Fouling control strategies for direct membrane ultrafiltration: Physical cleanings assisted by membrane rotational movement

Abstract

Recently, interest in the application of direct membrane filtration (DMF) in wastewater treatment has increased as an alternative to conventional activated sludge technologies. However, membrane fouling remains the main limitation for its application on an industrial scale. Dynamic shear-enhanced membrane systems, such rotating hollow fibre membranes (R-HFM), have shown to be effective in fouling control and mitigation with complex suspensions (e.g. anaerobic membrane bioreactors). In fact, previous works with DMF demonstrated the capability of membrane rotation to prevent matter deposition during the filtration cycles. Nevertheless, physical cleanings aided by membrane rotation in systems where there is no biological organic matter degradation has not been previously explored.Therefore, the current study addresses a physical cleaning strategy based on membrane module rotation for DMF. The effect of rotational speed (0 – 340 rpm) and relaxation time (0 – 270 s) on filtration performance was studied at dead-end lab scale trials. Pre-treatment effectiveness by in-situ and ex-situ sewage coagulation on fouling mitigation has been also assessed. DMF improved with pre-coagulation, reducing significantly membrane fouling. Ex-situ coagulation was identified as the best pre-treatment mode since flocs breakage and foulants releasing are avoided. In addition, the rotational speed increase during the backwashing stage enhanced erosion and dispersion of detached foulants. Besides, the addition of a relaxation stage (tR ≥ 180 s) enhanced the effectiveness in fouling re-dispersion. The physical cleanings demonstrated their effectiveness during long term trials, allowing operation at high permeate fluxes (24 L·h−1·m−2) with moderate organics up-concentration.