Purified water (PW) is the pharmacopoeial grade used for most non-sterile products and for early stages of cleaning, while water for injections (WFI) is the higher grade used for parenteral products, with tighter microbial limits and an endotoxin limit. Both are defined in pharmacopoeial monographs, such as those in the European Pharmacopoeia (Ph. Eur.) and the USP, which set chemical and microbial quality and permitted methods of production.
Water is often a site's most-used raw material, made continuously by a system you design and operate. A poorly controlled loop can contaminate every batch that draws from it, so water systems are a standard stop on any GMP inspection.
Pharmacopoeial grades
The main grades you will meet are:
- Purified water: for non-parenteral products, preparation of reagents and many cleaning steps.
- Water for injections: for parenteral products and final rinsing of equipment and components for parenteral manufacture. The USP calls it Water for Injection.
- Highly purified water: a historical Ph. Eur. grade with WFI-like quality made by non-distillation methods. Its monograph has since been withdrawn, so check the current Ph. Eur. if you meet it in older documents.
Sterile packaged forms, such as sterile water for injections, have their own monographs and are not the same as bulk water from a loop.
How PW and WFI are generated
Most systems start with pretreatment: multimedia filtration, softening and removal of chlorine, often by activated carbon or chemical dosing, to protect downstream membranes. Reverse osmosis (RO) then removes most ions, organics, microorganisms and endotoxin, and electrodeionisation (EDI) polishes the water to meet conductivity requirements. RO plus EDI is the typical route to purified water.
WFI has traditionally been produced by distillation, using multiple-effect or vapour compression stills. Since 2017, the Ph. Eur. WFI monograph also permits production by a purification process equivalent to distillation, such as RO combined with other suitable techniques, subject to conditions. EU GMP Annex 1 (2022) and EMA guidance on water quality set further expectations, and the USP has long allowed non-distillation routes. Check current texts and your regulator's position before choosing technology.
Storage and distribution loops
Generation is only half the system. Water is stored in a tank and circulated continuously through a loop to points of use. Good design principles include:
- Continuous circulation at a velocity that keeps flow turbulent, which discourages biofilm.
- Temperature control: WFI loops are usually kept hot; Annex 1 gives circulation above 70 °C as an example. PW loops are often ambient and need other controls.
- Minimal dead legs: branches where water stagnates should be as short as practicable, following current ISPE and ASME BPE design guidance.
- Sanitary construction: polished stainless steel, orbital welds, sloped lines for drainage and sanitary diaphragm valves.
- Vent filters on tanks, with integrity testing and protection against condensation.
Sanitisation
Every system needs a validated sanitisation method and frequency. Hot water sanitisation and continuous hot circulation are common for WFI. Ambient PW loops often use ozone in the storage tank, with ultraviolet (UV) lamps at 254 nm to destroy ozone before water reaches points of use, plus periodic hot water or chemical sanitisation.
Base the frequency on trend data. If counts climb between sanitisations, the interval is too long or the design has a problem.
Monitoring and commonly cited limits
Routine monitoring combines online and laboratory testing:
- Total organic carbon (TOC), often measured online, indicates organic contamination. A limit of 0.5 mg/L is commonly cited for both grades.
- Conductivity, measured online and temperature-compensated or at defined temperatures, indicates ionic contamination. Ph. Eur. and USP apply different conditions, so follow the text for your market.
- Microbial counts, by membrane filtration onto low-nutrient media with extended incubation, from points of use sampled the way production uses them.
- Bacterial endotoxin for WFI, using the methods in our guide to endotoxin testing.
Commonly cited pharmacopoeial and guidance action levels are 100 cfu/mL for purified water and 10 cfu/100 mL for WFI, with a WFI endotoxin limit of less than 0.25 EU/mL. Always check the current monographs and your regulator's guidance. Sites also set lower alert levels from their own trend data so drift is caught before an action level is reached.
| Attribute | Purified water | Water for injections |
|---|---|---|
| Main use | Non-sterile products, reagents, cleaning | Parenteral products, final rinse for parenteral equipment |
| Typical generation | Pretreatment, RO, EDI | Distillation, or RO with other techniques where permitted |
| Microbial action level (commonly cited) | 100 cfu/mL | 10 cfu/100 mL |
| Endotoxin | Not specified for general use | Less than 0.25 EU/mL (commonly cited) |
| TOC (commonly cited) | 0.5 mg/L | 0.5 mg/L |
| Typical loop | Ambient with ozone and UV, or hot | Hot circulation |
Qualification and trending
New water systems are usually qualified in phases, starting with intensive daily sampling of every point, then a period showing consistent control, then a full year to capture seasonal variation in feed water. Treat these as common practice and agree the plan with QA, using current WHO, EMA and pharmacopoeial guidance.
