Influence of a prolonged solid retention time environment on nitrification/denitrification and sludge production in a submerged membrane bioreactor

A three-stage submerged MBR was operated under a prolonged SRT condition—more than 800 days—to treat high strength industrial wastewater. Throughout the entire experiment, chemical and biological effects on sludge production and nitrification/denitrification were investigated. The trend of MLSS growth was found to be divided into four phases. An initial growth phase followed by a pseudo stable phase which the biomass growth stabilised in the region of 13,000 mg.L −1 and then a transformation phase which caused the biomass concentration to increase again before finally stabilising at 18,000 mg.L −1 in a final stable phase. Total COD and TN removal in the three-stage submerged MBR were maintained consistently at 98% and 97%, respectively. Specific nitrification rate in the pseudo-growth phase stabilised in the region of 1.42 mgN.gMLVSS −1.h −1, after which it started to increase to 1.71 mgN.gMLVSS −1.h −1 when the system enter the final stable phase. Specific denitrification rate follows a “flatout” trend and stabilised in the region of 2.11 mg N.g MLVSS −1.h −1. Low sludge yield (0.22 gVSS.gCOD −1) and endogenous decay coefficient (0.05 day −1) were achieved in 300 days SRT MBR compared to CAS process. The experimental results revealed that both MBRs with SRT of 10 days and 300 days have an identical ratio of dead to live cells (three dead cells:seven live cells). Nevertheless, 300 days SRT MBR had large cell population (3.93×10 6 cell.ml −1) compared to 10 days SRT MBR (3.56×10 5 cell.ml −1). The high viability of microbial population in 300 days SRT MBR explained the high COD removal efficiency (97%) under a prolonged SRT condition. Additionally, the results revealed that the occurrence of protozoa in 300 days SRT MBR may also be responsible for the low sludge production.