Bean curd wastewater treatment by membrane separation

Wastewater from a preserved bean curd manufacture was treated using the membrane separation method. The wastewater had chemical oxygen demand (COD) of about 10 000 mg/l. Chemical coagulation was rejected due to its poor efficiency in COD removal. Biological oxidation was not considered because of its requirement of large space which is not available in the factory. Microfiltration and ultrafiltration were tested but found inadequate for COD removal to meet the government foul sewer discharge permit, 3000 mg/l. Nanofiltration was found to be a feasible technology for treating this kind of wastewater. The permeate flux and COD were investigated in relation to the transmembrane pressure drop, temperature, and feed solution COD concentration. The permeate flux was found to increase significantly with transmembrane pressure drop and to decrease with feed concentration. The permeate COD for all the conditions tested meets the discharge permit. The permeate COD decreases with the increase of transmembrane pressure drop and increases with the operation temperature, which is consistent with the theory. The permeate can be reused in the bean processing and the concentrate may be returned to the production line. This method has been adopted by a bean curd manufacturing factory in Hong Kong.