In this process, discs equipped with two flat sheet membranes in both sides of each disc are rotating in feed to improve the membrane performance by producing shear stress on the membrane surface. During the Aqua Renaissance 90 project in Japan in later 80’s, the prototype systems equipped with 0.1 μm ceramic plate membranes were tested with anaerobic digester broth. The rotating disc module was encapsulated in a pressure vessel and permeate was obtained by pressure. Since the primary moving object was not waster, the energy cost of developing shear stress on the membrane surface was low. However, the benefit of the energy cost savings was offset by the capital cost increases created by the complexity of the rotating disc module.
Recently, the process had major improvements and commercialized as VRM® (Vacuum Rotation Membrane by Huber Technology Inc. as shown in Fig. 1 and 2.
- Ceramic membrane was replaced with less expensive polymeric membrane (PES)
- Vacuum filtration through the axis to eliminate the costly pressure vessel
- Air supply through the axis to scour membrane surface at a lower rotating speed at 0.9-2.5 rpm (Komesli, 2007;Melcer, 2009).
- SADm at typical condition is around 0.25 m3/m2/hr, which is substantially lower than other flat sheet membranes, but marginally higher than some hollow fiber membranes (Melcer, 2009).
The literature elucidating the hydrodynamics around the module are rare to date, but the rotation of the disc does not appear a major mechanism of shear stress generation unlike in the earlier rotating disc modules used in Aqua Renaissance 90 project. The maximum linear velocity in the edge of the disc is only 0.16 m/s, if discs with 3 m diameter rotate at 1 rpm. Moreover, when the discs with small spaces among them are rotating, the liquids in between the discs tend to move to the same direction, which makes relative linear velocity even lower. Therefore, scouring air appears a major mechanism of generating shear stress on membrane surface.
Other interesting feature is that the membranes are scoured by air only when it comes to the top of the axis since the air supplied through the two coarse bubble pipes move upward through the disc bundle. If approximately a quarter of the disc is within the aeration zone in each moment, the aeration frequency will be 15 seconds ON and 45 seconds OFF when discs revolve at 1rpm.
Due to the scarcity of literature, it is difficult to know actual field performance of rotating disc modules, but the small contribution of disc rotation to membrane scouring and the low effective aeration time suggest lower sustainable flux than the other flat sheet membranes run with more scouring air.
Variable intermittent suction cycles can be used depending on the membrane fouling potential in each application. Energy demand for the membrane rotation is negligible compared to that for aeration due to the low rotating speed. Although specific air demand (SADm) is about same as other membranes, aeration energy demand can be lower since air is supplied in the middle of the aeration basin at a low head pressure (<30kPa).
Fig. 1. VRM® membrane unit from Huber Technology Inc.; 1) Aeration tank or ﬁltration, 2) chamber, 3) Permeate discharge, 3) Scouring air inlet, 4) Membrane, 5) Membrane segment, 6) Hollow shaft, 7) Permeate collecting pipe.
Fig. 2. VRM® membrane unit from Huber Technology Inc.
© Seong Hoon Yoon