Effect of mixed liquor pH on the removal of trace organic contaminants in a membrane bioreactor

Experiments were conducted over approximately 7 months to investigate the effects of mixed liquor pH (between pH 5 and 9) on the removal of trace organics by a submerged MBR system. Removal efficiencies of ionisable trace organics (sulfamethoxazole, ibuprofen, ketoprofen, and diclofenac) were strongly pH dependent. However, the underlying removal mechanisms are different for ionisable and non-ionisable compounds. High removal efficiencies of these ionisable trace organics at pH 5 could possibly be attributed to their speciation behaviour. At this pH, these compounds exist predominantly in their hydrophobic form. Consequently, they could readily adsorb to the activated sludge, resulting in higher removal efficiency in comparison to under less acidic conditions in the reactor. Removal efficiencies of the two non-ionisable compounds bisphenol A and carbamazepine were relatively independent of the mixed liquor pH. Results reported here suggest an apparent connection between physicochemical properties of the compounds and their removal efficiencies by MBRs.