Cooling Tower Water Treatment: The Plant Engineer's Complete Chemical Guide
Table of Contents
What you will learn
💡 Frequently Asked Questions
Find quick answers to common questions about cooling tower water treatment: the plant engineer's complete chemical guide.
Cooling Tower Water Treatment: The Plant Engineer's Complete Chemical Guide
Stop overpaying for water treatment contracts. Here are the chemicals, the dosing math, and the monitoring protocols — everything you need to run your own program.
Cooling tower water treatment prevents three problems: scale buildup (calcium/magnesium deposits that choke heat transfer), corrosion (rust and metal loss that destroys equipment), and biological growth (bacteria, algae, and Legionella). The core chemicals are:
- Sulfuric acid for pH control (target 7.0–8.5)
- Sodium hypochlorite as an oxidizing biocide (maintain 1–3 ppm free chlorine)
- Scale and corrosion inhibitors (phosphonates, polymers, azoles)
- Sodium bisulfite for dechlorination before discharge
Most facilities can run their own chemical program for 40–60% less than a full-service contract.
1 Why Cooling Towers Need Chemical Treatment
Cooling towers are evaporative heat rejection devices. Water absorbs heat from a process — a chiller condenser, an industrial process, a data center cooling loop — and releases it to the atmosphere through evaporation. The water that evaporates is pure. The dissolved minerals stay behind and concentrate.
A tower running at 5 cycles of concentration has 5x the mineral content of the makeup water feeding it. Without chemical treatment, this concentrated water causes:
- Scale deposits on fill media and heat exchange surfaces — reducing efficiency by 20–30%
- Corrosion of piping, basins, and heat exchangers — shortening equipment life
- Biological growth including Legionella pneumophila — creating health hazards and liability
- Fouled equipment and unplanned downtime — costing tens of thousands in emergency repairs
2 The Three Enemies — Scale, Corrosion, Biofilm
Scale
Minerals like calcium carbonate and calcium sulfate precipitate out of solution as water concentrates through evaporation. They form hard, insulating deposits on heat transfer surfaces, fill media, and piping. The hotter the surface, the faster scale forms — calcium carbonate has inverse solubility, meaning it becomes less soluble as temperature rises.
Scale reduces heat transfer efficiency, increases energy consumption, and can eventually restrict or block water flow through the system.
Corrosion
Dissolved oxygen, low pH, chloride ions, and microbiologically influenced corrosion (MIC) attack metal surfaces throughout the cooling system. Mild steel piping, copper condenser tubes, and galvanized components are all vulnerable.
Corrosion thins pipe walls, creates pinhole leaks, and generates iron oxide deposits (rust) that further reduce heat transfer and clog distribution nozzles. Left unchecked, corrosion leads to catastrophic failures and expensive tube replacements.
Biological Growth
Warm (typically 85–95°F), aerated, nutrient-rich cooling tower water is an ideal growth environment for bacteria, algae, and fungi. Biofilm — a slimy layer of microorganisms — coats wetted surfaces with an insulating barrier that reduces heat transfer. Algae clogs fill packing and distribution decks, reducing airflow and water distribution.
Most critically, Legionella pneumophila — the bacterium that causes Legionnaires' disease — thrives in cooling tower water between 77–113°F. Cooling towers are the number one identified source of Legionnaires' disease outbreaks in the United States.
This is what neglect looks like. Scale and biological fouling in cooling water piping reduce flow, increase pumping energy, and accelerate corrosion underneath the deposits. By the time you can see deposits like these, the efficiency loss has been compounding for months.
3 Your Chemical Arsenal
A cooling tower treatment program is built around five categories of chemicals. Each one addresses a specific threat. Here's what they do, how to dose them, and which Alliance Chemical products to use.
📸 Sulfuric Acid for pH Control
Role: Lowers pH and alkalinity to prevent calcium carbonate scale.
Why sulfuric acid specifically: It's the industry standard for cooling tower pH control because it doesn't introduce chlorides the way hydrochloric acid does. Chlorides accelerate corrosion — particularly stress corrosion cracking of stainless steel. Sulfuric acid converts bicarbonate alkalinity to sulfate, which is far less likely to form scale.
Target pH: 7.0–8.5, determined by your Langelier Saturation Index (LSI) calculation. The LSI accounts for pH, temperature, calcium hardness, alkalinity, and TDS to predict whether your water will scale or corrode. A positive LSI means the water wants to deposit scale. A negative LSI means it's corrosive. The goal is to keep LSI near zero — slightly positive for mild steel systems (a thin protective scale layer), slightly negative for systems with corrosion inhibitors.
