Membrane attached biofilms for waste treatment - fundamentals and applications

Membrane Attached Biofilms (MABs) are being used in an increasing variety of bioreactors. Extractive Membrane Bioreactors (EMB) have been developed at Imperial College (1,2) for the aerobic biotreatment of toxic organics which employ MABs for treating Volatile Organic Compounds (VOCs)-containing wastewaters without incurring air-stripping problems. Investigations of the key factors controlling the optimal operating conditions for the EMB system have shown that process efficiency is highly dependent on the development of these MABs. Therefore MAB development and its influence on the flux across the membrane over time has been studied and is presented here. Two MAB model systems have been studied; Xanthobacter autotrophicus GJ10 growing on 1,2-Dichloroethane (DCE) and Pseudomonas JS150 growing on Monochlorobenzene (MCB). The results show that there is a problem in this system with excess biofilm growth on the membrane surface, resulting in reduced flux of organic substrate across the membrane. At the same time, a diffusion-reaction model has been developed to explain the experimental results, and to describe the behaviour of the EMB. It was theoretically concluded that an optimal biofilm thickness could be found from a compromise between the level of air-stripping and flux of pollutant across the membrane, and that cell endogenous decay could be used to manipulate the biofilm thickness. Methods of controlling excessive growth of biomass have been investigated, and the addition of sodium chloride to the biomedium to control excessive biofilm development has been shown to be effective.