Surging outbreaks and hard numbers are forcing cooling tower operators to run tighter programs: continuous biocide control, scheduled deep cleans, and routine culture testing with clear action thresholds.
Legionella thrives in warm, stagnant water — exactly the conditions found in many cooling towers — and the public-health stakes are no longer abstract. Reported U.S. cases hit ≈10,000 in 2018 (asm.org), a 2015 New York outbreak tied to a single tower caused 138 cases and 16 deaths (asm.org), and a 2025 Harlem cluster saw 111 cases and 6 deaths from 12 contaminated towers (apnews.com).
Prevalence surveys confirm the exposure risk: a Chinese study found 35.7% of 255 industrial cooling-tower samples positive for Legionella, averaging 9,100 CFU/L (CFU/L: colony-forming units per liter; a culture-based measure of bacteria) (pmc.ncbi.nlm.nih.gov). In Indonesia’s warm, humid climate (20–50 °C year-round), cooling towers similarly offer ideal conditions (www.slideshare.net).
The resulting playbook is aggressive by design: maintain biocide residuals around the clock, execute rigorous cleanings, and test routinely to keep counts low.
Continuous biocide control and monitoring
A robust biocide regimen, with automated dosing to hold a constant disinfectant residual (for example, free chlorine), is central to suppression (cdc.gov). When chlorine levels fall, Legionella can surge; one study recorded levels above 10^4 CFU/L under low chlorine (pubmed.ncbi.nlm.nih.gov). In practice, many operators target ~1–3 ppm free chlorine continuously, with automated controllers adjusting dosing based on sensor feedback, consistent with CDC guidance (cdc.gov).
In power stations, chemical feeds are often delivered through metered equipment; specifying dosing pumps and validated biocides aligns the day-to-day operation with these residual targets.
Shock dosing and chemistry sequencing
Periodic “shock” cycles — high-dose oxidant pulses — provide needed knock-down. CDC advises raising free available oxidant to ≥20 ppm during a shock (cdc.gov). In one cooling tower, a strong chlorine shock plus continuous feed cut Legionella from ~10^6.14 to 10^1.77 CFU/L, a ~4.4-log (≈99.99%) reduction (pmc.ncbi.nlm.nih.gov). By contrast, a peroxide+silver shock in the same system increased counts from 5.06 to 6.14 log CFU/L (pmc.ncbi.nlm.nih.gov), underscoring the risk of ineffective chemistries.
Plans typically specify which oxidants to deploy and how to rotate oxidizing and non-oxidizing agents to limit resistance and target biofilms (as recommended by water treatment experts). Plants looking to avoid gas handling may incorporate on-site generation, such as electrochlorination, within the defined shock program.
Auxiliary controls and quality checks
Biocide efficacy hinges on water chemistry. During shock treatment, CDC guidance sets pH bounds — pH less than 8.0 for chlorine or less than 10.5 for bromine (pH: a measure of acidity/alkalinity) (cdc.gov). Dispersants and antifoaming agents improve contact with water and biofilm (cdc.gov), making products such as dispersant chemicals and antifoam relevant adjuncts.
Heterotrophic plate counts (HPC; a general indicator of bacterial load) serve as indirect early warnings. In the cited case, HPC at 36 °C dropped ~1.95 log after proper treatment (pmc.ncbi.nlm.nih.gov). Continual logging of residuals, pH, and microbial indicators verifies that the biocide program operates as intended.
Offline cleaning and disinfection cycles
Biofilms, scale, and debris provide shelter for Legionella; routine physical cleaning is therefore critical. CDC and industry guidance call for taking towers offline at least annually for deep cleaning (cdc.gov; cdc.gov), with some operators performing this semi-annually at high-risk sites.
