Biodegradation of nitrophenol compounds and the membrane fouling trends in different submerged membrane bioreactors

Phenolic compounds are commonly used in industry and discharged into water. The objectives of this study were to evaluate the performance of submerged membrane bioreactors (MBRs) to simultaneously remove nitrophenols (metabolic uncouplers) and inorganic nitrogen compounds from wastewater and to determine the membrane fouling trends in two different types of MBRs: CSTR-MBR (continuous stirred tank reactor MBR) and MLE-MBR (modified Ludzack Ettinger MBR). At a target solids retention time of 110 day, a mixture of nitrophenol compounds (2-nitrophenol, 3-nitrophenol, 4-nitrophenol, and 2,4-dinitrophenol) was continuously dosed to the MBRs at the influent concentrations of 0.05 mM each. The nitrophenol removal efficiencies ranged from 87% to 96% while the total inorganic nitrogen removal efficiencies were 52% and 75% for the CSTR-MBR and MLE-MBR, respectively. The metabolic uncoupling by nitrophenols resulted in lower adenosine triphosphate (ATP) yield of 0.06 mg/g VSS and bound extracellular polymeric substances (EPS) of 20.5 mg/g VSS from the CSTR-MBR compared to the values of 0.10 mg ATP/g VSS and 30.7 mg EPS/g VSS from the MLE-MBR. There was a direct relationship between the EPS content and fouling, as membranes were less susceptible to fouling in the CSTR-MBR because of the reduced release of EPS due to the metabolic uncoupling by nitrophenols. These results suggest that both MBRs are effective in removing nitrophenols. While the MLE operation resulted in more efficient nitrogen removal, the CSTR-MBR configuration appears to be superior with less membrane fouling, an added benefit to the CSTR process, which is capable of reducing the toxicity of chemicals due to rapid mixing and dilution and widely used in industrial operations.