Establish the program team
Name the people responsible: facility management, the water-treatment provider, operators and, where needed, outside Legionella expertise. Record who can act when a limit is missed.
How to Control Legionella in Cooling Towers (ASHRAE 188 and CDC Toolkit)
Why towers grow Legionella, the water management program steps behind ASHRAE 188 and the CDC toolkit, the control measures that matter, and where ORP-controlled mineral oxychloride fits.
Control Legionella in a cooling tower with a written water management program that follows ASHRAE Standard 188 and the CDC toolkit. Keep a measurable oxidizing biocide residual every day, remove biofilm and sediment, eliminate stagnant piping, maintain drift eliminators, monitor continuously, test for Legionella to validate control, and act on any result outside your limits.
Approved, certified & independently validated
EPA FIFRARegistered
USDA NOPOrganic Allowed
NSF/ANSIStandard 60
UC DavisUniversity of WashingtonSpecial Pathogen LaboratoryMontana State UniversitySawtooth Ag ResearchGreenAgri SolutionsSouthern California EdisonGoleta Water District
~90%Of Legionella lives inside biofilm, not the water (Montana State University)
20–50 °CLegionella growth range, optimum near 37 °C
6-logLegionella reduction in under 10 seconds above +700 mV (Special Pathogen Laboratory)
400–500 mVRoutine cooling-tower ORP band for biofilm and MIC control
The Hazard
Why do cooling towers grow Legionella?
Short answer: a cooling tower combines warm water, nutrients, low-flow areas, sediment and biofilm with a fan that throws aerosol into the air. Legionella multiplies in the biofilm, often inside amoebae, and spreads in drift. CDC notes that tower aerosol can carry the bacteria over miles.
Legionella occurs naturally in fresh water, usually at low levels. It becomes a hazard when it gets into a building water system and multiplies. Cooling towers give it most of what it needs:
- Temperature. Legionella grows between about 20 and 50 °C, with an optimum near 37 °C. CDC puts the growth-favoring range at 77–113 °F (25–45 °C). Most towers run inside that band by design for much of the year.
- Stagnation and water age. Dead legs, bypass lines, idle standby cells and towers that cycle on and off let water sit and lose its disinfectant residual.
- Biofilm. Work at the Center for Biofilm Engineering at Montana State University puts roughly 90% of the Legionella in a system inside biofilm rather than in the water. Biofilm-grown Legionella is more resistant to disinfectants than free-floating cells.
- Sediment and scale. Basin sludge and deposits feed the film and shelter it from biocide.
- Amoebae and protozoa. Legionella infects free-living amoebae and multiplies inside them, which gives it a second protected reservoir inside the biofilm.
- Aerosol. Infection occurs when contaminated droplets are inhaled, and towers make aerosol continuously. Drift eliminators and tower siting decide how much of it leaves the tower.
This is why a clean bulk-water sample does not prove a clean tower. A grab sample sees the cells that have sloughed off the film, not the reservoir on the fill and in the basin. More on that in Legionellosis: breaking the chain of infection.
The Framework
What do ASHRAE Standard 188 and the CDC toolkit ask for?
Short answer: both ask for a written, maintained water management program. ANSI/ASHRAE Standard 188, Legionellosis: Risk Management for Building Water Systems, sets the minimum requirements. CDC’s toolkit, Developing a Legionella Water Management Program, explains how to build one step by step and has a dedicated cooling-tower module.
ASHRAE 188 applies to human-occupied commercial, institutional, multi-unit residential and industrial buildings, and it is written in enforceable language so jurisdictions can adopt it. ASHRAE Guideline 12 supplements it with practical guidance for specific systems. The CDC toolkit sets out seven steps. The sequence below follows those steps, with monitoring split out as its own step because it is where cooling-tower programs most often fail.
Step by Step
How do you build a cooling-tower water management program?
Describe the system and draw a flow diagram
Map makeup water, towers and cells, basins, pumps, exchangers, bypasses, side-stream filters, chemical feed points and blowdown. Mark standby equipment and seasonal operation.
Identify hazardous conditions
Find where Legionella could grow and spread: warm water, low-flow and dead-leg piping, sediment, biofilm, loss of residual, and drift reaching air intakes or occupied areas.
