Microbial responses of activated sludge under high salinity environment in a nanofiltration membrane bioreactor (NF-MBR)

Abstract

A lab-scaled side stream NF-MBR system was employed in this experiment to investigate the effects of varying salinity on the microbial responses in the activated sludge mixture. The experiment was carried out using differing concentrations of 0, 200 mg/L Ca2+, 400 mg/L Ca2+, 100 mg/L Mg2+ and 200 mg/L Mg2+. The experiment was conducted over 5 weeks with each cycle per week. The microbial responses to the differing concentration of cations was deduced via the testing of parameters like total organic carbon (TOC), chemical oxygen demand (COD), ammonia, adenosine triphosphate (ATP), specific oxygen uptake rate (SOUR), extracellular polymeric substances (EPS), mixed liquor suspended solids (MLSS), mixed liquor volatile suspended solids (MLVSS), etc. In most cases, the effects of Ca2+ and Mg2+ on the parameters were compared with each other and with the control unit. The results showed varying effects of each cation on the microbial responses as indicated by the parameters tested. Notable results include, Ca2+ showed better COD removal abilities than Mg2+ and Mg2+ promoted greater EPS production than Ca2+. The rest of the results include trends such as the MLVSS/MLSS ratio having stable values regardless of Ca2+ or Mg2+ ions addition and the MLVSS/MLSS ratio had an average value of approximately 0.8. Also, the SOUR levels were generally decreased with the addition of divalent cations in the bioreactor but microbial cell viability did not decrease with an increase in divalent cations. As a result, these trends did not reveal firm establishment with regards to whether Ca2+ or Mg2+ was the superior cation for promoting microbial activities within the activated sludge.