The permeability decline associated with particle deposition on the membrane surface is a major hurdle in membrane filtration. In traditional crossflow filtrations using pressure vessels and circulation pumps, membrane fouling could be controlled reasonably using high cross flow velocity, but excessive energy costs and negative impacts on microorganisms by pump shear stress were the concerns (Brockmann, 1996; Tardieu, 1998).
An alternative to crossflow filtration is the immersed (or submerged) membranes, where vacuum pressure is applied to obtain permeate while air scouring is performed to keep the membrane clean (Tajima, 1988; Yamamoto, 1989). It has been proven that air scouring is much more energy efficient than the traditional single-phase crossflow for membrane fouling control. In addition, unlike the circulation pump used for traditional crossflow filtration, air scouring does not produce excessive shear stress that disrupts biological floc. As a result, air sparging is used not only for immersed membranes, but also for the hollow fiber membranes housed in pressure vessel (Bérubé, 2006a).
In terms of cost, immersed membranes have advantages over crossflow membranes in both CAPEX (capital expenditure) and OPEX (operational expenditure). Due to the lack of pressure vessel (or housing) and the low operating pressure, system costs are generally low for a given permeate flow rate. As mentioned above, energy costs for aeration are also much lower than those for liquid circulation in crossflow filtration. As a result, vast majority, perhaps over 99%, of MBR capacities installed today are based on immersed membrane technologies.
Immersed modules tend to suffer from membrane fouling and clogging (or sludging) especially in MBR due to the high membrane packing density and high MLSS. Aeration plays a key role in mass transfer on membrane surface. It is crucial to design membrane module that allows efficient use of scouring air while maximizing mass transfer in the internal spaces of membrane module.
This section is dedicated to review the current understandings about the mechanisms of air scouring.
© Seong Hoon Yoon