Chlorine
The lowest-cost choice for clean makeup water at near-neutral pH. Operators, test kits and CDC's emergency procedures are all built around a free-chlorine residual. Compare the details on mineral oxychloride vs. chlorine.
Alternatives to Chlorine in Cooling Towers: Oxidizing Biocides Compared
Chlorine, bromine, chlorine dioxide, ozone, peracetic acid, hydrogen peroxide and mineral oxychloride compared on chemistry, pH, by-products, biofilm and real-world drawbacks – including where each one is the better buy.
Chlorine remains the cheapest cooling-tower biocide but weakens above pH 7.5 and struggles against biofilm. Bromine holds up better in alkaline water, chlorine dioxide penetrates biofilm, and ozone, peracetic acid and peroxide avoid halogens but leave little residual. Mineral oxychloride oxidizes at 2.8 to 2.9 V and removes biofilm under ORP control, at a higher unit price.
Approved, certified & independently validated
EPA FIFRARegistered
USDA NOPOrganic Allowed
NSF/ANSIStandard 60
UC DavisUniversity of WashingtonSpecial Pathogen LaboratoryMontana State UniversitySawtooth Ag ResearchGreenAgri SolutionsSouthern California EdisonGoleta Water District
2.8–2.9 VMineral oxychloride oxidation potential vs 1.36 V for chlorine (Cl₂)
~90%Of Legionella lives in biofilm, not the bulk water (Montana State University)
300%Biofilm is worse than calcium-carbonate scale for heat transfer
400–500 mVRoutine cooling-tower ORP band for biofilm and MIC control
Side by Side
How do the main oxidizing biocides for cooling towers compare?
Short answer: chlorine is still the cheapest and best understood oxidizing biocide, bromine holds up better in alkaline towers, chlorine dioxide penetrates biofilm and ignores pH, and ozone, peracetic acid and hydrogen peroxide leave no halogen behind but give little lasting residual. Mineral oxychloride oxidizes at 2.8–2.9 V and targets biofilm, but costs more per unit and needs ORP control.
| Oxidant | How it works | Oxidation potential (V) | pH sensitivity | By-products |
|---|---|---|---|---|
| Chlorine (Cl2 gas / sodium hypochlorite) | Chlorination and oxidation by hypochlorous acid | 1.36 (Cl2); 1.49 (HOCl); 0.94 (NaOCl) | High — above about pH 7.5 most HOCl shifts to the weaker hypochlorite ion | THMs, HAAs and other chlorinated organics |
| Bromine (BCDMH or NaBr activated with hypochlorite) | Oxidation by hypobromous acid | About 1.34 (HOBr); 1.09 (Br2) | Moderate — keeps its active acid form to a higher pH than chlorine | Brominated organics; bromide and HOBr in blowdown |
| Chlorine dioxide (ClO2) | Selective one-electron oxidation by a dissolved gas made on site | 1.57 | Low — effective across roughly pH 5–10 | Chlorite and chlorate |
| Ozone (O3) | Dissolved ozone and the hydroxyl radicals it forms | 2.07 | Decomposes faster as pH and temperature rise | Bromate in bromide-bearing water |
| Peracetic acid (PAA) | Peroxyacid oxidation, in equilibrium with peroxide and acetic acid | 1.81 | Works best slightly acidic to neutral (about pH 5–8) | Acetic acid, oxygen, water |
| Hydrogen peroxide (H2O2) | Molecular oxidation; radicals only with a catalyst, UV or ozone | 1.78 | Decomposes faster at high pH, heat and with dissolved metals | Water and oxygen |
| Mineral oxychloride (JC 9465 / JC 9450) | Liquid mineral–oxygen complex releasing reactive oxygen species on contact | 2.8–2.9 (effective) | Works across pH 4–9; dosed to ORP | Mineral oxides; no chlorinated DBPs added |
| Oxidant | Handling / equipment | Biofilm performance | Residual | Typical drawbacks |
|---|---|---|---|---|
| Chlorine | Gas cylinders with leak detection, or bulk bleach that loses strength in about 30 days | Spent at the surface of the biofilm gel; slow on established film | Free-chlorine residual, easy to test | Weak in alkaline, high-cycle water; high residuals attack fill, gaskets and mild steel; chlorinated by-products |
| Bromine | Solid tablets in an erosion feeder, or liquid bromide plus a bleach activator | Similar to chlorine; better kill at higher pH | Total halogen residual, easy to test | Costs more than chlorine; aquatic toxicity governs blowdown permits; two-chemical feed with activated bromide |
| Chlorine dioxide | On-site generator and precursor chemicals; gas hazards | Good — penetrates biofilm | Yes, measurable | Generator capital; volatile in an open tower; chlorite by-product |
| Ozone | Generator, contactor, off-gas destruction and power | Strong at the point of contact | None — dissipates within minutes | High capital and energy; no protection in remote parts of the loop |
| Peracetic acid | Corrosive, pungent concentrate; irritant to eyes, skin and lungs | Used to strip biofilm from surfaces; short contact | Short-lived | Little lasting protection in remote parts of the loop; acetic acid adds biodegradable carbon |
| Hydrogen peroxide | Liquid oxidizer; serious eye hazard; not compatible with some copper alloys and zinc | Weak — catalase in biofilm breaks it down | Little at practical doses | High doses needed; slow as a biocide on its own |
| Mineral oxychloride | Ready-to-use liquid; metering pump, tank and ORP controller; corrosive undiluted and will freeze | Oxidizes the EPS matrix; 600–800 mV band for biofilm removal | Measurable ORP residual; mildly biocidal mineral oxides | Higher unit price; needs ORP control and probe upkeep; specialty product |
Why Look Beyond Chlorine
Why do cooling-tower operators look for chlorine alternatives?
