Water Purification

No other compound is so widely used in the laboratory, but is your water of sufficient quality? Here, Simon Charlesworth guides us through the process of water purification.

Purified water is an essential commodity in almost all laboratories, being used for tasks as wide ranging as the washing of glassware, formulation of buffers, stains and reagents, and in specialised processes such as high performance liquid chromatography and atomic absorption spectrophotometry. In each case, there is likely to be a different requirement for both the quality and quantity of water needed on demand, from a few litres to many hundreds of litres per day.

Before looking at the various purification technologies that are available it is worth noting that water purity is governed by the BS EN ISO 3696 standard, which essentially defines three grades of purity for laboratory applications. Grade 1 is the highest level of purity, of between 10 and 18.2MΩ.cm with Grade 2 being between 1 and 10MΩ.cm, and grade 3 being between 5 and 1µS/cm.

Traditionally, many smaller laboratories have either purchased bottled supplies of distilled water or used a distillation unit to produce a supply on-site. Distillation is, however, a relatively slow process, so purified water cannot be produced on demand, and uses a disproportionate level of energy - typically, around one kilowatt of power is required for each litre of water produced. Perhaps as importantly, distillation can only produce water of a certain quality, normally to 1.0MΩ.cm at best, and requires careful storage to ensure that it remains sterile. 

In addition, distillate can frequently be contaminated with impurities such as silica, carbon dioxide and organic compounds, all of which can affect the performance of subsequent laboratory procedures, while distillation stills used in hard water areas require regular de-scaling and cleaning or some form of additional softening equipment.

One extremely cost effective option for producing small volumes of purified water on demand is the use of deionisation or ion exchange cartridges, which are connected to a normal mains water supply via a tap or stopcock. Deionisation cartridges typically provide flow rates of between 30 and 60 litres per hour, can generate water of a purity better than 1µS/cm, and are easy to fit and use as they operate solely under water pressure, so no electrical supply is required. 

Each cartridge incorporates specially developed colour-change resin beads that effectively remove anionic and cationic contaminants from the feedwater, exchanging them with active hydrogen and hydroxyl ions, which combine to form water molecules.  Over time, the active hydrogen and hydroxyl ions will be gradually consumed as contaminants are exchanged. When all the active sites are exhausted the resins will need to be replaced or recharged and this is indicated by a change in resin colour, from blue to brown. In some cartridge systems a simple display window is built into the cartridge to make it easy for users to check the status of the resin, while others feature a two window display showing when water purity falls to 1µS/cm and then to 10µS/cm, at which point the cartridge is considered exhausted.

Deionisation cartridges using high purity resins are a simple and cost effective method of producing purified water to a consistent level whenever it is required and in low volumes.  It should, however, be noted that they will only remove charged ions and can become fouled if the feedwater contains a large concentration of dissolved organic materials, and may therefore require additional filtration. 

In laboratory applications where larger volumes of water are required or where high purity water up to its theoretical maximum of 18.2MΩ.cm is required then specialised reverse osmosis (RO) systems are normally used. These are often supplied as self contained wall or bench mounted units, although they can be floor standing in larger versions; additionally, they can be configured as part of centralised ring main systems with multiple take-off points. In each case, the primary RO unit can be supplied with complementary technologies such as pre-treatment using base-exchange softening and activated carbon filters, with deionisation and photo-oxidising ultraviolet light being used for secondary polishing and disinfection.

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 feedwater passing through the membrane as a purified permeate, and impurities being removed in a residual concentrate stream that is run to drain.

Although extremely effective as a purification technology, the reverse osmosis process generally operates relatively slowly and RO systems are therefore normally supplied with separate pure water storage tanks. Similarly, depending on the nature of the feedwater it may be necessary to pre-treat the feed stream to protect the RO membrane, especially in areas where feedwater has high levels of organic contamination, hardness and free chlorine. 

A stand-alone RO unit can produce a level of purity that meets Grade 3 standards.  For higher levels of purity, a combination of RO and deionisation is often used, with water making a single pass through the system. In practice, larger laboratories will often install a ringmain with Grade 2 or 3 water being recirculated, with additional polishing at the point of use, if required.

A similar method of purification is used to achieve Grade 1 water, but rather than a single pass through the system, water is continually circulated and purified through the deionisation resin until the required level of purity is reached. Larger integrated systems often incorporate an electro-deionisation system (Edi), which provides second level purification when fed with permeate from the reverse osmosis system, producing water with a quality of 10 to 15MΩ.cm depending on flow rates and the quality of the feed water.

For applications where Grade I water with enhanced microbial quality is required, the RO purified water undergoes additional processes, such as UV photo-oxidisation at either 185 or 254nm and sub-micron filtration between 0.2 and 0.05μm to remove deactivated bacteria and fine particles. UV at 185nm can be used to cleave chemical bonds and, when used in conjunction with ion exchange, can produce a feed-stream with extremely high resistivity and low TOC (total organic carbon). Similarly, UV photo-oxidisation at 254nm produces a strong disinfection effect, significantly reducing levels of bacteria.

As most laboratory applications require an extremely high standard of purified water, the challenges in providing a suitable water purification system lie in combining high unit performance with compact design. The purified feed-stream normally has to comply with BS EN ISO 3696 and must also be available on demand, wherever in the lab it is needed. This can mean that one or more self contained units need to be positioned at different locations throughout the laboratory or a centralised system feeding a ringmain distribution network is required. With space being a valuable commodity in most laboratories, the chosen water purification solution needs to be as unobtrusive as possible while still offering the required quantity and quality of water.

In addition, it is essential to consider the quality of water needed and if the same quality of water will be required throughout the laboratory, or whether different purity levels are needed in each work area. Similarly, the quantity of water needed should be considered based on an analysis of the patterns of daily usage, as peaks and troughs in water requirements are often overlooked in favour of total consumption levels, over a daily, weekly or monthly period. It should be remembered that although an RO system may be able to deliver the total volume of purified water required, it may not be able to generate the 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 deliver 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 RO being 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 an oversized system.

A further factor to consider is the need to maintain systems as easily as possible. Routine cleaning and maintenance are essential to achieve consistently 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.

Ultimately, the process of selecting and using the best water purification technology should not be any more complex than that for any other item of high value laboratory equipment. Perhaps the biggest decision, once factors such as required quality, quantity and choice of supplier have been determined, is the level of complexity required between the use of a stand alone or centralised ringmain based system.

In many instances, a centralised system can prove to be a cost effective solution, in terms of both capital and operating cost, with a number of suppliers being able to provide systems that are based on modular design and construction yet can be easily customised to suit the needs of individual laboratories.

Whatever the solution, the latest water purification technologies offer the flexibility, reliability and performance to provide laboratories with an essential commodity that supports and enhances the efficiency of their day to day operations.

Simon Charlesworth. Simon is Sales and Marketing Manager – Laboratory – UK and International at Purite. He has a total of 12 years experience in the water treatment industry.