Membrane and Desalination Technologies - TCE Moodle Website

686 K. Mohanty and R. Ghosh
Fig. 16.12. Schematic representation of Zenon submerged membrane module (ZeeWeed 500).
Figure 16.12 describes the schematic representation of Zenon submerged membrane module
(ZeeWeed 500). ZeeWeed 500 membrane utilizes “Outside-In” flow, through a reinforced,
hollow-fiber membrane that has nominal and absolute pore sizes of 0.04 and 0.1 mm respectively
which excludes particulate matter including solids, bacteria, pathogens and certain
viruses. Zenon recommends some specific requirements like the need for vigorous coarse
bubbling and the advantage of loose fibres in the ZeeWeed 500 system. Mitsubishi has also
developed a submerged hollow fibre (Sterapore series) MBR with bubble injection to control
fouling.
Leonard et al. (68) studied the impact of gas–liquid two-phase flow on both permeate flux
and oxygen transfer rate in a submerged MBR. The flux enhancement was found to last for an
extended period even though the biomass concentration increased. This was due to the
permanent effect of the unsteadiness generated by the two-phase flow (see Fig. 16.13). Lee
et al. (69) studied the flux enhancement in a submerged membrane system fitted with a
stainless steel prefilter and reported that this system produced a constant permeate flux for
50 days of operation. The prefilter helped in reducing the cake resistance of the membrane.
The effect of the alignment of submerged hollow fibres has been looked at. Horizontal
alignment provided transverse flow for the bubbles whereas vertical alignment provided axial
flow for the bubbles. For single-phase liquid flow it has been reported that transverse flow
across hollow fibres gave better mass transfer than external axial flow (70).
Bouhabila et al. (71) has demonstrated that the combined use of gas sparging and periodic
backwashing increased permeate flux significantly without seriously increasing fouling
resistance. Gas sparging alone proved to be less efficient due to its limited effects on internal
fouling. It was reported that under optimal conditions for backwashing (15 s backwash every
5 min) along with gas sparging, the fouling resistance decreased by 3.5 fold. In a recent work,
a similar finding was reported by Posch and Schiewer (72).
Guibert et al. (73) has proposed a new technique of “air cycling” with air injected
sequentially on either side of a group of hollow fibre bundles instead of being injected
continuously. Under this strategy, the local bubbling intensity can be increased to a higher
value than for continuous bubbling and due to the density difference of aerated and unaerated
column of water. The efficiency of the process depends on the cycle frequency.