Biocides are added to cooling-tower water to prevent the growth of bacteria, algae, and other microorganisms that would otherwise thrive in the warm, nutrient-rich environment inside the tower. Without chemical treatment, these organisms form biofilm on heat-exchange surfaces, reduce cooling efficiency, accelerate corrosion, and can release dangerous pathogens into the air. The most serious of those pathogens is Legionella pneumophila, the bacterium responsible for Legionnaires’ disease.
Cooling Towers Are Ideal for Microbial Growth
A cooling tower works by evaporating water to pull heat out of a building or industrial process. That constant circulation of warm water, exposed to sunlight and ambient air, creates near-perfect conditions for microorganisms. Dust, pollen, and organic debris blow in through the open top, supplying nutrients. The water temperature in many systems sits squarely within the range where Legionella multiplies fastest: 77°F to 113°F (25°C to 45°C), according to the CDC.
Left untreated, bacteria colonize pipe walls and heat-exchange surfaces within days, secreting a sticky matrix of sugars and proteins called biofilm. Biofilm is more than a surface nuisance. It insulates metal from the water flowing over it, shields the organisms living inside it from temperature swings, and makes the colony far harder to kill than free-floating bacteria in the water column.
Protecting Heat Transfer Efficiency
The entire purpose of a cooling tower is to move heat from one place to another, and biofilm directly undermines that job. Biofilm has a thermal conductivity of roughly 0.6 W/m·K, about the same as still water, which makes it a surprisingly effective insulator when it coats metal surfaces designed to conduct heat rapidly. Even a thin layer forces the system to work harder to achieve the same cooling, raising energy costs and shortening equipment life.
Research using laboratory heat exchangers has measured clean-surface heat transfer coefficients around 660 W/m²·°C. As biofilm accumulates, that number drops steadily. The practical result is higher discharge temperatures, longer run times for chillers and compressors, and greater electricity consumption. For large commercial or industrial systems, the energy penalty from unchecked biofilm growth can dwarf the cost of a chemical treatment program.
Preventing Legionnaires’ Disease
Legionella bacteria are naturally present in freshwater at low concentrations. Problems start when they multiply inside a warm, stagnant system and then get aerosolized by the cooling tower’s fan, sending contaminated water droplets into the surrounding air. People nearby inhale those droplets, and some develop Legionnaires’ disease, a severe form of pneumonia that can be fatal, particularly for older adults, smokers, and people with weakened immune systems.
The CDC identifies four key factors that drive Legionella growth in cooling towers: sediment and biofilm buildup, water temperature within the 77°F to 113°F range, water age (how long water sits in the system between cycles), and lack of disinfectant residual. Biocide treatment targets the last factor directly and helps control biofilm, which addresses the first.
Industry guidelines from the American Industrial Hygiene Association set action levels for Legionella in cooling-tower water. A sample above 1,000 CFU/mL requires the tower to be taken offline for remedial treatment. The CDC’s position is even more conservative: there is no known safe level of Legionella, which is why maintaining a consistent biocide residual is considered essential rather than optional.
How Oxidizing Biocides Work
The two most common oxidizing biocides in cooling towers are chlorine and bromine. Both kill microorganisms by chemically reacting with cell membranes and internal proteins, breaking them apart through oxidation. Chlorine does this through hypochlorous acid, a small, neutral molecule that penetrates bacterial cell walls effectively. Bromine works through a similar mechanism, producing an acid form that is actually a stronger biocide than its chlorine equivalent at equal concentrations.
The critical difference between them shows up at higher pH levels. Cooling-tower water often runs alkaline because of mineral concentration from evaporation. At a pH of 8.5, only about 5% of sodium hypochlorite (chlorine) remains in its effective acid form. Bromine retains roughly 50% effectiveness at the same pH. This is why many operators choose bromine-based programs for towers that run at higher pH, or use bromine in combination with chlorine to maintain reliable disinfection across a wider operating range.
Oxidizing biocides are typically fed continuously into the recirculating water to maintain a steady residual concentration, commonly targeted around 0.2 to 0.5 mg/L depending on the chemical. That constant low-level presence prevents organisms from gaining a foothold between treatment cycles.
How Non-Oxidizing Biocides Work
Non-oxidizing biocides kill microorganisms through different chemical pathways. Glutaraldehyde, one of the most widely used, works by permanently bonding to proteins in the bacterial cell membrane, changing its structure so the cell can no longer regulate what passes in and out. The cell ruptures and dies. It also deactivates enzymes just inside the cell wall, accelerating the kill.
Other common non-oxidizing options include isothiazolins and quaternary amines, each targeting different cellular structures. These chemicals are typically “slug dosed,” meaning a concentrated batch is added to the system on a scheduled basis rather than fed continuously. The goal is to reach a target concentration high enough to penetrate biofilm and kill organisms that the continuous oxidizing program may have missed.
One important consideration with non-oxidizing biocides is resistance. Over time, microbial populations can adapt, and the same chemical becomes less effective. Water treatment programs rotate between different non-oxidizing chemistries when monitoring shows a drop in performance, preventing any single population of resistant organisms from dominating the system.
Why One Biocide Type Is Not Enough
Most well-managed cooling towers use both oxidizing and non-oxidizing biocides together. The continuous oxidizing feed handles day-to-day microbial control and maintains a measurable disinfectant residual in the bulk water. Periodic slug doses of a non-oxidizing biocide provide deeper penetration into established biofilm, reaching organisms that the oxidizer cannot.
This layered approach matters because oxidizing biocides, while fast-acting, react readily with the sticky outer matrix of biofilm and get consumed before they reach the living cells underneath. Non-oxidizing biocides are less reactive with that outer layer and can diffuse deeper into the colony. Using both in a coordinated program gives the best shot at controlling both planktonic (free-floating) bacteria and the more stubborn sessile (surface-attached) populations.
Supporting Measures That Make Biocides Effective
Biocides work best as part of a broader water management program. Operators also use scale and corrosion inhibitors to keep heat-exchange surfaces clean and intact, since rough or corroded metal gives biofilm more places to anchor. Automated blowdown systems control the concentration of dissolved minerals by regularly draining a portion of the recirculating water and replacing it with fresh makeup water, which also dilutes microbial populations.
Physical cleaning on a regular schedule removes accumulated sediment and sludge from basins and fill media. No amount of chemical treatment can fully compensate for heavy sediment buildup, which shelters organisms and consumes biocide before it reaches the water column. The CDC recommends adjusting cleaning frequency based on local conditions, water source, and system design, with the goal of minimizing the sediment and stagnant zones where Legionella and other pathogens concentrate.
Water temperature management also plays a role. When operationally feasible, running the system below 77°F moves the water outside Legionella’s preferred growth range, reducing the biological load that biocides need to handle. In practice, many systems cannot achieve this because the incoming process heat keeps water temperatures elevated, which makes consistent chemical treatment all the more important.