White Paper

Impact of Water Quality on Sterilisation Efficacy and Equipment Longevity in Australian Healthcare Facilities

October 2026

Disclaimer

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, microbial quality and dissolved contaminants on cleaning, disinfection and sterilisation efficacy and on the service life of washer-disinfectors and steam sterilisers. It covers treatment technologies, materials selection, and the water quality requirements of AS 5369:2023.

Summary

• Hardness: Calcium and magnesium salts form limescale on heated surfaces and reduce detergent performance. AS 5369:2023 limits hardness to 150 mg/L for wash stages, 10 mg/L for instrument washer-disinfector final rinse and 0.02 mmol/L for dedicated steam generator feedwater.
• Microbial Quality: Bacteria and endotoxin in rinse water recontaminate devices, and biofilm in storage and distribution sustains contamination. AS 5369:2023 sets total viable count and endotoxin limits for final rinse water, with tighter microbial limits for endoscope washer-disinfectors.
• Treatment: Sediment filtration, carbon filtration, softening, reverse osmosis (RO), ultraviolet (UV) disinfection and a sanitisable distribution loop produce compliant final rinse and feedwater. Performance depends on monitoring and maintenance.
• Corrosion: Chloride causes pitting of stainless steel, and iron, copper and manganese cause staining. Low-ionic water is aggressive to copper and brass, so purified water lines use stainless steel.
• Compliance: AS 5369:2023 requires water testing before installation, routine monitoring at defined frequencies, and corrective action when results exceed limits.

Water Hardness

Definition: Hardness is the concentration of dissolved calcium and magnesium salts, expressed in mg/L as calcium carbonate (CaCO3). 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. Australian supplies range across all four classes, and bore and rainwater supplies differ from reticulated supplies. Obtain the supply analysis from the local water authority and test at the point of use before installation.

Effects on Equipment: Heating precipitates calcium carbonate on heating elements, chamber walls, steam generator surfaces, spray nozzles, valves and sensor probes. Scale insulates heating elements, raises element sheath temperature, extends heat-up time and shortens element life. Scale reduces spray flow and pressure, causes valves to stick, and causes level and temperature sensors to read incorrectly. A level sensing fault in a steam generator can expose heating elements and cause burnout.

Effects on Cleaning and Disinfection: Calcium and magnesium consume detergent builders and chelating agents and reduce soil removal. Detergents formulated only for soft water precipitate calcium and magnesium salts onto devices and chamber surfaces. Mineral deposits encrust device surfaces, shield microorganisms and soil from the process, and prevent reliable visual inspection. Dilute chemical disinfectants with water of the quality specified in their IFU, because hardness can reduce disinfectant activity.

Treatment: Consider softening where supply hardness exceeds 60 mg/L, and soften where it exceeds the 150 mg/L wash stage limit. Potable supplies do not normally meet the final rinse conductivity limit of 30 µS/cm, so final rinse and steam generator feedwater require RO, deionisation (DI), electrodeionisation (EDI) or distillation. Do not dose polyphosphate on final rinse water, because the final rinse phosphate limit is 0.2 mg/L. Supply small steam sterilisers with feedwater to EN 13060 and the IFU, e.g. distilled, RO or DI water. Where the steriliser uses conductivity-based level sensing, use feedwater within the conductivity range stated in the IFU.

Microbial Quality

Risks: Microorganisms in rinse water increase the bioburden on cleaned devices. Steam sterilisation inactivates the organisms but does not inactivate endotoxin, which remains on the device and can cause pyrogenic reactions. Flexible endoscopes receive high-level disinfection without subsequent sterilisation, so organisms in the final rinse transfer directly to the device. Pseudomonas aeruginosa, non-tuberculous mycobacteria and Legionella colonise water systems and are associated with device-related infection.

Biofilm: Biofilm forms on the internal surfaces of pipework, storage tanks and filter housings. Biofilm organisms are substantially more resistant to disinfectants than free organisms, and biofilm sheds organisms into the water continuously. Bacteria can colonise the downstream side of 0.2 µm [0.000008 in] filters. Stagnant water and dead legs promote biofilm.

