Author
Will Fitzgerald, Water Safety Business Manager
A critical water system for sterile processing treats, stores and distributes water that meets the ANSI/AAMI ST108 Critical Water limits, including conductivity < 10 µS/cm, bacteria < 10 CFU/mL, endotoxin < 10 EU/mL and hardness < 1 mg/L. It delivers that water to the final rinse and steam generation points in a sterile processing department (SPD). To choose one, start with the equipment it serves and its peak demand, analyze the incoming water, and require a fully recirculating, disinfectable loop with a defined testing and service plan.
This guide explains what an ST108-compliant system includes, how hospitals and ambulatory surgery centers (ASCs) should size and specify one, and the questions to ask every vendor. It was written by the water treatment team at Apex Water + Process, which designs, installs, services and tests SPD water systems.
What is Critical Water?
Critical Water is one of three water categories defined in ANSI/AAMI ST108:2023. ST108 describes it as water that has been extensively treated, always including RO and/or DI, plus storage, distribution and usually final treatment, so that microorganisms and inorganic and organic material are removed. The other two categories are Utility Water (tap water, treated if needed) and Steam.
| Critical Water parameter | ST108 limit |
|---|---|
| pH at 25 °C | 5.0 – 7.5 |
| Conductivity | < 10 µS/cm |
| Total hardness | < 1 mg/L as CaCO₃ |
| Total alkalinity | < 8 mg/L as CaCO₃ |
| Bacteria | < 10 CFU/mL |
| Endotoxin | < 10 EU/mL |
| Total organic carbon (qualification) | < 1.0 mg/L |
| Metals and anions (qualification) | Al, Cu, Fe, Mn, Zn < 0.1 mg/L each; chloride, nitrate, phosphate, sulfate, silicate < 1 mg/L each |
Source: ANSI/AAMI ST108:2023, Tables 2 and 4. TOC, metals and anions are part of performance qualification. The other parameters are monitored routinely.
Where an SPD needs Critical Water, and where it does not
| Application | Water ST108 calls for |
|---|---|
| Final rinse of critical devices before sterilization | Critical Water |
| Final rinse after manual high-level disinfection | Critical Water |
| Final rinse in an automated endoscope reprocessor (AER) | Utility or Critical Water, per the AER manufacturer’s IFU. If Utility Water is used, bacteria < 10 CFU/mL and endotoxin < 10 EU/mL. |
| Feedwater for steam generators | Critical Water |
| Flushing, washing and intermediate rinsing | Utility Water |
| Thermal disinfection and pasteurization, e.g. bedpans or respiratory tubing | Utility Water. Critical Water is optional for the final rinse if hardness deposits are a concern. |
| Sterilizer vacuum pump supply | Utility Water only |
Source: ANSI/AAMI ST108:2023, Clauses 6.2 and 7.4, Table 3, Annex B.
This mapping matters for sizing. ST108 warns that using Critical Water for every stage can be unnecessary and costly, and it can damage equipment. A system sized only for the true Critical Water points is smaller, cheaper to run and easier to keep in control.
What is inside an ST108-compliant Critical Water system
| Stage | Typical components | What it does |
|---|---|---|
| Pretreatment | Sediment filter, water softener, carbon filter or chlorine-destruct chemical | Removes particles, hardness, chlorine and chloramine to protect the RO membranes |
| Primary treatment | Reverse osmosis, with DI or EDI polishing as needed | Removes dissolved solids and most microorganisms. The RO should divert poor-quality water to drain. |
| Storage | Closed tank with a conical or bowl-shaped bottom and a 0.2 µm hydrophobic vent filter | Buffers peak demand and drains completely for disinfection |
| Distribution and final treatment | Recirculation pump, optional UV, 0.2 µm absolute final filters or ultrafilter, recirculating loop | Controls bacteria and endotoxin up to the point of use. UV must be followed by a 0.2 µm filter because it can release endotoxin from killed bacteria. |
| Monitoring | In-line conductivity, pressure, flow, level and temperature sensors; alarms; sample ports | Detects drift early and supports routine testing |
Six steps to specify the right system
1. Build an equipment schedule
List every connection the system will serve, both now and in planned expansion: washer-disinfectors, cart washers, ultrasonic cleaners, manual final-rinse sinks, AERs and steam generators. For each one, record the water category required, the fill volume, cycle timing, temperature, inlet pressure and how many units can draw at once. Check the device, equipment and detergent IFUs, and resolve any conflicts with the manufacturers before procurement, not during startup.
