The determination and origin of fibre clogging in membrane bioreactors

► First study of membrane channel clogging in membrane bioreactors (MBRs) by coarse suspended solids (CSS) based on empirical measurement of 10-12 full-scale MBR plants. ► New on-site methodology developed for assessing “ragging” propensity: the agglomeration of fine fibres to form long “rags” in the MBR tanks. ► Behaviour of washed CSS filtered from the MBR sludge using a 4 mm - aperture net found to be identical to that of cotton wool fibres. ► Correlation between filterability and the concentration of CSS closer than that between filterability and classical mixed liquor suspended solids (MLSS) concentration. ► Heuristic observations revealed plants using macerating screens to be more prone to clogging than those using classical screen removal, consistent with the supernatant from the macerators returned to the process containing the fine cotton wool fibres responsible for ragging.