Page 22 - Membranes for Industrial Wastewater Recovery and Re-Use
P. 22
Introduc~troti 3
repurification plant and, more directly, on the reuse options. Hence, reclaimed
greywater for reuse in buildings is invariably employed exclusively for toilet
flushing, where the risk to human health through exposure to the contaminated
water is considered minimal. Large-scale municipal wastewater reclamation and
reuse schemes are predominantly aquifer recharge, such as the well-established,
flagship Water Factory 2 1 plant in Orange County, California (Wesner, 1987) or
turf grass irrigation schemes.
In the case of industrial water recycling the issue is notionally more
straightforward. As long as recycling can reliably and cost-effectively provide
water of a quality appropriate to its designated use, then there should be no
barrier to its introduction and deposition of the existing freshwater supply. Of
course, and as already mentioned, the cost benefit may be largely or wholly
determined by statutory requirements: there are, for example, a number of power
plants throughout the world that have a zero liquid discharge imposed upon
their operation (Section 5.3). Under such extreme conditions, wastewater
recovery and reuse is no longer an option but an absolute necessity - and
inevitably incurs cost penalties due to solid waste generation and energy
expenditure. In other cases, the decision to recycle is motivated entirely by
economics; the total cost of purification to provide a water of a quality
commensurate with or better than that of the freshwater supply is perceived as
being less than the total cost of freshwater and waste discharge. Perception in
this instance relates entirely to the acceptance of the technology, rather than to
the use of water employed for duties considered unsavoury.
Given the apparently less restrictive constraints imposed on the recycling of
industrial wastewater compared to that of domestic wastewater, it is perhaps
surprising that the former has received far more attention, has attained a much
higher profile and has achieved more widespread implementation than the
latter. There exist a number of domestic and municipal wastewater recycling
schemes, from single households through to large buildings, as well as municipal
or industrial wastewater reclamation for irrigation. Similarly, a number of
municipal wastewater reclamation schemes exist whereby the polished effluent
is reused for cooling, which represents around two-thirds of all industrial water
use, or other activities demanding low-grade process water such as for washing
or transporting. Direct “closed-loop” industrial water recycling, whilst limited in
technical and economic viability in many instances, is nonetheless attracting
greater interest and being more widely applied. Indeed, it is already established
practice for specific industrial processes where other resources are recovered in
addition to the water, such as pulp solids in paper manufacturing (Sections 3.2.5
and 5.4-5.6) and paint pigments (Section 5.10) in electrophoretic painting.
Two factors mitigate against widespread “closed-loop” industrial effluent
reuse. Firstly, most industrial processes involve a number of individual
operations that give rise to wastewaters of a certain compositional range. These
individual effluent streams are generally combined to give a wastewater whose
resultant temporal variation in quality is immense, representing a significant
challenge to any treatment process that is to provide water of a reliably high
quality. Secondly, it is invariably the case that conventional sewage treatment