[Effects of Free Ammonia Regulation on the Performance of High Solid Anaerobic Digesters with Dewatered Sludge]

High free ammonia nitrogen (FAN) is regarded as a main factor that inhibits biogas production in high solid anaerobic digestion systems with dewatered sludge. Two mesophilic semi-continuous anaerobic digesters fed with dewatered sludge (input total solids=15%) were operated and their performance under different FAN concentrations by two regulation methods including pH reduction and total ammonia nitrogen (TAN) promotion were investigated. Results showed that when FAN was elevated from (400±174) mg·L^-1 to (526±25) mg·L^-1 with the increase of TAN, the daily biogas yield declined from (11.9±0.3) L·d^-1 to (10.3±0.2) L·d^-1. Meanwhile, volatile solids (VS) reduction rate decreased by 33.7%. When FAN dropped from (330±99) mg·L^-1 to (47±13) mg·L^-1 due to pH reduction, daily biogas yield declined from (14.4±1.1) L·d^-1 to (10.8±0.3) L·d^-1 and VS reduction rate decreased by 26.9%. The system performance was not enhanced with the reduction of FAN. With the stop of pH regulation, FAN gradually raised again to 300 mg·L^-1 and digester performance recovered to the same as the situation before regulation. The result suggested that inhibition occurred during pH reduction. By 16S rRNA pyrosequencing analysis on bacterial and archaeal diversity and quantitative PCR analysis aiming at methanogens, it was found that bacterial community structure both changed obviously with obvious performance decline in the two digesters. Excessive FAN (>500 mg·L^-1) could change the main bacteria species which degrade protein from Tepidimicrobium and Proteiniborus to Anaerobranca. On the other hand, some carbohydrate and protein degrading bacteria were also inhibited by pH reduction for FAN control, leading to the decline of hydrolysis rates of organic matters. As a result, the shortage of substrate supply for the methanogens weakened the biogas production of the digestion system.