Biocides for Circulating Cooling Systems: Types, Dosing, and Control

The Cooling Tower as an Ideal Bioreactor

A circulating cooling system combines everything microorganisms need: a temperature of 25–45 °C, constant humidity, aeration, and a continuous supply of nutrients from the air. A cooling tower that cools 500 m³/h of water pumps up to 360,000 m³ of air per hour through itself — along with dust, spores, bacteria, and organic contaminants.

Over the past year, the SVK laboratory has performed over 200 analyses of water samples from circulating systems — in 40% of cases, the problem was not the biocide, but the incorrect pH of the makeup water, which reduced the efficiency of chlorination to a minimum.

Without chemical control, the biomass in the system doubles every 20–30 minutes under optimal conditions. In a week without biocide treatment, the total microbial count (TMC) grows from an acceptable 10³ to 10⁷–10⁸ CFU/ml, biofilm covers heat exchange surfaces, and with it come real risks — from a drop in cooling efficiency to outbreaks of legionellosis.

This article is a practical guide to selecting biocides for circulating systems, dosing, monitoring, and regulatory compliance. If you have not yet read the general overview of chemistry for water systems, start with the article "Reagents for Industrial Water Treatment".

Microbiological Threats in Cooling Systems

Legionella pneumophila

Legionella is a Gram-negative bacterium, the causative agent of legionellosis (lethality 5–30 % in nosocomial form). Cooling towers are one of the most frequent sources of outbreaks: an aerosol with contaminated water spreads up to 6 km from the installation.

The optimal reproduction range for Legionella is 25–42 °C, which exactly matches the operating temperature of most circulating systems. At the same time, the bacterium survives inside amoebas and biofilms, where the biocide concentration can be 100–1000 times lower than in the bulk water.

EU Directive 2020/2184 (Drinking Water Directive) establishes mandatory monitoring of Legionella in building water systems, and for industrial cooling towers, most EU countries have national regulations (for example, VDI 2047-2 in Germany). The threshold value is < 100 CFU/l; at > 1000 CFU/l, immediate disinfection is required, and at > 10,000 CFU/l, a system shutdown is required.

Biofilm and Reduced Heat Transfer

Biofilm is a structured community of microorganisms embedded in a matrix of extracellular polymeric substances (EPS). The thickness of industrial biofilm ranges from 25 µm to several millimeters.

The thermal conductivity of biofilm is 0.6 W/(m·K) — 25–80 times lower than steel. A 250 µm thick biofilm layer reduces heat exchange efficiency by 20–30 %, which is equivalent to energy overconsumption for cooling and an increase in process temperature by 3–7 °C. For steam turbine condensers, even a 1 °C increase in condensing temperature means a 0.3–0.5 % decrease in efficiency.

Additional risk: biofilm creates anaerobic zones underneath, where sulfate-reducing bacteria develop and pitting corrosion centers form. In parallel with biocide control, chemical protection against corrosion and scale inhibition are critically important — a comprehensive protection program is discussed in the article "Corrosion and Scale Inhibitors for Heat Exchangers".

Sulfate-Reducing Bacteria (SRB) and MIC Corrosion

SRB (Desulfovibrio, Desulfobacter, etc.) reduce sulfates to hydrogen sulfide (H₂S) in anaerobic zones under the biofilm. Microbiologically influenced corrosion (MIC) causes local metal destruction at a rate of 3–10 mm/year — 10–100 times faster than standard electrochemical corrosion.

A characteristic sign of MIC is black deposits of iron sulfide (FeS) under the biofilm and deep pitting with undercut edges. SRB are found in 60–70 % of cases of unexpected pipeline perforation in cooling systems. More about corrosion mechanisms and inhibition can be found in the article "Corrosion Inhibitors for Oil and Gas Pipelines".

Algae in Open Systems

In open-circuit cooling towers, sunlight stimulates the growth of green and blue-green algae (cyanobacteria). Algae themselves are not dangerous to the equipment, but their biomass clogs distribution nozzles, increases suspended solids, and serves as a nutrient medium for heterotrophic bacteria, including Legionella.

Types of Biocides

Oxidizing Biocides

Oxidizing biocides destroy the cell membranes of microorganisms through oxidation. They act quickly (minutes), have a broad spectrum, and allow for real-time monitoring of residual concentration.

Chlorine (NaOCl, Cl₂, Ca(OCl)₂) is the most common biocide for cooling towers. It is effective at pH 6.5–7.5, where the active form HOCl predominates. At pH > 8.0, efficiency drops by 80 %: HOCl dissociates into OCl⁻, whose biocidal activity is 80–100 times lower. Typical dosing: 0.3–1.0 mg/l as free chlorine. Disadvantages: formation of organochlorine compounds (THM), corrosiveness upon overdosing, and rapid degradation at high temperatures and organic loads.

