Ultrafiltration of natural organic matter

Increasingly stringent regulations for drinking water quality have stimulat ed the application of ultrafiltration to water treatment. In addition to re moving particulate materials from water (including microorganisms, bacteria and viruses), the use of membrane treatment also meets purification requir ements. However, irreversible fouling curtails the economic viability of su ch a process. Experiments in stirred-cells were conducted to evaluate the e ffects of surface water composition on rejection and fouling of two ultrafi ltration membranes with different molecular weight cut-offs (10 and 100 kDa ). Experimental solutions consisted of natural organic matter or humic subs tances in a background electrolyte. The effect of calcium concentration dec reased rejection of humic acid under certain circumstances. This is believe d due ro reduced molecular size with an initial increase in calcium concent ration. However, at about 2.5 mM CaCl2, IHSS humic acid aggregates. This ag gregation increased rejection, and also caused irreversible fouling of the 100 kDa membrane, presumably as a result of pore size reduction due to inte rnal deposition of aggregates. This was confirmed by blocking law analysis. The variation of transmembrane pressure indicated the importance of a 'cri tical flux' effect. The organics and their various fractions showed differe nces both in rejection and flux decline. The larger and more UV-absorbing f raction of humic acid was shown to be responsible for irreversible pore ads orption and plugging. The fulvic acid and the hydrophilic fraction showed a smaller and mostly reversible flux decline. (C) 2001 Elsevier Science B.V. All rights reserved.