The Principles of Medical Device Decontamination

If using Reverse Osmosis (RO) water treatment, hardness also has an impact as it causes fouling of the membrane resulting in less membrane space for the water to pass through. Hardness is easily controlled using a water softener. However, softeners require maintenance, back washing and salt dosing and are an essential component of water treatment.

Chlorides and Ionic Contaminant

To prevent corrosion, water used in decontamination processes should have a chloride concentration of less than 120 mg/L chlorine. Chloride concentrations greater than 240 mg/ L can cause pitting of some stainless steel and plastic components. Current Health Technical Memorandum (HTM) 01-01 part D guidance requires a final rinse water chloride level of no higher than 10 mg/L chloride and the Welsh Health Technical Memorandum (WHTM) 01-01 Part D states less than 50mg/l. These concentrations stem from the limits in sterile water for irrigation and are far below the levels needed to prevent corrosion. Where necessary chloride levels can be reduced using a carbon filter. As discussed earlier ionic contamination (and hence chloride levels) of water can be measured by conductivity.

Silicates

Silicates (minerals with silicon) are found in water that is taken from sandy locations and increased sharing of water supplies in the UK means that this may be a wider problem. Deposits on the instruments are opaque at first and turn dark blue when the layers grow thicker. Much of this is cosmetic, however when silicates interact with high chloride concentrations there maybe pitting and crevice corrosion. If high silicates are a concern (for instance in combination with high chlorides) then they can be reduced either by twin pass RO (e.g. two RO plants in series) or by polishing filters. If utilising the latter, then careful control of microbiological contamination of the polishing filter is needed.

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