High-solids thermophilic anaerobic digestion of sewage sludge: effect of ammonia concentration

High-solids thermophilic anaerobic digestion fed with sewage sludge of 7–8% and 9–10% total solids was investigated at the hydraulic retention time of 20 days in this lab-scale study. The anaerobic digester fed with 7–8% sewage sludge was successfully operated, resulting in volatile solids (VS) destruction of 57.8% and methane production of 0.327 L/gVS (at standard temperature and pressure). When fed with 9–10% sewage sludge, the control digester was upset after about 80 days of operation. Total ammonia nitrogen (TAN) of 2500 mgN/L or more caused inhibition to anaerobic microorganisms. Another digester combined with weekly batch ammonia stripping of digested sludge in a side-stream configuration (80 °C, 2 h, initial pH of 9 with NaOH and return ratio of 70%), was maintained at the average TAN of 1720 mgN/L, and was able to continue without ammonia inhibition. TAN was a better indicator on inhibition of ammonia than free ammonia nitrogen (FAN). Microbial analysis revealed poor microbial diversity of digested sludge, that is, Class Clostridia for Bacteria and Geneus Methanothermobacter and Methanosarcina for Archaea were predominated. In conclusion, high-solids thermophilic anaerobic digestion can accommodate the influent sewage sludge up to 10%, as far as TAN is properly controlled in digester.