Application of polyacrylamide flocculation with and without alum coagulation for mitigating ultrafiltration membrane fouling: role of floc structure and bacterial activity

There is a growing interest in the use of ultrafiltration (UF) for the treatment of micro-polluted surface waters for drinking water supplies. Effective pretreatment is required to mitigate membrane fouling and in this paper we have evaluated the application of polyacrylamide (PAM) flocculation with alum coagulation. Bench scale tests were conducted over extended periods with two types of PAM (different molecular weights (MW)) applied with, and without alum coagulation, in order to investigate their impact on membrane fouling. The structure of the resulting flocs formed in the process and the activity of bacteria within the membrane tank were identified as two key factors influencing UF system performance. It was found that development of the cake layer and hydraulic resistance of the membrane were influenced by the floc properties, which were in turn related to the MW and dose of the PAM. Coagulation and flocculation using the larger MW PAM formed amorphous flocs with a lower fractal dimension, which contributed to a lower density of the cake layer and lower rate of increase in trans-membrane pressure. PAM flocculation without alum coagulation induced severe membrane fouling by forming a continuous gel-like layer on the membrane surface. By alum-PAM dosing it was found that the concentration of bacteria present in the membrane tank and adhering to the cake layer, was sufficient to remove nearly all of the ammonia and around 80% phosphorus in the raw water. These results demonstrate that the combination of a high MW PAM with alum as a pretreatment method of UF process can effectively improve the floc properties and cake layer structure for controlling membrane fouling and producing high quality treated water