Removal of soluble COD by a biofilm formed on a membrane in a jet loop type membrane bioreactor

The soluble chemical oxygen demand (COD) removal efficiency through a cake layer (biofilm) deposited on the surfaces of a membrane was investigated as a function of biofilm thickness in a jet loop type membrane bioreactor (JL-MBR). The mechanisms for the removal were investigated based on the microbial characteristics of the biofilm. Polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) was used to identify the microbial community and a Specific oxygen uptake rate (SOUR) analysis was applied to determine the activities of microbial communities in a biofilm. Both the activities and community of microbial communities in the biofilm were similar to those found in a mixed liquor since, in JL-MBR all the substrates, dissolved oxygen, and nutrients are forced to flow through the biofilm, which differs from the biofilms grown on a non-permeable substratum in conventional biofilm process. The removal efficiency of soluble COD in a reactor through the active biofilm increased, reaching a constant value of 92% despite the continuous increase in the thickness of active biofilm with the operation time. This might be attributed to (i) the presence of soluble COD that is not readily biodegraded, (ii) the presence of small and non-biodegradable organic molecules that could easily pass through the biofilm as well as the membrane, and (iii) too short a contact time of soluble solutes with the active microorganisms in the biofilm.