The authors studied the performance of a hybrid MF membrane system combining activated carbon adsorption and biological oxidation for the removal of turbidity, natural organic matter, ammonia and manganese from river water in pilot-plant experiments. In this system, part of the soluble organic matter was adsorbed by the activated carbon. Ammonia and soluble manganese were oxidized by microorganisms that were concentrated in a submerged tank. In preliminary experiments, it became clear that powdered activated carbon (PAC) addition improved membrane permeability, because PAC has a large volume of macro-pores, which can adsorb larger molecular-weight organic matter. In the pilot-plant experiment, however, it was difficult to slow the rate of membrane permeability loss by PAC addition only. This may be due to biological fouling, caused by operating the system at a high recovery rate in order to maintain a high concentration of microorganisms in the submerged tank. Therefore, backwashing using hypochlorite solution (50-200 mg/l of Cl2) was carried out every few hours. This simple chemical washing was effective in preventing bio-fouling of the membrane.

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