Application of coagulation and conventional filtration in raw water pretreatment before microfiltration membranes

Two step coagulation using iron coagulant (FeCl₃) with anionic polyelectrolyte in the first step and aluminum coagulant in the second one was made to reduce the negative effect of the fouling phenomenon. The process was operated on the commercial scale. The raw water was the blowdown water from the power plant cooling system. The dead-end microfiltration units provided with 0.2 μm polypropylene membranes produced about 210 m³/h of water. The results of statistical tests show a significant difference between the concentrations of turbidity, total hardness, sulfates (VI), total iron, aluminum, silica, nitrate nitrogen, ammonia nitrogen, oxygen consumption and absorbance (254 nm) in the water during the whole process. The results of previous observations showed that the formation of the cake layer had been the main reason for the fouling phenomenon occurring on the investigated membranes. It is significant that 75% of the suspension has been removed during the pretreatment stages. The hydrobiological observations show the presence of Bacillariophyceae and Cyanophyta. It is possible to remove them during the coagulation and prefiltration stages. The investigation results show that it is possible to protect the polypropylene membrane from fouling occurring inside the membrane pores as well as the formation of the cake layer. The use of a hybrid process combining the traditional raw water pretreatment with a membrane process may be well-founded.