Set control points and limits
For each hazard, decide where it is controlled and the acceptable range. Examples: oxidizing residual or ORP band, pH, conductivity and cycles, dead-leg flushing frequency, and drift eliminator condition.
Monitor the control points
Decide who measures what, how often and with what instrument. Automated feed and continuous logging of residual or ORP catch the overnight and weekend excursions that spot checks miss.
Define corrective actions
Write down what happens when a limit is not met, from adjusting feed to escalating disinfection, and who authorizes each step. Include the response to a positive Legionella result.
Verify and validate
Verify that the program is carried out as written. Validate that it works, including routine Legionella testing by a qualified laboratory, and revise the plan when results or the system change.
Document and communicate
Keep the plan, logs, service reports and test results on site. Train staff when anything changes, and keep the records that your jurisdiction requires.
Control Measures
Which control measures matter most in a cooling tower?
Short answer: a continuously maintained oxidizing biocide program, biofilm and sediment removal, automated monitoring, good drift elimination, and scheduled cleaning and disinfection. CDC’s cooling-tower module calls for a measurable oxidizing residual throughout each day and for automating disinfectant, anti-scale and anti-corrosion feed.
| Hazard | Control measure | How it is monitored |
|---|---|---|
| Microbial growth in bulk water | Oxidizing biocide with a measurable residual every day | Free-halogen test or continuous ORP; heterotrophic plate counts |
| Biofilm on fill, basin and exchangers | Biofilm-removing oxidizer or biodispersant; physical cleaning | Visual inspection, approach temperature, ORP recovery after feed |
| Sediment and suspended solids | Automated blowdown; side-stream filtration; basin cleaning | Conductivity, turbidity, inspection |
| Stagnation and dead legs | Piping without dead ends; recirculation during idle periods; flush low-flow runs at least weekly | Flushing log |
| Scale and corrosion | Inhibitor program; pH control suited to the biocide | pH, corrosion coupons, inhibitor residual |
| Aerosol drift | High-efficiency drift eliminators; tower at least 25 ft from air intakes | Inspection of eliminator condition and fit |
| Startup, idle and shutdown | Clean and disinfect before commissioning and startup, when idle and after shutdown, plus offline cleaning at least annually | Service records |
Why ORP
Why monitor ORP in a cooling tower?
Oxidation-reduction potential (ORP) measures the oxidizing power of the water in millivolts. It reflects the condition that actually kills the organism, rather than how much chemical went in. A free-chlorine reading in ppm means different things at pH 7 and pH 8.5, because pH shifts how much chlorine is in its active form. ORP folds that effect into one number a controller can hold.
In the benchmark Jenfitch uses, surviving bacteria fall from about 300 CFU per 100 mL at +200 mV to 3 at +400 mV and zero at +600 mV. The practical rules are simple. Trend ORP continuously on the recirculating line rather than taking spot readings. Log every excursion below setpoint, because that is when the film rebuilds. Keep culture testing as the verification, and use the ORP trend as the control loop. Typical setpoints are on the ORP reference chart.
Corrective Action
What should you do after a positive Legionella result?
Short answer: follow the corrective actions in your water management program and any notification your jurisdiction requires, then escalate disinfection and cleaning in proportion to the result. CDC’s cooling-tower module describes four levels of response and an emergency procedure for when public health authorities suspect illness.
In order of increasing intensity, the CDC cooling-tower module lists online remedial treatment, online disinfection, offline cleaning and disinfection, and offline emergency cleaning and disinfection. The emergency procedure includes:
- removing heat load, shutting off fans and closing nearby air intakes;
- raising the oxidizing disinfectant to a specified level and holding it;
- controlling pH so the disinfectant stays effective;
- draining the tower, physically cleaning fill, drift eliminators and strainers, then refilling and disinfecting again;
- testing for Legionella with public health involvement before and after remediation.
Use the CDC text and your local health authority’s instructions as written. Do not adapt either from a summary. Afterward, find out why control was lost, whether through a missed residual, a dead leg, a fouled basin or a failed eliminator, and fix that in the plan, not only in the tower.
Where Jenfitch Fits
Where does mineral oxychloride fit in a Legionella program?