Typical outages involve draining the system, brushing or pressure-washing fill, nozzles, and sumps, then flooding with a high-dose oxidizer. CDC’s offline protocol circulates water, brings oxidant to ≥20 ppm, and maintains contact while scrubbing (cdc.gov). Lowering cycles of concentration (the ratio of dissolved solids in circulating water to makeup water) — i.e., increasing bleed-off — helps remove sediments during cleaning (cdc.gov). Plants often rely on specialist providers; a scheduled cooling tower cleaning service builds this into the maintenance calendar.
Post-clean chemistry and nutrient control
After cleaning, systems are reassembled and refilled under controlled chemistry. Removing sludge and organic residues starves Legionella of nutrients; in the noted case study, mandated cleaning and a chlorination ramp-up drove Legionella and heterotrophic counts toward background levels (pmc.ncbi.nlm.nih.gov). Skipping physical scrubbing leaves pockets for regrowth, which is why plans mirror step-by-step procedures consistent with ASHRAE or CDC outlines.
Once back in service, replenishing inhibitors protects metallurgy and heat-transfer surfaces; programmatic use of scale inhibitors and corrosion inhibitors is paired with the biocide strategy defined above.
Emergency cleaning and response hierarchy
Loss of control — whether indicated by routine tests or an outbreak — triggers expedited shutdown and cleaning. CDC provides a hierarchy of emergency response protocols. In practice, this means performing an offline disinfection outside the normal schedule, followed by reaffirmation testing to confirm success (per CDC advice embedded throughout the cooling towers toolkit).
Routine culture testing and thresholds
Environmental monitoring anchors the plan. Sampling frequency is risk-based; common practice is monthly or quarterly testing of cooling tower basins for Legionella by culture (some jurisdictions mandate quarterly; quarterly is a reasonable minimum for high-use systems). Samples from the sump or distribution basin are cultured per standard methods to establish a baseline; one survey reported positive-sample ranges from 10^2 to 8.8×10^4 CFU/L (pmc.ncbi.nlm.nih.gov).
Action thresholds are set in advance. Many water-safety guidelines regard less than 100 CFU/L as acceptable and greater than 1,000 CFU/L as triggering immediate remediation (ncbi.nlm.nih.gov). If results exceed the threshold, the plan’s corrective steps — higher biocide dose, cleaning, increased ventilation purge — are implemented. Per CDC advice, even one confirmed case of Legionnaires’ disease linked to a tower or any high-level positive typically requires public health notification and intensive control actions.
Performance indicators and seasonal trends
Beyond direct Legionella tests, correlated parameters are logged frequently. HPC and residual chlorine are tracked at least weekly or monthly; sudden deviations can prompt accelerated Legionella testing. Seasonal patterns matter: the Chinese study found significantly higher contamination in summer, peaking at 27,100 CFU/L in August (pmc.ncbi.nlm.nih.gov). Over multiple tests, a well-run system shows consistently low or undetectable Legionella; repeated positives or rising levels prompt a review of the control program (метricolation changes, or if CC> monitoring).
Data trends are operationalized through the same equipment that executes the program. For example, automated oxidant feed systems and their controllers are often integrated with the chemical package; specifying a coherent set of cooling tower chemicals and compatible controls improves response fidelity.
Treatment verification and documentation
Verification closes the loop. After a shock dose, immediate follow-up sampling should reflect a steep drop in counts, as seen after chlorination in the cited study (pmc.ncbi.nlm.nih.gov). Staying below action levels over time becomes an objective metric; facilities sometimes record a “legionella-free” streak to track sustained performance and demonstrate regulatory compliance.
Each element — biocide dosing, cleaning, testing — is data-driven. One power plant’s conversion to a chlorine-based program with annual cleaning produced a 4+ log drop in Legionella counts (pmc.ncbi.nlm.nih.gov). Neglecting any part invites regrowth; without routine shocks or cleaning, towers have returned CFU counts in the 10^3–10^5 range (pmc.ncbi.nlm.nih.gov). The management plan integrates all three into a cycle of monitor–act–verify, with clear documentation — chemicals added, dates of cleaning, test results — to ensure continuity and accountability.
