CHARACTERIZATION OF FOULING OF NANOFILTRATION MEMBRANES USED TO TREATSURFACE WATERS

The formation of a membrane fouling layer was monitored and characteri sed during treatment of a polluted surface water to potable standard u sing spirally wound nanofiltration membranes. These were housed in a l ow pressure reverse osmosis system (6 bar, 5 elements in series) opera ted 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 v alue during a period lasting from 8 - 38 days after which flux stabili sation occurred. Membranes operated intermittently stabilised more qui ckly after 8 - 16 days. Salt rejection and membrane feed channel diffe rential pressure were found to display no trend as a result of fouling of these membranes. Destructive membrane analysis revealed significan t fouling even after short periods of operation (14 days). initial dep osits of foulant were found to accumulate alongside the membrane feed channel spacer and with time these deposits encroached upon the remain ing free membrane area. The fouling layer was a mixture of biological, organic and inorganic species. Calcium and phosphorus were the main i norganic components present with minor iron and aluminium. The high le vels of phosphorus found were probably due to the use of sodium hexame taphosphate as a pretreatment.