The parameters measuring the air utilization efficiency in membrane scouring are defined as follow (Judd, 2011).
- Specific air demand based on membrane area (SADm) is defined as scouring air flow rate per membrane area. It ranges 0.18-0.60 m3 air/m2 membrane area/hr as shown in the table here. The SADm close to 0.18 m3/m2/hr are obtained only with intermittent aeration. For instance, SADm of ZW500d is 0.36 m3/m2/hr, but intermittent aeration at 10-10 mode (10 s ON and 10s OFF) reduces effective SADm to 0.18 m3/m2/hr (Côté, 2004). Under ideal condition, intermittent aeration can be performed at 10-30 mode, where aeration is performed only 25% of the total operating time. Then, SADm can be reduced further. Finally, SADmdoes not reflect the cost performance of a specific membrane since it is not sensitive to flux and the air pressure required.
where Qair is air flow rate (m3/hr) and Am is total membrane surface area (m2)
- Specific air demand based on permeate volume (SADp) is defined as scouring air volume per permeate volume. It ranges 7-24 or higher as shown in the table here. The low end can be only obtained at favorable biological conditions with intermittent aeration or with the membrane cassettes with stacked membrane modules upward. This parameter is a more direct indication of cost performance of membrane than SADm. But it still does not directly indicate the cost performance of membrane because it does not include the head pressure effect. Energy consumption increases as membrane modules are stacked up and the height of membrane cassettes increases. Since obtaining SED described below is not always easy, SADP is often used to compare the air utilization efficiencies of membranes.
where Qair is air flow rate (m3/hr), J is flux (m/hr) and Am is total membrane surface area (m2)
- Specific energy demand (SED) is the direct indication of cost performance. It is defined as the energy spent per unit permeate volume. If membrane tank is separated from aeration tank as illustrated in Fig. 1 here, energy for mixed liquor circulation can be included in SED. The state of the art membrane demonstrated SED of slightly below 0.1 kWh/m3 permeate.