Cooling tower water treatment is a 7-component chemical program of scale inhibitors, corrosion inhibitors, oxidizing and non-oxidizing biocides, acid, filtration and cleaning chemicals that keeps an open evaporative tower free of scale, corrosion, biofilm and Legionella. About 1 % of the circulating water evaporates for every 10 °F (5.6 °C) of cooling range and leaves its minerals behind, so how far can a tower concentrate its water before it scales or corrodes?
The program draws on 43 chemicals in 8 families: scale inhibitors, phosphates, corrosion inhibitors, biocides, oxidants, pH adjusters, dechlorination chemicals and filter media. Inhibitors are dosed in proportion to blowdown, never to the recirculation rate, and all of them come from the 16 families of water treatment chemicals.
The program holds pH by program (6.8 to 7.8 stabilized phosphate, 8.7 to 9.2 all-organic), free halogen at 0.5 to 1.0 mg/L as Cl2, carbon steel corrosion at or below 1 mpy (0.025 mm/y) and Legionella below 10 CFU/mL, under FIFRA, ASHRAE 188-2021, New York City's monthly testing and 40 CFR 423. US DOE compares programs by the cost to treat 1,000 gal (3.785 m³) of make-up.
Every value on this page is checked against primary sources (EPA, eCFR, CDC, NYC Health Code, EUR-Lex, ASTM, ASHRAE and engineering handbooks); see our methodology and fact-checking process.
What Is Cooling Tower Water Treatment?#
Cooling tower water treatment is the continuous feed and control of inhibitors and biocides in an open evaporative tower, while blowdown removes the minerals that evaporation concentrates. US federal rules define a cooling tower as an open water recirculating device that uses fans or natural draft to cool warm water by direct contact with air (40 CFR 749.68), and US DOE names corrosion, scale and biological inhibitors as the program elements.
Towers cool water across a range of 10 to 25 °F (5.6 to 13.9 °C) (Veolia Water Technologies Handbook, chapter 31). Fan types, fill and drift eliminators are described under how cooling towers work.
Why do cooling towers need water treatment?#
Cooling towers need water treatment because evaporation concentrates 3 problems: scale, corrosion and bacteria. The 3 problems are listed below.
- Scale: calcium carbonate, the most common deposit, becomes less soluble as water warms.
- Corrosion: steel pits under oxygen attack, with the cathode 1 to 2 pH units above the bulk water.
- Bacteria: they double every 20 to 30 minutes under optimal conditions, and Legionella grows at 25 to 45 °C (77 to 113 °F).
The 7 program components below answer these problems one by one.
How does evaporation concentrate cooling tower water?#
Evaporation concentrates cooling tower water because each pound evaporated removes about 1,000 Btu and leaves every dissolved solid behind, at about 1 % of the circulation per 10 °F (5.6 °C) of range. Drift adds a small loss: up to 0.2 % of recirculation in older towers, 0.02 % or less in new ones.
US DOE reports that many systems run at 2 to 4 cycles and that moving from 3 to 6 cycles cuts make-up by 20 % and blowdown by 50 %. How cycles are measured with chloride, sulfate or conductivity is explained under cycles of concentration.
What Goes Wrong in Cooling Tower Water?#
Four things go wrong in cooling tower water: scale, corrosion, biofilm and fouling, and Legionella growth. Each one rises with cycles.
Scale and deposits#
Calcium carbonate is the limiting scale in most towers: at 10 cycles, make-up with 50 mg/L calcium and 40 mg/L alkalinity (as CaCO3) reaches 500 mg/L calcium, 400 mg/L alkalinity, pH 8.9 and an LSI of +2.5 (Veolia Water Technologies Handbook, Table 31-2). Calcium and magnesium silicate and calcium sulfate follow.
Four controls limit scale: blowdown, acid feed, softening and scale inhibitors. The chemistry of each scale is explained under scale formation in water systems.
Corrosion of steel, copper and galvanized surfaces#
Corrosion attacks 3 metal groups: carbon steel pits under oxygen, copper alloys corrode under ammonia, excess chlorine or low pH, and galvanized steel forms white rust when unpassivated water exceeds about pH 8.2. Brass dezincifies below pH 6.0 and above 1.0 mg/L free chlorine (same handbook, chapter 24).
Corrosion control grows harder as conductivity approaches 10,000 µS/cm. Mechanisms and measurement are covered under corrosion in water systems.
