There are four major liquid pumpings in the flow diagram shown in Fig. 1: wastewater feeding (Q), internal mixed liquor recycle (QIR), mixed liquor recycle to membrane tank (Qr-Q), and permeate suction (Qp). In mechanical screen operation, majority of the energy is consumed to pump wastewater to compensate the head loss by the screen while screen operation itself does not need too much power. Therefore, the power consumption by mechanical screen is neglected in calculation. Pumping power (PP) is calculated using equation (1) elsewhere.
Fig. 1. Flow diagram of typical iMBR with anoxic tank
1) Wastewater pumping from holding tank to anoxic tank: Due to the variations of water level in the holding tank, the differential pressure of the pump is variable. Here, the average pressure difference between inlet and outlet of feed pump () is assumed at 3 m H2O. Flow rate equals to the wastewater flow rate, . Pump efficiency () is assumed at 0.6 while motor efficiency () at 0.9
2) Internal recycle: The average head pressure exerted by the recycle pump () is assumed at 2 m H2O including the head loss in pipelines. Internal recycle flow rate is assumed at . Since the pump handles a liquid with high MLSS, pump efficiency () is assumed somewhat low at 0.4 due to the viscosity of the mixed liquor.
3) Recycle from membrane tank: Due to the variations of water level in the aeration tank, the head pressure of the pump is variable. The average differential pressure of the recycle pump () is assumed at 2 m H2O. Recycle flow rate to membrane tank, , is assumed at while it ranges between and typically (Crawford, 2006). Due to the flow arrangement, actual flow to be pumped is. . Pump efficiency () is assumed at 0.4 as for the internal recycle pump.
4) Permeate suction: Permeate suction is performed intermittently in iMBR. The frequent on and off cycle is responsible for low pump efficiency, which is assumed at 0.3 here. The efficiency of VFD that controls permeate flow is assumed at 0.7. The flow rate is virtually same as influent flow rate, , because the water loss to the excess sludge removal (Qx) is less than 0.01Q. Suction pressure is assumed at 2 m H2O while it typically ranges 10-30kPa. It is also assumed that additional head of 2 m H2O is required to send the permeate to different location.
Finally SED for each unit process is calculated using the following equation.
= specific energy demand (kWh/m3)
= power consumption (kW)
= flow rate (m3/d)
© Seong Hoon Yoon