Efficacy of relaxation, backflushing, chemical cleaning and clogging removal for an immersed hollow fibre membrane bioreactor.

A pilot-scale hollow fibre immersed MBR, challenged with real municipal wastewater, was studied and operated under conditions identical to those prevailing at full-scale to assess the relative influence of backflushing, relaxation, chemical enhanced backflushing (CEB) and declogging on permeability decline and recovery. The influence of relaxation and backflushing was initially assessed using the conventional flux step method; results indicated reversible fouling to be similar for each method, whilst the irreversible fouling rate was significantly reduced by backflushing. For a given total backflush volume, fouling mitigation was found to be marginally better through employing higher backflush fluxes than longer backflush durations. The impact of the CEB on permeability recovery assessed at low and high fluxes indicated operation at more conservative fluxes to yield more sustained permeability. Under more aggressive operating conditions--fluxes of up to 35 L m⁻² h⁻¹ at specific aeration demand values of 0.25 Nm³/(m² h)--long-term permeability decline took place which was not significantly ameliorated by chemical cleaning. On declogging the membrane through gentle agitation permeability recovery was significant, but was followed by a rapid permeability decline over the course of a few hours. Results suggested control of clogging to be of greater importance than that of fouling in sustaining permeability.