Optimization and Evaluation of Operational Performance for Membrane Bioreactor (MBR)

Abstract

This study evaluates the operational performance of a membrane bioreactor (MBR) system at the Khaanzaa Wastewater Treatment Plant (WWTP), operated under subcontract with the Oyu Tolgoi mining project in the Gobi region of Mongolia. The assessment is based on laboratory experiments and statistical analysis of long-term operational data collected between 2024 and 2025. The results demonstrate that the MBR system achieves high treatment efficiency, with effective removal of chemical oxygen demand (COD) and ammonia under the studied conditions. A key finding is the strong inverse relationship between sludge wasting rate (Qw) and sludge retention time (SRT). Insufficient or irregular sludge wasting resulted in excessively high SRT, leading to aged biomass, increasing MLSS, poor sludge settleability, and unstable process performance. Statistical analysis further showed significant variability in MLSS and dissolved oxygen (DO), with frequent deviations from optimal ranges. Excessive DO levels contributed to unnecessary energy consumption, while low DO conditions risked reduced nitrification efficiency. Membrane performance analysis demonstrated that permeability was strongly affected by fouling and operational conditions. Although periodic cleaning temporarily restored performance, rapid fouling recurrence indicated the need for more stable biological control. Based on these findings, optimization of sludge wasting and aeration control is recommended to maintain stable MLSS and SRT conditions. Maintaining an SRT of approximately 20 days and DO within the optimal range (1.5–2.5 mg/L) is essential for improving system stability, reducing fouling, and enhancing long-term MBR performance.