Dynamic Simulation is Necessary to Understand Differnces in Design of Biological Nutrient Removal MBR Processes

The increasingly stringent quality of effluent and the need to reclaim and reuse water are promoting application of biological nutrient removal (BNR) membrane reactor (MBR) systems. At a given site, selection of a BNR process depends on influent characteristics and effluent quality expected from the treatment plant. Ideally, dynamic influent data should be used for process design. However, in most cases, average wastewater quality and flow is available at the design stage, without a detailed understanding of the dynamic nature of the specific source. This is a limitation in practice that can be addressed, to an extent, by incorporating sufficient safety in the design.This paper examined the design of BNR-MBR processes using BIOWIN™ process simulation software under average and dynamic wastewater conditions. The intent is to demonstrate the necessity of using dynamic quality to arrive at a proper design, since the actual influent concentrations can vary by orders of magnitude with respect to average values. Regardless, the plant is supposed to produce uniform effluent quality at all times. The MBR processes evaluated, modified UCT (conventional), modified UCT (membrane) and modified Johannesburg, produced different design and operating conditions at steady state simulation compared to dynamic simulation for the same target effluent quality. The dynamic simulation, using actual flows and wastewater quality data, showed significant differences in the process design and process responses, confirming the limitation of using the average wastewater quality for design. Furthermore, the evaluation indicated that modified Johannesburg (MJ) process, where a deoxygenation and denitrification reactor was introduced, to utilize the excess dissolved oxygen (DO) in mixed liquor from the membrane tank, resulted in an economical design for the specific wastewater and effluent criteria considered in this study.