Cooling Tower Water Treatment: Chemistry, Problems & Solutions

Updated April 2026

Cooling towers are the workhorses of industrial and commercial HVAC heat rejection. They operate by evaporating a small percentage of recirculating water to remove heat from the remaining water. This evaporation process concentrates dissolved minerals, creating a challenging environment where scaling, corrosion, and microbiological growth occur simultaneously. Without proper water treatment, cooling tower systems suffer reduced heat transfer efficiency, equipment damage, increased energy consumption, and potential public health risks from Legionella and other waterborne pathogens.

This guide covers the fundamental chemistry of cooling tower water treatment, the three primary problems (scaling, corrosion, and biological growth), practical solutions for each, and strategies for optimizing blowdown to reduce water and chemical consumption.

Cycles of Concentration: The Foundation of Cooling Tower Chemistry

As water evaporates in a cooling tower, dissolved minerals remain behind and concentrate in the recirculating water. The ratio of dissolved solids in the recirculating water to those in the makeup water is called cycles of concentration (CoC). For example, if makeup water has 200 ppm TDS and the recirculating water has 800 ppm TDS, the system is operating at 4 cycles of concentration.

Increasing cycles of concentration reduces water consumption and chemical usage because less makeup water is required and less blowdown is discharged. However, higher cycles also mean higher mineral concentrations, which increases scaling and corrosion potential. The optimal cycles of concentration balance water savings against treatment costs and equipment protection.

Cycles of Concentration	Makeup Water Savings vs. Once-Through	Blowdown as % of Evaporation	Typical TDS Multiplier
2	50%	100%	2x
3	67%	50%	3x
4	75%	33%	4x
5	80%	25%	5x
6	83%	20%	6x
8	88%	14%	8x

Most cooling towers operate between 3 and 6 cycles of concentration. The diminishing returns above 5–6 cycles (each additional cycle saves less water) combined with increasing scaling risk makes operation above 6–8 cycles impractical for most makeup water sources without advanced pre-treatment such as softening or reverse osmosis.

Problem 1: Scaling and LSI Control

Scaling occurs when the concentration of scale-forming minerals exceeds their solubility limits. Calcium carbonate (CaCO₃) is the most common scale in cooling towers due to its inverse solubility: CaCO₃ becomes less soluble as temperature increases, which means it preferentially deposits on the hottest heat transfer surfaces where you least want it.

The Langelier Saturation Index (LSI)

The Langelier Saturation Index is the standard tool for predicting CaCO₃ scaling tendency. LSI is calculated as:

LSI = pH − pH_s

Where pH is the measured water pH and pH_s is the pH at which water is saturated with CaCO₃, calculated from temperature, TDS, calcium hardness, and total alkalinity.

• LSI > 0: Water is supersaturated; CaCO₃ tends to precipitate (scaling tendency)
• LSI = 0: Water is at saturation equilibrium
• LSI < 0: Water is undersaturated; CaCO₃ tends to dissolve (corrosive tendency)

For cooling tower operation, the target LSI in the recirculating water should be maintained between −0.5 and +0.5. Use our online LSI calculator to evaluate your cooling tower water chemistry.

Scale Control Methods

• pH adjustment: Sulfuric acid addition lowers pH and alkalinity, shifting LSI toward negative values. This is the most cost-effective scale control method for many systems but requires careful control to avoid corrosion from over-acidification.
• Chemical scale inhibitors: Phosphonates (such as HEDP, ATMP, and PBTC) and polymer dispersants (polyacrylates, polymaleic acid) prevent crystal nucleation and growth at substoichiometric concentrations. These are typically fed at 5–25 ppm active product.
• Makeup water softening: Removing calcium and magnesium from makeup water allows higher cycles of concentration without scaling risk. Side-stream softening treats a portion of the recirculating water.
• Silica management: Silica scaling becomes a concern above 150 ppm SiO₂ in recirculating water (lower at higher pH). Silica scale is extremely hard to remove chemically and often requires mechanical cleaning.

