PW and WFI Loop Sanitization: Hot Water, Ozone, and Chemical Comparison
Selection, limitations, validation, and measurement of microbial control methods in purified water and water for injection distribution loops.
Three Methods, Three Different System Regimes
- Hot water (and steam): uses no chemicals and is straightforward to validate, but all materials must withstand heat. Applied as continuous hot circulation or periodic thermal sanitization.
- Ozone: intended for ambient-temperature and cold-stored systems; residuals convert to oxygen via UV, but it must be applied so that it does not reach non-ozone-resistant units such as RO membranes.
- Chemical (hydrogen peroxide, peracetic acid): applicable where materials are not heat-resistant; requires rinsing and residual verification.
Method selection depends on the storage temperature of the system, material compatibility, and ease of validation. No numerical target outside the source references is given; concentration, duration, and temperature are established through system validation.
Comparison of Sanitization Methods
Figures are taken from USP ⟨1231⟩, Practical Pharmaceutics, the ISPE ozone guide presentation, and the ISO 22519 presentation; all represent examples or common practice and do not replace system validation.
| Method | Parameters cited in sources | Advantages | Limitations |
|---|---|---|---|
| Continuous hot circulation | USP: at least 80 °C most common; continuous at least 65 °C also used effectively in insulated stainless steel. Hot storage described in literature as continuously above 70 °C, in practice above 85 °C. ISPE presentation: permanent sanitization 65 to 85 °C | Chemical-free; safe; accepted; easy control; continuously inhibits growth | Operating cost; costly cooling for cold points of use; hot storage carries rouging risk |
| Periodic hot water | Literature: 90 – 95 °C, at least 2 hours, validated. ISO 22519 presentation example: lowest system temperature 80 °C for at least 30 minutes; softened water (below 20 ppm as CaCO₃) prevents scaling on heating surfaces; activated carbon filter with hot water at least 85 °C for 1 hour, or with steam at least twice a week | Suitable for infrequent sanitization in cold storage; no added chemicals | All materials and units must withstand heat; provides no permanent protection |
| Steam | ISPE presentation: sterilization with 121 °C steam or alternatively pressurized hot water | Chemical-free; proven | Production stops; system must be drained and distribution sloped; no permanent protection. USP: in steam sanitization, slope and low-point drainage are critical for condensate removal |
| Ozone | ISPE presentation: daily or weekly 20 – 50 ppb; above 40 ppb for biofilm penetration; above 200 ppb for extended periods in persistent biofilm; typical sanitization around 25 ppb. Half-life in purified water at pH 7, 25 °C approximately 15 – 20 minutes | Permanent or periodic protection; protects the distribution network; residual converts to oxygen; no wastewater cost from chemical sanitization; can be automated; TOC and pyrogen reduction observed | Cost of ozone generator, UV lamp, and residual measurement; ozone-resistant materials required; ozone destruction on tank vent; RO membranes must not be exposed to ozone |
| Chemical (H₂O₂, peracetic acid) | Literature: hydrogen peroxide and peracetic acid solution diluted to 8 – 10% is fed into the system followed by thorough rinsing. USP: these oxidants oxidize bacteria and biofilm via reactive peroxides and free radicals | Effective; easy application; low investment; can also be used on non-heat-resistant materials | Rinsing must be monitored; production stops; no permanent protection; can damage membranes and resins; storage and handling burden |
| 254 nm UV (supplementary) | USP: inactivates a high percentage of microorganisms (not one hundred percent) | Chemical-free; supplementary step that destroys residual ozone | Does not control existing biofilm; does not provide protection inside storage tanks or throughout the network |
Which Method in Which System?
Hot storage and distribution
Continuous hot circulation is cited as the most effective way to prevent microbial growth. In Mikron's WFI loop design, water is circulated continuously above 80 °C. Supplemental chemical sanitization may be considered for established biofilm. For details, see water for injection (WFI) systems.
Ambient temperature
Storage and distribution at ambient temperature (15 – 30 °C) is economical and reasonably reliable, but biofilm risk is high. Frequent sanitization by hot water flushing or adding ozone is therefore recommended. Harsh methods such as steam require durable materials.
Cold storage
Because microorganisms multiply very slowly at 4 to 10 °C, sanitization is infrequent and can be performed with hot water; the cost and energy consumption of the chiller plant are disadvantages. In Mikron's WFI systems, cold loops are configured with UV-integrated and ozone-assisted storage.
The method must be compatible with every unit in the system: the ISO 22519 presentation requires that electrolytic scale reduction, UV, filter, RO, EDI, and UF units be compatible with thermal sanitization when hot water sanitization is used. Ozone requires ozone-resistant material selection, an ozone generator, UV ozone destruction, and residual measurement.
Ozone Sanitization: Generation, Destruction, and Measurement
Generation and dose calculation
The ISPE presentation lists three generation methods: corona discharge, UV (185 nm), and electrolytic generation. Corona discharge is cited as the single economical option for large capacities above 100 g/h in drinking and softened water, 185 nm UV in small laboratory systems, and electrolytic generation in pharmaceutical water treatment; electrolytic generation requires water conductivity below 20 µS/cm. Generator capacity is calculated from make-up and recirculation flow rates, target concentration, design factor, and transfer efficiency; in the sample calculation, 3.59 g/h is obtained for a 50 ppb target.
In Mikron's ozone systems, corona discharge and electrolysis methods, dissolution via venturi injector, diffuser, or static mixer, and dose calculation using the Ct approach are applied. See our engineering solutions page for details.
