Effect of salinity variation on treatment performance and sludge characteristics in a membrane bioreactor

Abstract

Saline wastewaters, generated from food processing, canning, petroleum, and petrochemical industries, are usually difficult to treat with conventional wastewater treatment processes. Application of membrane bioreactors (MBRs) in high salinity wastewater treatment may overcome low biodegradation rates and poor sludge settling in the secondary sedimentation basin. We investigated the influence of salinity variation on the treatment performance of biological reactor and biomass characteristics using four 7-L submerged MBRs. The MBRs were subjected to four different NaCl loadings of 0, 5, 10, and 20 g/L. Two different types of shock load scenarios-stepwise and instant shock mode-- were compared. Removal efficiencies of dissolved organic carbon (DOC) and NH3-N were monitored to evaluate the overall performance of the MBRs at different salt loading rates. Saline inhibition due to the high salt loadings significantly reduced NH3-N removals in the initial phase of the MBR operation. However, removal efficiencies of NH3-N were gradually recovered in 10-50 days of operation (i.e., recovery periods). The recovery periods were longer in MBRs with higher salt loadings. Several physicochemical and biological characteristics of biomass (e.g., extracellular polymeric substances, zeta potential, MLVSS, particle size, and molecular weight distribution) were measured to assess the effect of high salinity on biomass characteristics and their relationships with membrane fouling. High salinity adversely impacted the properties of biomass and membrane filtration: decreased MLVSS concentration, increased EPS concentration, and accelerated membrane fouling. PCR-DGGE analysis results indicated that microbial diversity was decreased in MBRs at high salt concentration. But, Nitrosomonas eutropha and Nitrosomonas europaea were still present at the 20 g/L of salt and its intensity was especially increased with increasing salt concentration. This result may be a direct cause of recovery of ...