Natural organic matter (NOM) fouling in low pressure membrane filtration — effect of membranes and operation modes

The objective of this study was to evaluate the effects of operating modes, membrane materials and pore size on natural organic matter (NOM) fouling. A range of flat sheet microfiltration (MF) and ultrafiltration (UF) membranes were tested under conditions of various constant pressure and constant flux filtration modes. Based on experimental filtration profiles, molecular weight (MW) distributions of NOM obtained using high performance size exclusion chromatography (HPSEC) and autopsies of fouled membranes using force emission scanning electron microscopy (FESEM), it was concluded that medium to low MW component of NOM (300–1,000 Da) is responsible for the initiation of fouling, where bulk of the fouling observed is due to very high MW ‘colloidal’ NOM (>50,000 Da). This two stage fouling phenomenon was in good agreement with classical blocking laws. As a general observation hydrophilic membranes were less prone to NOM fouling. A comparison of constant pressure and constant flux tests confirmed that modest constant flux, as used in industry, provided the most beneficial conditions.

In this work, none of the membranes showed no significant removal of medium to low MW NOM (300–1,000 Da) although significant removal of colloidal NOM in the 50–100 kDa range was observed with the ultrafilters. The fouling components appear to be colloidal and hydrophilic neutral fraction of NOM as evidenced by the pretreatment and resin fractionation results. High filtration resistance was observed with UF, and the trends are consistent with cake formation almost immediately at the commencement of the filtra- tion. A two-stage pore closure followed by cake filtration mechanism was observed for MF. Low constant flux filtration reduced fouling, especially irreversible fouling substantially, but results were less clear and consistent when comparing constant pressure filtrations in the 10–90 kPa range. Also irreversible fouling was dominant on larger pore MF membrane due to the deposition of low MW weight NOM on the internal structure of the membrane. Hydrophobic membrane was observed to be more prone to fouling than hydrophilic membrane.