Role of coagulation in membrane filtration of wastewater for reuse

This study explains the role of coagulation in membrane filtration of wastewater for reuse. For this purpose, a coagulation–ultrafiltration (UF) membrane system was used to treat secondary effluent from a nearby wastewater treatment plant using a rotating biological contactor. The study proceeded with the hypothesis that coagulation could affect membrane filtration through two phenomena: change in particle characteristics and contaminant loading reduction. If fouling reduction were observed at a low alum dosage, coagulation would affect membrane performance by changing particle characteristics because contaminant reduction could not be possible at low dosage. If fouling reduction were observed only at a high alum dosage, the role of coagulation would be contaminant loading reduction. Results showed that both phenomena were important. Coagulation improved the membrane performance by changing particle characteristics at a low alum dosage. The improvement was achieved through both a change in particle characteristics and contaminant loading reduction at a high alum dosage. Particle size among various characteristics was found the most important for membrane fouling. Coagulation increased particle size, which led to a reduction of fouling. The beneficial effect from coagulation was observed at both fouling steps of pore blocking/adsorption and cake formation. Coagulation pretreatment was also beneficial for the improvement of the permeate quality in terms of organic matter.

According to this study, coagulation pretreatment effectively improved membrane performance. Coagulation reduced membrane fouling by changing particle characteristics and removing fouling-causing contaminants. Fouling reduction was observed both at low and high alum dosages. When alum was added at a low dosage so that contaminant reduction was not possible, coagulation pretreatment reduced membrane fouling through a change in particle characteristics. Coagulation changed particle characteristics, which led to fouling reduction. When alum was added at a high dosage so that contaminant reduction was possible, both change in particle characteristics and reduced contaminant loading were important for fouling reduction. Of various characteristics, particle size was found the most significant for fouling reduction. An increase in particle size by coagulation was the main cause for fouling reduction. Coagulation was involved in both stages of fouling steps. Coagulation increased particle size, which interfered with particle entry into the membrane matrix. The cake layer, composed of large particles resulting from coagulation, yielded less resistance to flow. Coagulation pretreatment also improved the organic removal performance of the membrane system because coagulation effectively removed organics in raw water. It seemed that coagulation and membrane filtration preferentially removed large organics.