Page 26 - Membranes for Industrial Wastewater Recovery and Re-Use

Introduction 7

operations within the factory are conducted so as, in this instance, to limit the
freshwater demand. Actual examples of demand management, reuse and
recycling (Table 1.5) demonstrate that the degree of sophistication of the
solution to achieve a significant cost benefit depends upon the existing state of
water management.
It is apparent from Tables 1.2 and 1.3 that the quality of water demanded by
industry varies considerably from one duty to another. There exist certain
determinants, such as the Silt Density Index or SDI (Section 2.4.3), which are of
critical importance for some applications but are meaningless in others.
Moreover, it is generally the case that the volume of water demanded varies
inversely with that of its purity across any one industrial sector. For example,
according to the figures in Table 1.1, about 20 times as much water - making up
around 90% of the total water demand of the plant - is used for cooling in power
generation as that required for boiler feed, which demands a “high-purity’’
water. Such water can only be produced through a combination of adsorptive
and membrane separation processes. Water for once-through, non-intrusive
cooling, on the other hand, may not be required to meet any specification based
on chemical and biological constituents, needing only to be below a certain
temperature.
For some industrial processes the quality of the discharged water does not
substantially differ from that of the feedwater. Cooling towers, for example,
concentrate the water as a result of the evaporative cooling process, but do not
add significantly, if at all, to the chemical loading rate, in terms of mass flow rate
of solutes, of the efllluent. For most industrial sectors, however, there is a
significant pollutant load resulting from their activity. As already stated, the
large temporal variation in effluent water quality can preclude water recovery
and reuse in many cases due to the high cost of treatment to produce water of a
reliable quality, particularly by the more established non-barrier technologies
where treatment process performance varies with hydraulic and/or pollutant
load. On the other hand, membrane processes, which can offer a highly selective
barrier to the water being processed, are far more robust to changes in feedwater
quality and can provide water of reliably high quality.

1.3 Membrane technology
Membrane processes are designed to carry out physical or physicochemical
separations. Although most membrane applications are water based, there also
exist gas-liquid and gas-gas separation processes, although these are more
recent developments and have not yet achieved widespread implementation. In
terms of membrane sales, the most important application by far is hemodialysis,
as carried out in kidney dialysis machines: almost half of all membrane sales
are accounted for by this one application. The development of membrane-based
bulk water and wastewater treatment processes, as defined in Table 1.6, is
nonetheless significant, since they offer three clear advantages over
conventional techniques: