Closing the loop
By Simon Judd, Chair in Membrane Technology, School of Water Sciences, Cranfield University
The often-stated motivation for recycling of wastewater is the increasing pressure on natural water resources. Reuse of wastewater conserves the supply of freshwater, and this presents clear advantages with respect to environmental protection.
More pragmatically, however, wastewater reuse tends to arise either from the exigencies of environmental legislation or from the related supply and discharge costs. The increase in the latter has been significant: between 1980 and 1995 the cost of water supply and discharge trebled in the UK water and increased by more than an order of magnitude in California.
Reuse of effluent is particularly attractive in cases where the quality of the supplied freshwater, predominantly mains water which has therefore been treated to potable quality standards, either exceeds that demanded by the industrial process or else demands further processing. This is very often the case, since the stringent microbial quality requirements of potable water are largely immaterial for a large number of industrial processes.
Identifying opportunities for the recovery of water for direct reuse without purification is best conducted by a thorough auditing of the water throughout an industrial installation. Analysis of water volumes can then be conducted by Water pinch analysis. Many CAD packages exist for such an analysis, and they are becoming increasingly sophisticated.
On the other hand, these analytical tools have not yet reached a level of sophistication where water purification unit processes can be incorporated, since the performance of such processes can rarely be reliably predicted when industrial effluent quality is subject to such wide process-related, diurnal and/or seasonal fluctuations.
Indeed, it is always more desirable to reduce water demand rather than recycle effluent, and general awareness of this has led to dramatic reductions in water use over the past 10 years within many industrial sectors.
The application of recycling, that is the reuse of water for the same or similar duty, normally relies on some suitable process technology for water purification. The wide fluctuations in industrial effluent quality coupled by the requirement for process water of reliable quality tend to favour the application of membrane processes, since these can provide permeate product water of a reliably high quality.
Membrane systems are already established for specific industrial processes where other resources are recovered in addition to the water, such as pulp solids in paper manufacturing and paint pigments in electrophoretic painting, but are generally considered too expensive for wastewater recycling for most industrial processes.
However, this may not remain the case. Advances in membrane technology and significant improvements in its efficiency, and so cost effectiveness, have greatly increased the competitiveness of recycling over discharge. Data from eleven world-wide case studies from a broad spectrum of industries1 reveal the pivotal role played by membrane processes for both open (recycled polished sewage effluent) and closed water recycling loops.
Throughputs for the installations vary from 140 (microprocessor fabrication) to 3750 m³ day-1 (power generation), with the whole spectrum of membrane technologies (reverse osmosis through to microfiltration) being employed. Payback times for these examples varies from 1.7 to 5.2 years for the MDF manufacture to power generation respectively, although examples exist of payback periods of as low as 8 months in cases such as electrophoretic paint recovery where substantial resource recovery exists in addition to the water.
Industrial effluent recycling is technically feasible, and the economic viability is likely to depend on imposed existing or projected regulatory restrictions on supply and discharge combined with the perceived or actual technical and financial risk.
While it is clear that economic profitability is required for any reuse scheme to be implemented it appears to be an insufficient driver in its own right in most cases. Instead legislative or water scarcity issues are generally cited as being the primary drivers, which then impact on economics.
Notwithstanding this, there are a growing number of examples of recycle applications where the initial capital expenditure has been recovered and the schemes have operated successfully over extended periods of time. It remains to be seen whether increasing freshwater scarcity leads to more widespread membrane technology-based industrial effluent recycling, and the extent to which the ensuing concentrate management issues are effectively addressed.
1Judd, S. J. and Jefferson, B. (2003) Membranes for Industrial Wastewater Recovery and Reuse. Elsevier Science Publishers Ltd, ISBN 1-85617-389-5.
To find out more about industrial water recycling at a meeting on the subject being held at Cranfield University on 4/5 March 2004 contact the Short Course Office on +44(0)1234754176,or
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