Simon Charlesworth, Laboratory Sales and Marketing Manager - Purite Ltd

An on demnd supply of purified water is crucial for the smooth operation of any laboratory, being used in processes ranging from glassware washing to critical analyses. For the required quality and quantity of water to be consistently available, it is essential that laboratory water purification systems are both correctly specified and regularly maintained.

Purified water in the laboratory
Purified water is in constant use in laboratories, for processes as routine as glassware washing and rinsing, through to critical applications in molecular biological buffers, growth media for microbiological work, and tissue culture solutions. Different applications require varying levels of water quality; for example, British Standard, BS EN ISO 3696, defines three measurable grades of water purity for use in defined laboratory applications, with specific requirements and test methods for pH, electrical conductivity, TOC, absorbance, residue after evaporation, and silica content.

Grade one is the most pure of the three BS defined grades, and grade three the lowest purity. While grade two or three water is adequate for general laboratory duties such as glassware cleaning, a higher level of purity is required for analytical experimentation and testing. A furth er degree of purity can also be achieved, known as ultrapure water (UPW), and this is required for critical laboratory applications such as AAS and cell culture.

Purification techniques
The methods of purifying water for laboratory use depend on how the water is to be utilised. Most systems generally include a pre-treatment package, including some form of base-exchange softening to remove hardness that would otherwise scale downstream membranes. Further protection is provided by passing the water through activated carbon, to remove free chlorine and organic contaminants, with any remaining particulates being removed by filtration before the pre- treated water enters a R everse Osmosis ( RO) module.

To achieve grade three water, the minimum level of purity required for general use in laboratories, Reverse Osmosis (RO) is commonly used. Reverse Osmosis is an effective purification process where a pre-treated water supply is fed 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, with almost 70% of the feed-water passing through the membrane as a purified permeate, with impurities being removed in a residual concentrate stream that is run to drain.

For grade two water, used in processes such as glassware cleaning, a combination of RO and deionisation (DI) is often used, with water making a single pass through the system. In practice, larger laboratories will often install a ringmain with grade two or three water being recirculated, with additional "polishing" at the point of use, if required.

A similar method of purification is used to achieve grade one water, for use in laboratory applications such as ion chromatography and clinical analyser feed, but rather than a single pass through the system, water is continually circulated and polished through the DI resin until the required level of purity is reached. Larger integrated systems often incorporate a Continuous- or Electro-Deionisation system (CDi or EDi), which acts as a polishing de-ioniser when fed with permeate from the Reverse Osmosis system, and can produce water with a quality of 10 to 15 megohm depending on flow rates and the quality of the feed water. While this is an effective method of purification for larger systems, for desktop use ion exchange resins in disposable cartridges provide a more economic and convenient solution. For applications where grade one water with enhanced microbial quality is required, the water undergoes additional processes, such as UV disinfection and sub-micron (typical 0.2 microns) filtration.

Where ultrapure water at 18.2 Mohm.cm is required, additional treatment process are employed to remove the few remaining ionic contaminants. This would typically include additional deionisation with high grade ion exchange resins, in combination with photo-oxidising UV and additional fine filtration .

The challenges of developing laboratory water purification systems
With many laboratory applications demanding an extremely high standard of purified water, the challenges in providing a suitable water purification system lie in combining high performance with compact design. The purified water must be of a quality to comply with BS EN ISO 3696, in order to be compatible with laboratory applications, and also be consistently available. Likewise, it needs to be accessible to technicians wherever in the lab it is needed, introducing the requirement for either a number of self contained ind ividual units at different positions throughout the laboratory or an extensive ringmain distribution system, integrated into a lab's design. With space a precious commodity in laboratory environments, either option needs to be as unobtrusive as possible while still offering the required quantity and quality of water.

How to choose the right system
The best way to ensure that the system you choose is the right solution for your laboratory is to think carefully about what your purified water requirements are, or will be. It is crucial to choose a supplier that can work closely with you to build a solution designed around your lab and your specific requirements, without you having to make compromises to accommodate a 'one size fits all' system.

Important points to consider include the quality of water needed; is the same high quality of water necessary throughout the lab, or are different purity levels needed for various applications? It may make sense to employ a number of systems of varying purification capabilities throughout the laboratory, accessible where each specified level of water quality is needed.

