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Clear Solutions

Reverse osmosis systems offer a highly efficient method of water purification for laboratories, if the right specification and maintenance factors are considered

When specifying a water purification system for the laboratory, there are a number of issues to consider. What level of purity is required? How much water do you need? What are the costs and operational demands of maintenance? The water purification system that arguably delivers the most options for users is reverse osmosis (RO), but finding the right answers to the questions above will determine the ultimate level of efficiency to be gained from using the most recent technology.

Firstly, we need to look at the grades of water purity that have been standardised and the available methods of delivering them. There are three grades defined in the current standard BS EN ISO 3696: 1995 ‘Water for analytical laboratory use’, and expressed in terms of conductivity, Grade 1 represents the highest level of purity, 0.01mS/m (0.1μs/cm), with Grade 2 being 0.1mS/cm (1μs/cm) and Grade 3 at a level of 0.5mS/m (5μs/cm).

Not all water purification systems can deliver all three of these grades. For example, distilled water or on-site distillation units, as used by smaller laboratories, can only supply water typically to Grade 2 standards at best. Disposable deionisation or ion exchange cartridges, which can be connected directly to a mains water supply, are capable of delivering a Grade 1 flow of purified water, but can be uneconomical if the feedwater contains a high level of dissolved solids and the demand for purified water is more than 10 to 20 litres per day. However, RO systems are, depending on the quality of the feedwater, able to provide typically Grade 3 purity purified water, in large volumes, from a water supply fed under pressure into a module containing a semi-permeable membrane, which removes up to 98 per cent of inorganic ions, plus virtually all colloids, micro-organisms, endotoxins and organic macromolecules.

Reverse osmosis is therefore an excellent option for laboratory use, but, having determined that you require the high volume and purity that RO can provide, there are still a number of important factors to consider when specifying your water purification system. For example, how do you choose between stand-alone and centralised systems? Are there any recent technological developments that you should be aware of that can improve both water quality and laboratory productivity? And how do you select a system that satisfies your regular requirements without making excessive financial demands on installation, maintenance and running costs?

Stand-Alone versus Centralised Systems

It is important to be aware that a stand-alone RO unit can produce a large volume, but only at a level of purity that meets Grade 3 standards. It is, however, possible to achieve higher levels of purity with a stand-alone unit by employing a combination of RO and deionisation.

To achieve the purity of Grade 1 standard, the product water (permeate) is either continually circulated or treated on a single cycle through a bed of deionisation resin until the required level of purity is reached. As an alternative to resin based deionisation systems, larger integrated laboratory systems can incorporate an electrodeionisation system (EDi) for secondary purification when fed with permeate from the RO system, producing water with a quality of greater than 10MΩ cm. EDI is a purification technology that uses a combination of ion-exchange membranes/resin and electricity to deionise water.

There are further measures that can be taken to enhance the quality of supplies delivered by stand-alone systems. For example, where Grade 1 water with enhanced microbial quality is required, the RO/deionised purified water is further processed using UV irradiation at 254nm and sub-micron filtration between 0.2 and 0.05 microns to remove bacteria and fine particulate matter. However, although the selection of the deionisation cartridges is relatively straightforward, the specification of RO systems that can achieve these enhanced levels of performance can be complex if optimum levels of performance, energy efficiency and operating costs are to be achieved.

Even though there is potential to raise the quality of purified water supplies delivered by stand-alone units, there are situations where it is more efficient and economical to install a centralised system. For example, as most purification systems will have to comply with BS EN ISO 3696 and provide water on demand wherever they are needed throughout the lab, the nature of the work may dictate that one or more self-contained units need to be positioned at different locations. Space being a valuable commodity in most laboratories, the chosen water purification solution needs to be as unobtrusive as possible while still delivering the required quantity and quality of water. In these situations, a centralised system feeding a ring-main may be more appropriate.

Semi-Permeable Membrane Elements

Whether you choose a stand-alone or a centralised system, there is one aspect of RO water purification technology that is continually being enhanced to the benefit of customers: the ever-increasing refinements in the construction of semi-permeable membrane elements. For example, the many advances made in membrane technology have allowed pump speeds, and correspondingly pump pressures to be significantly lowered, thus reducing energy levels. In centralised systems, it may be possible to make further gains in efficiency if pumps are linked to variable speed drives, enabling the speed of each pump to be matched exactly to the output demands of the process and water treatment system.

As a result, the feed pressure, compared with a traditional high rejection RO element, has been significantly reduced, with lower fouling potential and less pressure drop, while flow rates have increased dramatically. This also means that the operating life of many applications can be significantly extended, while the creation of a far higher active membrane surface area has given designers the option to reduce the total number of modules in larger reverse osmosis systems and reduce the need for repairs and maintenance.

Getting the Specification Right

Having considered the benefits of stand-alone or centralised water purification systems and the remarkable results that have been achieved in refining the quality of semi-permeable membranes, it is important to consider the ways in which configuring the system for your needs can determine its overall efficiency.

It is essential to consider the quality of water needed. It is also crucial to consider whether that quality is required throughout the laboratory, or if different levels of purity are needed in each work area. Similarly, the volume of water needed should be analysed based on the patterns of daily use to highlight peaks and troughs in water requirements over extended periods.

A common cause of inefficiency in water purification systems is oversizing, so it is important to specify equipment that can deliver only the volume of purified water that you need. Oversized systems require extra space, greater expense and, because RO is potentially less efficient when the laboratory is only operational for short periods, can suffer degraded performance. Likewise, a realistic estimate must be made of the number of takeoff points that will be in use at any one time; if it is simply assumed at the installation stage that all points will be in use at once, the result can be a dramatically oversized and expensive system.

Depending on the nature of the raw water, it may be necessary to pre-treat the feed stream to protect the RO membrane, especially in areas where the feedwater contains high levels of organic contamination, hardness and free chlorine. It is also important to consider the fact that all water purification systems require routine cleaning and maintenance to ensure consistent levels of performance and reliability. The key is to choose a unit that is quick and easy to maintain with easyto- change consumable parts. Similarly, the cost of consumables should be taken into account, as systems that use high volumes of resins, chemicals and cleaning solutions can quickly become uneconomical. It should be remembered that, although an RO system may be able to deliver the total volume of water required over time, it may not be able to meet sudden surges in demand, so sufficient purified water storage facilities should also be considered.

Conclusion

To help you make the right decisions regarding specification and maintenance factors from the outset, choose a supplier who is willing to work with you on-site and help you to specify the best solution for your needs. A well-specified system, coupled with on-going technical support from an experienced supplier, will ensure that your laboratory receives an efficient, economical and reliable supply of purified water.


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Mark Bosley is Business Support Divisional Manager at Purite, where he has worked for over 15 years. He heads up the company’s Technical and Industrial/Laboratory Systems contracting. Mark has over 30 years’ experience in the water treatment industry. This has included work in the development of both stand-alone laboratory water purification units and the design of specialised purified water systems for a variety of market sectors, including brewing and distilling, food processing, healthcare and pharmaceuticals. Email: contactus@purite.com
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