Cooling Tower Water Treatment: A Complete Program Guide for Industrial Facilities

A cooling tower is one of the hardest-working pieces of equipment in an industrial facility — and one of the most neglected from a water chemistry standpoint. When the water treatment program is working, nobody notices. When it fails, the consequences range from expensive (heat exchanger fouling, corrosion damage, unplanned downtime) to serious (Legionella outbreaks, regulatory citations).

This guide covers the fundamentals of cooling tower water treatment: the three problems you are solving, the chemicals that solve them, how to manage blowdown and cycles of concentration, and the Legionella compliance requirements that apply to Texas industrial facilities.

Every cooling tower water treatment program is designed to control three interrelated problems:

As water evaporates in the cooling tower, dissolved minerals — primarily calcium carbonate, calcium sulfate, and silica — become increasingly concentrated. When concentration exceeds solubility limits, these minerals precipitate and deposit on heat transfer surfaces as scale.

Scale is an excellent insulator. Even a thin layer (1/32 inch) of calcium carbonate scale on a heat exchanger surface reduces heat transfer efficiency by 10–15%. Heavier scale deposits can reduce efficiency by 30–40%, forcing chillers and process coolers to work harder to achieve the same cooling effect — directly increasing energy costs.

Scale also creates conditions for under-deposit corrosion, where the chemistry beneath a scale deposit differs from the bulk water, accelerating metal attack.

Cooling water is corrosive to the metals it contacts — carbon steel, copper, brass, galvanized steel, and aluminum are all vulnerable to varying degrees. Corrosion is driven by dissolved oxygen, chlorides, low pH, and galvanic effects between dissimilar metals.

Corrosion damage manifests as pitting in heat exchanger tubes, thinning of pipe walls, and failure of pump impellers and other wetted components. A single pinhole leak in a heat exchanger tube can contaminate process streams and require expensive tube replacement or bundle replacement.

Cooling towers are ideal environments for microbial growth — warm water, sunlight, nutrients from makeup water and atmospheric contamination, and large surface areas for biofilm attachment. Uncontrolled biological growth creates:

• Biofilm on heat transfer surfaces — a highly insulating layer that reduces heat transfer efficiency even more than mineral scale
• Microbiologically influenced corrosion (MIC) — certain bacteria produce acids and other corrosive byproducts that attack metal surfaces beneath biofilm
• Legionella pneumophila — the bacterium that causes Legionnaires' disease, which can be aerosolized by cooling tower drift and inhaled by people in the vicinity

Legionella is the most serious biological risk in cooling tower management. Texas has seen multiple Legionnaires' disease outbreaks linked to inadequately maintained cooling towers, and regulatory requirements for Legionella risk management have increased significantly in recent years.

Scale inhibitors work by interfering with crystal growth — they adsorb onto the surface of forming mineral crystals and distort their structure, preventing them from growing large enough to deposit. Modern scale inhibitors also disperse existing deposits, keeping them suspended in the bulk water where they can be removed by blowdown.

Phosphonates (HEDP, PBTC, ATMP) are the workhorses of cooling water scale inhibition. They are effective against calcium carbonate and calcium sulfate scale at low dosages and provide some corrosion inhibition benefit.

Polyacrylates and polymaleic acid copolymers are effective dispersants for calcium carbonate, calcium phosphate, and silica. They are often used in combination with phosphonates for broad-spectrum scale and deposit control.

Silica inhibitors are specifically formulated for systems with high silica in the makeup water — a common challenge in Texas where groundwater silica levels can be elevated.

Molybdate-based inhibitors form a protective oxide film on steel surfaces. Molybdate is effective, low-toxicity, and compatible with most other treatment chemicals. It is the preferred corrosion inhibitor for systems with copper alloys.

Phosphonate/zinc combinations provide corrosion protection for steel through a combination of anodic and cathodic inhibition. Zinc is an effective cathodic inhibitor but is regulated as an environmental concern — discharge limits apply.

Azole compounds (benzotriazole, tolyltriazole) are specifically formulated to protect copper and copper alloys. They form a stable complex with copper that prevents corrosion. Any system with copper heat exchangers or brass fittings should include an azole in the treatment program.

pH control is foundational to corrosion management. Most cooling water programs target a pH of 7.0–9.0. Below 7.0, corrosion rates increase sharply; above 9.5, calcium carbonate scale risk increases. Sulfuric acid is the most common pH adjustment chemical for cooling water; sodium hydroxide or sodium bicarbonate is used to raise pH when needed.

Biocide programs for cooling towers use two categories of chemistry:

Oxidizing biocides provide continuous residual control. Sodium hypochlorite (liquid chlorine) is the most widely used — it is effective, inexpensive, and easy to monitor with standard test kits. Bromine-releasing compounds (sodium bromide activated with an oxidizer) are more effective at higher pH and are preferred in systems operating above pH 8.0. Target residual: 0.2–0.5 ppm free chlorine (or equivalent bromine) in the basin.