Most operators change oxidizers for one of three reasons: alkaline water, biofilm or by-products. Chlorine still works well in clean, near-neutral water; the trouble starts at high cycles, when pH climbs and the residual has to be pushed up for the same kill.
The first problem is pH. Chlorine kills mainly as hypochlorous acid at 1.49 V. Above about pH 7.5 most of it turns into hypochlorite ion at 0.94 V, so a tower running at pH 8.5 needs much more feed for the same result. CDC's cooling-tower guidance reflects this: during emergency disinfection it says to keep pH below 8.0 for chlorine-based disinfectants and below 8.5 for bromine-based ones.
The second problem is biofilm. The extracellular polymeric substance (EPS) holding a film together uses up oxidant at its surface, so a basin residual can read in range while the film on fill and tubes stays intact. Roughly 90% of the Legionella in a system lives in that film, according to Montana State University's Center for Biofilm Engineering. On a heat-transfer surface the film is also about 300% worse than the same thickness of calcium-carbonate scale.
The third problem is by-products and corrosion. Raising free chlorine to force it through biofilm attacks wood and plastic fill, elastomers and mild steel, and it adds chlorinated organics to the blowdown. For the Legionella side of this, see how to control Legionella in cooling towers.
Fair Comparison
Where is each alternative the better choice?
Short answer: each oxidant has a tower where it is the sensible pick. Chlorine wins on price, bromine on alkaline water, chlorine dioxide on dirty process water, and the peroxygens where discharge limits rule out halogens. Mineral oxychloride makes sense where biofilm, rather than bulk-water kill, is the problem.
Bromine
The usual step up for alkaline, high-cycle towers, because hypobromous acid stays active at a higher pH than hypochlorous acid. It is a common way to keep a halogen program working at pH 8–9.
Chlorine dioxide
Strong where process leaks bring in ammonia or organics. Chlorine dioxide does not form chloramines, penetrates biofilm well and is largely unaffected by pH. It suits large systems that can justify a generator. See mineral oxychloride vs. chlorine dioxide.
Ozone
Fits sites that already run a generator and want very little chemical in the blowdown. It needs a backup residual for parts of the loop far from the contactor. See mineral oxychloride vs. ozone.
Peracetic acid and hydrogen peroxide
Useful where halogen discharge is restricted, or as periodic cleaning shocks. Peroxide is a cheap commodity and leaves only water and oxygen. See vs. peracetic acid and vs. hydrogen peroxide.
Mineral oxychloride
Fits towers where biofilm, fouling or Legionella risk persists despite a residual that reads in range. It also suits plants that want to replace a separate oxidizer, dispersant and algaecide with one ORP-controlled feed. The trade-off is a higher unit price and the need for an ORP controller.
Selection
How do you choose an oxidizing biocide for a cooling tower?
Start with the water and the tower, not the chemistry. The right oxidant depends on pH, organic and ammonia load, existing biofilm, discharge limits and the equipment you already own. Work through these questions with your water-treatment provider before you request quotes:
- What pH does the tower actually run at? Above about pH 8, plain chlorine loses much of its strength. Bromine, chlorine dioxide or an ORP-controlled program will hold up better.
- Is the problem in the water or on the surfaces? If plate counts and residuals look fine but approach temperature keeps drifting or Legionella results come back positive, treat it as a biofilm problem.
- What is in the makeup and process water? Ammonia, organics, bromide and iron all consume or change oxidants differently.
- What do your discharge permit and local rules require? Halogen residual, bromide, aquatic toxicity and any oxidant specified in a local Legionella rule can decide the choice for you.
- What can your site handle? Gas cylinders, generators, corrosive concentrates and freeze protection all carry different safety and staffing costs.
- How will you verify control? A free-halogen test kit, an ORP controller, or both, plus routine culture results.
- What is the total cost? Include chemical, equipment, power, labor, heat-transfer losses and cleaning, not only price per drum.
For mineral oxychloride specifically, the cooling-tower targets are on the ORP reference chart: 400–500 mV for routine biofilm and microbiologically influenced corrosion (MIC) control, and 600–800 mV to remove established biofilm.