Chlorine dioxide (ClO₂) is effective over a wider pH range (6.0–10.0), does not form THMs, and penetrates biofilm better due to lower reactivity with organics. Residual dosing: 0.1–0.5 mg/l. The cost is higher than chlorine, and it requires on-site generation (reaction of NaClO₂ + Cl₂ or NaClO₂ + HCl).

Bromine (NaBr + NaOCl, BCDMH) is an alternative to chlorine for systems with pH > 8.0. Hypobromous acid (HOBr) retains biocidal activity at pH up to 9.0. It is effective against Legionella. Stabilized bromine (BCDMH — bromochloro-5,5-dimethylhydantoin) is a tablet form for convenient dosing. Typical concentration: 0.5–1.5 mg/l as total bromine.

Ozone (O₃) is the strongest oxidizer (oxidation potential 2.07 V versus 1.36 V for chlorine). It completely destroys biofilm and leaves no persistent by-products. Limitations: requires on-site generation, leaves no residual protection in the system (half-life 20–30 minutes), and high equipment cost. It is mainly used in large installations (>5000 m³/h).

Non-Oxidizing Biocides

Non-oxidizing biocides act on specific cellular processes: they inhibit enzymes, destroy membranes, and block metabolism. The main advantage is the ability to penetrate biofilm and destroy microorganisms protected by the EPS matrix.

Glutaraldehyde (GA) is effective against a wide spectrum of bacteria, including SRB. It penetrates biofilm by cross-linking cell wall proteins. Dosing: 50–200 mg/l during shock treatment (2–4 hours). Limitations: toxic (maximum permissible concentration in wastewater is 0.5–1.0 mg/l), polymerizes and loses activity at pH > 8.5. Requires neutralization with sodium bisulfite before discharge.

Isothiazolinone (CMIT/MIT) is a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one in a 3:1 ratio. It is effective at low concentrations: 20–75 mg/l (by active substance). Broad spectrum, including algae. Limitations: inactivated by reducing agents (Na₂SO₃, N₂H₄), sensitizer — requires caution when handling.

DBNPA (2,2-dibromo-3-nitrilopropionamide) is a fast-acting biocide with a half-life of 2–4 hours at pH 7.0–8.0. Ideal for shock treatment: destroys planktonic bacteria in 15–30 minutes at a dosage of 10–50 mg/l. Rapidly hydrolyzes into non-toxic products — convenient for systems with discharge restrictions. Not effective against established biofilm — used in combination with a biodispersant.

THPS (tetrakis(hydroxymethyl)phosphonium sulfate) is a biocide with the additional ability to dissolve iron sulfide (FeS). Particularly effective against SRB in systems with MIC corrosion. Dosing: 50–200 mg/l. Biodegradable (>60 % in 28 days, OECD 301B). Advantages: low toxicity to aquatic organisms, non-foaming.

Comparison of Biocides for Cooling Systems

\* Chlorine oxidizes phosphonate scale inhibitors at concentrations > 2 mg/l.

\** Isothiazolinone is inactivated by sulfite oxygen scavengers.

Dosing Strategies

Continuous Dosing

Maintaining a constant residual concentration of an oxidizing biocide is the baseline strategy for most systems. Typical parameters: free chlorine 0.3–0.5 mg/l or total bromine 0.5–1.0 mg/l. An automatic controller (ORP meter or amperometric sensor) regulates the feed.

Advantages: stable microbiological control, minimization of growth peaks. Disadvantages: constant reagent consumption, gradual adaptation of microflora to low concentrations.

Periodic Shock Treatment

A short-term increase in biocide concentration to a bactericidal level. Oxidizing shock: 3–5 mg/l of free chlorine for 2–4 hours, 1–3 times a week. Non-oxidizing shock: 100–200 mg/l of glutaraldehyde or 30–50 mg/l of DBNPA for 2–6 hours, once a week.

Shock treatment is more effective against biofilm: the high concentration overcomes the diffusion barrier of the EPS matrix. The optimal strategy is a combination of continuous and shock dosing.

Slug Dosing for Heavy Contamination

With contamination > 10⁶ CFU/ml or detection of Legionella > 1000 CFU/l, a shock dose is required: 5–10 mg/l of ClO₂ or 200–500 mg/l of a non-oxidizing biocide with the prior addition of a biodispersant. The dispersant (usually a surfactant based on dodecylbenzenesulfonate or alkyl polyglucoside) destroys the biofilm structure and provides biocide access to protected cells.

Biocide Rotation

The monotonous use of a single biocide forms a resistant microflora. The recommended strategy is alternating an oxidizing (continuous) and a non-oxidizing (shock, weekly) biocide, changing the non-oxidizing product every 3–6 months.

Example protocol: continuous feed of stabilized bromine (BCDMH) + shock treatment with glutaraldehyde (April–September), switching to THPS (October–March).

