Influence of mixed liquid suspended solids and hydraulic retention time on oxygen transfer efficiency and viscosity in a submerged membrane bioreactor using pure oxygen to supply aerobic conditions

The performance of a wastewater bench-scale membrane bioreactor (MBR) treatment plant using pure oxygen conditions was monitored daily to provide aerobic conditions. The results showed the capacity of the MBR system to remove organic material over at hydraulic retention time (HRT) of 12 h and 18 h. The alpha-factors of aeration were determined at three different mixed liquid suspended solids (MLSS) concentrations, in order to understand the system performance when pure oxygen was used to provide the aerobic conditions of the MBR system. Under these working conditions, an alpha-factor in the range 0.90–0.12 was obtained when the HRT of 18 h was performed, and the MLSS concentration increased from 4300 to 10,275 mg/L. Additionally, an alpha-factor in the range of 0.28–0.02 was obtained at HRT of 12 h and the MLSS increased from 4071 to 11,192 mg/L. Our study suggested significant changes in the behaviour of the biological system (viscosity, aeration efficiency, organic matter removal) when the concentration of MLSS was increased and the HRT decreased in the bioreactor, decreasing the aeration efficiency in both cases. Furthermore, the efficiency of aeration seems to be more affected by MLSS concentration than by HRT under our operating conditions over the range studied.

► Alpha-factor significantly decreases with MLSS concentration.
► Changes in the behaviour of the biological system happen with different MLSS and HRT.
► Kinetic coefficients have been changed with different MLSS and HRT in MBR system.
► The efficiency of oxygen transfers is highly influenced by MLSS and HRT.