Biofouling, biofilm and Legionella#
Biofouling is the growth of bacteria, algae and fungi as a slime layer, and a biofilm 1 mm thick on low-carbon steel resists heat flow like 80 mm of extra tube wall. Legionella multiplies in the same water at 25 to 45 °C (77 to 113 °F) (CDC).
Biofilm also shelters Legionella from biocides, so microbial control is the third pillar of every program, and Legionella control in cooling towers has its own page, linked from Table 1.
How Is Cooling Tower Water Treated? The 7 Components of a Chemical Program#
Cooling tower water is treated with 7 program components: scale inhibitors and dispersants, corrosion inhibitors, oxidizing biocides, non-oxidizing biocides, acid feed, side-stream filtration with make-up pretreatment, and periodic cleaning, passivation and blowdown dechlorination. Inhibitors run continuously in proportion to blowdown or make-up, while biocides are shot-fed.
Table 1 compares the 7 components by what each controls, typical chemicals and dose with its basis.
| # | Component | Controls | Typical chemicals | Dose and basis | Family hub | Program or topic node |
|---|---|---|---|---|---|---|
| 1 | Scale inhibitors and dispersants | CaCO3, calcium phosphate, silica, suspended solids | HEDP, ATMP, PBTC, polyacrylate, polymaleic acid | HEDP 2 to 5, ATMP 2 to 3, PBTC 3 to 5 mg/L as active | scale inhibitors; phosphates in water treatment | scale and deposit control in cooling towers |
| 2 | Corrosion inhibitors | carbon steel, copper, galvanized steel | orthophosphate, zinc, molybdate, tolyltriazole | molybdate about 3 mg/L as Mo; TTA 1 to 2 mg/L | corrosion inhibitors | corrosion control in cooling water systems; white rust and galvanized steel passivation |
| 3 | Oxidizing biocides | bacteria, algae, biofilm, Legionella | sodium hypochlorite, activated bromide, BCDMH, chlorine dioxide | 0.5 to 1.0 mg/L free as Cl2 in the hot return | oxidants and oxidizing biocides | cooling tower biocides; Legionella control in cooling towers |
| 4 | Non-oxidizing biocides | resistant organisms, biofilm, sulfate-reducing bacteria | glutaraldehyde, isothiazolinone, DBNPA, quats | slug dose, at least 4 h contact, 2 biocides alternated | biocides for water treatment | see row 3 |
| 5 | Acid feed and pH control | alkalinity, LSI, CaCO3 | sulfuric acid, hydrochloric acid | to the pH or LSI target | pH adjustment chemicals | none |
| 6 | Side-stream filtration and make-up pretreatment | suspended solids, hardness | filter sand, anthracite, lime softening | per filter design | adsorbents and filter media | none |
| 7 | Cleaning, passivation, dechlorination, foam control | deposits, white rust, blowdown chlorine, foam | descaling acids, polyphosphate, sodium bisulfite, antifoam | polyphosphate 3 to 5 mg/L as PO4 after 5 to 6 days of high dose | descaling and chemical cleaning agents; dechlorination chemicals; defoamers and antifoams | cooling tower cleaning and disinfection chemicals |
Doses are typical program values; confirm them against the product label and the system analysis.
1. Scale inhibitors and dispersants#
Scale inhibitors hold calcium carbonate in solution in cooling water: HEDP at 2 to 5 mg/L, ATMP at 2 to 3 mg/L, PBTC at 3 to 5 mg/L and HPAA at 5 to 15 mg/L (ppm), each as active (Standish, 2024). HEDP and ATMP hold calcite at about 125 to 150 times saturation, PBTC at about 225 times (EPRI).
ATMP is chlorine-sensitive, HPAA degrades rapidly, and PBTC is virtually immune to halogen. Low-phosphorus programs stay below 1.0 mg/L total P. HEDP, ATMP, PBTC and DTPMP are compared on phosphonate scale inhibitors.
What disperses solids and calcium phosphate? Polymers: polyacrylate at 1 to 15 mg/L as product alone and phosphinocarboxylic acid at 5 to 20 mg/L alone (supplier figures).
2. Corrosion inhibitors#
Corrosion inhibitors protect steel, copper and galvanized metal in cooling water: orthophosphate, zinc at about 0.5 mg/L insoluble above the soluble level, molybdate at about 3 mg/L as Mo and tolyltriazole at 1 to 2 mg/L. Silicate needs 2 to 3 weeks to build its film.
