pH effects on the adherence and fouling propensity of extracellular polymeric substances in a membrane bioreactor

Both conformation and adsorption characteristics of extracellular polymeric substances (EPS) are affected by water chemistry in general and by the pH in particular. Therefore, changes in pH during operation of membrane bioreactor (MBR) systems are likely to affect fouling of ultrafiltration (UF) membranes. Pore clogging of UF membranes attributed to EPS can be affected by the pH in a complex manner: since the heterogeneous nature of the EPS mixture consists of both aromatic and aliphatic components with carboxylic acids (COOH) and phenolic alcohols (OH), at lower pH, adsorption of EPS to negatively charges surfaces is usually increasing. However, EPS conformation is also affected by pH changes that induce either linear (stretched) or coiled (compact) structure at high and low pH values, respectively. This study shows an increased adsorption of MBR originated EPS to Polyvinylidene Fluoride (PVDF) surfaces at lower pH, probably due to smaller deprotonated chemical functional groups that are in charge of repulsive electrostatic interactions. In contrast to the EPS adsorption results, the fouling rate of UF membrane by similar EPS at pH value of 6.3 was significantly lower than the fouling rate at higher pH value of 8.3. The relation between pH elevation and flux decline during EPS fouling experiment is shown to be a result of EPS conformation and swelling characteristics, which affect water and EPS permeation through the UF membrane pores. When EPS structure is stretched, linear chains are more accessible to the membrane pores that eventually clog the UF membrane at higher pH values.