Problem 2: Corrosion

Cooling tower systems contain multiple metals including carbon steel piping, copper alloy condenser tubes, galvanized steel tower components, and sometimes stainless steel. Each metal has different corrosion characteristics, and treatment must protect all metals simultaneously.

Corrosion Mechanisms

• Dissolved oxygen corrosion: Unlike boiler systems, cooling towers constantly saturate recirculating water with dissolved oxygen through air contact. This makes oxygen corrosion a continuous concern, particularly for carbon steel components.
• Galvanic corrosion: Contact between dissimilar metals (such as copper tubes and steel pipe) in an electrolytic solution drives accelerated corrosion of the less noble metal.
• Under-deposit corrosion: Scale, sludge, and biofilm deposits create differential oxygen cells that accelerate localized corrosion beneath the deposit.
• Microbiologically influenced corrosion (MIC): Sulfate-reducing bacteria (SRB) and other microorganisms create highly corrosive conditions under biofilm, generating hydrogen sulfide and organic acids.

Corrosion Inhibitor Programs

• Molybdate-based programs: Sodium molybdate is an effective anodic inhibitor for carbon steel, typically fed at 5–15 ppm as MoO₄. It has low toxicity and does not interfere with biological treatment but is relatively expensive.
• Phosphate-based programs: Orthophosphate and polyphosphate blends provide both cathodic and anodic protection. These programs require careful pH control (7.0–8.5) and can contribute to biological growth if overfed.
• Azole programs (for copper alloys): Tolyltriazole (TTA) and benzotriazole (BTA) form a protective film on copper and copper alloy surfaces. Typically fed at 2–5 ppm active azole alongside the primary inhibitor program.
• Zinc-based programs: Zinc is an effective cathodic inhibitor but faces increasing discharge restrictions due to aquatic toxicity limits. Many facilities have moved away from zinc-based programs.

Corrosion monitoring should include corrosion coupon testing (with 90-day exposure periods), corrosion rate probes for real-time data, and periodic inspection of system components. Target corrosion rates are below 3 mpy (mils per year) for carbon steel and below 0.1 mpy for copper alloys.

Problem 3: Microbiological Growth and Legionella Risk

Cooling towers provide an ideal environment for microbiological growth: warm water (85–105°F), oxygen, sunlight exposure, and nutrients from airborne debris. Uncontrolled biological growth creates several serious problems.

Legionella and Public Health

Cooling towers are recognized as a primary source of Legionella pneumophila, the bacterium responsible for Legionnaires' disease. The warm, aerated water in cooling towers promotes Legionella growth, and the tower drift (water droplets carried into the air) can spread bacteria over significant distances. Outbreaks linked to cooling towers have caused hundreds of cases and multiple fatalities in documented incidents worldwide.

ASHRAE Standard 188 (Legionellosis: Risk Management for Building Water Systems) and many local jurisdictions now require written water management programs that include Legionella risk assessment, monitoring, and response protocols for cooling tower systems.

Biofilm and Its Consequences

Biofilm is a complex matrix of bacteria, algae, fungi, and their extracellular polymeric substances (EPS) that adheres to surfaces. Biofilm in cooling systems insulates heat transfer surfaces (reducing efficiency by 5–15%), accelerates under-deposit corrosion, protects pathogens including Legionella from biocides, and harbors sulfate-reducing bacteria that cause MIC.

Biocide Programs

Oxidizing biocides provide broad-spectrum kill and some biofilm penetration:

• Chlorine (sodium hypochlorite or gas): Effective and inexpensive. Maintain 0.5–1.0 ppm free chlorine residual. Effectiveness decreases above pH 8.0 as HOCl converts to less-effective OCl⁻.
• Bromine (stabilized liquid or activated by chlorine): Effective across a wider pH range than chlorine. Preferred for systems operating above pH 8.0.
• Chlorine dioxide (ClO₂): Excellent biofilm penetration and effectiveness independent of pH. Does not form trihalomethanes. Generated on-site.

