To clarify the effect of sludge retention time (SRT) on performance and microbial behaviour in a submerged membrane bioreactor for the treatment of domestic wastewater, four runs of a laboratory scale reactor with hydraulic retention time of 5h and SRTs of 5, 10, 20 and 40 days, respectively, were conducted. The membrane bioreactor process was capable of achieving over 90% removals both for chemical oxygen demand and ammonia nitrogen, on average, almost independent of SRT. With prolonged SRT, concentrations of suspended solids and volatile suspended solids in the bioreactor increased accordingly, whereas sludge growth kinetic parameters, sludge yield and endogenous decay coefficients, declined slightly and exponentially, respectively. The mean sludge particle sizes at different SRTs were in the range 14.82-48.24m, providing a favourable environment for enhancement of mass transport. Characterized by an oxygen consumption rate, sludge specific activities both for organic decomposition and nitrification of ammonia nitrogen varied with SRT. However, volumetric oxygen consumption rates of the membrane bioreactor, representing the whole ability of the process for decomposing pollutants, were enhanced as SRT increased.

In the present study, an orthogonal array design was adopted to investigate effects of operational parameters including aeration intensity, membrane flux, suction time and non-suction time on sludge accumulation on membrane surfaces respectively at a high SS concentration of 10g 1-l and a low SS concentration of 1g I-1 in a submerged membrane bioreactor. Average transmembmne pressure (TMP) increase rate over the operation time was used to evaluate sludge accumulation. Among the four factors, membrane flux was found to influence Th4P the most obviously. The effect of aeration intensity became significant only at a high SS concentration of 10g I-1TMP increased with suction time and decreased with non-suction time. There was a critical membrane flu over which sludge particles were deposited, and accordingly, ThW increased sharply. Two zones, predicting whether sludge particles are deposited or not, could be comprehensively determined by the critical flux, correspondent aeration intensity and SS concentration. For long-term stable operation, it is suggested that a membrane bioreactor be operated in the zone with prediction of no obvious sludge deposit on membrane surfaces.

A self-forming dynamic membrane (SFDM) method that used the biomass layer formed on a coarse mesh to effect solid-liquid separation was proposed. A 100-

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)\100000 accounted for 34%. A decrease in the supernatant TOC corresponded to a shift from large to small molecules. The organic substances with a MW\100000 decreased from 34 to 16%, whereas those with a MWB30000 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.