High-purity water reference sheets

Resources

High-purity water reference sheets.

The figures used most often in the field, one page each. Limits are drawn from the same standards source Eighteen2 checks designs against. Print them for the mechanical room.

Seven one-page references for the people who walk the mechanical room. Each prints on its own page.

Sterile processing

AAMI ST108 water quality

ST108 defines two water qualities for device reprocessing. Critical water is for the final rinse and anywhere water touches a device after cleaning. Utility water is for washing, flushing, and intermediate rinses.

Critical water
Parameter	Limit
pH @ 25 °C	5.0 – 7.5
Conductivity	< 10 µS/cm
Total alkalinity	< 8 mg/L as CaCO₃
Total hardness	< 1 mg/L as CaCO₃
Bacteria	< 10 CFU/mL
Endotoxin	< 10 EU/mL
TOC	< 1.0 mg/L
Chloride	< 1 mg/L
Iron, copper, manganese, zinc, aluminum	< 0.1 mg/L each
Nitrate, phosphate, sulfate, silicate	< 1 mg/L each
Color and turbidity	Colorless, clear, no sediment

Utility water (washing, flushing, intermediate rinse)
Parameter	Limit
pH @ 25 °C	6.5 – 9.5
Conductivity	< 500 µS/cm
Total alkalinity	< 400 mg/L as CaCO₃
Total hardness	< 150 mg/L as CaCO₃Soften above this unless detergent chemistry tolerates it.
Bacteria	< 500 CFU/mL< 10 CFU/mL and < 10 EU/mL when used as final rinse after chemical HLD.
Chloride	< 250 mg/L
Silicate	< 50 mg/L

Steam

ST108 also addresses the water used to make steam for sterilization. Size the steam generator's feed to the sterilizer maker's instructions for use and the facility's water management plan, and record which source feeds which sterilizer.

Monitoring at a glance

Conductivity · Daily · After last treatment step
pH, alkalinity, hardness · Monthly · After last treatment step
Bacteria (HPC) and endotoxin (LAL) · Monthly · Loop out and loop return
pH, conductivity, alkalinity, hardness; bacteria and endotoxin · Monthly · First point of use (chemistry); each point of use (bacteria, endotoxin)
Utility water chemistry and bacteria · Quarterly · Treatment system and point of use

Frequencies are the standard's recommended minimums. A water management team and a documented risk assessment are expected.

Laboratory

ASTM D1193 reagent water, Types I–III

Types I, II, and III are defined by ionic, organic, and silica limits. Bacterial subtypes A, B, and C are optional add-ons; ask whether the customer's spec names one.

Parameter	Type I	Type II	Type III
Resistivity @ 25 °C	≥ 18.0 MΩ·cm	≥ 1.0 MΩ·cm	≥ 4.0 MΩ·cm
Conductivity @ 25 °C	≤ 0.056 µS/cm	≤ 1.0 µS/cm	≤ 0.25 µS/cm
TOC	≤ 50 µg/L	≤ 50 µg/L	≤ 200 µg/L
Sodium	≤ 1 µg/L	≤ 5 µg/L	≤ 10 µg/L
Chloride	≤ 1 µg/L	≤ 5 µg/L	≤ 10 µg/L
Silica	≤ 3 µg/L	≤ 3 µg/L	≤ 500 µg/L
Final filtration	0.2 µm	Not specified	0.45 µm

Rules of thumb

Type I is usually made at the point of use: an RO or Type II feed, then a mixed-bed polish, UV, and a 0.2 µm final filter.
Type II: older editions called for distillation; current editions accept any process that meets the limits. Confirm the customer spec does not still require distilled water.
Type III at 4 MΩ·cm is beyond RO alone. Plan a DI polish.

Clinical lab · Pharma

CLSI GP40 and USP Purified Water

CLSI clinical laboratory reagent water (CLRW)
Parameter	Limit
Resistivity @ 25 °C	> 10 MΩ·cm
TOC	< 500 ppbOnline TOC not required; referral-lab testing acceptable.
Bacteria	< 10 CFU/mLViable plate count; endotoxin optional.
Particulates	0.2 µm filtration or finer

Special reagent water (SRW) is set per test by the method manufacturer. Analyzer feed specs can be tighter than CLRW; carry both.

USP Purified Water
Parameter	Limit
Conductivity (Stage 1 @ 25 °C)	≤ 1.3 µS/cm
TOC	≤ 500 ppb
Bacteria (action level)	100 CFU/mL

Conductivity is a three-stage test; Stage 1 is read without temperature compensation, and 1.3 µS/cm applies at 25 °C. Purified Water systems need design qualification and IQ/OQ/PQ.

