6.2 Water Quality in Instrument Processing
Key Takeaways
- Water quality directly impacts cleaning effectiveness, instrument longevity, and sterilization efficacy
- Three categories per AAMI ST108: utility water, critical water, and steam
- Utility water (tap water) is used for initial rinsing and general washing — not for final rinses
- Critical water is treated water (RO, DI, or distilled) used for final rinses and preparation of HLD solutions
- Steam quality affects sterilization — contaminants in steam can deposit on instruments and prevent proper sterilization
- Hard water causes mineral deposits (scaling) on instruments and inside sterilizers
- Water treatment systems (reverse osmosis, deionization, distillation) remove contaminants to produce critical water
- ANSI/AAMI ST108 is the standard for water quality in medical device processing
Last updated: March 2026
Water Quality in Instrument Processing
Water is used in nearly every step of instrument processing — cleaning, rinsing, steam sterilization, and HLD solution preparation. Poor water quality can damage instruments, leave deposits, and compromise sterilization.
ANSI/AAMI ST108: Water Quality Standard
AAMI ST108 (Water for the Processing of Medical Devices) defines the water quality needed at each step of reprocessing:
Three Categories of Water:
| Category | Description | Source | Uses |
|---|---|---|---|
| Utility Water | Standard treated water (tap water from municipal supply) | Municipal water system | Pre-rinsing, initial cleaning, washer-disinfector pre-wash |
| Critical Water | Treated to remove microorganisms, minerals, and organic contaminants | RO (Reverse Osmosis), DI (Deionized), Distilled | Final rinses, HLD solution preparation, steam generator feed |
| Steam | Water heated to vapor phase | Hospital boiler or sterilizer steam generator | Steam sterilization |
Problems Caused by Poor Water Quality
| Problem | Cause | Effect |
|---|---|---|
| Mineral deposits (scaling) | Hard water (high calcium, magnesium) | White deposits on instruments; scale in sterilizers |
| Staining/spotting | Dissolved minerals, chlorine | Cosmetic damage; potential corrosion sites |
| Corrosion | Low pH water, chlorides, dissolved minerals | Pitting, rusting, instrument failure |
| Biofilm in water lines | Bacterial contamination of water supply | Instruments contaminated during rinsing |
| Poor steam quality | Contaminants in boiler water | Wet packs, non-condensable gases, instrument damage |
| Impaired cleaning | Hard water reduces detergent effectiveness | Inadequate soil removal |
Water Treatment Methods
| Method | How It Works | Removes |
|---|---|---|
| Reverse Osmosis (RO) | Forces water through semi-permeable membrane | 90-99% of dissolved minerals, organisms, particles |
| Deionization (DI) | Ion exchange resins remove charged particles | Dissolved minerals (ions); NOT bacteria or endotoxins |
| Distillation | Boils water and collects condensed steam | Most contaminants; highly pure output |
| Filtration | Physical barrier removes particles | Particulates; pore size determines what is filtered |
| UV treatment | Ultraviolet light kills microorganisms | Bacteria, viruses; does NOT remove minerals |
Typical Water Treatment System:
Municipal Water → Sediment Filter → Carbon Filter → Water Softener → RO System → DI Polish → UV Sterilizer → Critical Water
Steam Quality Considerations
Poor steam quality is a common cause of wet packs and sterilization failures:
| Steam Quality Issue | Cause | Solution |
|---|---|---|
| Wet steam (excessive moisture) | Inadequate boiler maintenance; long piping runs | Maintain boilers; install steam traps and separators |
| Superheated steam (too dry) | Pressure reduction without moisture addition | Install desuperheaters; adjust boiler settings |
| Non-condensable gases | Air in steam lines; chemical additives | Proper air removal; monitor boiler chemistry |
| Chemical contaminants | Boiler treatment chemicals carried into steam | Use proper chemical treatment; install chemical separators |
Saturated steam (steam at its maximum moisture-holding capacity for a given temperature) is required for effective sterilization. Both wet steam and superheated steam are problematic.
Monitoring Water Quality
| Test | What It Measures | Frequency |
|---|---|---|
| Conductivity/Resistivity | Dissolved mineral content | Daily or continuous |
| Bacterial counts | Microbial contamination | Weekly to monthly |
| Endotoxin testing | Gram-negative bacterial toxins | Per facility policy |
| pH | Acidity/alkalinity | Daily to weekly |
| Hardness | Calcium and magnesium content | Weekly |
| Total dissolved solids (TDS) | Overall mineral content | Daily to weekly |
Test Your Knowledge
According to AAMI ST108, critical water is used for:
A
B
C
D
Test Your Knowledge
Hard water in the CS department causes:
A
B
C
D