Submerged Configuration

A special configuration of a two-end module is the submerged mode because a cross flow near membrane surface is applied without producing an additional stream (Fig. 12.8b). To minimize the building of a sludge cake layer and to avoid the high energy cost for high turbulence cross flow, a submerged system is used. The membranes are immersed directly in the activated sludge tank, where intensified aeration produces cross flow at the membrane surface (see Fig. 12.6b). Investigations have been performed on the influence of shear stress induced by bubbles (see Section 12.3.5). Permeate is removed by vacuum at transmembrane pressures of about 0.05 bar to 0.6 bar (Gunder 1999; ATV-DVWK 2002b).

Transmembrane pressure is remarkably lower than 1 bar because of the drop in pressure which results from the water column above the submerged membrane. The rejected biomass remains in the bioreactor and the purified water passes through the membranes, usually from the outside in. Hollow fibers are flexible and move slightly to reduce gel layer formation because they are slightly longer than the module length and are therefore free to move (see Fig. 12.8b).

One significant characteristic of the submerged system is its operation in quasi steady state, in contrast to dead-end mode, and its higher frequency of back washing. Compared to the cross-flow configuration, there is less energy used (per m3 permeate) but a greater membrane surface area is needed because of the low attainable fluxes in submerged systems. Recently, submerged systems have become highly significant in the field of aerobic biological wastewater treatment. The advantages and disadvantages of dead-end and submerged mode are summarized in Table 12.5.

Enhanced permeate flux was proven to be a result of two-phase flow. Gas-sparged ultrafiltration with tubular membranes has been investigated experimentally with different suspensions as well as with river water (Chang and Fane 2000; Cabbasud et al. 2001) although computational fluid dynamics (CFD) were used to model this two-phase flow conditions (Taha and Cui 2002).

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