Fouling and accumulation of dissolved organic matter in membrane bioreactors

Abstract

The primary objective of this thesis is to contribute towards a more fundamental understanding of dissolved organic matter (DOM) fouling and accumulation in submerged MBR systems. The complex DOM mixture was fractionated into four more homogeneous components, namely, hydrophobic aquatic humic substances (AHS), hydrophilic acids (HiA), hydrophilic bases (HiB), and hydrophilic neutrals (HiN). The order of fouling potential of the fractional DOM components, evaluated at comparable conditions, was observed to be AHS > HiN > HiB > HiA. Fouling caused by HiN and AHS was mainly irreversible. Simulation results of a newly developed fouling model indicate that DOM is the key contributor to the long-term irreversible fouling of the MBR system. Accumulation of DOM in the MBR was found to be more pronounced at short sludge retention times (SRTs). Carbohydrates and proteins appeared to be the components of DOM prone to accumulate in the MBR compared with aromatic compounds. The fouling potential of DOM was observed to increase considerably as SRT shortened. Size exclusion or sieving of the microfiltration membrane alone was experimentally demonstrated inadequate to explain DOM accumulation. The retarded transport of DOM through porous membranes was postulated as a new mechanism and shown to be a better explanation for DOM accumulation in MBR systems.