In this process, compressed gas is injected to the inlet side of the multi-tube module while liquid circulation is performed by circulation pump as shown in Fig. 1. Since only small amount of gas is injected relative to the liquid flow rate, the system performance is not significantly affected whether the module is vertically mounted or horizontally mounted.
However, two conflicting effects exist internally in the system:
1) flux enhancement due to the stronger turbulence by two phase flow and
2) shortened channel length due to the increased pressure loss per membrane channel length
While the maximum length of channel is dominated by the pressure loss in the channel as discussed here, two phase flow only allows shorter channel length relative to one phase flow. As a result, the net effect of gas injection to tubular membrane does not necessarily improve the system performance in terms of energy efficiency.
Interestingly, the oxygen injection simultaneously contribute to the oxygen supply to the aeration tank in addition to the membrane fouling mitigation. In Linde’s AXENIS™ process, pure oxygen is injected to the module in order to control DO in aeration tank especially when high strength wastewater is fed to the aeration basin.
Fig. 1. Schematic diagrams of membrane bioreactors running with multi-tube module.
© Seong Hoon Yoon