Excess chlorination deactivates azoles. Tolyltriazole, benzotriazole and MBT are compared on copper and yellow-metal corrosion inhibitors. Chromate has been banned in US comfort cooling towers since 1990 (40 CFR 749.68).
3. Oxidizing biocides#
Oxidizing biocides kill bacteria, algae and Legionella in cooling water: chlorine at 0.5 to 1.0 mg/L (ppm) free as Cl2 in the hot return, and bromine where pH runs above about 7.5 to 8. Hypochlorous acid dominates below pH 7.6 and is up to 80 times more effective than hypochlorite ion; hypobromous acid dominates up to pH 8.7 (Veolia Water Technologies Handbook, chapter 27).
Every mg/L of chlorine consumes 1.4 mg/L of alkalinity, chlorine dioxide does not react with ammonia, and oxidizer and inhibitor dose points stay apart. Activated bromide, BCDMH and bromine chemistry sit under bromine water treatment.
4. Non-oxidizing biocides#
Non-oxidizing biocides control organisms that halogen misses: glutaraldehyde, isothiazolinone, DBNPA, bronopol, quats and dodecylguanidine salts, slug-fed for at least 4 h of contact. Glutaraldehyde labels allow slugs of 40 to 200 ppm as product, DBNPA under 1 to 24 ppm as active.
Two biocides with different kill mechanisms are alternated. Kill mechanisms and compatibility of every class are compared on non-oxidizing biocides.
5. Acid feed and pH control#
Acid feed lowers pH and alkalinity so a tower can run more cycles: at 15 cycles the example water reaches LSI +3.2 and cannot operate, while acid to pH 8.7 brings it to +2.5, operable with an inhibitor (same handbook, Table 31-2). Sulfuric acid raises sulfate, hydrochloric acid raises chloride, and carbon dioxide is generally not practical.
6. Side-stream filtration and make-up pretreatment#
Side-stream filtration and make-up softening let HVAC towers run 7 to 10 cycles instead of 3 to 6. US DOE lists both as retrofits, plus alternative make-up such as air-handler condensate and municipal reclaimed water, while New York City requires an approved plan for non-potable make-up.
7. Cleaning, passivation and blowdown dechlorination#
Three periodic treatments complete the program: offline cleaning and disinfection (at least once a year under CDC guidance, twice a year in New York City), passivation of new galvanized steel, and blowdown dechlorination. Fouled fill can gain up to 10 times its initial weight.
Passivation runs 4 to 12 weeks across 3 tower makers' guidance at about pH 6.5 to 8.2 (AWT compilation), or uses polyphosphate at 3 to 5 mg/L as PO4 after a 5 to 6 day high dose. Cleaning chemicals have their own page, linked from Table 1.
Cooling Tower Water Control Limits#
Cooling tower water is controlled by 6 groups of limits: pH, cycles and conductivity, inhibitor residuals, free halogen, corrosion rate and microbial counts, each set by the program and the metals. Table 2 lists each limit with its unit and source.
