Energy Consumption of MBR for Municipal Wastewater Treatment: Current Situation and Potential

Energy consumption factors were developed for membrane bioreactor (MBR) and conventional activated sludge (CAS) flowsheets using consistent assumptions based on a Metcalf & Eddy average-strength domestic wastewater, for a plant with an average daily flow (ADF) rate of 18,925 m3/d (5 MGD) and a peak hourly flow factor of 2.0.For a flowsheet without primary clarification, the energy consumption factors for MBR and CAS plants were 476 Wh/m3 and 406 Wh/m3, respectively; so the energy consumption for the MBR plant was 17% higher than for the CAS plant. The fraction of the energy consumption factor for the MBR plant attributed to the membrane system was 92 Wh/m3, or about 20% of the total plant energy consumption.The energy consumption factors estimated for MBR plants in this paper are lower by about 50% when compared to most values reported in the literature. One reason is the rapid evolution of MBR technology which has led to a reduction in the energy required for membrane air scouring by a factor of 15 over the past 15 years. Another reason is the plant utilization factor which penalizes MBR when the flow is less than design because membrane scouring and RAS flow rate are typically constant and independent of the feed flow rate. It is suggested that the mode of operation of the membranes could be modified to make energy consumption proportional to the feed flow rate. With this method it is shown that the energy penalty for MBR could be reduced to as little as 10% over CAS when the plant utilization factor is 60%.