Monitoring and Control

Methods of Microbiological Control

Dip Slides — immersion agar plates for rapid TMC assessment. Results after 24–48 hours of incubation at 30 °C. Accuracy: ±0.5 log. Recommended frequency: 1–2 times a week. The method is simple, requires no laboratory, but does not identify microorganism species. Definitions of terms and methods of industrial control can be found in the "Glossary of Industrial Chemistry".

ATP Testing — measures adenosine triphosphate as an indicator of total biomass. Results in 5–10 minutes (luminometer). Thresholds: < 100 RLU — clean system, 100–500 RLU — moderate contamination, > 500 RLU — shock treatment required. Advantages: speed, objectivity. Limitations: does not distinguish between live and dead cells (after shock treatment, the ATP reading may be artificially high).

Legionella Analysis — culture method on BCYE agar (ISO 11731). Results in 7–14 days. Alternatives: PCR (Polymerase Chain Reaction) — results in 4–6 hours, but also detects non-viable cells. Testing frequency: monthly for high-risk systems, quarterly for systems with stable control.

Key Threshold Values

Control Frequency

• Daily: residual biocide concentration (ORP or colorimetry), pH, temperature, conductivity
• Weekly: dip slides or ATP, visual inspection of the cooling tower
• Monthly: full microbiological analysis, including Legionella (for high-risk systems)
• Quarterly: Legionella analysis (standard systems), biofilm condition assessment (coupons or endoscopy)

Regulatory Requirements

European Union

Directive 2020/2184 (updated Drinking Water Directive) establishes a maximum permissible concentration of Legionella pneumophila < 250 CFU/l for drinking water systems. For industrial cooling towers, national standards apply: VDI 2047-2 (Germany), L8 HSE Guidance (UK), Arrêté du 14/12/2013 (France).

The EU BPR (Biocidal Products Regulation) 528/2012 requires the registration of biocidal products with proven efficacy and environmental risk assessment. Products containing active substances from Annex I undergo a simplified procedure. More details on chemical regulation can be found in the article "Industrial Disinfectants".

Ukraine

The Law of Ukraine "On Biocidal Products" (implementation of the EU BPR) is currently under development. Currently, ДСанПіН 2.2.4-171-10 (drinking water quality requirements) and ДБН В.2.5-64:2012 (design of water supply systems) are in effect. Legionella monitoring is regulated by an order of the МОЗ (Ministry of Health), but mandatory control applies primarily to medical and hotel facilities.

With the gradual implementation of the Association Agreement with the EU, the requirements for microbiological control of industrial water systems will become stricter. Enterprises working for export or with European partners already adhere to European standards.

FAQ

Which biocide is best for combating Legionella in a cooling tower?

For Legionella control, the optimal combination is: continuous feed of an oxidizing biocide (chlorine dioxide or stabilized bromine) to maintain residual protection + periodic shock treatment with a non-oxidizing biocide (DBNPA or THPS) to destroy Legionella inside the biofilm and amoebas. The sole use of chlorine is insufficient due to weak penetration into the biofilm and loss of efficacy at pH > 7.5.

How often should water be tested for Legionella?

For high-risk cooling towers (located near residential areas, hospitals, shopping centers) — monthly. For standard industrial systems with stable biocide control — quarterly. If > 1000 CFU/l is detected — immediate shock disinfection and a repeat analysis 48 hours after treatment.

Why shouldn't a single biocide be used continuously?

The monotonous application of a single product forms a resistant microflora within 3–6 months. Bacteria adapt: they produce neutralizing enzymes, change membrane permeability, and form a thicker EPS matrix. Rotating between oxidizing and non-oxidizing biocides and changing the non-oxidizing product every 3–6 months prevents the development of resistance.

How do you determine if a biocide program is ineffective?

Signs of ineffectiveness: TMC is consistently > 10⁴ CFU/ml despite dosing, appearance of slime on heat exchanger surfaces, an increase in condenser pressure drop > 15 %, a decrease in ORP < 500 mV at nominal dosing, detection of SRB > 10 CFU/ml. In this case, the program needs to be reviewed: analysis of the causes (insufficient dosing, incompatibility with water, microflora resistance) and adjustment.

SVK Biocides for Cooling Systems

SVK manufactures a line of biocidal products for circulating cooling systems: oxidizing and non-oxidizing biocides, biodispersants, and combined "biocide + corrosion inhibitor" formulas. Each product has been tested for efficacy in accordance with EN 1276, EN 13623, and EN 14885.

Our approach is not just selling a reagent, but developing a comprehensive biocide program: analyzing the microflora of your system, selecting the optimal combination of biocides, calculating the dosage and treatment schedule, and setting threshold values for monitoring.

The SVK laboratory conducts microbiological analysis of water samples, including the determination of TMC, SRB, and Legionella spp. Order a water analysis from your cooling system — we will prepare recommendations for biocide control tailored to the specific parameters of your installation.

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Read also:

• Reagents for Industrial Water Treatment
• Industrial Disinfectants
• Corrosion Inhibitors for Oil and Gas Pipelines
• Glossary of Industrial Chemistry: 30 Terms