CHARACTERIZATION OF FOULING OF NANOFILTRATION MEMBRANES USED TO TREAT SURFACE WATERS

The formation of a membrane fouling layer was monitored and characterised during treatment of a polluted surface water to potable standard using spirally wound nanofiltration membranes. These were housed in a low pressure reverse osmosis system (6 bar, 5 elements in series) operated at 80% recovery through concentrate recirculation. The plant feed was not disinfected and displayed a high fouling tendency. Normalised product flows of the membranes decreased to about 50% of its initial value during a period lasting from 8 - 38 days after which flux stabilisation occurred. Membranes operated intermittently stabilised more quickly after 8 - 16 days. Salt rejection and membrane feed channel differential pressure were found to display no trend as a result of fouling of these membranes. Destructive membrane analysis revealed significant fouling even after short periods of operation (14 days). initial deposits of foulant were found to accumulate alongside the membrane feed channel spacer and with time these deposits encroached upon the remaining free membrane area. The fouling layer was a mixture of biological, organic and inorganic species. Calcium and phosphorus were the main inorganic components present with minor iron and aluminium. The high levels of phosphorus found were probably due to the use of sodium hexametaphosphate as a pretreatment.