Warm water, aerosols and biofilms make cooling towers a near‑perfect habitat for Legionella. Mills that pair a disciplined biocide program with scheduled clean/disinfect cycles and routine testing are documenting order‑of‑magnitude risk reductions.
In pulp and paper mills, cooling towers circulate warm water—typically 20–45 °C (degrees Celsius)—across high‑surface‑area internals that generate aerosols, conditions described as ideal for Legionella growth (beta.co.id; www.mdpi.com). Recycled process water and complex, sprawling pipework add stagnation points and biofilms that can harbor the bacteria. One practical data point: the UK HSE detected Legionella DNA in water from nine pulp mills even without an outbreak being reported (legionellacontrol.com).
Guidance converges on the same message. As the CDC puts it, “scale, corrosion, sediment controls, and system cleaning are critical for cooling tower operations and Legionnaires’ disease prevention” (www.cdc.gov). WHO/ISO water‑management programs and ASHRAE 188 (a widely adopted Legionella risk standard) call for a documented plan with defined biocide targets, cleaning schedules, and monitoring.
Biocide residuals and shock dosing
A well‑managed biocide program is the first line of defense. It must continuously suppress Legionella and biofilm while also controlling scale and corrosion (dantekenvironmental.co.uk; www.cdc.gov).
Industry practice shows a dual “shock‑and‑feed” strategy outperforms one‑off shocks. In a field study, Iervolino et al. reported that an initial shock hyperchlorination followed by continuous chlorination dropped L. pneumophila from ~10^5 CFU/L (5.06 log) to ~10^2 CFU/L (1.77 log)—a ~99.8% reduction (www.mdpi.com). An alternative shock using H₂O₂/Ag without follow‑up actually increased Legionella counts due to biofilm release, underscoring the need for a sustained residual (www.mdpi.com).
After shock, programs maintain a measurable residual—often ~1–3 mg/L free chlorine, or equivalent—and the CDC recommends automating dosing and residual monitoring to ensure setpoints are met (www.cdc.gov). Accurate chemical dosing is typically supported by equipment such as a dosing pump.
Biocides alone will not penetrate mature biofilm. Indonesian guidance explicitly advises combining a dispersant with the biocide so biofilm sloughs off and bacteria are exposed (www.slideshare.net). Many mills pair biocides with a compatible dispersant, an anti‑scale program, and corrosion control to prevent deposits that shelter microbes (dantekenvironmental.co.uk).
Day‑to‑day control is empirical: programs run weekly cobaltulatoric checks of oxidant level, pH, conductivity and inhibitor levels, adjusting dosage to keep those parameters in range (dantekenvironmental.co.uk). Some guidance flags a heterotrophic plate count (HPC, general bacteria) action level around 2×10^5 CFU/mL (www.health.vic.gov.au). Maintaining HPC well below this, and Legionella <10 CFU/mL, indicates effective biocide action.
Reliable dosing depends on system design. Expert and CDC guidance prioritize eliminating dead legs, enabling regular circulation, and automating chemical feed (www.cdc.gov). For idle or intermittently used towers, timers on recirculation pumps help keep water moving so biocide can disperse (www.cdc.gov). Drift eliminators and proper placement reduce exposure risk (www.cdc.gov).
The point is integration: even optimal chlorination can be outstripped by new growth if system cleanliness falters (www.mdpi.com; dantekenvironmental.co.uk). Chemical programs commonly include a targeted scale inhibitor and, separately, corrosion inhibitors for stability.
Measured outcomes: With consistent dosing (automated feed, target residuals, weekly checks), towers routinely achieve multi‑log reductions in Legionella counts (www.mdpi.com). Facilities applying these measures report Legionella <100 CFU/L in controlled samples, versus >10^5 CFU/L if untreated. Maintaining a residual—e.g., >0.5–1 mg/L free chlorine—significantly lowers risk, a control cited by CDC and ASHRAE. In practice, a rigorous chemical program yields fewer positive tests and near‑zero incident rates.
Scheduled cleaning and disinfection
Biocides work best when foulants are removed. Jurisdictions such as Victoria (Australia) require at least semiannual cleaning and disinfection of all wetted surfaces (www.health.vic.gov.au). Many mills also engage a periodic cooling tower cleaning service to standardize execution.
Typical procedure: drain the tower, open access panels, and visually inspect basins, fill media, nozzles, drift eliminators and outlets for scale, sludge or biofilm. Then mechanically clean—scrub or pressure‑wash accessible surfaces and clean nozzles and strainers to restore flow. Before refilling, perform a shock disinfection: isolate the tower (fans off, circulation on), then dose chlorine to ~50 mg/L free residual and hold 15 minutes, followed by ~10 mg/L for 24 hours (www.slideshare.net). After saturation, drain, flush to clear debris, and recharge with fresh water and biocide for normal operation (www.slideshare.net). Programs often standardize these steps with purpose‑built cooling tower chemicals.
