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This content is provided for information only. The authors make no representation or warranty regarding the accuracy, completeness or currency of the content. No information in this whitepaper should be construed as medical advice. Readers should seek appropriate professional guidance before acting on any information contained in this document. The authors expressly disclaim all liability for any direct or indirect loss or damage arising from the use of or reliance on this information.

Introduction

This paper sets out the effects of water hardness and other dissolved contaminants on washer-disinfector cleaning performance, equipment maintenance and device life. It covers treatment options, the water quality requirements of AS 5369:2023, and application to hospitals, day procedure units, dental practices and laboratories.

Water Hardness

Hardness is the concentration of dissolved calcium and magnesium salts, expressed in mg/L as calcium carbonate (CaCO3) equivalent. AS 5369:2023 classifies water as soft from 0 to 60 mg/L, moderately hard from 60 to 120 mg/L, hard from 120 to 180 mg/L and very hard greater than 180 mg/L. Hardness salts precipitate as limescale when water is heated or evaporates.

AS 5369:2023 limits water for pre-cleaning, cleaning and all rinses before the final rinse to a hardness not greater than 150 mg/L and chloride not greater than 120 mg/L. Specify a lower hardness where a device requires it. Obtain the supply water analysis from the local water authority, and arrange notification of events that affect supply quality, e.g. mains works and supply shutdowns.

Effect on Cleaning Performance

Detergent Performance: Calcium and magnesium consume the builders and chelating agents in a detergent and reduce the capacity available to disperse soil. Detergents formulated only for soft water can precipitate calcium and magnesium salts, and some surfactants, onto devices and chamber surfaces. Precipitation increases with water temperature, and detergent precipitate is difficult to remove. Select a detergent rated for the measured supply hardness and set the dose to the detergent IFU.

Residue and Encrustation: Mineral deposits encrust device surfaces, shield microorganisms and organic soil from the cleaning process, and prevent reliable visual inspection. A cleaning failure compromises the subsequent disinfection and sterilisation processes. Hard rinse water leaves spots and films as it evaporates during drying.

Final Rinse: Dissolved solids in the final rinse remain on devices after drying. Supply final rinse water that meets the AS 5369:2023 final rinse limits for the washer-disinfector type.

Effect on Maintenance and Service Life

Limescale: Heating hard water precipitates calcium carbonate on heating elements, chamber walls, spray arm nozzles, pipework and valves. Scale reduces spray flow and pressure, insulates heating elements, raises element sheath temperature, extends heat-up time and shortens element life. Scale on temperature probes, level sensors and conductivity cells causes measurement error. Descale with the agent and procedure in the washer-disinfector IFU, and set the descaling interval from inspection of nozzles, elements and chamber surfaces.

Corrosion and Staining: Chloride causes pitting corrosion of stainless steel devices and equipment, and pitted surfaces retain soil and microorganisms. Iron, copper and manganese tarnish and stain stainless steel. Silicates in the final rinse can cause discolouration of devices.

Device Life: Encrustation can permanently damage a device and shorten its serviceable life. Hinged instruments, lumens and handpieces are most exposed because deposits accumulate in crevices and channels.

Warranty: Confirm the water quality conditions in the equipment IFU and warranty before installation, and record supply water test results as evidence of conformance.

Water Treatment Options

Ion-Exchange Softening: A cation resin exchanges calcium and magnesium for sodium and is regenerated with brine. Softening removes hardness only. It does not reduce chloride, silicate or conductivity, so softened water alone does not normally meet a final rinse conductivity limit of 30 µS/cm. Use softened water for wash stages and as pre-treatment ahead of reverse osmosis. Size the resin volume for the supply hardness and daily demand, maintain the brine salt level, set regeneration by volume or measured hardness, and test outlet hardness at defined intervals.

Reverse Osmosis (RO): A semi-permeable membrane rejects dissolved salts, typically 95 to 99 % depending on the membrane and feed water. Pre-treat with sediment filtration, activated carbon to remove chlorine (catalytic carbon where chloramine is present) to protect polyamide membranes, and softening or antiscalant dosing to prevent membrane scaling. Monitor permeate conductivity continuously with an alarm, and record feed and permeate pressures and recovery. Replace pre-filters on differential pressure or chlorine breakthrough. Clean or replace membranes when normalised permeate flow or salt rejection falls below the manufacturer limit. Treated water supports microbial growth in storage, so design the storage tank and distribution loop for sanitisation, sanitise at defined intervals, and test total viable count and endotoxin at the point of use.

Deionisation (DI): Mixed-bed resin cartridges remove cations and anions. Feed with softened or RO water to extend cartridge life. Fit a conductivity or resistivity monitor at the outlet and replace the cartridge at a set conductivity. Resin beds can support microbial growth, so include DI outlets in microbial testing.

Distillation: Distillation produces water with very low dissolved solids at high energy input and low output. It suits small steriliser reservoirs and low-volume final rinse demand. Descale the boiling chamber at the interval in the IFU.

Filtration and Conditioning: Fit sediment filtration upstream of softeners, RO and washer-disinfectors. Where ultraviolet units control microbial growth in storage, replace lamps at their rated operating hours and clean quartz sleeves. Do not use polyphosphate dosing on final rinse water because AS 5369:2023 limits final rinse phosphate to 0.2 mg/L. Do not rely on magnetic or electronic conditioners for hardness control because they do not reduce measured hardness.

Monitoring: Test hardness downstream of each softener, monitor conductivity on every RO and DI outlet, and record all results. Set filter, resin and membrane replacement intervals from monitoring data and the manufacturer IFU.

