Purified water

Published: 13-May-2004

Mark Bosley, development manager for Purite, considers the options for purified water

Water is an essential raw material for a wide range of cleanroom applications, being used in low and high volume production operations and for research and development. In many applications, such as semiconductor or pharmaceutical manufacturing, the effective removal of particles, suspended colloidal silica, total organic carbons and other contaminants is essential for optimum product quality and can only be achieved using Ultra Pure Water (UPW) with a theoretical maximum resistivity of up to 18.2M?.cm. UPW plants are, however, relatively complex, have a high capital cost and can be expensive to operate and maintain. Perhaps as importantly, for a large number of cleanroom applications ranging from glass rinsing and reagent mixing to the development of optical and analytical devices, the use of UPW is often unnecessary; indeed, water purity of around 15.0M?.cm is generally sufficient.

Simple with cylinders High quality purified water can be produced using a number of methods, of which the simplest is the use of stand alone deionising cylinders that can be connected directly to a mains or process water supply. Cylinders are easy to install, require little space and can be run until they are exhausted and then simply exchanged for a new unit by the supplier, which regenerates each cylinder using specialised plant. Cylinders are available with a choice of mixed bed deionising resins and activated carbons giving a range of water purities. Water purification cylinders offer a quick, simple and cost effective method of producing reasonable volumes of high quality water. In addition, there is no need for regeneration chemicals to be stored on site or for disposal of chemical effluent, as these issues are dealt with by the cylinder supplier. For larger volume applications, or where a continuous output is required, a single pass Reverse Osmosis (RO) system is generally the preferred choice. This uses a specialised semi-permeable membrane through which the pressurised feed water is passed. The process can remove typically 98% of the minerals or salts contained in the raw water supply, together with silica, organic compounds and bacteria. If higher purity water is required, then a double pass RO system can be used, with the feed water being passed through a sequence of membranes, giving a removal rate in excess of 99%. In addition, a separate electro-deionisation (EDI) unit can be used to polish the permeate from the RO system. EDI uses ion selective membranes and ion exchange resins sandwiched together in combination with an applied voltage to remove ions continuously from the water supply and produce an extremely high purity feed stream. To enhance the performance of the EDI unit and optimise water quality, a further option is to add a degassing stage between the RO and EDI units, to remove dissolved CO2 from the feed stream.

Membrane degassers evolve Although chemical dosing or degassing towers have traditionally been used to remove CO2, recent developments in membrane technology now mean that membrane degassers with hydrophobic hollow fibres can be used. The hydrophobic nature of these materials prevents liquid from penetrating the membrane, but will allow gases to diffuse through. By having the purified water on one side of the membrane and applying air, vacuum or nitrogen to the other side, CO2 is drawn across the membrane and out of the purified water stream. Although this approach is more expensive than conventional processes, membrane degassers offer a number of benefits over chemical dosing and degassing towers. In particular, chemical dosing requires careful monitoring and regular chemical top-ups. There are, however, obvious health and safety considerations associated with the use of chemical dosing agents, such as Sodium hydroxide, for degassing purposes. Unlike degassing towers, which often require a large amount of space, the new membrane degassers have been designed to be modular in construction, and so can be more easily situated in confined areas or installed into existing pipe work systems. Other advantages of membrane degassing are that the design is inherently more sanitary than previously employed technologies; the process is continuous and has low maintenance requirements; and the modular design means that the system can be easily expanded in the event that demands change. Membrane degassers are already being used to replace traditional processes and with continued development are likely to become the standard system for degassing procedures in pharmaceutical and semiconductor manufacturing. Additionally, of paramount importance in both sectors is the maintenance of sanitary conditions throughout the water system. Within the water purification plant this has traditionally been achieved using chemical sanitation and cleaning. A highly effective alternative is the recent introduction of heat sanitisable ROs. Although the capital cost can be high, they have been specially developed for use in sanitary conditions and are designed so that the entire reverse osmosis plant can be regularly sanitised in-situ at temperatures of up to 80°C.

Next generation EDI systems EDI systems generate an inherently biostatic environment within the EDI cell and can be chemically sanitised. How effective this is, however, is questionable as the materials within the EDI cell will not tolerate conventional concentrations of sanitising chemicals. For microbiologically sensitive applications, a new generation of heat sanitisable EDI systems is now available. Significant developments have also been made in the use of ozone, in terms of process system operation, reliability and safety, enabling this gas to be used for routine sanitising of purified water distribution pipe work and systems. Keeping UPW systems sanitised is considerably easier than it ever has been with the introduction of heat sanitisable solutions and ozone for pipe work. The disinfection of membranes using heat, however, will never fully replace the requirement for chemical cleaning, as heat sanitising will not remove organic fouling and scaling. Although RO or combined RO/EDI systems have a far higher capital cost than exchangeable cylinders, they can provide a more effective long term solution, requiring less day to day monitoring while potentially giving a more consistent source of supply. Despite developments in technology, they will not, however, be totally maintenance-free as both the RO membranes and EDI systems will continue to require chemical cleaning and periodic replacement, the timing of which is dependent on feedwater composition, pretreatment, system design and maintenance schedules. Ultimately, a balance has to be found between water quality and capital and operating cost. In practice, it may be necessary to provide separate UPW and high purity feed streams, using a combination of systems to provide the requisite solution.

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