CAP inspections

CAP inspects against CLSI GP40 or the analyzer maker's spec. The lab defines the water type for each test, reviews resistivity daily, keeps bacterial counts on schedule, and documents every out-of-spec result and its corrective action. Confirm item numbers against the lab's current Laboratory General checklist.

Technology

RO, DI, or CEDI

	Reverse osmosis	Exchange DI	CEDI
What it removes	Most dissolved salts, plus organics, particles, bacteria, and endotoxin	Ions, down to very high resistivity	Ions remaining after RO, continuously
Typical role	Primary treatment ahead of storage	Low flows, polishing, or labs without drain capacity	Chemical-free polish after RO
Needs ahead of it	Sediment filtration, softening or antiscalant, and chlorine removal	Ideally RO; consumption scales with feed TDS	RO permeate within the maker's hardness, CO₂, and chlorine limits
Drain	Yes, continuous reject; recovery set by feed chemistry	None on site	Small concentrate stream
Watch for	Scaling, chlorine damage to thin-film membranes, cold-feed derate	Exhaustion between exchanges, resin fines, bacterial growth; add a post filter	Hardness and CO₂ breakthrough from upstream

Single-pass RO permeate is rarely below a few µS/cm. If the spec needs MΩ·cm, plan a polish.

System design

A typical pretreatment and distribution system

Backflow preventer (RPZ)Required by plumbing code where treatment connects to potable water.
Pressure-reducing valveHolds a steady inlet for the equipment downstream.
Sediment filterCommonly 5 µm. Protects softener valves and carbon beds.
SoftenerTwin-tank for continuous service. Removes hardness that would scale the RO.
CarbonRemoves chlorine. Chloramine needs catalytic carbon and more contact time; size on empty-bed contact time.
ROPump, membranes, and a reject line to drain.
Storage tankWith a hydrophobic vent filter, sized to cover peak demand.
Distribution pumpOften duty/standby. Keeps the distribution loop moving.
PolishDI or CEDI when the grade needs it.
UV254 nm for bacterial control; 185 nm when TOC reduction is the goal.
Final filter0.2 µm where the standard calls for it.
Loop and points of useReturn to the tank, with sample valves at the loop out, the return, and each point of use.

Before you select anything, get a feed-water analysis: hardness, alkalinity, iron, silica, TDS, chlorine and chloramine, temperature range, and pressure.

Units

Conductivity, resistivity, and hardness

Resistivity (MΩ·cm) = 1 ÷ conductivity (µS/cm). Use conductivity for treated and utility water, and resistivity once the water is high-purity.

Resistivity	Conductivity	Where it shows up
18.2 MΩ·cm	0.055 µS/cm	Theoretical pure water at 25 °C
18.0 MΩ·cm	0.056 µS/cm	ASTM Type I floor
10 MΩ·cm	0.1 µS/cm	CLSI CLRW floor
4 MΩ·cm	0.25 µS/cm	ASTM Type III floor
1 MΩ·cm	1 µS/cm	ASTM Type II floor
0.77 MΩ·cm	1.3 µS/cm	USP <645> Stage 1 limit at 25 °C
0.1 MΩ·cm (100 kΩ·cm)	10 µS/cm	AAMI ST108 critical water limit
0.002 MΩ·cm (2 kΩ·cm)	500 µS/cm	AAMI ST108 utility water limit

Handy conversions

Hardness: 1 grain per gallon (gpg) = 17.1 mg/L as CaCO₃.
TDS estimate: mg/L ≈ 0.5 to 0.7 × conductivity in µS/cm, depending on the ions present. Use a lab analysis for design.
Temperature: conductivity rises roughly 2% per °C in ordinary water, and faster in high-purity water. Always compare compensated readings.
ppm and ppb: in dilute water, 1 ppm ≈ 1 mg/L and 1 ppb ≈ 1 µg/L.

Sterile processing

SPD equipment: what to ask

For each machine the system will feed, get these before you size anything.

Make and model, and the water spec in its instructions for use
Water quality per connection: utility, critical, or softened; hot or cold
Gallons per cycle and cycles per day, at today's load and planned growth
Peak flow (gpm) and how many machines can fill at once
Inlet pressure range and connection size
Inlet temperature, and whether the machine heats its own water
Drain capacity and location for the treatment equipment
Power available at the equipment room

Equipment that usually needs water

Washer-disinfectors
Steam sterilizers and steam generators
Cart and rack washers
Ultrasonic cleaners
Automated endoscope reprocessors
Manual cleaning sinks and rinse stations

These sheets are working summaries for reps and engineers. They are not a substitute for the current edition of each standard or for the equipment maker's instructions for use, and they are not legal or regulatory advice. Eighteen2 designs are budgetary and are not stamped; a licensed engineer or the manufacturer confirms the final design.