Structural characteristics of cake layer in membrane bioreactor with chromate exposure

Chromate (CrO₄^2-) exposure, especially high concentration (mg/L), still probably occurs in the industrial and mining area due to industrial accidents or even illegal discharge, though CrO₄^2- has been restricted to be discharged into wastewater treatment system (WWTS). Therefore, this study was applied to better understand the structural characteristics of cake layer in membrane bioreactor (MBR), which is one of best alternative for WWTS of industrial or mining area, with CrO₄^2- exposure. Three submerged MBRs with CrO₄^2- exposure (10 mg/L was normal high concentration CrO₄^2-; 50 mg/L as extreme level for better identification; 0 mg/L as control condition) were applied in this study. Results showed that CrO₄^2- exposure caused an obvious variation of cake layer structure. Because of organic component variation, cake layer structure with CrO₄^2- exposure was re-constructed into loose and porous with biomicromolecules, and resulted in the rapid cake layer thickness increase, finally leading to severe membrane biofouling. Additionally, CrO₄^2- distributed evenly along the cross-sectional cake layer. CrO₄^2- only induced the inorganic structure variations of cake layer, but without any obvious effects on the other inorganic elements structure. CrO₄^2- exposure induced the bacterial community structure variation and led to tolerated-CrO₄^2- microorganisms as the majority in cake layer community, but had no obvious effects on the population diversity.