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Not only boiler feed, but a wide range of other applications require a high degree of influent water quality, much purer than available from mains water distribution. In the electronics industry, components (especially semi-conductor materials)
must be washed in water that is sufficiently pure as to leave no residue behind after the washing process, even from evaporated droplets. The pharmaceutical industry needs pure water as a base for growth cultures, as a constituent of parenteral drugs or many other types of medicine, which is guaranteed to be free of pathogens or any other micro-organisms, which occur in sizes well below 0.1um – a condition that relates especially to the biotechnological component. The cosmetic and toiletries sector uses water as an ingredient for the majority of its products, and demands absence of colour and dissolved materials as important conditions in its feed water. Similarly, many parts of the food and beverage industries demand highly pure water as an ingredient, as do many sectors of the chemicals industry.
The qualities that determine the suitability of a feed water include the absence of suspended particulate material, determined by microscopic examination and/or membrane filtration, and the absence of dissolved material, especially organic colour and mineral salts, determined by visual observation and electrical resistivity measurements.
The optimum process for ultra-pure water production combines membrane separations and deionization processes, and the combination can be tailored to meet the required purity of the product water. Microfiltration would be the first stage, to
remove much of the residual suspended material and most bacteria. Ultrafiltration can then be used to remove viruses and other pathogens, large organic molecules, colour and other colloidal material. A deionization process using acid-base ion
exchange can finally be used to remove all remaining dissolved material, although a reverse osmosis process can also do this, and a combination of these last two processes may provide the best solution.