Hydrophobicity and molecular weight of humic substances on ultrafiltration fouling and resistance

Humic substances are the major foulant during ultrafiltration (UF) of wastewater. This study evaluates the effects of hydrophobicity and fractionated humic substances on UF fouling and permeation resistance. A commercial humic acid (HA) obtained from Aldrich was subjected to DAX-8 resin for fractionation of hydrophobic and hydrophilic fractions. These fractions were fractionated further into different molecular weight groups using gel filtration chromatography. The hydrophilic fraction exhibited the greatest flux decline, revealing that hydrophobicity decreases fouling. Since the size of particle fractions was significantly smaller than the pore size of membranes, fouling was greatest for the largest fraction with a 100-kDa membrane and the smallest fraction with a 10-kDa membrane. Severe fouling was due to adsorption and pore blocking. For the first 300 min of filtration, the fouling rate was high in all fractions and is more for 100-kDa than 10-kDa membrane due to greater hydraulic resistance of the HA deposit on the membrane surface. The effect of resistances was also investigated. Each operationally defined resistance depended on membrane pore size, pressure, and HA characteristics. For hydrophobicity and the molecular weight effect, the hydrophilic fraction has the strongest resistance. This study suggested that strong resistance is responsible for irreversible fouling and that is primarily due to pore adsorption and pore blocking.