Tim Lambert of Purite Ltd summarises the current situation
The growing incidence of infections acquired in hospital has focused attention on the need for effective decontamination techniques in which water plays a crucial part. Improved decontamination processes could lead to the control of diseases such as vCJD and Hepatitis B. According to researchers at the London School of Hygiene and Tropical Medicine, 1 in 10 patients pick up a disease while in hospital.
Washer-disinfectors (WDs) are the devices used in hospitals to decontaminate soiled items so that they can be safely reused in clinical practice. WDs are normally located in sterile services departments (SSDs), where they are able to decontaminate a broad selection of articles ranging from human-waste containers to surgical instruments.
The role of water
Decontamination entails subjecting the soiled articles to a combination of processes - including cleaning, disinfection and sterilisation - that will remove or destroy micro-organisms and other contaminants. Water plays a pivotal role in every aspect of the decontamination process. For instance, cleaning is a vital prerequisite for both disinfection and sterilisation, and often involves initial flushing with water to remove the gross contamination, followed by washing with detergents or enzymic cleaners to remove residual dirt.
Other roles for water include acting as a carrier for cleaning agents, and serving as a suspension medium to prevent dislodged particulate contaminants from being redeposited on clean surfaces. Of particular importance is the need for high-purity water both to act as a final rinsing agent to remove any remaining traces of dirt or detergent from WDs, and to provide steam for thermal disinfection and steam sterilisation.
Thermal disinfection entails exposing the articles to moist heat at a particular temperature for a specified time. Although disinfection destroys or removes virtually all the pathogenic micro-organisms present, it may not deactivate certain viruses and bacterial spores. By contrast, steam sterilisation destroys all micro-organisms, including spores, and is widely used as a final sterilisation stage for items that have already been cleaned and disinfected. The process is normally carried out in an autoclave using dry, saturated steam under pressure.
Waterborne contaminants
Many of the contaminants found in water supplies can affect the operation of WDs. For example, hardness salts can cause problems ranging from fouling of heating elements (as per section 7.7 of HTM 2030), resulting in a sharp increase in running costs, to partial blocking of spray nozzles and the deposition of a white powder on load items. Other inorganic salts - such as chlorides - can cause pitting and corrosion in metallic items, especially stainless steel instruments.
Chemical impurities apart, however, it is bacteria and endotoxins present in the final rinse water, which can create a potential health hazard to patients, either through infection or erroneous diagnosis. (Endotoxins are fragments derived from bacterial cell walls which, when introduced into the human body, can induce a febrile reaction ranging from mild fever to shock or even death). It is especially important to minimise microbial contamination of rinse water when load items are given a final rinse after disinfection.
Regulatory aspects
Two documents published by National Health Service Estates provide SSD managers with comprehensive guidelines on all aspects of the operation and maintenance of washer-disinfectors and steam sterilisers. Health Technical Memorandum (HTM) 2030 covers washer-disinfectors while HTM 2031 deals with clean steam for sterilisation, and provides guidelines for both steam purity and the quality of feedwater for steam generators.
A harmonised European Union standard for washer-disinfectors is currently being finalised (pr EN 15883), and is expected to incorporate guidelines that are broadly similar to those specified in HTM 2030. Washer-disinfectors that are used to clean and disinfect medical devices are themselves classified as medical devices. They must comply with the essential requirements of EU Medical Devices Directive (93/42/EEC) and bear a CE (Conformité Européan) mark indicating conformity with the demands of the Directive.
The quality of the feedwater used at various stages in the decontamination process is described in HTM 2030 and NHS guidelines C30. Guidelines are provided in terms of recommended limits for various parameters of water purity. These include electrical conductivity, total dissolved solids (TDS) content and water hardness; as well as acceptable concentrations of chloride, heavy metals, iron, phosphate and silicate; together with maximum levels of bacterial endotoxin and the total viable count of micro-organisms.
Water Technology
Because of the adverse effects of waterborne impurities, feedwater for WDs must normally be either treated or purified, depending on the operational requirements of the machine. Likewise, because steam may contain chemical impurities that could be deposited on sterile surfaces, 'clean steam' derived from purified water must normally be used for sterilisation.
The principal water technology processes currently used include base-exchange softening, deionisation, reverse osmosis together with ancillary technologies such as ultra-violet irradiation, particle filtration and adsorption.
Both base-exchange softening and deionisation are ion-exchange processes. In softening, hardness cations in the water are replaced by sodium ions released by a cation-exchange resin; in deionisation, impurity cations and anions are replaced by additional water molecules derived from a combination of cation-exchange and anion-exchange resins. Once the ion-exchange sites in the resins are fully occupied by impurity ions, the resins must be chemically regenerated.
Although deionisation is a highly effective means of removing ionic contaminants from water, it does not remove micro-organisms or endotoxins; in fact, bacteria tend to proliferate on resin beds. Nevertheless, deionised water can be used for thermal disinfection in all WDs and as final rinse water in certain cases.
Reverse osmosis (RO) is a membrane process in which feedwater under pressure flows into a module containing a semipermeable membrane. A proportion of the feedwater passes through the membrane - which retains most of the impurities - to form the purified permeate. The impurities accumulate in the residual ' concentrate stream which is continually run to drain. RO membranes are able to remove up to 98% of inorganic ions from the feedwater, together with virtually all the colloids, micro-organisms, endotoxins and other macromolecules.
