Power Plants’ Hidden Health Risk: A Cooling‑Tower Playbook for Beating Legionella

Cooling towers are ideal environments for Legionella—think 20–50 °C water, neutral pH, and biofilm niches—according to technical notes and public‑health guidance (Lautan Air Indonesia; Indonesia Ministry of Health alert).

In tropical Indonesia, year‑round warmth and humidity prevail, and circulating power‑plant cooling water often contains scale, corrosion and organic deposits that fuel bacterial growth. Studies confirm high contamination rates in such towers: 35.7% of industrial towers in one survey were Legionella‑positive, with 71.8% of positives exceeding 10^3 CFU/L (CFU/L: colony‑forming units per liter) (PMCID: PMC4626988).

For context, in developed regions Legionella accounts for ~1–13% of all pneumonia; US estimates suggest 25,000–100,000 legionellosis cases annually (PMCID: PMC11746996). In Indonesia, the Ministry of Health has flagged cooling towers as high‑risk sources (alert link), effectively mandating—by regulatory spirit if not explicit law—stringent controls (as do many international codes).

The plan is straightforward and data‑driven: identify hazards, prevent growth, and monitor performance. The US CDC emphasizes active control of scale, corrosion, and sediment—and thorough system cleaning—as critical to prevention (CDC toolkit), and UK HSE guidance mandates risk assessment and written control schemes with documented inspections (HSE L8).

Biocide residuals and automation

Continuous biocide dosing keeps a residual disinfectant present at all times. Oxidizing biocides (e.g., chlorine, bromine, chloramine, hydrogen peroxide) and non‑oxidizing biocides (e.g., glutaraldehyde, isothiazolines) are widely used (Iervolino et al., 2017). One industrial study applied a continuous NaOCl (sodium hypochlorite) feed at 2–3 mg/L (mg/L: milligrams per liter), supplemented by daytime boosts (study details).

CDC recommends automated monitoring and adjustment of disinfectant residual to keep it in range—typically ≥0.5–1.0 mg/L free Cl₂ in cold climates, higher in warm climates—and to log process data such as flow, conductivity, pH, and biocide feed‑rate (CDC toolkit). Automated feed using a dosing pump supports steady residuals. Tower chemistry programs routinely draw on dedicated biocides to suppress biofilm.

Some operators generate chlorine on site from salt solution—safer than storing chlorine gas—via electrochlorination. The outcome target is clear: keep total microbial counts low and Legionella undetectable. Aggressive chlorination in a contaminated tower has cut L. pneumophila from ~10^5 to ~10^1 CFU/L (over sequential shocks and maintenance feed) (Iervolino et al.).

Shock disinfection cycles

Periodic high‑dose “shock” treatments break down resilient biofilm. Industry guidance often calls for 10–50 mg/L free chlorine (or equivalent) for several hours (Indonesian technical manual). One Indonesian reference recommends an initial free‑chlorine residual ≥50 mg/L during shock disinfection (manual details). In practice, ~20–30 mg/L for 2–4 hours is common during shutdowns or suspected contamination.

After shock, systems are flushed and routine dosing resumes. Alternating oxidizing and non‑oxidizing agents helps prevent resistant biofilm regrowth (biocide classes). A measurable outcome is expected: each shock should produce a ≥1–2 log drop in viable Legionella (culture) and heterotrophic counts.

Scaling and corrosion control

Scale and corrosion create rough, rusty surfaces that harbor biofilm; CDC calls their control “critical” for prevention (CDC toolkit). Programs target low scale formation—e.g., a Langelier Saturation Index (LSI) ≤0—and manage conductivity by makeup rates.

In tropical climates, blowdown is optimized (e.g., 5–10% of flow or more) to keep cycles <5. Chemical aids are standard: plants deploy scale inhibitors to minimize deposition, and corrosion inhibitors to protect metallurgy so biocides can access all surfaces.

Mechanical cleaning and disinfection

Scheduled cleaning—often quarterly or aligned with extended outages—removes biofilm, sediment, and algae nests that chemicals alone cannot reach (CDC toolkit). Standard practice includes draining, scrubbing/pressure‑washing hot‑ and cold‑water basins, fill media, drifts, and nozzles, and replacing heavily scaled fill. Drift eliminators are kept intact to limit droplet entrainment. Many facilities rely on a professional cooling‑tower cleaning service to execute this regimen.