Non-oxidizing biocides are used for periodic slug dosing to penetrate and remove biofilm that oxidizing biocides cannot reach. Common non-oxidizing biocides include:

• Isothiazolinones (CMIT/MIT, BIT) — broad-spectrum, effective against bacteria, algae, and fungi
• Glutaraldehyde — effective against biofilm, compatible with most system materials
• DBNPA (2,2-dibromo-3-nitrilopropionamide) — fast-acting, rapidly biodegrades

A complete biocide program uses both categories: continuous oxidizing biocide for residual control plus periodic non-oxidizing biocide slug doses (typically every 1–2 weeks) for biofilm management.

As water evaporates from the cooling tower, dissolved minerals concentrate in the remaining water. The ratio of mineral concentration in the circulating water to mineral concentration in the makeup water is called cycles of concentration (CoC).

Higher cycles of concentration mean less makeup water consumption and less blowdown — which reduces water and chemical costs. But higher cycles also mean higher mineral concentrations, which increases scale and corrosion risk.

The maximum practical cycles of concentration for a given system depends on the makeup water chemistry and the treatment program. For most Texas cooling systems with municipal or well water makeup, 4–6 cycles is a reasonable target range.

Blowdown is the deliberate discharge of a portion of the circulating water to control cycles of concentration. Blowdown rate is calculated as:

Blowdown (gpm) = Evaporation Rate (gpm) ÷ (Target CoC − 1)

Automatic blowdown controllers — which measure conductivity (a proxy for dissolved solids concentration) and open a blowdown valve when conductivity exceeds a setpoint — are the most reliable way to maintain consistent cycles of concentration.

Legionella pneumophila is a naturally occurring bacterium that thrives in warm water (77–113°F / 25–45°C) — exactly the temperature range of cooling tower water. When aerosolized by cooling tower drift and inhaled, Legionella can cause Legionnaires' disease, a severe pneumonia with a fatality rate of 10–15% in the general population and higher in immunocompromised individuals.

ASHRAE Standard 188-2018 establishes the minimum requirements for Legionella risk management in building water systems, including cooling towers. It requires a Water Management Plan (WMP) that includes:

• System description and flow diagrams
• Identification of hazardous conditions (water temperature, stagnation, aerosol generation)
• Control measures and monitoring procedures
• Corrective action procedures
• Documentation and recordkeeping

While ASHRAE 188 is a voluntary standard, it is increasingly referenced by regulators, insurers, and liability attorneys as the standard of care. Texas DSHS has issued guidance recommending ASHRAE 188 compliance for cooling towers.

Texas DSHS Cooling Tower Registration: Texas requires cooling towers to be registered with the Department of State Health Services and to maintain a Legionella water management plan. Facilities that have experienced a Legionella-associated illness must notify DSHS.

Effective Legionella control in cooling towers requires:

1. Maintain oxidizing biocide residual — a measurable free chlorine or bromine residual in the basin at all times is the primary Legionella control measure. Test and log residuals at least weekly; daily testing is recommended.

2. Control system temperature — Legionella grows most rapidly between 77–113°F. Cooling tower basin temperatures above 68°F (20°C) support growth; design and operate the system to minimize warm stagnant zones.

3. Eliminate stagnation — dead legs, infrequently used sections, and low-flow areas allow Legionella to proliferate. Flush infrequently used sections regularly; eliminate dead legs where possible.

4. Periodic system cleaning and disinfection — at minimum annually, and after any system shutdown of more than 5 days. Hyperchlorination (10–50 ppm free chlorine for 24 hours) followed by mechanical cleaning removes biofilm that harbors Legionella.

5. Drift eliminators — properly maintained drift eliminators reduce aerosol generation from the tower. Inspect and replace damaged eliminators promptly.

6. Legionella testing — periodic culture testing (quarterly is common) provides verification that the control program is working. Positive results above action levels (typically 1,000 CFU/mL) require immediate corrective action.

Program design:

• Makeup water analysis (hardness, alkalinity, silica, chlorides, conductivity, pH)
• System metallurgy inventory (steel, copper, galvanized, aluminum)
• Cooling tower capacity and recirculation rate
• Target cycles of concentration
• Chemical selection: scale inhibitor, corrosion inhibitor(s), oxidizing biocide, non-oxidizing biocide

Monitoring (weekly minimum):

• pH
• Conductivity (cycles of concentration)
• Free oxidizing biocide residual
• Inhibitor residual (molybdate, phosphonate, or zinc depending on program)
• Visual inspection of basin, fill, and drift eliminators

Monitoring (monthly):

• Corrosion coupon inspection (if installed)
• Deposit inspection on accessible heat transfer surfaces

Annual:

• System cleaning and disinfection
• Legionella culture testing
• Water management plan review and update
• Drift eliminator inspection

Texas-Chem Texas supplies the full range of cooling tower treatment chemicals:

• Scale inhibitors: phosphonates, polyacrylates, silica inhibitors
• Corrosion inhibitors: molybdate, zinc, azoles (benzotriazole, tolyltriazole)
• Oxidizing biocides: sodium hypochlorite (various concentrations), sodium bromide
• Non-oxidizing biocides: isothiazolinones, glutaraldehyde, DBNPA
• pH adjustment: sulfuric acid, sodium hydroxide, sodium bicarbonate
• Coagulants/flocculants: for side-stream filtration programs

All products are available in drums, totes, and bulk quantities with SDS and CoA documentation.

Contact Chemical and Filtration Products of Texas at texas-chem.com/contact-us or call 855-839-2436 for cooling tower chemical supply and technical assistance.