| Parameter | Typical limit or target | Basis and unit | Source |
|---|---|---|---|
| Cycles of concentration | 2 to 4 common, 6 or more possible; HVAC 3 to 6, 7 to 10 with pretreatment | ratio | US DOE FEMP BMP #10; supplier guidance |
| pH | 6.8 to 7.8 / 8.0 to 9.2 / 8.7 to 9.2 by program (Table 3); galvanized passivation about 6.5 to 8.2 | pH units | Veolia ch. 31; AWT compilation |
| Conductivity | corrosion control harder approaching 10,000; galvanized guidance below 2,400 | µS/cm | Veolia ch. 31; AWT compilation |
| Calcium hardness (phosphate programs) | 75 to 1,200; zinc added below 75 | mg/L as CaCO3 | Veolia ch. 31 |
| LSI (inhibited) | about +2.0 to +3.0 with PBTC; +2.5 operable with acid at 15 cycles | index | EPRI; Veolia Table 31-2 |
| Free halogen, continuous | 0.5 to 1.0 in the hot return (CTI); 0.1 to 0.5 recirculating (Veolia) | mg/L (ppm) as Cl2 | CTI; Veolia ch. 27 |
| Free halogen, intermittent | 1.0 to 2.0 held at least 1 h daily | mg/L as Cl2 | CTI |
| Halogen pH ceiling | HOCl dominant below 7.6; HOBr below 8.7 | pH | Veolia ch. 27 |
| Carbon steel corrosion | 1 mpy (0.025 mm/y) or less excellent; federal facility example below 3 mpy (0.076 mm/y) | mpy (mm/y) | AWT 2000; GSA example |
| Copper alloy corrosion | 0.1 mpy (0.0025 mm/y) or less excellent; federal facility example below 0.2 mpy (0.005 mm/y) | mpy (mm/y) | AWT 2000; GSA example |
| Bulk heterotrophic count | below 10,000 | CFU/mL | CTI WTB-148 (2008) |
| Legionella (culture) | below 10, well controlled | CFU/mL | CDC (2025-01-03) |
| Blowdown free available chlorine (steam electric plants only) | 0.5 maximum, 0.2 average, 2 h a day per unit | mg/L | 40 CFR 423.13(d) |
Control ranges by treatment program#
The pH of cooling tower water depends on the program: 6.8 to 7.8 for stabilized phosphate, 8.0 to 9.2 for alkaline phosphate, 8.7 to 9.2 for all-organic and 7.6 or higher for alkaline zinc. Table 3 compares the 4 programs.
| Program | pH | Phosphate | Calcium | Note |
|---|---|---|---|---|
| Stabilized phosphate | 6.8 to 7.8 | total inorganic PO4 10 to 25 mg/L | 75 to 1,200 mg/L as CaCO3 | polymer controls calcium phosphate |
| Alkaline phosphate | 8.0 to 9.2 | inorganic PO4 2 to 10 mg/L plus organic phosphate 3 to 8 mg/L | 75 to 1,200 mg/L as CaCO3 | zinc where calcium is below 75 mg/L |
| All-organic | 8.7 to 9.2 | phosphonate (Table 1, row 1) | n/a | phosphonate plus polymer |
| Alkaline zinc | 7.6 or higher | phosphate added below pH about 8.4 | n/a | about 0.5 mg/L insoluble zinc above the soluble level |
Sources: Veolia Water Technologies Handbook, chapter 31; NACE 07061 for the zinc program.
Scaling indices and cycles limits#
Inhibited cooling programs run at an LSI of about +2.0 to +3.0 with PBTC. LSI equals actual pH minus saturation pH, a positive value predicts carbonate deposition, and the index is valid from pH 6.5 to 9.5.
Make-up chemistry sets the ceiling: 15 cycles without acid reach +3.2 and cannot operate. The Ryznar, Puckorius and Stiff-Davis variants are worked through on Langelier saturation index.
Corrosion-rate targets in mpy and mm/y#
The Association of Water Technologies rates open cooling corrosion from coupons exposed about 90 days: 1 mpy (0.025 mm/y) or less is excellent for carbon steel, 0.1 mpy (0.0025 mm/y) or less for copper alloys (Boffardi, 2000). A federal facility example (GSA) caps mild steel at 3 mpy and copper at 0.2 mpy. Table 4 lists the bands.
| Rating | Carbon steel, mpy (mm/y) | Copper alloys, mpy (mm/y) |
|---|---|---|
| Excellent | 1 or less (0.025 or less) | 0.1 or less (0.0025 or less) |
| Very good | 1 to 3 (0.025 to 0.076) | 0.1 to 0.25 (0.0025 to 0.0064) |
| Good | 3 to 5 (0.076 to 0.127) | 0.25 to 0.35 (0.0064 to 0.0089) |
| Moderate | 5 to 8 (0.127 to 0.203) | 0.35 to 0.5 (0.0089 to 0.0127) |
| Poor | 8 to 10 (0.203 to 0.254) | 0.5 to 1 (0.0127 to 0.0254) |
| Very poor | above 10 (above 0.254) | above 1 (above 0.0254) |
Source: AWT, Boffardi 2000; 1 mpy = 0.0254 mm/y (conversions calculated); coupons about 90 days, ASTM D2688-23.
Microbiological and Legionella targets#
CDC rates a cooling tower below 10 CFU/mL of Legionella as well controlled, and the Cooling Technology Institute targets bulk heterotrophic counts below 10,000 CFU/mL. CDC treats a 10 to 100-fold increase as poorly controlled (2025-01-03). New York City's Table 8-1 sets 4 levels up to 1,000 CFU/mL or more, which requires remediation within 48 h.