Changeover
How do you switch a cooling tower from chlorine to another oxidizer?
Short answer: treat the switch as a change to your water management program. Baseline the system, bench-test the new chemistry, stop feeding the old oxidizer before the new one starts, bring the control point up in stages, and verify with culture results before you call it done.
Baseline the tower
Record pH, ORP, conductivity, cycles, temperature, the existing residual, heterotrophic plate counts, recent Legionella results and approach temperature on the exchangers.
Bench-test the new oxidant
Test it on your makeup and recirculating water to set the target residual or ORP and confirm material compatibility with fill, gaskets and metallurgy.
Update the water management plan
Write in the new control limits, monitoring method and corrective actions so the team, the plan and any regulator’s requirements stay aligned.
Separate the chemistries
Stop the old feed and keep the oxidizers apart. Mineral oxychloride, like most oxidizers, must not be mixed with acids or other oxidizers in tanks or feed lines.
Install feed and control
For mineral oxychloride: a metering pump, a storage tank and an ORP controller on the recirculating line. A typical install takes under 30 minutes.
Step up the setpoint
A fouled tower will hold a low ORP while existing biofilm is oxidized. Raise the setpoint in stages, expect sloughing and higher blowdown, and clean strainers.
Verify and settle
Confirm with culture results and heat-transfer data, then drop to the maintenance band (400–500 mV) and trend ORP continuously.
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The takeaway
No single oxidant is best for every tower. Chlorine and bromine remain sound, economical choices where pH and biofilm are under control. If the problem is biofilm that a halogen residual is not reaching, mineral oxychloride gives the highest oxidation potential on this list (2.8–2.9 V) and an ORP-controlled program that also handles algae and fouling. You pay for it with a higher unit price and ORP equipment to maintain.
Oxidation potentials are standard reference values; field performance depends on dose, contact time, pH, temperature, demand and system design, and should be confirmed by a bench study and pilot. Use any biocide only as its product label allows, and follow your jurisdiction’s cooling-tower rules. JC 9465 is EPA FIFRA registered; JC 9465 and JC 9450 are NSF/ANSI/CAN 60 certified. Terms are defined in the glossary.
Frequently Asked Questions
Common questions.
Is bromine better than chlorine in a cooling tower?
Often, in alkaline towers. Hypobromous acid stays in its active form at a higher pH than hypochlorous acid, so bromine keeps working in high-cycle water running around pH 8 to 9 where chlorine loses much of its strength. It costs more than chlorine, still forms halogenated by-products, and its aquatic toxicity is managed through discharge permits. In clean, near-neutral water, chlorine is usually the cheaper choice.
Can hydrogen peroxide replace chlorine in a cooling tower?
Rarely on its own. Hydrogen peroxide oxidizes at 1.78 V and breaks down to water and oxygen, which is attractive for discharge. But many bacteria produce catalase, an enzyme that destroys peroxide, and Montana State University work found a flowing peroxide stream could not penetrate or kill a catalase-producing biofilm in an hour. Peroxide is better suited to periodic cleaning, odor or sulfide work than to continuous biocide duty.
Does a non-chlorine oxidizer still need a measurable residual in the tower?
Yes. CDC guidance for cooling towers calls for maintaining a measurable oxidizing residual throughout each day and following the manufacturer's recommendations. Ozone, peracetic acid and hydrogen peroxide decay quickly, so they usually need a backup. Mineral oxychloride leaves a measurable ORP residual and mildly biocidal mineral-oxide by-products. Whatever the chemistry, write the control limit and how it is measured into your water management plan.
Is mineral oxychloride more expensive than chlorine for a cooling tower?
Per unit, yes. It is a specialty product with a higher unit price than commodity chlorine or bleach. The lower dose offsets part of that: below 1.0 mg/L per mg/L of inorganic demand against roughly 6 mg/L for chlorine. One ORP-controlled feed can also replace a separate oxidizer, dispersant and algaecide. Whether total cost comes out lower depends on the tower, so price a trial against your current program.
Can ORP be used to control a cooling tower instead of a free-chlorine test?
ORP measures the oxidizing power of the water in millivolts, so it tracks kill efficacy for any oxidizer and suits automatic feed control. For mineral oxychloride, Jenfitch uses 400 to 500 mV for routine control and 600 to 800 mV for biofilm removal. Where a rule or CDC procedure specifies a free available oxidant level in ppm, keep that test too. ORP probes need regular cleaning and calibration.
Which oxidizing biocide works best against cooling-tower biofilm?
Halogens are largely spent on the surface of the biofilm gel, so they often fail on established film. Chlorine dioxide penetrates biofilm well. Peracetic acid can strip films during cleaning, while hydrogen peroxide is weakened by catalase. Mineral oxychloride is designed to oxidize the polymer matrix itself, with Jenfitch targeting 600 to 800 mV ORP for biofilm removal. Physical cleaning is still needed for heavy deposits.