Surface water treatment using nanofiltration—pilot testing results and design considerations

Abstract

Integrated membrane systems (IMS) were pilot tested for treatment of a high TOC river water. Pretreatment methods included microfiltration, indashline coagulationdashmicrofiltration, and coagulationdashsedimentationdashfiltration. Fouling was minimized at lower flux, lower recovery and with the addition of a biocide. Such experiments incurred no fouling. A cellulose acetate nanofilter was susceptible to biological degradation therefore a biocide was necessary to ensure membrane integrity. A polyamide membrane was sensitive to oxidation by monochloramine and rejection characteristics were compromised. The rate of fouling between the three nanofilters tested increased with increasingly negative surface charge and increasing surface roughness. Organic adsorption therefore did not follow the charge-repulsion theory but may have been negatively influenced by the greater surface area associated with a rougher surface. Modeling of productivity decline by a form of the resistance model using permeate volume and TOC concentration provided a better fit than that of the linear model of productivity with time. Rejection of organic and inorganic parameters increased with decreasing nanofilter molecular weight cut-off (MWCO). Rejection was influenced by diffusion and size exclusion mechanisms. Log removals of Bacillus subtilis spores by IMS ranged from 5.4 to 10.7 log with the highest removals achieved by microfiltration pretreatment followed by a low (200 dalton) MWCO polyamide nanofilter.