How Is a Cooling Tower Chemical Program Dosed and Controlled?#
A cooling tower program is dosed against blowdown: inhibitors on blowdown or make-up volume, biocides shot-fed on a timer or ORP, acid on pH, and bleed on conductivity. Feeding on recirculation overdoses every inhibitor.
Water balance: evaporation, blowdown and make-up#
Blowdown equals evaporation divided by cycles minus one, BD = E / (C - 1), and make-up equals E x C / (C - 1). For a 50,000 gpm (11,355 m³/h) tower with a 20 °F (11.1 °C) range, the Veolia Water Technologies Handbook (Table 31-1) gives these flows.
| Cycles | Make-up, gpm (m³/h) | Blowdown, gpm (m³/h) |
|---|---|---|
| 4 | 1,330 (302) | 333 (75.6) |
| 8 | 1,140 (259) | 143 (32.5) |
Doubling cycles from 4 to 8 cuts blowdown by more than half. Conductivity and timer control are covered under cooling tower blowdown.
Feed methods: continuous, proportional and shot feed#
Cooling tower chemicals are fed 3 ways: continuous inhibitor feed proportional to blowdown or make-up, shot feed for biocides, and acid feed on a pH signal. US DOE recommends automated feed above 100 tons (352 kW). A controller drives the 4 outputs listed below.
- Bleed valve, on conductivity.
- Inhibitor pump, on bleed or make-up volume.
- Acid pump, on pH.
- Biocide pumps, on a timer or ORP.
A shot-fed biocide decays with a half-life of 0.693 V/BD. Setpoints and control modes are covered under water treatment controllers and conductivity control.
Worked example: inhibitor feed rate from blowdown#
Worked example: a tower with 100 gpm (22.7 m³/h) of blowdown and an inhibitor target of 100 ppm (mg/L) as product needs 120 lb/day (54.5 kg/d) of product. The 3 steps are listed below.
- Measure blowdown from meters: 100 gpm (22.7 m³/h).
- Multiply target ppm as product by blowdown and the constant: 100 x 100 x 0.012 = 120 lb/day, or 100 x 22.7 x 0.024 = 54.5 kg/d.
- Add an initial slug of system volume times target ppm.
The chemical dosage calculator converts mg/L into kg/d, lb/day and pump settings.
How Is Cooling Tower Water Tested and Monitored?#
Cooling tower water is tested on 3 time scales: chemistry several times a week, Legionella monthly in New York City, and corrosion coupons over about 90 days.
Routine water tests and their frequency#
New York City requires pH, temperature, conductivity and biocide indicators at least 3 times a week, no more than 2 days apart, and a bacteriological indicator weekly (24 RCNY 8-05(f)).
Chloride and sulfate are the preferred cycles tracers but fail where chlorine or sulfuric acid is fed, and contacting conductivity electrodes read accurately from about 50 to 8,000 µS/cm (Veolia Water Technologies Handbook, chapter 36). Test methods, kits and tracers are covered under cooling tower water testing.
Corrosion coupons and corrosion probes#
Corrosion coupons give a time-weighted metal loss over about 90 days, while linear polarization resistance probes give an instantaneous rate in mpy. Coupons sit in the riser or warmest water, away from injection points.
Electrical resistance probes and test heat exchangers complete the toolkit (ASTM D4778-24). Racks, exposure and weight-loss arithmetic are covered under corrosion coupons and corrosion monitoring.
Microbiological and Legionella sampling#
Legionella is measured by culture, with a detection limit of about 10 CFU/mL and 7 to 14 days to result, or by PCR, which reports genomic units, not CFU, in 2 to 48 h (CDC). New York City requires culture at least monthly, no more than 31 days apart, by an ELAP laboratory since 2026-05-08 (24 RCNY 8-05(f)(3)).
Samples are taken just before a non-oxidizer shot, and CDC emergency disinfection holds 10 ppm free halogen for at least 24 h. Dip slides, ATP and plate counts are compared under microbiological monitoring.
Which Regulations Apply to Cooling Tower Water Treatment?#
Three layers of rules apply to cooling tower water treatment: biocide registration, Legionella management and blowdown discharge limits. Each binds a different party: the biocide maker, the building owner and the discharger.
