Brian Stother, Managing Director at Purite, describes the technologies used to provide high purity water in hospital laboratories.

The use of purified water is vital for the successful operation of many hospital departments, in particular, infection control and pathology laboratories, where water is used both for cleaning and disinfection, and as the basis for making up cell cultures, broths and reagents.  In each case, the systems used for generating purified water have to function reliably and efficiently, producing water to rigorous standards and qualities of up to 18.2MΩ.cm, yet be simple to setup and use. Typically, a water purification system will be built around a reverse osmosis (RO) filtration unit.  This takes potable water, normally from a mains supply, and feeds it under pressure into a module containing a semi-permeable membrane.  The membrane removes a high proportion of impurities, including up to 98% of inorganic ions, together with virtually all colloids, micro-organisms, endotoxins and macromolecules; almost 50% of the feed-water passes through the membrane as a purified permeate, with impurities being removed in a residual concentrate stream that is run to drain.

Larger or high volume RO systems generally include a pre-treatment package designed to meet the characteristics of the feed-water, especially in areas with hard water or high levels of organic contamination. By comparison, smaller stand alone laboratory units are generally fitted with carbon cartridges for the removal of chlorine.  This equipment can also include a base-exchange softener to remove hardness that would otherwise create scale in downstream membranes, plus activated carbon filters to remove free chlorine and organic contaminants, with any remaining particulates being removed by a fine filter before the pre-treated water enters the RO unit.

Although the latest RO systems are extremely sophisticated and efficient, they are only capable of producing water to a quality specified as Grade 3 (up to 1.0MΩ.cm) under the relevant BS EN ISO 3696 standards.  For higher levels of purity, to Grade 2 (up to 10.0MΩ.cm) for applications such as glassware cleaning, or Grade 1 (up to 18.2MΩ.cm), for applications such as ion chromatography and clinical analyser feed, it is then necessary to introduce further process operations. These include a deionisation stage, introduced after the RO membrane, with water making a single pass through the system, while for high qualities a recirculation and polishing stage is added, with water being continually circulated through the deionisation resin until the required level of purity is reached. Additionally, processes such as UV disinfection and sub-micron (typically 0.2 microns) filtration can also be used where Grade 1 water with enhanced microbial quality is required.

When specifying the most suitable system, although the needs of laboratory departments such as infection control, pathology and chemistry may vary in terms of the water qualities and qualities that are required, the same key criteria need to be considered. For example, there is a choice between a number of independent self-contained water purification units at different locations throughout a laboratory, or a ring main distribution system, which should ideally be considered at the time of laboratory design and build.  In practice, smaller laboratories will generally use bench-top units, while larger laboratories will install a ring main with Grade 2 or 3 water being recirculated, with additional polishing at the point of use, if required.

Most suppliers will be able to offer standard water purification units, while others offer customer design and build services, that include both the water purification system and the associated pipework and services.
Similarly, it is important to consider factors such as ease of use and maintenance.  These are generally simplified by integrated microprocessor control, linked to various system monitoring sensors and alarms.  It should be noted that, regardless of supplier, routine cleaning and maintenance are essential if consistent levels of performance are to be achieved; systems that require minimal downtime are therefore preferable. The cost of consumables should also be considered, as systems that use high volumes of resins, chemicals and cleaning solutions can quickly become uneconomical.

Finally, it is important to assess laboratory water consumption and to establish if the required volume of purified water is likely to be consistent or if water demand will fluctuate. This analysis of the pattern of daily usage is essential if a system is to be specified correctly, as all too often the only factor that is considered is the total consumption level, over a daily, weekly or monthly period; as a result, systems can easily be over sized and used at less than optimum capacity for the majority of the time.

A typical example of how the latest purification technology can benefit a laboratory can be seen at pathology department a leading NHS trust, which recently installed a self-contained water purification system as part of an ongoing development programme to improve the performance of its blood analyser test units. The blood analyser automatically performs a range of clinical tests including complete blood cell counts, detection of acids or viruses and quantification of glucose and depends for the accuracy and repeatability of its test results on high quality water.  This is used for sterilising cuvetts, probes, dilution and reaction cups, and onboard probes within the analyser. The system chosen by the laboratory uses a Purite Reverse Osmosis unit to remove up to 95% of dissolved Calcium, Magnesium and Sodium salts, as well as more than 99% of bacteria; an exchangeable deionisation cylinder is also incorporated to polish the water further and remove the remaining 5% of residual salts. This ensures that the water quality is consistently in excess of Grade 1. In addition, a separate ultra-violet disinfection unit is incorporated in the ring main distribution system to ensure that disease-causing micro-organisms such as E. coli are neutralised. The system is completed by a 0.2µm bacteria filter to remove fine particulates and microbial contaminants.

Installations such as the one above highlight the importance that purified water plays in hospital laboratory departments.  Such a critical resource demands that purification systems meet, and can operate consistently to, the highest standards, ensuring a consistent supply of water efficiently, reliably and at realistic cost, and enabling laboratory staff to concentrate on the critical aspects of their jobs.