Mesophilic High-Solid Anaerobic Digestion of Municipal Wastewater Sludge-Effects of Influent Solids Content and Ammonia Stripping

This laboratory-scale study evaluated mesophilic high-solid anaerobic digestion (HSAD) of 9‒11%, 12‒15% and 17‒21% municipal wastewater sludge. At the hydraulic retention time of 20‒30 days, volatile solids (VS) reduction of 53.4‒56.6% and methane generation of 0.275‒0.296 NL/g-VS were obtained on average. At 12‒15% and 17‒21%, ammonia stripping of digested sludge and its return to the digester reduced the average total ammonia nitrogen (TAN) to 2,690 and 3,270 mg-N/L in digester, respectively, mitigating ammonia inhibition. The ammonia stripping, performed at 80℃, initial pH of 9 for 2‒3 hours and the return ratio of 100‒120%, showed TAN removal of 63.4‒74.5%. Microbial diversity became poorer with the influent level. Dewatering was achieved by the two-chemical conditioning with ferric chloride and an amphoteric polymer. Overall, the mesophilic HSAD in the wide and high solid range is feasible, as far as measures against high TAN and poor mixing is properly taken.