2. Analyze the incoming water
Get a recent analysis of the feed water, covering at least hardness, alkalinity, conductivity, chlorine or chloramine, silica, iron and manganese. Review your municipality’s annual water quality report as well. Many sources change by season, so plan for the worst season, not the average. These results determine the softener size, whether carbon or chemical dechlorination is needed, and whether RO alone will reach < 10 µS/cm or a DI or EDI polishing step is needed.
3. Size for peak demand, not daily average
RO makes water slowly, so a storage tank covers the moments when several machines fill at once. Size production and storage together, using simultaneous demand, operating hours, tank recovery time and planned growth.
4. Specify the distribution loop
- Fully recirculating. ST108 says all newly designed and installed systems shall be on a fully recirculating loop.
- 3–5 ft/s velocity on the Critical Water loop, confirmed by a flow transmitter.
- No dead legs, meaning no branches longer than 3–5 times the pipe’s internal diameter.
- Compatible materials. ST108 lists PVDF, polypropylene, stainless steel and other plastics for water distribution. Apex typically specifies Schedule 80 PVC, polypropylene or HDPE for SPD Critical Water loops.
- Required sample ports at the loop out after final filtration and just before the return to the tank, plus one ahead of each washer where feasible.
- Alarms that people will notice, audible or visual, in both the treatment area and the SPD.
- A dedicated SPD system. Apex recommends against sharing the loop with dialysis, labs or other high-purity users.
- A short route. Place the treatment equipment next to the SPD where possible, and leave room for service access.
- Labeled piping and a posted, as-built process flow diagram.
5. Plan disinfection and maintenance up front
ST108 names monthly disinfection of the loop and storage tank as best practice. The system should be designed for it, with a fully drainable tank, a loop that can be disinfected end to end, and an RO that allows low- and high-pH cleaning. Common disinfectants include ozone, chlorine, hydrogen peroxide and peracetic acid. Ask each vendor which method the system uses, how long processing is interrupted, and how water quality is verified before the system returns to service. Also plan for a daily checklist and replacement of filters, membranes and UV lamps on the OEM schedule.
6. Require a qualification and testing plan
Before routine use, the system needs performance qualification. That means intensive daily sampling at the treatment outlets and points of use for a defined period, then reduced-frequency testing for at least a year to capture seasonal changes. Routine monitoring then follows ST108 Tables 5 and 6: daily conductivity, plus monthly pH, alkalinity, hardness, bacteria and endotoxin at the loop and at each point of use. Our companion post, ST108 Water Testing and Gap Analysis, covers the full schedule.
Hospital vs. ASC: how the right system differs
Hospitals and ASCs have to meet the same water quality limits. What changes is scale, redundancy and who does the day-to-day work.
| Planning factor | Hospital or health system | Ambulatory surgery center |
|---|---|---|
| Typical layout | Central RO/DI system with a longer recirculating loop serving many users, sometimes several departments | Compact skid next to the SPD with a short recirculating loop |
| Demand | Multiple washers, cart washers and steam generators; future expansion | Fewer machines, but peak draws can still be sharp. Size for simultaneous rinses. |
| Continuity | Redundant RO trains or a backup supply, so maintenance does not stop processing | A written plan for cases and reprocessing during service or a failure, such as a backup unit or rental |
| Day-to-day support | Facilities team, building automation integration, alarm escalation, multi-site reporting | Often no on-site water specialist, so clear daily checks, remote alarms and a service partner are essential |
| Biggest risk | Dead legs and stagnant branches in long or modified loops | Direct-feed units without a loop, and gaps in testing and disinfection |
For ASCs, one point needs emphasis. Under ST108, a compact system is fine, but a new direct-feed unit without a recirculating loop is not. If you already have a direct-feed system, ST108 recommends converting it to a loop when possible and monitoring and disinfecting it more often until then.
Questions to ask every vendor
- What feed water analysis did you base this design on, and how does it handle seasonal changes?
- What Critical Water quality will be delivered at the point of use, not just at the RO outlet?