Biocide registration: FIFRA and the EU Biocidal Products Regulation#
In the US, a cooling tower biocide is a pesticide that must carry an EPA registration before sale (40 CFR 152.15), because slimicide and algicide claims are pesticide claims.
Cooling tower products are non-public-health antimicrobial pesticides, among more than 4,000 antimicrobial products registered with EPA. Labels, EPA numbers and state registration are covered under FIFRA registration of water treatment biocides.
EPA's final efficacy guidance for Legionella claims in cooling towers (August 2024) asks for a mean reduction of at least 5.0 log for remediation and 3 log for routine maintenance.
In the EU, cooling biocides fall under product types 11 and 12 of Regulation (EU) 528/2012. Glutaraldehyde is approved until 2029-03-31 (Decision (EU) 2026/1109) and CMIT/MIT until 2027-06-30 (Regulation (EU) 2016/131), while DBNPA gains PT11 approval only from 2027-07-01, for preservation of up to 14 days (Regulation (EU) 2026/577). PT11 approvals are listed under the EU Biocidal Products Regulation.
Legionella rules: ASHRAE 188, CDC and New York City#
ANSI/ASHRAE Standard 188-2021, the edition listed by ASHRAE on 2026-10-20, sets the framework of a Legionella water management program, with Guideline 12-2023 as its companion. CDC's Legionella Control Toolkit adds the cooling tower module.
New York City requires automated treatment at least daily, an oxidizing primary biocide approved by the state, certified applicators, monthly Legionella culture and records kept for at least 3 years. Program elements are set out under ASHRAE 188 and Legionella water management programs.
Blowdown discharge limits and the chromate ban#
Steam electric plants may discharge cooling tower blowdown with at most 0.5 mg/L free available chlorine (0.2 mg/L average) for 2 h a day per unit, 0.2 mg/L chromium and 1.0 mg/L zinc (40 CFR 423.13(d)). Chromate is banned in comfort cooling towers (use from 1990-05-18, 40 CFR 749.68) and industrial process cooling towers (40 CFR 63.402).
Other facilities follow the limits in their NPDES permits. Industry categories are covered under effluent limitations guidelines.
Complete List of Cooling Tower Chemicals: 43 Chemicals by Function#
The complete list of cooling tower chemicals holds 43 chemicals in 8 families, each with CAS number, role and typical cooling dose with its basis.
| # | Chemical | CAS | Family | Role in the tower | Typical cooling dose and basis |
|---|---|---|---|---|---|
| 1 | HEDP | 2809-21-4 | scale inhibitors | scale and steel corrosion | 2 to 5 mg/L as active |
| 2 | ATMP | 6419-19-8 | scale inhibitors | CaCO3 scale; chlorine-sensitive | 2 to 3 mg/L as active |
| 3 | PBTC | 37971-36-1 | scale inhibitors | CaCO3 at high LSI; halogen-stable | 3 to 5 mg/L as active |
| 4 | DTPMP | 15827-60-8 | scale inhibitors | alkaline and high-barium water (manufacturer) | no verified cooling dose |
| 5 | HPAA | 23783-26-8 | scale inhibitors | carbon steel corrosion, low-halogen systems | 5 to 15 mg/L as active |
| 6 | polyacrylic acid (sodium polyacrylate) | 9003-01-4 | scale inhibitors | dispersant, calcite threshold | 1 to 15 mg/L as product alone (supplier figure) |
| 7 | polymaleic acid | 26099-09-2 | scale inhibitors | calcite at high LSI | 1 to 15 mg/L with a phosphonate (supplier figure) |
| 8 | AA/AMPS copolymer | 40623-75-4 | scale inhibitors | calcium phosphate, zinc and iron dispersion | no verified cooling dose |
| 9 | phosphinocarboxylic acid | 71050-62-9 | scale inhibitors | CaCO3 and calcium phosphate | 5 to 20 mg/L alone (supplier figure) |
| 10 | polyaspartic acid | 25608-40-6 | scale inhibitors | phosphorus-free programs | 5 mg/L in static tests (Liu 2011) |
| 11 | polyepoxysuccinic acid (PESA) | 51274-37-4 | scale inhibitors | phosphorus-free programs | no verified cooling dose |