Non-oxidizing biocides supplement oxidizing programs and target specific organisms:

• Isothiazolinones: Broad-spectrum microbiocide effective against bacteria, algae, and fungi. Typically slug-fed 2–3 times per week.
• DBNPA (2,2-dibromo-3-nitrilopropionamide): Fast-acting, rapidly degrading biocide. Good for systems with short retention times or strict discharge limits.
• Glutaraldehyde: Effective against SRB and biofilm. Widely used in industrial cooling systems. Requires careful handling due to health hazards.

For comprehensive biocide and chemical treatment options, browse our chemicals and cleaning solutions catalog.

Makeup Water Quality Requirements

The quality of makeup water directly determines the maximum achievable cycles of concentration and the complexity of the chemical treatment program. General guidelines for cooling tower makeup water:

Parameter	Recommended Makeup Limit	Notes
Calcium hardness (as CaCO3)	<200 ppm	Lower allows higher cycles
Total alkalinity (as CaCO3)	<200 ppm	Drives LSI positive
Silica (as SiO2)	<30 ppm	Keeps recirculating <150 ppm at 5 cycles
Iron	<0.5 ppm	Prevents iron fouling
TSS	<25 ppm	Prevents sludge buildup
pH	7.0–9.0	Pre-treatment dependent

Where municipal or well water makeup quality is poor, pre-treatment filtration and softening can significantly improve cooling tower performance and reduce chemical costs.

Blowdown Optimization

Blowdown optimization is the process of finding the ideal cycles of concentration that minimize total operating cost (water + chemicals + energy + maintenance). Key strategies include:

• Conductivity-controlled automatic blowdown: Installing a conductivity controller and automatic blowdown valve maintains consistent cycles of concentration, reducing water waste from over-blowing and scaling risk from under-blowing.
• Makeup water pre-treatment: Softening or RO treatment of makeup water allows higher cycles of concentration. The investment in pre-treatment equipment is often recovered within 12–24 months through reduced water, chemical, and sewer discharge costs.
• Side-stream filtration: Filtering 5–15% of recirculating flow through a side-stream filter removes suspended solids, reducing biological nutrient load and under-deposit corrosion risk, allowing higher cycles.
• Water reuse: Using treated wastewater, RO reject water, or condensate as cooling tower makeup can substantially reduce freshwater consumption. These sources require careful evaluation for microbiological, scaling, and corrosion impacts.

Request a cooling tower water treatment assessment from ForeverPure →

Frequently Asked Questions

What are the ideal cycles of concentration for a cooling tower?

Most cooling towers should operate between 3 and 6 cycles of concentration, with the optimal value depending on makeup water quality and treatment program capability. Increasing from 3 to 5 cycles reduces makeup water consumption by 20% and blowdown by 40%. Above 5–6 cycles, water savings diminish while scaling and corrosion risks increase unless makeup water is pre-treated with softening or reverse osmosis.

How do I prevent Legionella in my cooling tower?

Legionella prevention requires a multi-barrier approach: maintain an effective oxidizing biocide residual (0.5–1.0 ppm free halogen), supplement with non-oxidizing biocides to control biofilm, keep the system clean and free of stagnant areas, maintain water temperature below 68°F where possible in the basin, implement drift eliminators to reduce aerosol release, and conduct regular Legionella testing using culture-based methods. A written water management program per ASHRAE Standard 188 is recommended for all cooling tower installations.

What is the LSI and what should it be for my cooling tower?

The Langelier Saturation Index (LSI) predicts whether water will tend to deposit calcium carbonate scale (positive LSI) or dissolve existing scale and corrode metal surfaces (negative LSI). For cooling towers, maintain the LSI of recirculating water between −0.5 and +0.5. Calculate your LSI using our online LSI calculator by entering pH, temperature, TDS, calcium hardness, and total alkalinity of your recirculating water.