Limits: Final rinse water for instrument washer-disinfectors and manual cleaning must have a total viable count not greater than 100 cfu/100 mL and endotoxin not greater than 0.25 EU/mL, which equals the pharmacopoeial endotoxin limit for water for injection. Final rinse water for endoscope washer-disinfectors to ISO 15883-4 must have a total viable count not greater than 10 cfu/100 mL, no Pseudomonas aeruginosa or atypical Mycobacterium species detected in 100 mL, and endotoxin not greater than 30 EU/mL.

Controls: Fit 0.2 µm [0.000008 in] point-of-use filters where specified and replace them at the IFU interval. Install UV disinfection on the recirculating loop and replace lamps at their rated operating hours. Sanitise the storage tank and loop thermally, e.g. by recirculating water at 80 to 90 deg C [176 to 194 deg F], at a frequency set from microbial results. Eliminate dead legs and keep water circulating. Test for Legionella where warm water systems are involved.

Response Plan: Maintain a water quality risk management plan that defines responsibilities, sampling points, frequencies, action levels and corrective actions. For any exceedance, define the immediate response, e.g. removing the affected washer-disinfector from service, sanitising the system, replacing filters, tracing the source and resampling until results are within limits.

Water Treatment

• Sediment Filtration: Cartridge filters of 5 µm [0.0002 in] or 1 µm [0.00004 in] remove particles that foul membranes, valves and nozzles. Replace on differential pressure, typically monthly to quarterly.

• Softening: Ion exchange replaces calcium and magnesium with sodium. It does not reduce chloride, silicate or conductivity. Use softened water for wash stages and as RO pre-treatment to prevent membrane scaling. Install duplex softeners so that one regenerates while the other is in service, and test outlet hardness at defined intervals.

• Activated Carbon: Carbon removes free chlorine, which degrades polyamide RO membranes and attacks stainless steel. Use catalytic carbon or extended contact time where chloramine is used. Monitor for chlorine breakthrough and replace carbon on breakthrough, typically every 6 to 12 months.

• Reverse Osmosis: A semi-permeable membrane rejects typically 95 to 99 % of dissolved salts. At these rejections, feed hardness of 100 mg/L gives permeate hardness of 1 to 5 mg/L, and feed chloride of 50 mg/L gives 0.5 to 2.5 mg/L. Single-pass RO normally meets the final rinse limits. Double-pass RO, or RO followed by DI or EDI, is used to meet the steam generator feedwater limits of 5 µS/cm conductivity and 0.5 mg/L chloride. Monitor permeate conductivity continuously with an alarm and divert to drain. Clean membranes when normalised permeate flow or salt rejection falls below the manufacturer limit, and replace them when cleaning no longer restores performance. Manage the reject stream to drain or to non-critical reuse.

• Deionisation and Electrodeionisation: DI resin or EDI modules polish RO permeate. Replace DI cartridges at a set outlet conductivity. Include DI outlets in microbial testing because resin beds support microbial growth.

• UV Disinfection: UV at 254 nm inactivates organisms in clear water. Size the unit for the required dose, monitor UV intensity, replace lamps at their rated operating hours, and clean quartz sleeves.

• Storage and Distribution: Size the storage tank to daily demand to limit residence time. Fit a hydrophobic vent filter and a spray ball for sanitisation. Use a continuously recirculating loop of hygienic design with no dead legs, and drain water held beyond the set residence time.

Monitoring and Maintenance: Install inline conductivity monitoring on RO and DI outlets and chlorine monitoring upstream of the RO. Calibrate conductivity, UV intensity and temperature instruments at defined intervals. Set filter, resin and membrane replacement intervals from monitoring data and the manufacturer IFU. Provide a bypass for maintenance that supplies untreated water to wash stages only, subject to risk assessment, and never to final rinse or steam generation. Hold a contingency supply of treated water for critical use during plant failure.

