Influence of mixed liquor suspended solids, aeration, and mixing on oxygen transfer parameters in a sequencing batch membrane bioreactor

Abstract

Aeration is utilized to supply dissolved oxygen to activated sludge in wastewater treatment facilities for the conversion and removal of carbon, phosphorous, and nitrogen. It accounts for 50-90% of a treatment facility’s energy demands. In membrane bioreactors (MBR), the traditional clarifier is replaced with a membrane separation, allowing for operation at higher mixed liquor suspended solids (MLSS) concentration in the bioreactor. While this approach results in a reduced system footprint and higher quality treated effluent suitable for different reuse applications, it comes with an energy penalty. With a higher energy demand than conventional activated sludge (CAS) processes, efforts to reduce the aeration are even more important. The most important parameter in transferring oxygen to the activated sludge biomass is α, defined as the ratio of the overall mass transfer coefficient (KLa) of oxygen from the supplied aeration to the bioreactor liquid in wastewater during treatment (process water) to the the KLa in clean water. Other important oxygen transfer parameters are β, the ratio of saturated oxygen concentration of process water in the bioreactor to clean water; SOTR, the oxygen transfer rate standardized to 20 °C, 101.3 KPa, and zero dissolved oxygen in the bioreactor; SOTE/Z, the oxygen transfer efficiency with respect to airflow and water depth; and SAE, the aeration efficiency with respect to energy consumption. Understanding the impact to these parameters of operating a system under different configurations and operating conditions will facilitate the optimization of a facility’s aeration practices. Over the last seven years, a hybrid MBR system at the Colorado School of Mines has been used to treat the wastewater of the Mines Park student housing complex. By adding the flexibility of a sequencing batch reactor (SBR) to the MBR treatment process, a sequencing batch membrane bioreactor (SBMBR) is able to treat wastewater to varying effluent qualities, tailoring it to specific onsite reuse opportunities. However, the energy footprint of the system currently makes widespread implementation cost-prohibitive. To reduce the energy demand of the SBMBR aeration, a study was performed to evaluate the impact of MLSS concentration, aeration diffuser type (fine bubble versus coarse bubble), air flowrate, and sludge mixing on the oxygen transfer parameters and overall aeration. With fine bubble diffusers, α is highest with mixing at low MLSS operation, and without mixing at mid and high MLSS. The commonly assumed β value of 0.95 is accurate for the SBMBR, but the results suggest that this may not be the case for the very high MLSS concentrations of aerobic digesters. SOTE/Z for the fine bubble diffusers is more than double that of the coarse bubble diffusers, with little impact from mixing or aeration flowrate. However, the SAE is much higher for coarse bubble diffusers operating at a high flowrate of 70 scfm. The blower used for the fine bubble aeration at 19 scfm, originally designed for coarse bubble aeration at high flow, is illsuited for this high-pressure, low-flow operation. A replacement blower has been ordered, and this provides quantitative evidence that if a system is upgraded from coarse bubble diffusers to fine bubble diffusers, the blowers may also need upgrading to realize energy savings from the modification.