Cleaning strategies for iron-fouled membranes from submerged membrane bioreactor treatment of wastewaters

Cleaning strategies for iron-fouled membranes from submerged membrane bioreactors (MBRs) dosed with iron salts are investigated here. Severe membrane fouling resulting from iron dosing was observed with amorphous ferric oxyhydroxide particles and gelatinous assemblages containing Fe(III) bound to polysaccharide materials responsible particularly for irreversible membrane fouling. Sodium hypochlorite (NaOCl) and citric acid, the most commonly used chemical cleaning agents, were not particularly effective in removing iron species from the membrane surface while the Fe(III) reducing agents ascorbic acid and sodium dithionite were very effective in removing iron-containing deposits under the conditions used in this study. In the presence of oxygen, the Fe(III)-catalysed oxidation of ascorbic acid and direct oxygen-mediated oxidation of dithionite control the dissolution rate constant. Use of NaOCl followed by ascorbic acid at concentrations of 10-20 mM is recommended as a reasonable balance between cleaning effectiveness and cost though further investigation of cleaning of fouled membranes from full scale MBRs is required.