Behaviour of soluble microbial products in a membrane bioreactor

Synthetic wastewater was treated with a submerged membrane bioreactor to investigate the organic removal performance, as well as the behaviour of soluble microbial products during long-term operation. Satisfactory chemical oxygen demand (COD), total organic carbon (TOC) and biological oxygen demand (BOD) removal efficiencies were achieved, averaging over 90, 94 and 95%, respectively. The accumulation of TOC in the supernatant of the bioreactor and its degradation after 5 months of operation was observed. Among this accumulation, macromolecules with a molecular weight (MW) >100 000 accounted for 34%. A decrease in the supernatant TOC corresponded to a shift from large to small molecules. The organic substances with a MW > 100 000 decreased from 34 to 16%, whereas those with a MW < 30 000 increased from 33 up to 52%. The accumulation proved to be inhibitory towards the metabolic activity of the activated sludge, as well as contributing to the poor membrane permeability of the mixed liquor. (C) 2000 Elsevier Science Ltd. All rights reserved.