- What are the production rate, storage volume and recovery time against our peak simultaneous demand?
- Is the loop fully recirculating? What is the design velocity, and how is it verified?
- Where are the sample ports, and are there any branches that could become dead legs?
- What disinfection method do you use, how often, and how long is the SPD affected?
- What alarms are included, and where do they sound?
- Who performs performance qualification, and what does the sampling plan include?
- Who performs routine testing, and how are results trended and reported?
- What is the preventive maintenance schedule, and what consumables does it include?
- What is the emergency service response time, and what is the plan if the system goes down?
- What is the five-year cost, including consumables, testing, disinfection, utilities and service?
Compare lifecycle cost, not just equipment price
Signs your existing system needs an upgrade
- Direct-feed piping with no recirculating loop, or no loop-return sample port
- Dead legs left behind by equipment moves or renovations
- Repeated bacteria or endotoxin excursions, even after disinfection
- Frequent conductivity alarms, or RO output declining over time
- A storage tank that cannot be fully drained or disinfected
- A loop shared with dialysis, labs or other departments
- Recurring instrument spotting or staining confirmed by water testing
- No current as-built drawing, or unknown piping materials
How Apex helps
Apex Water + Process brings together the expertise of Apollo, Aqua Treat, Guardian CSC and Weas Engineering to support SPD water from design through daily operation:
- Assessment: on-site review of your current system and water results against ST108
- Design and equipment: RO, RO/DI and EDI systems, storage, and high-purity recirculating loops sized for hospitals and ASCs
- Startup and qualification: installation, performance qualification sampling and documentation
- Lifecycle service: preventive maintenance, monthly loop and tank disinfection, Utility, Critical Water and steam testing, and trend reporting
Request a consultation. Bring your equipment list, a recent water analysis and your operating priorities: 1 (844) 603-4077 or info@teamapex.com.
Frequently asked questions
Is an RO system the same as a Critical Water system?
No. Reverse osmosis is the main treatment step, but a Critical Water system also needs pretreatment, a closed storage tank, a recirculating distribution loop, final filtration for bacteria and endotoxin, monitoring and alarms, and a disinfection plan. Water labeled “RO water” only counts as Critical Water if it meets the ST108 limits where it is used: conductivity < 10 µS/cm, bacteria < 10 CFU/mL and endotoxin < 10 EU/mL.
Does ST108 require a recirculating loop?
Yes, for new systems. ST108 states that all newly designed and installed systems shall be on a fully recirculating loop. Existing direct-feed systems should be converted when possible, and until then they may need more frequent monitoring and disinfection.
What flow velocity should a Critical Water loop have?
ST108 recommends 3 to 5 feet per second to limit biofilm, bacterial growth and endotoxin. Confirm it with a flow transmitter on the loop return, and avoid dead legs longer than 3–5 times the pipe’s internal diameter.
How often should a Critical Water loop and tank be disinfected?
Monthly, which ST108 names as best practice. Also disinfect after replacing in-line probes, sensors or recirculation pumps, after other service on the distribution system, and whenever bacteria or endotoxin results exceed limits.
Do I need DI as well as RO?
It depends on your incoming water. RO alone may not reach < 10 µS/cm on high-mineral feed water, so many SPD systems add DI or electrodeionization (EDI) as a polishing step. ST108 notes that systems using DI should also include carbon and ultrafiltration, because DI resin can grow bacteria and does not remove endotoxin.
Can an ASC use a compact Critical Water system?
Yes. A compact system placed next to the SPD can meet ST108 if it delivers Critical Water at the required flow, includes a short fully recirculating loop with sample ports, and can be disinfected and serviced easily. New installations should not be direct-feed units without a loop.
Can a water system upgrade fix instrument spotting?
Sometimes. White spots usually point to hardness or silica, and rust or rainbow staining can point to chloride, pH or metals. Test the water first, and review detergent, loading and rinsing practices at the same time. Upgrade the system only if the test results show the water is the cause.
Will Fitzgerald
Water Safety Business Manager, Apex Water + Process
Will has 13 years of experience in water management and risk mitigation pertaining to Legionella and other waterborne pathogens. He is also certified in ASSE 12080 Legionella Water Safety and Management.