AS 5369:2023 Water Requirements

AS 5369:2023 superseded AS/NZS 4187:2014 and AS/NZS 4815:2006. The Australian Commission on Safety and Quality in Health Care lists the water quality requirements as unchanged from AS/NZS 4187:2014. Test the water supplied before equipment installation and record the results. Testing laboratories should meet AS ISO/IEC 17025.

Wash Stages: Water for pre-cleaning, cleaning and rinses before the final rinse must have a hardness not greater than 150 mg/L and chloride not greater than 120 mg/L. Water of this quality may be suitable for the final rinse of washer-disinfectors to ISO 15883-3, ISO 15883-6 and ISO 15883-7, e.g. bedpan washer-disinfectors.

Final Rinse: For manual cleaning, manual disinfection and washer-disinfectors to ISO 15883-1 and ISO 15883-2, final rinse water must meet the following limits.

• pH 5.5 to 8.0.

• Conductivity not greater than 30 µS/cm at 20 deg C [68 deg F].

• Total hardness not greater than 10 mg/L as CaCO3.

• Chloride not greater than 10 mg/L.

• Iron not greater than 0.2 mg/L.

• Phosphates not greater than 0.2 mg/L.

• Silicates not greater than 1.0 mg/L.

• Total viable count not greater than 100 cfu/100 mL.

• Endotoxin not greater than 0.25 EU/mL.

For washer-disinfectors to ISO 15883-4 for thermolabile endoscopes, final rinse water must have a total viable count not greater than 10 cfu/100 mL, no Pseudomonas aeruginosa or atypical Mycobacterium species detected in 100 mL, endotoxin not greater than 30 EU/mL, and chemical purity to the washer-disinfector specification.

Routine Monitoring: For washer-disinfectors to ISO 15883-1 and ISO 15883-2, test supply water hardness and chloride monthly, final rinse conductivity monthly, final rinse total viable count monthly for the first 12 months and then at a reduced frequency of not less than annually where results remain within specification, and final rinse endotoxin annually. Adjust frequencies according to results. After any exceedance, increase the test frequency until results show the system is back under control. Where a facility does not yet meet the water requirements, document the improvement plan and timeline.

Treatment Selection: Determine by risk assessment, in consultation with equipment and service providers, whether the supply water is suitable. Where treatment is required, provide softened, filtered, demineralised, RO or distilled water to meet the limits. Where water is not treated, the risk assessment must define the response to significant excursions in supply quality.

Steriliser Feedwater: Feedwater to a dedicated steam generator must meet the AS 5369:2023 feedwater limits, including conductivity not greater than 5 µS/cm at 20 deg C [68 deg F] and hardness not greater than 0.02 mmol/L. Test at IQ or OQ and annually. Supply small sterilisers with feedwater to EN 13060.

Related Documents: ISO/TS 5111 gives guidance on water quality for sterilisers, sterilisation processes and washer-disinfectors. ANSI/AAMI ST108:2023 replaced AAMI TIR34:2014 and defines utility, critical and steam water categories.

Applications

Hospitals and CSSDs

Supply instrument washer-disinfector wash stages with softened water. Supply final rinse and steam generator feedwater from a central plant comprising sediment filtration, carbon filtration, softening, RO, storage and a sanitisable distribution loop. Interlock or alarm final rinse supply on high permeate conductivity. Specify duplex softeners and twin RO trains, or equivalent redundancy, so regeneration and maintenance do not interrupt supply. Document the contingency for loss of treated water, and do not bypass to untreated water for final rinse. Sample at the point of use. Supply bedpan washer-disinfectors with water that meets the wash stage limits, and soften where supply hardness exceeds 150 mg/L.

Dental Practices, Day Procedure Units and Aged Care

AS 5369:2023 applies the same water requirements to office-based practices as to hospitals. Fill small steriliser reservoirs only with water that meets EN 13060 and the IFU, e.g. distilled, RO or DI water, and drain and clean reservoirs at the interval in the IFU. Where a steriliser has feedwater conductivity monitoring, act on every alarm. Fit under-bench washer-disinfectors with an integrated or external softener set to the measured supply hardness, and supply the final rinse through a point of use RO or DI unit that meets the final rinse limits. At low usage, size treated water storage to daily demand to limit stagnation, and schedule filter and cartridge replacement with the annual equipment service.

Laboratories

AS 5369:2023 does not apply to laboratory glassware. Supply glassware washer wash stages with softened water to prevent scale and detergent precipitation. Specify the final rinse water grade required by the analytical methods and the laboratory quality system. Include temperature probes, flow sensors and dosing pumps in the descaling and calibration schedule because scale causes measurement and dosing error.

Key Actions

• Analyse: Test the supply water before installation and obtain the water authority analysis. Classify hardness and identify chloride, iron and silicate levels.

• Treat: Treat wash stage water to a hardness not greater than 150 mg/L and chloride not greater than 120 mg/L. Treat final rinse water and steriliser feedwater to the AS 5369:2023 limits for the equipment type.

• Select Detergent: Select detergents rated for the measured hardness and dose to the IFU.

• Monitor: Test at the AS 5369:2023 frequencies, increase frequency after any exceedance, and record all results.

• Maintain: Replace filters, resins, membranes and UV lamps on monitored performance and manufacturer intervals. Sanitise storage and distribution at defined intervals.

• Descale: Set descaling intervals from inspection of nozzles, heating elements, sensors and chamber surfaces.

Sources

1. Standards Australia. AS 5369:2023 Reprocessing of reusable medical devices and other devices in health and non-health related facilities.
2. Australian Commission on Safety and Quality in Health Care. Transitioning from AS/NZS 4187:2014 to AS 5369:2023, August 2024.
3. ISO/TS 5111 Guidance on quality of water for sterilizers, sterilization and washer-disinfectors for health care products.
4. ANSI/AAMI ST108:2023 Water for the processing of medical devices.