Also to be evaluated is the quantity of water needed, and, again, whether this requirement is consistent or if there are intermittent periods where extremely high volumes of water are required followed by a relative lull in usage? This analysis of the patterns of daily usage is important, as peaks and troughs in water requirements are not always considered, with technicians instead looking at total consumption levels, over a daily, weekly or mont hly period. While a system may be able to del iver this volume, it may not be able to make available the large volumes of water required during peak times, such as the filling cycle of a glass washing machine.

In the same way that a system needs to be large enough to del iver the required amount of purified water, it also shouldn't be any larger than necessary. Apart from the unnecessary space being taken up by an oversized system, performance can also be affected, with Reverse Osmosis potentially less efficient where the plant is only operational for relatively short periods. Likewise, diversity should be taken into account, with an estimate made of the likely number of points that will be in use at any one time. If it is assumed that all points will be in use at once, the result can be a hugely oversized system.

Another point to keep in mind when considering your purified water requirements, is the need to maintain and upgrade systems as easily as possible. Routine cleaning and maintenance are essen tial to achieving the highest levels of performance from any system, and in order to maximise productivity and avoid disruption, a system should be chosen that requires the lowest possible amount of downtime. The cost of consumables is another factor to consider, as systems that use high volumes of resins, chemicals and cleaning solutions can quickly become uneconomical.

Integrated solutions
The latest generation of water purification technology, from manufacturers such as Purite, offers a range of integrated solutions that require minimum space, but that also offer extremely high levels of performance and efficiency, in line with the relevant ISO quality standards. From small units that can sit on a worktop or be wall mounted, and provide small quantities of purified water, to extensive systems offering large volumes of water for routine use, a combination of systems can be put together to build a customised solution that works for you.

The importance of regular maintenance
Even the most suitably designed water purification system needs regular cleaning and maintenance in order for peak performance to be maintained. Simply plugging in a system and forgetting about it can have serious effects on a system's performance and ultimately on water quality. In addition, components such as RO membranes can fail prematurely if not adequately maintained, causing unexpected cost and downtime, while the service life of downstream deionisers can also be shortened through neglect, resulting in un-budgeted costs.

How to achieve the best from your system
A number of simple checks, if carried out regularly, can keep your purification system del ivering the quality level of water you require while minimising downtime and maximising service life. As well as regular membrane cleaning with specialised chemicals, components such as pre-filters should be regularly checked and replaced if necessary, as their performance directly affects the quality of purified water produced. Carbon pre-treatment is particularly important, as failure to replace or maintain this element can lead to organic contamination of the RO membrane, microbiological growth within the carbon itself, or chlorine breakthrough that will quickly and irreversibly destroy the RO membrane. Ion exchange resin cartridges or cylinders should be replaced or regenerated when the conductivity monitor within the system indicates that this is required. As contaminants are taken up by these resins during ion exchange, a gradual loss of deionising capacity occurs through use, however the drop-off in water quality as the resin becomes exhausted can be rapid.

Likewise, if UV disinfection is being utilised, UV lamps should also be changed as recommended by the manufacturer. While a UV lamp may still appear to be working, its disinfection performance will be greatly diminished if it is being used past its recommended replacement date, again having a negative affect on the quality of the water supplied.

As well as these maintenance measures, it is advisable to carry out regular water checks to make sure that equipment is maintaining its peak performance. This enables you to investigate quickly if performance drops rather than waiting for noticeably negative results or worse still complete shutdown of the equipment and resorting to emergency maintenance work. It is crucial for the best to be achieved from any system, that it is monitored regularly; if the purification equipment is sited some distance from the point of use, it is advisable that a remote ind icator or building management system be incorporated to continually check the equipment's performance.

Conclusion
A consistent quality and constant supply of purified water is essential to a range of laboratory techniques, from routine cleaning to critical analytical applications. Strict standards are in place for water quality, and systems need to be implemented to meet these and to cope as the standards evolve. A water purification solution designed for use in a laboratory needs to be carefully considered, specified and implemented for maximum efficiency and results, followed by regular and thorough cleaning and maintenance to ensure that the high level of water quality is retained. With a new generation of purification systems now combining performance with compact design, a suita ble solution is available for any laboratory's requirements.