UPW - essential questions answered
UPW is simply water at its maximum theoretical chemical purity. A range of technologies is applied to potable water to remove all ionic, trace organics and microbiological contaminants. The unit of measure most often used to assess water at such high purity is resistivity, expressed in Megohms centimetre (MW.cm). The higher the resistivity, the purer the water, owing to the lack of ionic contaminants present to conduct electrical charge. UPW has a theoretical maximum resistivity of 18.2 MW.cm at 25°C.
How is UPW produced? Mains water in the UK varies enormously in both chemical composition and concentration of ionic contamination. Due to this, the exact method used to produce UPW from mains water may vary geographically, however the core technologies used for removing ions from water are ion exchange and reverse osmosis (RO), or usually a combination of both where RO removes the bulk of the contaminants, followed by further purification stages utilising ion exchange. Most systems designed to generate UPW comprise a number of process stages: 1. Pre-treatment 2. Reverse osmosis (primary deionisation) 3. Secondary deionisation 4. Polishing deionisation
Pre-treatment Pre-treatment has two main functions: to remove particulate matter from the mains water, and to remove any contaminants that would impair the RO stage of the process. Particulate matter can be removed in a variety of ways including sand or multimedia filtration or using finer cartridge-type filters that would be used to remove particles down to 5 microns typically at this stage. It is normal to utilise a number of filters in series, with each filtration stage taking out smaller particles than the previous stage. For an RO plant to operate effectively and efficiently the feed water should ideally be softened to prevent the build up of calcium deposits on the RO membranes. This is normally achieved using softening resins that remove the hardness by exchanging calcium ions, (the main contributor to hardness), for sodium ions. This process is reversible, which allows the softener to be automatically regenerated in-situ by passing a brine solution over the exhausted softener resin. It is usual for a twin softener to be used which comprises two vessels containing softener resin; this facilitates 100% redundancy so that one vessel is always in service while the other is either in standby or regenerating. Regeneration occurs on either a time elapsed or volume of water treated basis. In addition to softening, carbon filtration is normally used in the pre-treatment stage. This has the dual function of removing the free chlorine, usually present as a biocide in drinking water, and reducing the level of dissolved organics. Both of these are important for RO treatment as chlorine damages the RO membranes and organic matter will foul them. Where raw water is particularly rich in organic matter, then a further process utilising organic scavenger ion-exchange resins is used to remove the organic contaminants. This is a resin-based technology, similar in operation to softening.
Reverse osmosis RO is a process that removes ions from water using membrane technology. Water under pressure is applied to the specialised membrane resulting in purified water passing through the membrane while up to 98% of the minerals/salts are rejected. The purified water passing through the membrane is termed permeate, while the water containing the rejected ionic salts is called concentrate. Typically, RO membranes can operate at an efficiency of up to 75% recovery, dependent on feed water quality. In addition to demineralising the RO membrane will also remove more than 99% of micro-organisms. RO water would then enter the first pure water storage tank. A typical Purite RO plant is shown in Fig. 1.
Secondary deionisation Having removed 98% of the ionic contaminants, the next stage of the process is to target the remaining ions. This involves ion exchange resins. These resins are cation or anion specific, meaning that they selectively exchange either positive or negatively charged ionic species for hydrogen or hydroxyl ions. When exhausted, these resins are regenerated using acid and caustic soda. This process can either be achieved on-site or by returning the cylinders to a specialist company which operates a collection, delivery and regeneration service for ion exchange resins. Purite operates such a service. A more recent variation on this theme is electrodeionisation (EDi), which combines ion exchange resins 'sandwiched' between charged membranes. Under an applied voltage the potential difference set up across the membranes attracts the charged ionic contaminants out of the water and towards their respective electrodes where they are rejected in a waste stream. The process is also self-regenerating due to the production, in-situ, of small amounts of hydrogen and hydroxyl ions, which help to maintain the ion exchange resins in a regenerated state. At this point, the water will have a resistivity in the range 10-18 MW.cm and would be stored in a second pure water storage tank and recirculated to maintain purity. The use of membrane degasser to remove dissolved CO2, prior to the EDi cell, has been shown to enhance system performance at this stage. After the first stages of deionisation, most of the ions will have been removed from the water; however there are still trace contaminants remaining. In order to remove these, water from the secondary loop is further treated by passing it through another set of deionising resins before being irradiated by ultra-violet (UV) light and further polishing deionisation. The UV commonly used in this process is designed to provide a high dose of short wavelength UV at 185nm. This has the effect of breaking-down the remaining contaminants and converting them into charged ionic species (photo-oxidation). These can then be passed through a final 'polishing' deionising resin bed to remove them. The resin used in the polishing part of the process should be of the highest purity, so as not to introduce contaminants back into the water, and would not be regenerable in-situ. At this point the water is UPW. The UPW is typically distributed around a ring main and returned to the pure water storage tank. A typical schematic for a system capable of generating UPW is shown as Fig. 2.
How is quality maintained? UPW is extremely difficult to store and any system should be designed to minimise the length of time/pipework between the point of UPW generation and the points of use. In the part of the system where the UPW is generated, careful consideration must be given to the material of all wetted parts, as contaminants will leach out of most materials. The norm is to use PVDF pipe work and fittings for the polishing system and either polypropylene or ABS for the primary loop and pre-treatment stage. The final pure water storage tank should also be fabricated of PVDF and should also include nitrogen blanketing to prevent the ingress of CO2 that would result in a marked reduction in water resistivity, and a shortening of the life of the deionising resins. This is because the removal of CO2 exhausts the resins prematurely.
Tackling biofilm growth Even within UPW systems, where theoretically there is little to support micro-organisms, biofilm growth on pipework remains an issue. There are a number of important design considerations that must be considered to minimise microbial contamination. The first line of defence is to take regular steps throughout the system to reduce the number of micro-organisms. The first of these is the RO membrane that will remove <99% of micro-organisms. UV at 254nm is commonly used at various stages of the process to disinfect the water, with a properly sized system capable of reducing the number of viable micro-organisms by more than 99.9%. Filtration in the range 0.2 microns is commonly used to remove inactivated bacteria, post UV. The system design is also fundamental in maintaining microbiological purity. For example, good hydraulic design will not include for dead-legs or static water and recirculating water should be maintained at sufficient pipe work velocity to inhibit the formation of biofilm on pipe work surfaces. Ultimately, regardless of the technologies employed, contamination with micro-organisms can occur, and because of this all systems will require periodic sanitisation using, for example, hot water, ozone or hydrogen peroxide. Generating and maintaining UPW is a careful process, but with the right system design, it is achievable and sustainable. Purite supplies water purification systems to a variety of sectors including healthcare and pharmaceutical. Systems can be supplied as standalone or bespoke, tailored to the needs of individual customers.
Subscribe now to Cleanroom Technology to get unrestricted online access to our exclusive content and receive our high quality magazine every month.