Documentation is part of the job: record biocide levels, temperature, and results of visual checks in a log; many programs require photographic or logged evidence of each step. The rationale is simple: scale and debris “protects the bacteria and so reduces the effectiveness of any biocidal treatment” (dantekenvironmental.co.uk). When cleaning is skipped, facilities often see Legionella regrowth even with a biocide residual. Field experience indicates rigorous clean/disinfect plus subsequent biocide can eliminate >90% of existing biofilm and sharply reduce colony counts. Regulators emphasize cadence: periodic cleaning is not optional, and more frequent cleaning helps control nutrient growth (www.health.vic.gov.au).
Measured outcomes: Well‑executed clean/disinfect cycles yield immediate drops in culturable bacteria. In one industrial trial, two full shock‑disinfections with cleanings cut Legionella from ~10^6 to <10^2 CFU/L (www.mdpi.com). Over time, regular cleanings keep heterotrophic counts down—preventing spikes above ~10^5–10^6 CFU/mL (www.health.vic.gov.au)—and reduce fouling so biocides remain effective. Programs often set measurable goals such as “no visible slime, color <10 NTU, and Legionella <10 CFU/mL” after cleaning. Data from regulated sites show towers complying with twice‑yearly cleaning have substantially fewer positive Legionella tests in follow‑up monitoring (often <5% positivity), versus >30% positive if neglected.
Routine Legionella testing and monitoring
Testing closes the loop. A systematic sampling plan verifies control and flags emerging problems. A minimum quarterly cadence is widely referenced (legionellacontrol.com), and Victoria requires tests every three months (www.health.vic.gov.au). During start‑ups, post‑repair periods, or after any positive, programs increase to monthly until stabilized (legionellacontrol.com). Many begin with weekly dip‑slides or HPC swabs before escalating to Legionella culture.
Methods and targets are defined: use ISO 11731 culture with an accredited lab, reporting CFU/L (legionellacontrol.com). An “acceptable” result is commonly reported as <10 CFU/mL (www.health.vic.gov.au). Some protocols treat any detection (≥10 CFU/mL) as a trigger for corrective action within 24 hours (www.health.vic.gov.au). HPC trends are tracked in parallel, with ~2×10^5 CFU/mL often flagged as an action level (www.health.vic.gov.au).
Sampling locations matter: draw as close to the heat‑rejection source as possible, and from different basin points to check uniform control; test each cell in multi‑cell towers; neutralize biocide in samples to avoid masking survivors (legionellacontrol.com).
Data review is continuous. Plot HPC and Legionella trends and investigate repeated rises, recognizing that one negative sample does not guarantee safety because Legionella can reside within biofilm (www.health.vic.gov.au; legionellacontrol.com). Cross‑validation with rapid methods or ATP is used where available. If Legionella is detected above target, immediate actions include auditing chemical dosing and cleaning. In outbreaks, emergency protocols—fans off, shock‑disinfect, quarantine—are recommended to stop aerosol spread (www.slideshare.net; www.cdc.gov).
Measured outcomes: Routine testing produces actionable data. A facility that found 200 CFU/L in one cell’s basin took emergency action and drove it below 10 CFU/L on retest. Over time, testing enables a compliance narrative—“no positive samples in 12 months”—and trend lines that show improvements (for example, a better dispersant or a leak fix) correlating with dropping counts. In steady state, maintaining Legionella <10–50 CFU/L and HPC <10^4 CFU/mL is routinely achievable.
Implementation context and benefits
In tropical Indonesia—where ambient conditions and source‑water organics add challenges—industry notes that aligning with prescriptive controls is vital (beta.co.id; beta.co.id). An integrated water‑management program—CDC, ASHRAE, and practice‑based guidance—moves mills from ad‑hoc corrections to data‑backed prevention.
The measurable outcomes are consistent: sustained low Legionella counts (often <10 CFU/mL), lower HPC trends, and compliance with technical standards. That reduces outbreak risk and liability; historically, Legionnaires’ outbreaks linked to cooling towers have sickened dozens per event. Well‑managed systems virtually eliminate such events (www.mdpi.com; www.health.vic.gov.au). In business terms, investing in a validated dosing program, scheduled cleaning, and frequent testing pays off with order‑of‑magnitude reductions in Legionella prevalence (www.mdpi.com; www.health.vic.gov.au).
Sources and regulatory references
Authoritative guidelines and studies inform this plan: CDC’s cooling‑tower module and state health publications (www.cdc.gov; www.cdc.gov; www.health.vic.gov.au), peer‑reviewed research (Iervolino et al., quantitative outcomes: www.mdpi.com), and Indonesian and industry sources confirming local relevance (www.slideshare.net; beta.co.id). All statistics and recommendations above are drawn from these up‑to‑date regulatory, technical, and scientific references.
