Mark Bosley explains all you need to know about choosing and operating a reverse osmosis water purification system
Water is one of the most important raw materials used in the laboratory. Modern reverse osmosis water purification technology can provide a reliable source of pure and ultrapure water, but the best quality and value for money requires care in specification, operation and maintenance.
The chemical composition of raw water even when drawn direct from mains supplies can vary considerably in terms of the quantity and the variety of dissolved minerals, particulate matter and organics. Water contamination comes from a number of different sources, and the importance of eliminating these different types of contaminant will vary depending on the specific equipment and processes in use in the lab.
Manufacturers of laboratory water purification equipment typically design their systems to meet one of the established European and international standards for water quality, such as ISO 3696 or ASTM D1193-06. The ISO standard defines three basic categories of water: Grade 3, or RO water, for general use, such as washing glassware; Grade 2, or deionised water, for most standard applications; and Grade 1, or ultrapure water, for critical applications where a high level of purity is required. The ASTM standard categorises water from Type IV, for general use, to Type I for very high purity applications.
The required level of purity in the standards is based on the measurement approach used to determine the level of these different types of contaminants. Ions increase the ability of the water to conduct electricity, for example, so are measured by determining the conductivity of the water. In purer water, due to the lack of ions, its inverse, resistivity, is used. While tap water may have a conductivity of 500 microsiemens/cm, some laboratory applications may call for a conductivity of 0.055 microsiemens/cm (a resistivity of 18MΩ-cm).
Laboratory water purification systems make use of a number of different technologies, or combinations of technologies, in their operation. One of the most effective modern approaches is reverse osmosis (RO), which uses a semipermeable membrane to remove particles. The best RO systems can remove more than 98% of dissolved mineral content and over 99% of bacteria from the feed water supply. RO systems generally include a pre-treatment package designed to meet the characteristics of the feed water. Typically, this equipment includes a base-exchange softener to remove hardness that would otherwise scale the membranes. Further protection is provided by passing the water through 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 plant.
Selection criteria
When selecting an RO system, labs need to think hard about their water needs, since both volume and quality requirements have a significant impact on the cost and design of the most appropriate system. There is little point using expensive 18MΩ-cm water when a lower grade would be sufficient for the application in question.
One common polishing approach uses deionisation, an excellent method of producing up to 18 MΩ-cm. An RO membrane is used for the main purification process, and this water is then passed through a disposable deionisation cylinder or cartridge before final use. These cartridges or cylinders use a mixture of resins to remove anionic and cationic contaminants from the feed water, exchanging them with active hydrogen and hydroxyl ions, which combine to form water molecules.
Some laboratory processes may be particularly sensitive to specific forms of contamination, requiring even higher levels of purity than those defined by the general standards. In these cases, laboratories may need to install additional equipment to produce water at an appropriate level of purity.
As well as determining the size of the purification system required, the lab’s demand patterns will affect its configuration. Central systems use a single, large purification plant, with water distributed through pipework to the entire laboratory. This approach is most appropriate when high volumes of a consistent grade of water are required. Point of use (POU) systems, by contrast, use smaller floor, wall or bench mounted equipment to supply the specific water requirements of each lab. This approach is useful for lower volume applications, or where some activities have highly specific water purity requirements. Point of use purification systems must also compete with other equipment for valuable lab space, however.
Between these two extremes, a number of alternative configurations are possible, for example a ‘floor by floor’ approach that uses smaller versions of the centralised systems distributing water locally via pipework, or the installation of point of use polishing equipment to generate higher purity water for specific applications.
Operation and maintenance
As soon as water leaves the purification system, it is exposed to potential sources of contamination. Chemicals, particulates, gases and bacteria can enter from the air in the laboratory or from containers used to store or transport the water to the point of use. As a result, proper selection of equipment and correct operating procedures are every bit as important to the final quality of the water as the technology used to produce it. It is surprisingly common, for example, to see technicians storing or transporting the purified water in open plastic containers.
Storing water for long periods after treatment increases the risk of contamination, so while it is important for labs to size their purification system to meet both current and expected future water needs, care should be taken not to significantly oversize the system. Where demand variability is high, it can be useful to select a system that features built-in recirculation to maintain the quality of water stored in internal tanks.
Even the best water purification systems will only perform well when supported by routine cleaning/disinfection and maintenance, and so to maximise efficiency labs should look for equipment that is straightforward to maintain, with easy to change consumable parts. Modern systems have made major strides in the area of usability. They may, for example, employ the use of colour indicators in ion exchange systems that provide a simple but clear visual indication that it is time to change the resin, or feature digital displays to indicate flow rates, overall water quality, specific contaminant levels and the condition of consumables. On-board data logging capabilities are also built into many higher-end systems, allowing operations and maintenance staff to monitor their performance over time.
Regular cleaning of RO membranes is relatively straightforward and is typically carried out using specialised cleaning solutions. Acid based cleaners are used to remove scale, and alkaline based solutions to remove organic matter. If required, special chemicals can be used for disinfecting, but should only be added once all scale and organic matter has been removed. Solutions are simply circulated through the RO system and tank, and then flushed to drain.
The cost of consumables is also an important consideration when comparing different options, as systems that use high volumes of resins, chemicals and cleaning solutions can quickly become uneconomical. Energy costs can vary considerably too, with the better systems able to save energy by putting pumps into standby mode when there is sufficient water in on-board storage tanks to meet current demand.
Laboratories will need to decide if they want to handle routine maintenance themselves, or enter a service agreement with their chosen provider. Whichever option they select, they will need to ensure they have service arrangements in place to suit their needs, for example ensuring that support is available overnight or at weekends to ensure continuity of supply.
Mark Bosley is with Suez Water Purification Systems.
