The determination and origin of fibre clogging in membrane bioreactors

Abstract

Membrane channel clogging in membrane bioreactors (MBRs) has been largely unexplored by the research community, despite it being widely recognised as the main impediment to sustainable MBR operation by practitioners, and reflects the difficulty of examining clogging with the same scientific rigour as membrane fouling.

The incidence of clogging/“ragging” has been assessed across 10–12 full-scale flat sheet immersed MBRs, equipped with inlet screens of similar rating, and physical characteristics of the sludge examined. Measured sludge quality determinants included classical mixed liquor suspended solids (MLSS) concentration and filterability measurement, as well as a series of novel sludge characterisations based on determination of coarse suspended solids (CSS) concentration using sieves or nets of aperture sizes between 150 and 4000 μm. A further rudimentary test was conducted on the coarse filtered solids to assess the mechanical integrity of the agglomerated material.

Results showed there to be a reasonable correlation between filterability and concentration of super-4 mm particles, whereas no such correlation existed between filterability and MLSS. This indicated the nature of the solids, and particularly the highly coarse particulate content, to be critical in determining filtration resistance. Both filterability and CSS concentration also correlated with heuristically assessed clogging propensity, with those plants fitted with screenings maceration being more prone to clogging than those with more conventional screenings treatment where screenings were removed entirely.

The coarse suspended solids were found to readily agglomerate to form characteristic “rags”, i.e. 10–15 cm-long braids of aggregated material, which were self-supporting when suspended in air. The mechanical integrity of these fibres was unaffected by chemical treatment either with detergent or hypochlorite, indicating a degree of robustness of the rags which were formed primarily of cellulosic matter. Identical properties were recorded from experiments conducted on cotton wool filaments, suggesting ragging to relate to disposal of cotton wool-based produ