| 12 | sodium hexametaphosphate | 68915-31-1 | phosphates | white-rust passivation of new galvanized towers | 3 to 5 mg/L as PO4 after 5 to 6 days of high dose |
| 13 | zinc orthophosphate | 7779-90-0 | phosphates | zinc-phosphate corrosion programs | program-specific |
| 14 | tolyltriazole (TTA) | 29385-43-1 | corrosion inhibitors | copper alloys | 1 to 2 mg/L in zinc programs |
| 15 | benzotriazole (BZT) | 95-14-7 | corrosion inhibitors | copper alloys | 1 mg/L as active |
| 16 | mercaptobenzothiazole (MBT) | 149-30-4 | corrosion inhibitors | copper alloys (largely replaced) | 1.5 to 2.5 mg/L |
| 17 | sodium molybdate | 7631-95-0 | corrosion inhibitors | anodic inhibitor; tracer | about 3 mg/L as Mo (tracer about 0.5 mg/L as Mo) |
| 18 | zinc salts (zinc sulfate) | 7733-02-0 | corrosion inhibitors | cathodic inhibitor | about 0.5 mg/L insoluble Zn above the soluble level |
| 19 | sodium silicate | 1344-09-8 | corrosion inhibitors | anodic film | film in 2 to 3 weeks |
| 20 | glutaraldehyde | 111-30-8 | biocides | slug biocide, SRB | 40 to 200 ppm as product (20 to 100 mg/L as active, 50 % product) |
| 21 | isothiazolinone (CMIT/MIT) | 55965-84-9 | biocides | slug biocide, algae | 100 ppm as product, 4 h contact (supplier figure) |
| 22 | DBNPA | 10222-01-2 | biocides | fast-kill slug | under 1 to 24 ppm as active (EPA label range) |
| 23 | bronopol | 52-51-7 | biocides | bactericide | 8.6 to 264 ppm as active (EPA label range) |
| 24 | quaternary ammonium compounds (ADBAC, DDAC) | 68424-85-1; 7173-51-5 | biocides | algae, biodispersant synergist | label: initial 39.8 ppm, maintenance 14.9 ppm |
| 25 | THPS | 55566-30-8 | biocides | SRB and aerobes | program-specific |
| 26 | polyquat (WSCP) | 31512-74-0 | biocides | algaecide | program-specific |
| 27 | TTPC | 81741-28-8 | biocides | biocide and biodispersant | program-specific |
| 28 | dodecylguanidine hydrochloride | 13590-97-1 | biocides | metabolic inhibitor | program-specific |
| 29 | methylene bis(thiocyanate) | 6317-18-6 | biocides | algaecide | program-specific |
| 30 | sodium hypochlorite | 7681-52-9 | oxidants | primary halogen | 0.5 to 1.0 mg/L free as Cl2 in the hot return |
| 31 | chlorine gas | 7782-50-5 | oxidants | once-through and power-plant halogenation | to the TRC or FAC limit (40 CFR 423.13) |
| 32 | chlorine dioxide | 10049-04-4 | oxidants | high-pH or ammonia-contaminated towers | dosed by microbial results |
| 33 | sodium chlorite | 7758-19-2 | oxidants | chlorine dioxide generation | 2.6 lb per lb of Cl2 reacted (theory) |
| 34 | sodium bromide (activated bromine) | 7647-15-6 | oxidants | bromine activation | 1.45 mg per mg/L Cl2 (calculated) |
| 35 | BCDMH | 16079-88-2 | oxidants | solid bromine | initial 0.24 to 0.72 kg per 10 m³ (0.53 to 1.6 lb per 2,640 gal) to reach 1 mg/L bromine |
| 36 | trichloroisocyanuric acid (TCCA) | 87-90-1 | oxidants | erosion-feeder chlorine | cyanurate lowers HOCl activity |
| 37 | peracetic acid | 79-21-0 | oxidants | biocide (EU PT11, PT12) | program-specific |
| 38 | sulfuric acid | 7664-93-9 | pH adjusters | alkalinity and LSI control | to the pH or LSI target |
| 39 | hydrochloric acid | 7647-01-0 | pH adjusters | alkalinity control where CaSO4 limits cycles | to the pH target |
| 40 | hydrated lime | 1305-62-0 | pH adjusters | make-up or side-stream softening | jar test |
| 41 | sodium bisulfite | 7631-90-5 | dechlorination | blowdown dechlorination | per mg/L residual chlorine |
| 42 | silica filter sand | 14808-60-7 | media | side-stream filtration | rate per filter design |
| 43 | anthracite filter media | 8029-10-5 | media | side-stream filtration | n/a |
Label and supplier figures are marked. None of the biocides in rows 20 to 29 is listed in 21 CFR 173.310. Every chemical is filterable by CAS and function in the water treatment chemical database.