Short answer: as the oxidizing biocide inside the plan, not as a replacement for the plan. JC 9465 and JC 9450 are dosed to an ORP setpoint and oxidize the biofilm where most Legionella lives. Special Pathogen Laboratory testing recorded a 6-log Legionella reduction in under 10 seconds with the water held above +700 mV.
Biofilm removal
The reactive oxygen species attack the EPS matrix that shelters Legionella and its amoebal hosts, not only free-floating cells. Run 600–800 mV to break down established film, then drop back. See JC 9465 as a biofilm biodispersant.
ORP-controlled dosing
Routine biofilm and MIC control runs at 400–500 mV on a metering pump and ORP controller. The same feed also controls algae and fouling, which simplifies steps 4 and 5 of the program. See the Legionella case study.
The limits matter as much as the strengths. Mineral oxychloride is controlled by ORP, not by a free-halogen reading, so a rule or CDC procedure that specifies free available oxidant in ppm still has to be met as written. Use the product only as its label allows. It is a specialty chemistry with a higher unit price than chlorine or bromine. Bench-test it on your water before changeover; chlorine alternatives for cooling towers compares the options.
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Rules vary by jurisdiction
Requirements differ by state and city. New York State’s cooling-tower regulation (Subpart 4-1) requires owners to register towers and have a maintenance program and plan that follows ASHRAE 188-2018. It also requires Legionella testing by a state-certified laboratory, inspections every 90 days while a tower is in use, and annual certification. New York City adds its own registration and testing rules under Local Law 77 of 2015 and later amendments, including monthly Legionella sampling since May 2026. Check the current rules wherever your towers operate.
This page is general technical information, not medical, legal or regulatory advice. Compliance requirements vary by jurisdiction and change over time; follow the current text of ASHRAE Standard 188, CDC guidance, your local health authority and the product label, and involve a qualified water-treatment professional. Terms are defined in the glossary.
Frequently Asked Questions
Common questions.
Is a cooling-tower water management program required by law?
It depends on where the tower is. ASHRAE Standard 188 is a consensus standard that becomes mandatory when a jurisdiction, payer or code adopts it. New York State, for example, requires every cooling tower owner to have a maintenance program and plan that follows ASHRAE 188-2018, and New York City has its own rules. Elsewhere, requirements may come from health departments, building codes or accreditation bodies, so check locally.
How often should a cooling tower be tested for Legionella?
Frequency is set by your water management program and any local regulation, not by a single national rule. New York City, for example, has required monthly Legionella sampling, with no more than 31 days between samples, since May 2026. Many programs test more often after startup, after a positive result or after a system change. Use a qualified laboratory, keep the reports with your program records, and treat routine results as validation that your control measures work.
What is the difference between verification and validation in a water management program?
Verification confirms the program is being carried out as written: readings are taken, logs are complete, and feed equipment and drift eliminators are maintained. Validation confirms that the program actually controls the hazard, usually through Legionella testing and other indicators such as plate counts. A program can pass verification and still fail validation, which is the signal to revise the control limits.
Do drift eliminators prevent Legionella?
No, they reduce exposure. Drift eliminators capture water droplets before they leave the tower, limiting how much aerosol escapes, and CDC recommends high-efficiency eliminators and siting towers away from building air intakes. They do nothing about growth inside the tower, so they must be paired with biocide control, biofilm removal and cleaning. Inspect them for gaps, damage and fouling, and clean them during offline disinfection.
What records should a cooling-tower water management program keep?
Keep the written plan and flow diagram, the team list, control limits, monitoring logs for residual or ORP, pH and conductivity, flushing and blowdown records, chemical delivery and feed records, inspection and service reports, cleaning and disinfection records, laboratory results, and every corrective action taken with its date and outcome. CDC recommends keeping logs, test procedures, service reports and results on site, and some jurisdictions set specific retention periods.
Can mineral oxychloride be used during emergency disinfection after a positive result?
Follow CDC's procedure and your health authority's instructions as written. CDC's emergency steps specify a free available oxidant level, which is measured differently from the ORP setpoint used to control mineral oxychloride. Mineral oxychloride's role is in routine control and biofilm remediation: holding 600 to 800 mV to break down established film, then returning to the 400 to 500 mV maintenance band, always within the product label.