Because of this wide-ranging performance, reverse osmosis is generally regarded as the most effective means of purifying feedwater for SSDs. It is important to remember, however, that RO is a percentage removal process and that the quality of the purified water will depend on both the characteristics of the raw water supply and the design of the RO system. If the feedwater has high TDS levels - or if a high degree of chemical purity is required - a downstream polishing deionisation cylinder can be installed.
The microbiological purity of the water can be enhanced by incorporating an ultra-violet disinfector into the distribution system. UV light at a germicidal wavelength of 254 nm can denature micro-organisms - including bacteria and viruses - without the addition of chemicals or application of heat. The radiation ruptures the organisms by interacting with the cell DNA - thereby preventing replication - and is most effective when the feedwater is largely free from contaminants that could shield the cells from the lethal rays.
The Purite approach
Purite Limited is a specialist water technology company, which has pioneered the development of water purification equipment for the healthcare sector. The company's approach to providing plant for SSDs has been two-fold: Purite has designed, manufactured and installed reverse osmosis systems capable of producing high-purity water that fully satisfies the requirements of HTM 2030 / 2031 and NHS C30. (In many cases, a single water purification system will provide feedwater for both washer-disinfectors and clean-steam generators).
Secondly, Purite has satisfied the regulatory requirements of the European Union in respect of water purification equipment designed for medical applications. For example, rigorous testing and evaluation of Purite equipment has demonstrated that the company has complied with Medical Devices Directive 93/42/EEC, thereby enabling it to use CE marking for certain products. In addition, by introducing a stringent quality assurance system, Purite has conformed to the requirements of quality system standards ISO 9001:2000 and BS EN ISO 13485:2001 for the design, manufacture and service of water purification equipment.
RO systems
RO systems designed to provide high-purity water for SSDs have certain elements in common. These include a pretreatment package geared to the characteristics of the feedwater; a particle prefilter (typically with a filter rating of 5µm); a reverse osmosis unit; a recirculation tank; and a ringmain distribution system incorporating a pump and ancillary items.
The pretreatment package normally features base-exchange softening, both to remove hardness cations that might otherwise foul downstream RO membranes and, where necessary, to provide a flow of softened water directly to the WDs. In some cases, a single-vessel softener provides softened water during the working day and regenerates out of hours. Alternatively, a continuous flow of softened water can be produced by a twin-vessel softener in which one vessel is on line while the other one is regenerating or on standby.
Further protection of the RO membranes is afforded by passing the water through an activated carbon filter which removes free chlorine, as well as conditioning the water by taking out organic contaminants. Any remaining fine particulates are then removed by the 5µm filter before the pretreated water enters the RO plant.
Virtually all the residual contaminants present in the water are removed by the RO membranes, after which the purified permeate is fed into the recirculation tank and constantly recycled around the distribution ringmain. The water is delivered to washer-disinfectors - and if necessary to steam generators - via take-off points in the ringmain.
Distribution systems
Distribution systems fall into two broad categories: those in which the RO permeate is heated to 60oC, sometimes with the option of occasional 5-minute periods of pasteurisation at 90ºC; and those in which the water is maintained at ambient temperature. Both types of distribution system normally incorporate a black, polypropylene recirculation tank with a conical base to facilitate good circulation. The tanks are sealed and fitted with a sub-micron vent filter to exclude airborne contaminants, and feature a sanitary overflow.
The benefits of using a heated distribution system are two-fold: first, the system is 'self-disinfecting' in that levels of bacteria and endotoxin are controlled; and second, the cycle times of WDs are shortened with corresponding increased efficiencies. The main disadvantage of heated systems is that capital and operating costs are significantly higher than those of ambient systems.
Figure 1 shows a typical system for producing heated RO water. In this example, the feedwater is drawn from the potable supply, so the first unit in the system is a break tank fitted with a type A air gap to prevent any possible backflow. The pretreatment base-exchange softener in this system is a single-vessel cabinet unit.
Key features of the distribution system include: a recirculation tank which has been insulated and clad; a steam-heated heat-exchanger for raising the temperature of the water to 60ºC (and periodically to 90ºC for sanitising); an insulated ringmain made from 316-grade stainless steel; and a 0.2µm filter housed in a stainless steel vessel. Smaller systems are usually heated by electric immersion heaters located horizontally in the recirculation tank. Electric heaters are more economical than steam-heated heat-exchangers, but are less efficient.
A system for producing RO water at ambient temperature is shown in Figure 2. The feedwater in this case is obtained from an on-site hospital tank, so although there is no need to include a break tank, a booster set may still be required. A twin-vessel base-exchange softener, coupled with a brine tank, ensures that there is a continuous supply of softened water.
Distinctive features of the distribution system include: distribution firework made from ABS plastic; an ultra-violet disinfector to destroy any residual micro-organisms in the recirculating water; and a 0.2µm filter to remove surviving organisms, dead cells and fine particulates from the water. Although the distribution system is unheated, the temperature of the recirculating water may rise - typically between 15-30°C - as a result of local conditions or heat generated by the recirculation pump and the UV lamp.
Conclusion
A supply of high-purity water is essential if items used in clinical practice are to be fully decontaminated and the risk of cross-contamination minimised. On balance, reverse osmosis is the most effective means of producing purified water that meets the demands of HTM 2030/2031 and NHS C30, and RO systems can be tailored to meet specific requirements. Increasingly, however, water purification equipment destined for medical applications must be manufactured, installed and serviced to the highest standards to ensure compliance with European Directives.
For further information please contact Purite Ltd, Tim Lambert, on 01844 217141 or
email: mail@purite.com . Alternatively the Purite website can be found at www.purite.com .