Water purification for decontamination services

All modern hospitals depend on decontamination services for the cleaning and disinfection of medical equipment to extremely high and consistent standards. To meet such stringent standards, these centres all depend on a consistent supply of high-quality purified water to carry out decontamination tasks.

Water plays a pivotal role in many areas of decontamination and has to be available in large volumes at specified levels of purity. In particular, it is vital that automated washer disinfectors use purified water for either all stages or the final rinse stage of the wash process. This removes potential contaminates from the feedwater entering the washer disinfector. It also ensures that there are no residual mineral deposits that could potentially harbour bacteria or endotoxins, as noted by the strict microbiological specification set by the HTM guidelines.

For autoclaves the requirement is somewhat different in that the water is not used for rinsing, but raised as steam for sterilisation, but as with washer disinfectors this final process must not result in instruments being left with unwanted surface contaminants.

HTM 01-01
Guidelines covering the standards of water purity that decontamination services should meet have been in existence for many years and can be found in their current form under Health Technical Memorandum (HTM) 01-01: management and decontamination of surgical instruments (medical devices) used in acute care.

HTM 01-01 is divided into four sections:

• HTM 01-01: management and decontamination of surgical instruments (medical devices) used in acute care
• HTM 01-04: decontamination of linen for health and social care
• HTM 01-05: decontamination in primary care dental practices
• HTM 01-06: decontamination of flexible endoscopes.

Meeting the criteria of, in particular, HTM 01-01 and 01-06, where high levels of water purity are especially important, can be achieved using a range of systems. Selecting the best solution, however, calls for a degree of expertise in designing water purification systems and an understanding of how each system meets the varying demands of decontamination services.

It is generally accepted that reverse osmosis (RO) is one of the most effective technologies for providing a continuous supply of high-quality water, to the HTM 01-01 requirements. RO systems are often combined with complementary technologies such as base-exchange softening, ultra-violet irradiation, micro-filtration and activated carbon adsorption. Some of these techniques operate as pre-treatment stages for others — base-exchange softening and particle filtration may be installed upstream of RO units, for example, to protect the RO membranes — while some, such as UV and micro-filtration, are designed into the system downstream of the RO unit to achieve and maintain the desired microbiological purity levels within the distribution and storage system.

How a Reverse Osmosis systems works
An RO system consists of a module containing a semi-permeable membrane into which incoming water is pumped under pressure. A proportion of the water passes through the membrane, which retains most of the impurities, to produce a purified permeate for downstream use. The impurities are then removed by a residual concentrate stream, which is continuously run to drain. RO efficiencies are such that up to 75% of the feedwater passes through the membrane as the purified permeate, with the membrane removing up to 98% of inorganic ions from the water, along with virtually all colloids, micro-organisms, endotoxins and dissolved organics.

Leaving to one side the incoming feedwater quality, typically the design of water treatment systems for decontamination applications, from the (RO) up to the point of take-off tend to follow the same format and are generally similar in their design irrespective of the supplier.

The selection of technologies used within the pre-treatment stage depends to a large degree on the quality of the incoming mains feedwater. Water quality and hardness or softness can vary greatly across the country and will correspond to the characteristics of the region from which the water is being abstracted.

Typically, in areas of hard water a base-exchange softener would be installed to remove the hardness that might otherwise deposit scale on the downstream RO membranes. Base-exchange softening is an ion-exchange process in which water is passed down through a bed of cation exchange resin in the Sodium form, to replace the hardness cations with Sodium ions. Once the bed is exhausted it will have to be automatically regenerated using brine solution as the source for replacing the depleted Sodium. To ensure continuity of supply, a typical system will have two units, with one in operation while the other is being regenerated off-line; this is generally referred to as duplexing.

Activated carbon filters also play an important role in pre-treatment for RO units as they remove ‘free Chlorine’, which is added at source by water companies to kill potentially harmful micro-organisms. If left untreated, free Chlorine will rapidly degrade the (RO) membranes and result in unacceptable levels of minerals in the permeate. The purified permeate from the RO modules is fed into a tank from where it is continuously recirculated around a distribution ring-main that feeds the washer-disinfectors and steam generators via various take-off points.

To preserve and enhance the microbiological purity of the water in the distribution ring-main an ultraviolet (UV) disinfection system can also be incorporated. UV light at a wavelength of 254nm will kill water-borne micro-organisms, including bacteria and viruses. This presupposes that the distribution ring-main system itself is free of contaminants and designed in such a way so as not to include any points in the pipework that could harbour contamination, such as crevices or dead-legs.

As validation of water quality is primarily performed on samples taken at the point of use, any contamination downstream of a UV system can be a problem. An increasingly popular method of avoiding this is to employ heat sanitisation of the distribution pipe work. In a heated system the RO permeate is maintained between 80º and 85ºC (sometimes with the possibility of occasional short periods at 90ºC). This effectively makes the system self-disinfecting by controlling levels of bacteria and endotoxins and has the added benefit of reducing the cycle times of washer-disinfectors.

Heated RO systems are important to meet the requirements of the HTM 01-01 guidelines; in particular, the need to reduce the total viable count of micro-organisms to less than 10cfu/100ml. In practice, this can only be consistently and practically achieved by elevating the temperature of the permeate to levels above 80º for a significant period of time to disinfect the whole distribution pipework system and prevent the build-up of biofilm on the internal surfaces of pipework.

In the case of washer-disinfectors the recirculating water can be run continually hot; however, this is not currently possible with endoscope reprocessors. This is because endoscopes are not generally able to withstand such high temperatures.

Nonetheless, heat still has a vital role to play in these systems, as the distribution pipe work can still be automatically heat sanitised when the reprocessors are not in use, to maintain extremely low levels of bacteria and endotoxin.

To date, the main drawback of heated systems has been their capital and operating costs, which can be considerably higher than those of ambient temperature systems. For example, operating at temperatures up to 90ºC may require a switch to stainless steel or the use of cross-linked polyethylene (PEX) piping, while raising and maintaining the system at elevated temperatures requires investment in heat generation and heat exchangers. Despite this, the increased microbiological integrity offered by automatically programmed heat sanitisation remains key in consistently achieving the highest standards.

Because of the need for a consistent supply of feedwater, RO-based systems are generally designed as complete packages. These will include the appropriate pre-treatment systems for the feedwater quality available in the area, together with a distribution system for the purified water.

In addition, many of the latest systems feature sophisticated data logging technology, for use as part of validation and traceability regimes. These allow users to provide local and remote monitoring of the purification system, and to capture in real time a wealth of data on flow rates, temperatures, pressures, alarm states and water quality.

Ultimately, these new developments mean that for all those responsible for decontamination can be assured that any future water purification equipment utilising integral heat sanitisation and data logging will be able to meet the steadily changing, and ever more rigorous, operating standards without the need for chemical sanitisation.