Water Quality Required for Washer Disinfectors and Steam Sterilisers

Washer Disinfectors

Washer disinfectors are devices that clean medical equipment by first removing heavy soiling from the equipment with a pre-rinse of cold water. Once the heavy soiling is complete, the washer disinfector moves to a cleaning cycle where the medical equipment is washed with the combination of hot water and surfactants/detergents. The cleaning agents are then rinsed off and the washer disinfector will thermally disinfect the medical instruments at 90 °C. In the final cycle of the washer disinfector the medical instruments undergo flash evaporation to remove any moisture.

AS4187 table 7.2 outlines the feedwater quality required for washer disinfectors. Ensuring the feedwater to the washer disinfectors meets these requirements is essential for them to operate correctly and also reduces ongoing maintenance costs. Feedwater with a pH above 8.0 and below 5.5 can cause corrosion of the mechanical parts of the washer disinfector and can damage the material being disinfected. Water with a conductivity over 30 µS/cm or the presence of chlorides in concentrations higher than 10 mg/L can also pitting of metal surfaces, a process which occurs faster when the water is heated. Dissolved iron concentrations over 0.2 mg/L in the feedwater can lead to the formation of an orange precipitate when exposed to air which can discolour the medical instruments and clog the machine.

From a microbiological perspective, the total viable count (which includes bacteria, yeast and mould spores) of the feedwater needs to be kept at a low enough level so that the disinfection process can render the medical equipment safe. Endotoxins are toxic substances which are released when bacteria disintegrates (for example during disinfection). Endotoxins can only be destroyed by dry heat treatment and require exposure to temperatures of 250 C for 30 minutes to be inactivated. This is why the endotoxin level needs to be kept low in the feedwater to the washer disinfector.

Steam Sterilisers

Steam sterilisers (or autoclaves) use pressurised steam to disinfect medical equipment. Medical equipment is loaded into the steam steriliser and the unit is sealed. Steam is then generated and pumped into the sterilisation chamber until a specific pressure is reached. The equipment is held at this temperature and pressure for a preset time before the steriliser is vented and the medical instruments can be removed. Steam sterilisers differ from washer disinfectors in that they operate at higher temperatures and don’t have a washing/rinsing step.

AS4187 table 7.4 outlines the quality required for steam sterilisers. The feedwater to steam sterilisers requires the removal of similar species as was required for the feedwater to washer disinfectors. These limits are again placed on the feedwater to avoid corrosion, scaling and contamination. The difference is that table 7.4 has more stringent limits as steam is being produced in steam sterilisers and the temperatures these devices can reach is commonly between 121 and 132°C. During the steam production process, these species will be concentrated more in steam sterilisers than in washer disinfectors which is why more stringent limits are required.