Cooling Tower Water Treatment

Why Cooling Towers Present the Highest Legionella Risk

Cooling towers reject heat by evaporation and can generate aerosols. Warm recirculating water, nutrients, deposits, stagnation and extensive wetted surfaces can favour microbial growth. If Legionella is present and control fails, aerosols create a route of exposure.

Biofilm and deposits can shelter microorganisms and increase oxidant demand, but their presence does not by itself establish that viable Legionella is present. The risk assessment should consider system design, operation, condition, water quality, exposure routes and the people who may be exposed.

HSG274 Part 1 therefore uses a multiple-barrier approach. Good engineering, drift control, effective water treatment, planned cleaning and disinfection, inspection, monitoring and timely corrective action all matter.

Assessing Chlorine Dioxide in Cooling-System Legionella Control

Chlorine dioxide is one oxidising-biocide option. Its performance depends on the specific formulation, delivered concentration, contact time, system demand, mixing, temperature, pH, deposits and the organisms present. Published evidence for one formulation or test condition cannot be treated as proof for every tower or product.

Claims about biofilm control require verification in the actual system. A measured residual confirms active at the sampling point and time; it does not prove that every wetted surface was reached or that attached growth was removed.

Selection must be made by competent persons within the full treatment programme, considering corrosion and scale control, materials, discharge constraints, safe dosing and interactions with other chemicals. Use only a product supplied under the applicable GB BPR position, whether authorised or lawfully available under relevant transitional arrangements, and follow its permitted label and Safety Data Sheet.

PT11 includes open and closed liquid-cooling and processing systems; it is not blanket evidence that Plus L20 is suitable for every circuit. Assess Plus L20 only where its current label and technical documents support the intended use. Review system design, existing corrosion inhibitors, glycol, materials, operating conditions, dosing and monitoring.

No routine recommendation is made for inhibited closed heating or chilled-water loops without expressly supported current product instructions and written compatibility evidence. In a closed-circuit evaporative cooler, assess the external spray-water circuit separately from the enclosed process-fluid loop. HSG274 Part 1 controls for evaporative spray water do not establish suitability for treatment of the enclosed loop.

HSG274 Part 1: Regulatory Requirements for Cooling Tower Treatment

HSG274 Part 1 is HSE technical guidance for controlling Legionella in evaporative cooling systems. It should be read with ACOP L8 and applied through a system-specific risk assessment and written scheme under competent oversight.

The written scheme should define responsibilities, safe operation, the water-treatment programme, inspection, cleaning and disinfection, sampling points, monitoring frequencies, control limits, corrective actions and records. Operators should use the current HSG274 Part 1 tables rather than relying on summary figures on a supplier page.

Microbiological results need interpretation with operating and treatment data. An adverse result requires the response in the written scheme and HSG274 Part 1; a satisfactory result does not remove the need to maintain all control measures.

A biocidal product must be supplied under the applicable GB BPR position, either authorised or lawfully available under relevant transitional arrangements, and used according to its permitted label. PT11 covers preservation of liquid-cooling and processing systems. Other disinfecting purposes may fall under a different product type, so PT11 should not be treated as blanket permission for all water or equipment disinfection.

Implementing Chlorine Dioxide in Cooling Tower Systems

Before changing chemistry, a competent person should review the risk assessment and written scheme, the intended regulatory purpose, applicable GB BPR product status and permitted label, system volume and turnover, demand, materials, discharge, chemical interactions and monitoring capability.

There is no universal chlorine dioxide dose or residual for every cooling tower. The permitted label and site-specific written scheme govern dosing. Commissioning should verify delivery and measurement at representative points, while microbiology, cleanliness and operating trends verify the wider programme.

Do not assume compatibility with every inhibitor, elastomer, metal or dosing arrangement. Obtain product and equipment documentation, assess injection-point reactions and material limits, and control exposure during storage, dosing, sampling and maintenance.

Any microbial or biofouling outcome, removal, energy saving or reduction in cleaning must be demonstrated for the system. Physical cleaning and maintenance remain necessary where the written scheme requires them.

2026 Biofilm Monitoring Study: Useful Evidence with Clear Limits

Kerry and colleagues monitored two operational cooling towers on one UK nuclear-waste site on eight sampling occasions from March to October, collecting five biofilm and three bulk-water samples per tower at each time point. Legionella pneumophila serogroup 1 DNA was detected by qPCR exclusively in tower A2 biofilms and never in bulk water. Broader Legionella signals in biofilm preceded or coincided with some bulk-water rises, but there was no universal lead time and no proof that L. pneumophila moved from biofilm into water.

qPCR did not establish viability, and culture confirmation was unavailable because of quarantine and sample-timing constraints. Total and intact cell counts were not reliable Legionella predictors. The study supports considering representative biofilm evidence alongside water and operational data, not replacing HSG274 monitoring. It was an observational field study, not a comparative biocide trial; ChloroKlean was not tested.

Read the dedicated cooling-tower biofilm monitoring guide for the study design, FAQs and sampling limits.

GB BPR status and verification

ChloroKlean products are supplied for their stated Great Britain uses under applicable GB BPR transitional arrangements while the relevant chlorine dioxide precursor-route and product-type combinations remain in the GB Review Programme. This is not a claim of full product authorisation. No EU or Northern Ireland authorisation is claimed.

Verify the exact precursor route and product type, current Review Programme status and deadlines, the applicable GB Article 95 supply chain, product label and other national requirements. Article 95 listing is a supply-chain requirement, not product authorisation, an efficacy assessment or an HSE endorsement.

A screening comparison only. Applicable GB BPR status, permitted label, site trials and the written scheme govern selection and use.

HSG274 Part 1 does not make a generic product universally compliant or effective. Competent selection and verification remain necessary.

A planning sequence for evaluating a lawfully supplied chlorine dioxide product within the complete HSG274 Part 1 programme.

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