A simple performance measure applies: visibly clear water with no floating biofilm or sediment after cleaning. Some jurisdictions mandate weekly water changes or disinfection schedules; New York City requires weekly bacteriological testing, effectively driving frequent blowdown and dosing (NYC cooling towers).

Post‑cleaning, guidance calls for a shock disinfection before restart—for example, fan off, basin pump on, and chlorination to 50 mg/L free Cl₂ with circulation for 1–2 hours (manual steps). After shock, towers are drained and refilled, normal feed re‑established, and a repeat shock applied if Legionella is detected on follow‑up testing.

System maintenance and make‑up water

Routine maintenance limits stagnation by eliminating deadlegs and keeping auxiliary equipment clean. Filters and strainers on makeup and recirculation lines are cleaned or replaced monthly; a robust strainer program helps keep solids out of the system.

Makeup water quality is managed with pre‑filtration or UV as needed; low‑chemical, low‑operating‑cost ultraviolet systems can provide a 99.99% pathogen kill rate. Air‑intake screens are cleaned to reduce organic loading. Implementation of an autosampler logger or IoT monitoring is recommended so readings can be trended over time.

Routine monitoring and testing

Leading indicators—pH, conductivity, and biocide residual—are monitored continuously. CDC advises adjusting measurement frequency based on stability; sampling increases if residuals dip or usage changes (CDC guidance). Ideal practice is automated data logging with alarms; all chemical doses and tower operations are recorded.

Microbiological testing is scheduled regularly for Legionella. International practice is 1–3 times per year at minimum, with higher frequency in warm seasons or after prior contamination. New York City requires a certified lab culture for Legionella at least every 90 days and notifies at ≥1,000 CFU/mL (CFU/mL: colony‑forming units per milliliter) (NYC requirements). Many operators target non‑detect or <10 CFU/mL; confirmed colonization (>1000 CFU/mL) triggers immediate remedial action (shock plus re‑test) (NYC threshold). Heterotrophic plate counts (HPC) are tracked weekly or monthly; rising HPC often precedes a Legionella bloom.

Data are used to verify program efficacy. If trends rise, controls are escalated—e.g., increased biocide dosage or cleaning frequency. Case data show seasonal peaks: one study observed Legionella concentrations spiking in autumn, when tower water reached ~32 °C (seasonal trend).

Documentation and accountability

A well‑documented control plan—combining continuous biocide dosing, routine shock/cleaning, and frequent testing—measurably suppresses bacterial levels. Specific targets (e.g., chemical residuals and culture CFU counts) and records of all actions create accountability. Regulators and insurers consider such programs best practice; documented compliance can avoid fines or liability if an outbreak occurs (CDC; NYC DOH). In summary, data from research and regulatory guidance make clear that only an integrated, strictly managed water program can keep cooling towers safe and minimize disease risk.

Sources and references

Scientific studies, guidelines and industry data support the recommendations (CDC toolkit; PMCID: PMC4626988; HSE L8; PMCID: PMC11746996; Iervolino et al., 2017; NYC cooling towers; Indonesian manual). All citations correspond to the references below.

Zhong YH et al. Global Burden and Trends of Legionella spp. Infection‑Associated Diseases from 1990 to 2021: An Observational Study. J. Epidemiol Glob. Health. 2025;15(1):3 (PMCID: PMC11746996).

Li L et al. Prevalence and Molecular Characteristics of Waterborne Pathogen Legionella in Industrial Cooling Tower Environments. Int. J. Environ. Res. Public Health. 2015 Oct;12(10):12605–12617 (PMCID: PMC4626988; study details).

CDC. Controlling Legionella in Cooling Towers (toolkit, 2025) (toolkit link; monitoring guidance).

UK Health & Safety Executive (HSE). Legionnaires’ disease: The control of legionella bacteria in water systems (Approved Code of Practice). HSE Books; 2023 (HSE L8).

NYC Dept. of Health. Cooling Tower Requirements – Environmental Sampling (NYC page).

Iervolino M et al. Industrial Cooling Tower Disinfection Treatment to Prevent Legionella spp. Int. J. Environ. Res. Public Health. 2017;14(10):1125 (biocides overview; dosing example).

Lautan Air Indonesia. “Combating Legionella in Cooling Towers: Maintaining Operational Safety and Efficiency.” (company technical article).

Darwel et al. Sanitasi Rumah Sakit, 1st ed. (Internal technical guide, Indonesia 2022) (chlorination guidance).