Destruction and safety
254 nm UV converts ozone into oxygen. The ISPE presentation notes the high oxidation potential of ozone (2.08 V; hydrogen peroxide 1.78 V, chlorine 1.36 V) and the risk of radical formation if concentration and UV intensity are improperly designed; the short half-life of ozone mitigates this risk. USP writes that because of ozone's short half-life and achievable concentration limits, it must be added continuously. An ozone loop also requires an ambient ozone detector and off-gas destruction, which are included in Mikron's system design.
ASME BPE notes that ozone sanitization has been observed to retard alloy corrosion in purified water and WFI systems. For its relationship with rouge, see our rouging and derouging guide.
| Measurement | Instrument | Range or specification | Note |
|---|---|---|---|
| Residual ozone | Hefer O-3000-C | 0.01 – 20 ppm; 2 control relays, 1 alarm relay, 4-20 mA | 20 ppb (0.02 ppm) is near the lower end of the range; consult manufacturer documentation for accuracy at low concentrations |
| Residual ozone, controller-connected | Walchem amperometric ozone sensor | 0 – 2 and 0 – 20 ppm | The sensor manual specifies a minimum sample conductivity of 10 µS/cm; PW and WFI conductivity is at the 1.3 µS/cm limit, so sensor selection on a purified water line must be made together with manufacturer documentation |
| Water quality | Hamilton Conducell UPW | 0.01 – 1,500 µS/cm; USP 645 compliant; CIP/SIP compatible | Conductivity monitoring on PW and WFI return loops |
| Temperature | Inline PT100 | Verification of the coldest point during thermal sanitization | Used for thermal mapping and loop monitoring |
The ISPE presentation cites membrane electrochemical (polarographic) sensors and UV photometers for ozone measurement and states that the reliable detection limit with current methods is approximately 5 ppb.
How Is Sanitization Validated?
- Thermal method: USP ⟨1231⟩ requires a heat distribution study demonstrating that the sanitization temperature is reached throughout the system, including point-of-use valve bodies.
- Chemical method: adequate chemical concentration throughout the system, exposure of all wetted surfaces (including point-of-use valve bodies), and complete removal of the sanitant at the end of the process must be demonstrated. The method for determining residuals of the sanitant and its degradation products is also part of validation.
- Frequency: sanitization frequency is supported—and even triggered—by the microbial monitoring results of the system; microbiological trend analysis serves as an early warning for maintenance.
- Why it matters: microorganisms within a well-developed biofilm are difficult to kill even with aggressive oxidizing biocides; the thinner the biofilm, the greater the efficacy. Frequent sanitization therefore aims to prevent significant biofilm development.
In system design, slope, drainage, and dead-leg rules govern the effectiveness of sanitization: hygienic design and dead legs. For CIP and SIP operations, see CIP and SIP explained; for passivation and surface verification, see passivation and derouging and passivation verification tests. IQ/OQ/PQ and sanitization validation documentation are covered under system validation and qualification, UV systems under UV disinfection systems, and purified water systems under purified water (PW) systems.
Sources: USP 37 ⟨1231⟩; Boeke and Le Brun, Practical Pharmaceutics, Chapter 27; ISPE Ozone Sanitization Guide presentation (2014); ISO 22519 presentation; ASME BPE-2019 Nonmandatory Appendix D.
Frequently Asked Questions About PW and WFI Sanitization
Hot water, ozone, chemical sanitization, residual removal, and validation.
How is a PW and WFI system sanitized?
USP ⟨1231⟩ states that microbial control is achieved primarily through sanitization practices and that this can be performed by thermal or chemical means. The thermal approach uses periodically or continuously circulating hot water and steam; the chemical approach uses oxidizing agents such as halogenated compounds, hydrogen peroxide, ozone, and peracetic acid. Selection depends on the thermal resistance of the materials, storage temperature, and how easily residuals can be removed.
At what temperature is hot water sanitization performed?
It varies by reference. USP ⟨1231⟩ writes that at least 80 °C is most commonly used, while continuously circulating water at a minimum of 65 °C has also been used effectively in insulated stainless steel distribution loops. In pharmaceutical engineering literature, 90 to 95 °C for at least 2 hours is cited for periodic hot water disinfection, and the process requires validation. The ISO 22519 presentation gives as an example a periodic thermal sanitization where the lowest temperature in the system is 80 °C for at least 30 minutes. Values are established through validation according to the system and materials.
What concentration is used in ozone sanitization?
In the ISPE ozone guide presentation, 20 to 50 ppb is cited for daily or weekly application, above 40 ppb to penetrate biofilm, and above 200 ppb for an extended period in persistent biofilms; sanitization is mostly carried out around 25 ppb. The half-life of ozone in purified water at pH 7 and 25 °C is approximately 15 to 20 minutes. Exact values are established by validating the system.
How is ozone removed from the system?
UV light at a wavelength of 254 nm converts ozone into oxygen. The ISPE presentation notes that 254 nm UV delivers a dose above 600 J/m² to destroy ozone and microorganisms, while 185 nm UV is used for TOC reduction. USP ⟨1231⟩ also notes that the ease with which ozone is degraded by 254 nm UV at points of use makes it well suited for continuous use.
Does ozone damage RO membranes?
Pharmaceutical engineering literature states that RO membranes must not be exposed to ozone and that the manufacturer should specify the proper disinfection method for the membrane. The comparison table in the ISPE presentation likewise notes that chemical sanitization can damage membranes and resins. In practice, ozone is applied in the storage tank and distribution loop, while the manufacturer's documentation is followed for disinfecting the membrane unit.
Does hot water sanitization remove biofilm?
USP ⟨1231⟩ writes that thermal methods control biofilm development either by inhibiting its growth or, in periodic applications, by killing microorganisms within the biofilm, but are not effective at removing established biofilm. Killed yet intact biofilm can serve as a nutrient source for rapid regrowth after sanitization ends. In such cases, combining routine thermal sanitization with periodic chemical sanitization may be more effective.
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