of 65.3%, and methane yield (MY) of 98.5 L-N (CH4) kg(-1) add On the other hand, the digester fed with the ternary mixture produced 21 L d(-1) of biogas with 43% methane, VS removal of 73.4%, and MY of 174.5 L-N (CH4) kg(-1) (VS add). The mass and energy balances showed that through methane recovery, the ternary mixture substrate contributed to the production of 12 times more energy than that of digesters fed with pure sewage sludge.