FOULING BY ORGANICS. CALCIUM AND COLLOIDS

It should be noted here that the pore size and membrane resistance are clean water characteristics. Fouling and concentration polarisation may alter their value, but the use of fouled membrane resistance for example seems unsuitable to predict filtration behaviour. Salt rejection for example, would shift the apparent resistance of TFC-S and TFC-SR membranes to larger values. 

where sieving is believed to dominate rejection, charge effects were observed. However, it is the coUoid stability that determines rejection for colloids which were smaller than membrane pore size. Aggregates that form in the absence of organics are retained, but exhibit reduced rejection at high fluxes, pressures and after a backwash. 

it is also charge and sieving that are important. While in MF colloid interactions and in UF organic interactions are important, in NF it is the speciation, ion dissociation, organic-cation complexation, as well as the structure and size of compound that have a significant impact on rejection. Solute-solute interactions of decreasing size scale are determining rejection as membrane pore size decreases. 

Fouling also affects rejection. In MF and UF pores gradually fill up, while in NF a deposit of a higher charge than the membrane may form and enhance rejection. Colloid fouling can also increase the thickness of the unstirred boundary layer and increase concentration polarisation, which adversely affects rejection. 

The general conclusion for all processes and their rejection mechanisms is that solute-solute interactions are important. The size of the solute of interest varies with process (or pore size). 

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