Fouling effects on rejection in the membrane filtration of natural waters

Membrane processes in drinking water applications are micro- (MF), ultra- ( UF) and nanofiltration (NF). These processes remove turbidity and bacteria (MF), viruses and macromolecules (UF) and small molecules and hardness (NF) . Of particular concern in water treatment is the removal of natural organi c matter (NOM) which contains potential disinfection by-product precursors. The presence of colloids, multivalent ions and organics in surface waters may cause substantial fouling of membranes. A study was carried out which l ooked at the rejection abilities of a range of membranes targeting hematite colloids (40-500 nm), NOM and cations, fouling conditions and cost of trea tment of these processes with consideration of chemical pretreatment with f erric chloride [1]. In this paper the effect of membrane fouling on rejecti on is presented. The study was based on experiments with two MF membranes ( GVWP, GVHP, 0.22 mum, Millipore), six UF membranes (1, 3, 5, 10, 30, 100 kD a, regenerated cellulose, Millipore), and four organic NF membranes (TFC-SR , TFC-S, TFC-ULP, CA-UF, Fluid Systems, US). Three different types of organ ics (IHSS humic acid, MSS fulvic acid and an Australian concentrated NOM) i n a carbonate buffer containing calcium chloride and a background electroly te were used. Experiments were carried out in perspex (MF, UF) and stainles s steel (NF) stirred cells of a volume of 110-185 mt and a membrane area of 15.2-21.2x10(-4) m(2) at transmembrane pressures of 1, 1-3, and 5 bar for MF, UF, and NF, respectively. UF removes 10-95% of NOM depending on the mol ecular weight cut-off (MWCO) of the membrane. Pore sizes of <6 nm are requi red to remove about 80% of NOM, where a 6 nm pore size corresponds to a MWC O of about 10 kDa. Colloids are fully rejected. NF removes NOM effectively (70-95% as dissolved organic carbon (DOC) and 85-98% as UV absorbance). Cat ion rejection is very membrane dependent and varies for the investigated me mbrane types between 13 and 96% for calcium and 10-87% for sodium. Fouling was also dependent on pore size and was caused by large colloids (250 nm) o r coagulant flocs in MF, small colloids, organic-calcium flocs and aggregat es with a dense structure (formed slowly) in UF, and by a calcium-organic p recipitate in NF. The fouling influenced the rejection of colloids in MF an d that of NOM in UF and NF. If a highly charged layer was deposited on the NF membranes, cation rejection tvas also influenced. The characterisation o f permeate organics revealed that low molecular weight acids passed through the NF membranes and that the rejection of these acids was also dependent on the deposit on the membrane. The mechanisms which can explain such an in crease in rejection are different for the three membrane processes. In MF, pore plugging and cake formation was found responsible for fouling. This re duces the pore size and increases rejection. In UF, internal pore adsorptio n of calcium-organic flocs reduces the internal pore diameter and subsequen tly increases rejection. In NF, the key factor appears to the charge of the deposit. This was investigated with the deposition of a ferric chloride pr ecipitate. If the precipitate was of high positive charge, the rejection of cations increased and that of negatively charged low molecular weight acid s decreased compared to more neutral or negative precipitates. In essence, the rejection characteristics of membranes depend more on the fouling state of the membranes and the nature of the foulants than on the initial membra ne characteristics.