Water purification for the laboratory

What are the key issues that technicians and laboratory staff must consider when specifying a water purification system for the laboratory?

A well-specified water purification system from a trusted and experienced supplier will provide an exceptionally high level of purity and, 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. With this goal on the horizon, how do technicians and laboratory staff achieve the right specification?

The first stage in the process is to ensure that everyone involved in the specification process fully understands the grades of water purity that have been standardised, the available methods of delivering them and the requirements of the laboratory itself. Having established the level of purity required, you then need to assess the volume needed on a regular basis. This is followed by the decision as to whether to install centralised or stand-alone units and this will be dictated by the overall costs and the operational demands of each option.

We need to consider each of the above stages in detail, beginning with water purity. There are three standardised grades, expressed in terms of conductivity, defined in the current standard, BS EN ISO 3696:1995 ‘Water for analytical laboratory use’. 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). There are several methods of delivering these grades and not all water purification systems offer the capability to deliver all three.

The practice of boiling water to create steam, thus leaving behind those contaminants with boiling points higher than that of water, is a hot, inefficient system for producing purified water, requiring frequent maintenance and high running costs. It has also been surpassed by newer technologies and is no longer the method that provides the highest purity. For example, on-site distillation units, as used by smaller laboratories, can only supply water typically to Grade 2 standards at best.

Another method of providing water purification is to connect disposable deionisation or ion exchange cartridges directly to a mains water supply. Each cartridge uses a mixture of resins to remove anionic and cationic contaminants from the feedwater, exchanging them with active hydrogen and hydroxyl ions, which combine to form water molecules. During usage, the capacity of the resin to exchange impurities and release active hydrogen and hydroxyl ions is gradually consumed with the resins changing colour as they become exhausted, to indicate when cartridges need changing. This method is 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 litres per day.

Depending on the quality of the feedwater available, reverse osmosis systems are able to provide large volumes of purified water. In this method, the water supply is fed under pressure into a module containing a semi-permeable membrane that is capable of removing up to 98% of inorganic ions, plus virtually all colloids, micro-organisms, endotoxins and organic macromolecules. The ever-increasing refinements in the construction of semi-permeable membrane elements have not only improved water quality but allowed pump speeds, and correspondingly pump pressures, to be significantly lowered, thus reducing energy consumption.

It is important to be aware, however, that although a stand-alone RO unit can produce a large volume of purified water it can only do so 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 reverse osmosis and deionisation.

Purity levels can be raised by degrees to the required specification by continually exposing the water supply to 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 electro-deionisation system (EDi), for secondary purification when fed with permeate from the RO system, producing water with a quality of greater than 10 M?.cm. EDi is a purification technology that uses a combination of ion-exchange membranes/resin and electricity to deionise water.

Deionisation can raise purity to Grade 1 level but there are additional measures that can enhance water quality further. 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.

While the measures detailed above can raise the purity of stand-alone reverse osmosis water purification to Grade 1 level, there are situations where it is more efficient and economical to install a centralised system. For example, if supplies are required at different locations a centralised system feeding a ring-main may be more appropriate, offering an unobtrusive and space-saving option that can deliver high volumes and high quality. In centralised systems, it may be possible to make 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.

If supplies are required at different locations a centralised system feeding a ring-main may be more appropriate, offering an unobtrusive and space-saving option that can deliver high volumes and high quality

Once you have considered the benefits of stand-alone or centralised water purification systems and the remarkable enhancements to both that have been achieved in refining the quality of semi-permeable membranes, it is time to choose and specify a system that will give you the right level of water quality and the most effective balance between cost and efficiency.

Having weighed up all the available options, you can now make the right specifications based on the available technology and the needs of your laboratory. We have already considered that it is essential to consider the quality of water needed but, to maximise efficiency, it is also essential to consider whether that quality is required throughout the laboratory, or only at a limited number of work areas. 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 typical specification error that occurs when end-users specify a system without the expert advice of a seasoned supplier such as Purite is oversizing, so it is important to specify equipment that can deliver only the volume of purified water that you need. Likewise, a realistic estimate must be made of the number of take-off 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 therefore unnecessarily expensive system.

Pre-treatment of the feed stream may be necessary to protect reverse osmosis membranes, especially in areas where the feedwater contains high levels of organic contamination, hardness and free chlorine, while maintenance can be streamlined by choosing a unit with parts that are quick and easy to maintain or replace.

To achieve the best specification it is advisable to work with a supplier who is willing to work with you on-site and help you to specify the best solution for your needs. Purite’s strength as a trusted and valuable supplier flows from its capacity not only to supply but to plan, specify and install a system, rather than delegate to outside contractors. With the right help and advice, an efficient, economical and reliable supply of purified water is available for your laboratory that will remain cost-effective throughout its lifespan.

Contact: 01844 217141 contactus@purite.com www.purite.com