How Is a Cooling Tower Water Treatment Program Bought?#
A cooling tower water treatment program is bought under one of 5 service models, and US DOE advises comparing vendors on the cost to treat 1,000 gal (3.785 m³) of make-up and the highest cycles each one commits to. The 5 service models are listed below.
- Full-service program: chemicals, feed equipment, visits, testing and reporting.
- Chemical supply with technical service: the buyer operates the system.
- Chemical-only commodity supply: a sealed bid on unit price and specification.
- Monitoring-led service: online controller, remote lab and alarms.
- Performance-based contract: payment linked to make-up cost, water saved or corrosion targets.
A quote needs tower size in tons (kW) or recirculation, system volume, make-up analysis, current cycles, blowdown control, metallurgy, meters, the ASHRAE 188 plan status and the water and sewer cost per 1,000 gal. A complete proposal states dose basis, a KPI table, visit frequency, equipment ownership, price basis and FIFRA-registered biocides. How to compare providers is covered under water treatment companies.
The committed cycles belong in the contract, because US DOE notes that some vendors resist raising cycles.
Programs are priced per wet ton delivered, per 1,000 gal of make-up, or as a monthly fee with per-unit chemical prices. How service contracts are priced is covered under water treatment program cost.
How Does Cooling Tower Treatment Differ from Other Cooling Water Systems?#
Cooling tower treatment differs from the 2 other cooling water systems by water loss: an open tower evaporates and bleeds water, a closed loop loses almost none, and once-through cooling discharges all of it after one pass. That loss decides the feed, as Table 6 shows.
| System | Water loss | Treatment approach | Key number | Page |
|---|---|---|---|---|
| Open recirculating cooling tower | evaporation, drift, blowdown | continuous inhibitor feed proportional to blowdown, halogen, shot biocides | blowdown = E / (C - 1) | this page |
| Closed loop | almost none | one-time inhibitor charge by system volume through a bypass feeder | slug by system volume | closed loop water treatment |
| Once-through cooling | all water discharged after one pass | intake chlorination, continuous flow-paced feed | TRC 0.20 mg/L maximum at plants of 25 MW or more (40 CFR 423.13(b)); 316(b) at 2 MGD (7,570 m³/d) or more | once-through cooling water treatment |
| Intake macrofouling | n/a | molluscicide programs at intakes | Asiatic clams and zebra mussels, the main US freshwater concern | macrofouling control |
Boilers, membranes and wastewater run their own chemical programs. Boiler, membrane and wastewater programs are compared under industrial water treatment.
What causes cooling tower foaming?#
Cooling tower foaming comes from tertiary-treated wastewater make-up, organics or oil, microbial growth and high dissolved solids, and the first fix is more blowdown, not an antifoam. Foam depressants are reserved for extreme cases (Veolia Water Technologies Handbook). Quaternary ammonium biocides can also foam.
Which cooling tower chemicals must never be mixed?#
Sodium hypochlorite must never meet an acid, because the mixture releases chlorine gas, and hypochlorite with hydrochloric acid caused 24 % of 72 analyzed chemical-unloading accidents (INERIS, 2025). Quats must not share a feed with anionic dispersant polymers, and ammonia causes a 50 % loss of glutaraldehyde in 8 h (100 ppm each, pH 9; supplier data). Every incompatible pair is listed under water treatment chemical compatibility.
What is the best cooling tower water treatment?#
No single program is best: the best cooling tower water treatment holds the limits of Tables 2 to 4 at the highest cycles and the lowest cost per 1,000 gal (3.785 m³) of make-up.
Can non-chemical devices replace cooling tower chemicals?#
Non-chemical devices cannot replace the chemical biocide in a New York City tower unless the city approves them, and the Cooling Technology Institute found little application data for them (2008). US DOE lists ozonation and ionization only after a life-cycle cost evaluation.
Is ozone used in cooling tower water treatment?#
Yes, but rarely: ozone sees limited cooling use because of its capital cost.