Filtration mechanisms occurring during ultrafiltration of coagulated and non-coagulated surface water were investigated using the general blocking filtration laws. The sharp decline in permeate flux in the first 10-20 minutes of filtration was largely due to a combination of the blocking filtration mechanisms. However, a distinction between “complete”, “standard” and “intermediate blocking” mechanisms was not possible as a large degree of overlap existed between the mechanisms and the transition from one type of blocking to another was very smooth. Coagulation appeared to retard blocking and consequently, the duration of the “blocking phase” was twice as long for coagulated water compared with non-coagulated water. The “transition phase”, where both blocking and cake filtration occurred simultaneously, was also extended in the case of coagulated water as continuous blocking of the membrane through the pores of the cake was observed. The modified fouling index-ultrafiltration (MFI-UF) was employed to examine the impact of coagulation on filter cake properties. Coagulation reduced the specific resistance of the filter cake by 50% at a TMP of 1.5 bar, and 35% at a TMP of 0.5 bar. Depth filtration was hardly evident at a TMP of 0.5 bar, but was very pronounced at a TMP of 1.5 bar as the specific resistance of the filter cake increased continuously, particularly in the absence of coagulant. The filter cake was sensitive to the applied TMP and the MFI-UF value increased by 130% for non-coagulated surface water and 55% for coagulated water due to compression of the cake, when the TMP increased from 0.5 to 1.5 bar.

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