After qualification, trend every parameter by point of use. A single result is less informative than a slow rise across several weeks.
Worked example: a loop excursion in a Hyderabad plant
A plant in Hyderabad makes oral liquids and tablets using an ambient PW loop with ozone in the tank and UV before the loop. A routine sample from the user point in the liquids manufacturing room returns a count above the 100 cfu/mL action level. Previous results at that point were well within the site alert level.
- Contain: QA raises a deviation, the user point is placed on hold and a resample is taken with the production hose and a sanitised sampling port.
- Identify: the isolates are Gram-negative non-fermenters. Identification rules out Burkholderia cepacia complex, which would be objectionable in an oral liquid, but QA still treats the excursion as significant.
- Check the system: ozone and UV logs are normal. Maintenance records show a valve was replaced near that user point two weeks earlier, and inspection finds the new branch creates a longer dead leg.
- Assess impact: batches made with water from that point since the last acceptable result are listed. Each is assessed using product microbial results, preservative efficacy and the nature of the organism, following the site's OOS investigation process.
- Correct and prevent: the branch is re-engineered to remove the dead leg, the loop is hot-sanitised, and the point is sampled daily until consecutive results meet limits. Change control is updated so water system maintenance requires engineering and QA review.
Common mistakes
- Sampling from a sanitised port with a fresh hose when production uses a different hose, which hides real contamination.
- Treating action levels as pass or fail specifications and ignoring adverse trends below them.
- Making small modifications without change control, creating dead legs.
- Assuming RO alone guarantees WFI quality without the conditions the monograph and regulators require.
- Using media and incubation conditions that do not recover stressed waterborne organisms.
Key takeaway: Purified water and WFI are defined by pharmacopoeias, but their quality is made by system design, sanitisation and trending. Check current monographs, sample like production uses water, and investigate drift before it becomes an excursion.
Frequently Asked Questions
What is the difference between purified water and WFI?
Purified water is used for non-sterile products, reagents and most cleaning, while WFI is used for parenteral products and final rinses of parenteral equipment. WFI has tighter microbial limits, an endotoxin limit and stricter production requirements, traditionally distillation. Both have chemical limits such as TOC and conductivity set out in the pharmacopoeias.
Can WFI be made by reverse osmosis?
Yes, under conditions. The Ph. Eur. has permitted WFI production by a process equivalent to distillation, such as reverse osmosis combined with other suitable techniques, since 2017. The USP has allowed non-distillation routes for longer. Regulators expect robust design, monitoring and control, so check current monographs, EU GMP Annex 1 and your regulator's position.
What is the microbial limit for purified water?
The commonly cited action level for purified water is 100 cfu/mL, and for WFI it is 10 cfu/100 mL. These are pharmacopoeial and guidance values intended for process control, and sites also set lower alert levels from trend data. Always check the current monograph and guidance for your market.
Why are WFI loops kept hot?
Hot circulation strongly discourages microbial growth and biofilm formation, which also helps control endotoxin. EU GMP Annex 1 gives circulation above 70 °C as an example of a suitable control. Some sites use cooled points of use or alternative controls, which must be justified and validated.
Next steps
Our Pharmaceutical Microbiology & Contamination Control course covers water system monitoring, objectionable organisms and investigations, and Root Cause Analysis & CAPA helps you close excursions properly. For related reading, see bacterial endotoxin testing: LAL methods, cleanroom environmental monitoring: grades A to D and IQ, OQ, PQ explained. For advice on where to start, speak to a counsellor.
Looking for structured certification?
Explore Innoryx courses or speak to a counsellor about the right program for you.