Fouling by humic acids during ultrafiltration and microfiltration for water treatment

Microfiltration and ultrafiltration are increasingly used in drinking water treatment for the removal of particles and microorganisms to meet more stringent water quality requirements. Natural organic matter (e.g., humic and fulvic acids) has been identified as a critical foulant, but its effect on fouling during membrane filtration is not clearly understood. The overall objective of this study was to develop a fundamental understanding of the underlying physical and chemical phenomena governing humic acid fouling during microfiltration and ultrafiltration.; Humic acid fouling during microfiltration is very well described using a modified form of a recently developed combined pore blockage and cake filtration model. The filtration data at long times suggest that there is a critical flux below which cake formation becomes negligible. The model is able to account for a wide range of experimental observations, including humic acid rejection as the pores become blocked during filtration. The four model parameters were all evaluated directly from the flux decline data; however, the results demonstrate that these parameters can also be estimated from independent measurements of the physical properties for the humic acid system.; Humic acid fouling during ultrafiltration is primarily due to the formation of a humic acid deposit on the membrane surface, with adsorption and concentration polarization having relatively small effects on the flux. The surface properties of the ultrafiltration membranes were significantly modified by the humic acid solutions.; Humic acid fouling is also influenced by the presence of inorganic particles (e.g., kaolin) and/or different pretreatment steps. Inorganic kaolin particles prevent humic acid aggregates from reaching the membrane surface, decreasing the total resistance of the resulting deposit. Pretreatment of the humic acid solution with aluminum sulfate significantly reduced the rate of fouling while powdered activated carbon gave little improvement in flux. Membrane cleaning by periodic backflushing was effective at removing the fouling deposits. These results provide important insights into the fouling characteristics of humic acids that can be used to develop and design improved membrane systems for water treatment.