Ascorbic acid-mediated reductive cleaning of iron-fouled membranes from submerged membrane bioreactors

• Ascorbic acid can be effective in removing iron from fouled membranes.
• Cleaning effectiveness of ascorbate compromised by Fe-catalysed ascorbate oxidation.
• Kinetic model of ascorbate-mediated iron oxide dissolution describes cleaning results.
• Oxygen removal, ascorbate replenishment or Fe(II) ligand prevent ascorbate oxidation.

The ascorbic acid (Asc)-mediated cleaning of iron-fouled membranes from submerged membrane bioreactors (MBRs) dosed with iron salts for facilitation of phosphorus removal is investigated here with the cleaning process, involving the reductive dissolution of iron oxides on the membrane surface and within membrane pores, conducted under both oxic and anoxic conditions and in the presence of 1,10-phenanthroline, a strong Fe(II)-binding ligand. A significant reduction in cleaning effectiveness was observed when oxygen was present with kinetic modelling of key processes operating in this system indicating that the reduced effectiveness could be attributed to the Fe(III)-catalysed oxidation of ascorbate with Fe(III) being replenished by the relatively rapid heterogeneous oxidation of Fe(II). While either removal of oxygen or addition of Fe(II)-binding agents such as phenanthroline are effective in preventing the Fe(III)-catalysed oxidation of ascorbate, cleaning of iron oxide-fouled membranes under oxic conditions with frequent replenishment of freshly prepared Asc is recommended as a reasonable balance between cleaning effectiveness and cost.