High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

Feasibility of high-solid anaerobic digestion of sewage sludge was investigated in single-stage completely stirred tank reactors at 35 ± 1 °C. System stability and the effect of organic loading rate (OLR), sludge retention time (SRT) and total solid (TS) content on the performance of high-solid system was examined. Experimental results showed that, with the concentration of free ammonia nitrogen (FAN) lower than 600 mg l−1, high-solid anaerobic digestion of sewage sludge could maintain satisfactory stability. Slight, moderate and significant inhibition was found with FAN concentration ranging from 250 to 400, 400 to 600 and 600 to 800 mg l−1, respectively. The VFA/TA criteria could not foresee system instability in significant ammonia inhibition system by its traditional ratio grades. High-solid system could support higher OLR (4–6 times as high) and obtain similar methane yield and VS reduction as conventional low-solid system at the same SRT, thus reach much higher volumetric methane production rate.