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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.

It has been observed that the application of coagulation and raw water prefiltration contribute to the rise of the retention coefficients of both water soluble substances and contaminants responsible for the colloidal suspension formation. Moreover the increase of the retention coefficient of solid suspension has been observed. It is significant especially in the case when the microfiltration permeate becomes the raw water for the reverse osmosis process. The investigation 7/9 34/63 87/76 13/24 9/15 74/82 29/40 38/49 5/11 68/75 — — — 67*** — 4ath step retention coefficient, % * to the water after the coagulation I and sedimentation ** to the raw water *** to the water after the coagulation II and filtration **** to the water after the activated carbon filter 80*/90** 7/14 50** Turbidity, NTU Total hardness, mg/dm3 CaCO3 Sulfates (VI), mg/dm3 SO4 Total iron, mg/dm3 Fe Aluminum, mg/dm3 Al Silica, mg/dm3 SiO2 Nitrate nitrogen, mg/dm3 NNO3 Ammonia nitrogen, mg/dm3 NNH4 Oxygen consumption KMnO4, mg/dm3 O2 Absorbance UV, λ = 254 nm TOC, mg/dm3 C Suspension, mg/dm3 3rd step retention coefficient, % 2nd step retention coefficient, % Parameter 5bth step retention coefficient, % 62/66.5 — 58/78 41/64 55/92 6.5/29 8/22 — 79/92 3.5/12 — — — 67***/97** 100**** 1.5/15 — 4bth step retention coefficient, % Table 7 Retention coefficients of contaminations present in the investigated water Total retention coefficient for the four step process, % — — Total retention coefficient for the five step process, % — — — — Total retention coefficient for the process without pretreatment, % results show that it is possible to protect the polypropylene membrane from the fouling occurring inside 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.