A Typical Story from the Field
Maria, a seasoned sterile processing manager at a suburban hospital, had just finished overseeing the installation of a reverse osmosis (RO) system in her department. The facilities team was confident. The vendor had checked the box. Leadership was pleased.
But three months later, something wasn’t right.
Rust spots began appearing on surgical instruments. Staff noticed cloudy residue in trays. Worse, one patient developed a post-op fever that couldn’t be explained. The infection prevention team began tracing back every possible cause — and that trail led straight to the new water system.
But how? They had an RO system. Wasn’t that enough?
According to ANSI/AAMI ST108:2023 — no, it’s not.
The Misconception: RO = Compliance?
A reverse osmosis system is a powerful tool in water purification. It forces water through a semi-permeable membrane, removing many dissolved salts, organics, and microorganisms. But it’s only one part of a multi-barrier system needed to meet the water quality requirements in ANSI/AAMI ST108.
The standard breaks water down into categories — Utility Water, Critical Water, and Steam — and each has its own strict performance requirements. Critical Water, used for final rinses and steam generation, has limits like:
<10 CFU/mL bacteria
<1.0 mg/L TOC (Total Organic Carbon)
<1 mg/L ionic contaminants like chloride and nitrate
<1 mg/L water hardness
An RO system alone might reduce conductivity or certain ions, but it can’t guarantee microbiological control, endotoxin removal, or chemical stability over time.
Why RO Alone Isn’t Enough: The Science
1. Bacteria and Biofilm
RO membranes can reject many microorganisms — but bacteria can colonize the membrane itself, especially in systems that aren’t continuously monitored and sanitized. ST108 specifically warns that gram-negative bacteria and nontuberculous mycobacteria can proliferate in even treated water due to nutrients, temperature, and stagnant conditions.
Once established, biofilm — a slimy matrix of microorganisms and extracellular polymers — becomes a persistent contaminant. RO can’t remove established biofilm downstream.
2. Endotoxins Slip Through
Even if the RO membrane captures bacteria, endotoxins (lipopolysaccharides from bacterial cell walls) can remain in the water. These molecules are heat-stable and pyrogenic — meaning they can cause fever even after sterilization. ST108 limits endotoxins in Critical Water to <10 EU/mL, and that requires ultrafiltration or final submicron filtration — not RO alone.
3. Chemical Corrosives
RO systems may still allow low levels of chlorides or other corrosive ions to pass through. Left unchecked, these contaminants can damage surgical instruments over time. ST108 mandates very low levels of chlorides, iron, and other metals in Critical Water to prevent pitting, staining, and surface degradation.
ST108 Requires a System — Not Just a Component
A compliant water system under ST108 includes:
Pretreatment: Carbon filtration, water softening
Primary Treatment: RO and/or deionization
Storage & Distribution: Covered, recirculating loops with sloped drains
Final Treatment: Submicron or ultrafiltration at the point-of-use
Routine Monitoring: Microbial, chemical, and physical testing at defined intervals
In Maria’s case, VERDA Water Quality Systems helped redesign the hospital’s water pathway to include final point-of-use filtration, better storage tank design, and monthly testing for TOC and endotoxins. The difference was clear — both in lab reports and in the absence of further complications.
Conclusion: Compliance Isn’t a Filter — It’s a Commitment
If your team believes that installing a single RO unit checks the box for water compliance, it’s time to think again. ANSI/AAMI ST108 elevates the conversation from equipment to outcomes — demanding a holistic, science-backed approach to water quality.
Because when water is the last thing touching your instruments before they touch a patient, it had better be right.