Physical processes achieve substantial clari®cation of
water. As such, they are reasonably eective in decreasing
the organic pollutant load of grey water prior
to reuse, since this is to some extent associated with
turbidity. The aesthetic quality of the product water is
thus increased and problems associated with downstream
disinfection encountered in the coarse ®ltration
systems substantially ameliorated. On the other hand,
simple ®ltration based on ®brous (cloth) or granular
depth ®lters presents no absolute barrier to suspended
matter, resulting in coliform breakthrough and a propensity
for solids' unloading whenever hydraulic shocks
occur.
Membrane systems oer a permanent barrier to suspended
particles greater than the size of membrane
material, which can range from 0.5 lm for micro®ltration
membranes down to molecular dimensions for reverse
osmosis. The treated water is thus generally
extremely low in turbidity below the limit of detection
for coliforms (Table 4). On the other hand, the energy
demand for membrane systems is substantially higher
than that for depth ®lters: the data shown in Table 4
refer to tubular MF/UF systems, the most commonly
used, which are operated at pressures up to 2.0 bar.
The key factor constraining the economic viability of
membrane systems is the fouling of the membrane surface
by pollutant species. This increases the hydraulic
resistance of the membrane, thereby commensurately increasing the energy demanded for membrane permeation
and/or decreasing the permeate ¯ux. Fouling can
be suppressed by operation at a lower membrane ¯ux, in
which case the membrane area requirement is increased,
and substantially removed by cleaning. Both increase
the overall process cost and cleaning also imparts an
undesirable chemical load on the waste stream. Fouling
can be such that the ¯ux declines by up to 90% after just
1 h of operation