Influence of suspended solids concentration on oxygen transfer in a membrane bioreactor operated at short solids retention time

The highest operational costs observed in activated sludge wastewater treatment plants are mainly due to the aeration costs. The aeration costs are even more pronounced in wastewater treatment systems such as membrane bioreactors (MBRs) designed to be operated at very high mixed liquor suspended solids (MLSS) concentrations (in the range of 10 to 20 g/L of MLSS). MBRs operated at such high MLSS concentrations exhibit certain advantages such as low footprint requirements. A crucial parameter to determine the efficiency of introducing air (or dissolved oxygen) into the activated sludge processes is known as the ""alpha factor"". The alpha factor is defined as the ratio of the oxygen transfer coefficient (KLa) in process water over the oxygen transfer coefficient (KLa) in clean water. Alpha factors are influenced by many different parameters including the mixed liquor suspended solids (MLSS) concentration, and the mixed liquor volatile suspended solids (MLVSS) concentration. There is not a clear correlation between the MLSS concentrations and alpha factor, although it is well known that the alpha factor decreases as the MLSS concentration increases. Most of the researches aiming at determining alpha factors have been performed with sludge taken from wastewater treatment plants operated at high biomass solid retention times. In this research the behavior of the oxygen transfer coefficient (KLa) and the alpha factors in activated sludge at different mixed liquor suspended solids (MLSS) concentrations were evaluated. The activated sludge was collected from a laboratory scale membrane bioreactor (MBR) operated at a short solids retention time (approximately 3.6 days). Short solids retention time conditions may lead to an increase in the oxygen transfer efficiency due to the low production of microbial by-products. Microbial by-products may highly impact the oxygen transfer coefficient (KLa). The activated sludge was concentrated up to MLSS concentrations ranging from 4,450 mg TSS/L to 20,489 mg TSS/L. Both the oxygen uptake rate (OUR), and the oxygen transfer rate (OTR) were evaluated at each experimental condition in a separate batch reactor. The results of these experiments demonstrated that the KLa and the alpha factor decreased as the MLSS concentration increased. At the experimental conditions of the present study, alpha factors of 1, 0.71, 0.50, 0.35, 0.30, and n.d. (non determined) were obtained at MLSS concentrations of 0,5.24, 9.27, 10.86, 16.90, and 20.49 g TSS/L, respectively. At the highest MLSS concentration of 20.49 g TSS/L, dissolved oxygen could not be supplied into the mixed liquor. Moreover, it was observed that the alpha factors at short solids retention times (SRTs) were higher than alpha factors obtained (from previous studies) at longer SRTs at the same mixed liquor volatile suspended solids (MLVSS) concentrations. MBRs operated at high MLSS concentrations (if not properly designed) may demand an excessive amount of air (and energy) resulting in extremely high operational costs. Knowing the alpha factors at the actual operational condition is crucial to better design MBRs processes at the highest economically feasible MLSS concentration