Advanced analysis of membrane-bioreactor performance with aerobic-anoxic cycling

Mathematical modeling and intensive chemical analysis are used to quantify the relationships among the heterotrophic bacteria, autotrophic bacteria, and key inorganic (NH₄⁺-N and NO₃⁻-N) and organic (COD) compounds of municipal wastewater treated in a pilot-scale membrane bioreactor (MBR) operated with aerobic-anoxic cycles. Key features of the model for MBR are no biomass in the effluent, partial removal of biomass-associated products by the membrane, and D.O. cycling with 9 mg/L during aeration period and 0.5 mg/L for the anoxic period. The model explains the key trends in the cyclic data: NH₄⁺-N is consumed only during aerobic periods and rises steadily during anoxic period; NO₃⁻-N is produced only during aerobic periods, but declines in anoxic periods; The soluble COD in treated water mainly consists of BAP and is relatively constant through the cycle. Advantages of introducing an anoxic cycle to the continuous-flow MBR process are reduction of total effluent nitrogen, oxygen consumption, and sludge production as a consequence of denitrification. On the other hand, the anoxic period causes an increase in the average effluent NH₄⁺-N.