Page 72 - Membranes for Industrial Wastewater Recovery and Re-Use

52 Membranesfor Industria2 Wastewater Recovery and Re-use

after allowance was made for differing mean particle size of the cake layer and
the bulk suspension, which required experimental verification. A similar
limitation appears to apply to the Song model (Song and Elimelech, 1995): an
excellent fit with theoretically predicted flux was obtained for CFMF of 0.06 pm
homodispersed colloidal silica once the specific cake resistance R’, and the cake
concentration C, had been surmised (Wang and Song, 1999).
It is generally the case that the modified concentration polarisation
models developed to define dynamic and steady-state behaviour during
microfiltration are very sensitive to key parameters pertaining to particle
transport and the hydraulic resistance of the cake. These parameters are either
only calculable for highly idealised systems or else must be determined
empirically. For real feedwaters, which may be physically, chemically and
biologically complex, and the useful employment of these models for predictive
purposes is probably restricted to defining trends. Even then, there are
pronounced differences in reported data. For example for a single matrix of
bovine blood, flux dependency on cross-flow appears to vary from U0.47 to direct
proportionality (Cheryan, 1998). Moreover, there appears to be a paucity of
mechanistic information on fouling and dynamic behaviour of key
heterogeneous matrices, such as natural organic matter (NOM) in surface
waters or biomass in membrane bioreactor liquors, that could be used to form the
basis of a predictive model.

2.4 Process design and operation
Key elements of process design that contribute to costs are:

e staging,
e power consumption,
0 fouling and pre-treatment, and
e backflushing and cleaning.

Staging is of critical importance in large-scale dense membrane processes, i.e.
reverse osmosis, nanofiltration and electrodialysis. Fouling is ubiquitous
throughout the entire gamut of membrane technologies, but pretreatment to
suppress or ameliorate fouling is only routinely practised in dense membrane
processes. Backwashing is always carried out when the module design permits
this, and cleaning is an essential part ofmembrane plant operation.

2.4.1 Staging
For most dense membrane processes the conversion of feed into product is limited
either by the membrane area or the rate of extraction attainable by passage
through a single module. It is for this reason that most reverse osmosis and
electrodialysis technologies employ staging, the use of sequential stages to
produce more product than that attainable by a single passage.