Novel membrane bioreactor for detoxification of VOC wastewaters: Biodegradation of 1,2-dichloroethane

In this work a novel extractive membrane bioreactor has been used to overcome air-stripping problems which occur during aerobic treatment of 1,2-dichloroethane (DCE) contaminated wastewaters. The operation of a conventional air-lift bioreactor at a wastewater residence time of 11.6 h led to 25–34% of the DCE supplied to the reactor being lost to the exit gas stream. In contrast, employing the novel membrane bioreactor resulted in negligible (1.5%) air stripping at the same wastewater flowrate. The membrane bioreactor operates by separating the DCE containing wastewater from the aerated biomedium. This is achieved by means of a silicone rubber membrane which is coiled around a perspex draft tube. DCE diffuses across the silicone rubber membrane and into a biofilm growing attached to the surface of the membrane, while oxygen diffuses into the biofilm from the biomedium side. Oxygen and DCE meet in the biofilm and degradation occurs without the DCE being directly exposed to the aerating gas stream. Of the DCE supplied to the membrane bioreactor, 94.5% was biodegraded during operation of this system, and approximately 65% of the carbon entering the system was evolved as CO2. A mathematical model has been used to describe the transfer of DCE across the membrane and subsequent diffusion and reaction of DCE and O2 in the biofilm attached to the membrane tubes. Parameters describing microbial growth kinetics on DCE were determined using a CSTR bioreactor. The results of the mathematical analysis confirmed that the biofilm has two major effects on system performance: (a) it prevents direct contact between DCE and the aerating gas, thus avoiding air stripping; and (b) it limits the flux of DCE across the membrane with consequent accumulation of DCE at the membrane-biofilm interface, which reduces the mass transfer driving force for DCE extraction from the wastewater.