Corrosion and Materials

Chloride: Chloride breaks down the passive film on stainless steel and causes pitting, crevice corrosion and stress corrosion cracking. The rate increases with temperature, so chloride carried into sterilisers at 134 deg C [273 deg F] is more aggressive than at ambient. Martensitic stainless steel instruments are more susceptible than austenitic grades. Pits roughen surfaces, retain soil and microorganisms, and weaken components. AS 5369:2023 limits chloride to 120 mg/L for wash stages, 10 mg/L for final rinse and 0.5 mg/L for steam generator feedwater.

pH: Acidic water leaches copper, lead and zinc from brass and copper components. RO and DI water absorb carbon dioxide and fall to a slightly acidic pH. AS 5369:2023 sets pH limits of 5.5 to 8.0 for final rinse and 5 to 7.5 for steam generator feedwater. Monitor pH, and where permeate falls below the final rinse limit, remove carbon dioxide by degassing rather than adding minerals that raise hardness and conductivity.

Other Ions: Iron, copper and manganese tarnish and stain stainless steel. Silicates cause discolouration and deposits, and phosphates leave films. AS 5369:2023 limits final rinse iron to 0.2 mg/L, silicates to 1.0 mg/L and phosphates to 0.2 mg/L.

Instrument Life: Chloride, hardness deposits and residual chemicals cause pitting, staining and stiff joints, and residual chemicals cause staining during sterilisation. Rinse devices with final rinse water that meets the limits so that no residue remains to concentrate as the water evaporates.

Materials: Use 316L stainless steel for purified water storage and distribution. Do not use copper or brass in purified water lines, because low-ionic water corrodes copper and introduces copper ions. Minimise mixed metals to avoid galvanic couples. Use plastics only where they are rated for purified water and the sanitisation temperature.

AS 5369:2023 Requirements

Scope: AS 5369:2023 superseded AS/NZS 4187:2014 and AS/NZS 4815:2006, and applies to hospitals, office-based practices and other facilities that reprocess medical devices. The Australian Commission on Safety and Quality in Health Care lists the water quality requirements as unchanged from AS/NZS 4187:2014.

Testing Before Installation: Test the water supplied before equipment installation against the applicable limits and record the results. Testing laboratories should meet AS ISO/IEC 17025. Determine by risk assessment whether treatment is required, and 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 excursions in supply quality.

Limits: The following limits apply. All values are maximums unless a range is given.

Steam generator feedwater limits also include evaporative residue of 10 mg/L, cadmium of 0.005 mg/L, lead of 0.05 mg/L, other heavy metals of 0.1 mg/L, and a colourless, clear appearance without sediment. Wash stage water of the quality shown 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. Supply small steam sterilisers with feedwater to EN 13060.

Routine Monitoring: For instrument washer-disinfectors, 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 endotoxin annually. For endoscope washer-disinfectors, test supply water hardness and chloride monthly, final rinse total viable count with Pseudomonas aeruginosa and atypical mycobacteria monthly, and endotoxin annually. Test dedicated steam generator feedwater at IQ or OQ and annually, or steam condensate where the supply is non-dedicated. After any exceedance, increase the test frequency until results show the system is back under control. Where a facility does not yet meet the limits, document the improvement plan and timeline.

Supply Changes: Arrange notification from the water authority of events that affect supply quality, e.g. mains works and supply shutdowns, and increase testing after such events.

Laboratories: Laboratory autoclaves and glassware washers that do not reprocess medical devices are outside the scope of AS 5369:2023. Supply them with water of the quality specified in the equipment IFU and the laboratory quality system.

Key Actions

• Analyse: Obtain the water authority analysis, test at the point of use before installation, and classify hardness.
• Treat: Soften wash stage water to 150 mg/L or less, and produce final rinse and steam generator feedwater by RO, DI, EDI or distillation to the AS 5369:2023 limits.
• Control Microbial Quality: Recirculate, disinfect by UV, sanitise thermally, eliminate dead legs and replace point-of-use filters at defined intervals.
• Monitor: Test at the AS 5369:2023 frequencies, monitor conductivity continuously, and increase test frequency after any exceedance.
• Select Materials: Use 316L stainless steel for purified water, and exclude copper and brass.
• Plan: Maintain a water quality risk management plan with action levels, corrective actions and a contingency supply.

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 15883-4 Washer-disinfectors, Part 4: Requirements and tests for washer-disinfectors employing chemical disinfection for thermolabile endoscopes.
4. ISO/TS 5111 Guidance on quality of water for sterilizers, sterilization and washer-disinfectors for health care products.
5. EN 13060 Small steam sterilizers.