Dosing: Controlled by a pH controller connected to a chemical metering pump. The controller monitors tower water pH continuously and feeds acid to maintain setpoint. Typical feed rates for a 200-ton tower range from 0.5 to 5 gallons per week of 93% sulfuric acid, depending on makeup water alkalinity.
🧪 Sodium Hypochlorite as Biocide
Role: Oxidizing biocide — kills planktonic bacteria, controls algae, and helps manage biofilm.
How it works: Sodium hypochlorite dissociates in water to form hypochlorous acid (HOCl), the active germicidal agent. HOCl is a powerful oxidizer that disrupts cell membranes and destroys enzymes in microorganisms.
Target: Maintain 1–3 ppm free chlorine residual in the recirculating water. Test with a DPD (N,N-diethyl-p-phenylenediamine) test kit — the same type used for swimming pools.
Feed strategy: Most towers use intermittent feed controlled by a timer (e.g., 30 minutes on / 2 hours off) or an ORP (oxidation-reduction potential) controller that maintains a target millivolt reading. Periodic shock doses of 5–10 ppm for 2–4 hours help penetrate and break up established biofilm.
🛡️ Scale and Corrosion Inhibitors
- Phosphonates (HEDP, ATMP, PBTC): Threshold scale inhibitors that work at low ppm to interfere with calcium carbonate crystal growth. They don't remove calcium — they prevent it from forming organized crystal structures that deposit on surfaces.
- Polymeric dispersants (polyacrylic acid, maleic copolymers): Keep suspended solids and precipitated minerals dispersed in the water so they can be removed through blowdown rather than depositing on surfaces.
- Azoles (tolyltriazole/TTA, benzotriazole/BTA): Form a thin protective film on copper and copper-alloy surfaces (condenser tubes, brazed plate heat exchangers). Without azole protection, copper alloys suffer accelerated corrosion in chlorinated cooling water.
- Molybdate: An anodic corrosion inhibitor for mild steel. Sodium molybdate forms a passive oxide film on steel surfaces. Used in some programs, particularly where phosphate discharge limits are strict.
Note: Most facilities purchase a pre-blended inhibitor product from a treatment chemical supplier. These proprietary blends combine phosphonates, polymers, and azoles in a single drum. Contact Alliance Chemical if you need guidance on inhibitor selection for your specific system.
♻️ Sodium Bisulfite for Dechlorination
Role: Neutralizes residual free chlorine in blowdown water before discharge.
When needed: Required when your blowdown discharges to a surface water body (river, lake, storm drain) or to a sewer system with chlorine limits. Most NPDES permits and POTW discharge permits have limits on residual chlorine.
Chemistry: Sodium bisulfite (NaHSO₃) reacts with hypochlorous acid on a 1:1 molar basis. In practical terms, 1.46 ppm of sodium bisulfite neutralizes 1 ppm of free chlorine. The reaction is nearly instantaneous.
Dosing: Feed proportionally to blowdown flow using a small metering pump triggered by the blowdown valve. Alternatively, install a dechlorination tablet holder on the blowdown discharge line.
🔼 Sodium Hydroxide for pH Adjustment Up
Role: Raises pH when makeup water is naturally acidic or when acid overfeed occurs.
Also used during system passivation procedures after cleaning, and for neutralization of acid-containing waste streams.
Less commonly needed than acid in cooling tower programs, but essential to have on hand for pH correction and emergency overfeed response.
📊 Chemical Comparison at a Glance
| Chemical | Role | Alliance Source | Typical Range | Feed Method |
|---|---|---|---|---|
| Sulfuric Acid 93% | pH reduction / alkalinity control | Acids Collection | pH 7.0–8.5 target | Metering pump, pH controlled |
| Sodium Hypochlorite 12.5% | Oxidizing biocide | Sodium Hypochlorite Collection | 1–3 ppm free Cl₂ | Timer or ORP controlled |
| Phosphonate blend | Scale inhibition | Contact Alliance | 5–15 ppm as PO₄ | Proportional to makeup flow |
| Polymer dispersant | Deposit control | Contact Alliance | 5–20 ppm active | Blended with inhibitor |
| Azole (TTA/BTA) | Copper alloy protection | Contact Alliance | 2–5 ppm active | Blended with inhibitor |
| Sodium Bisulfite | Dechlorination before discharge | Wastewater Collection | 1.46 ppm per ppm Cl₂ | Proportional to blowdown |
| Sodium Hydroxide 50% | pH increase | Bases Collection | As needed | Metering pump, manual or auto |
A typical chemical feed system for cooling tower treatment includes metering pumps, controllers, and chemical storage tanks. Proper sizing and installation ensure consistent dosing and reliable treatment — the difference between a program that works on paper and one that actually protects your equipment.
4 Cycles of Concentration — The Math
Cycles of Concentration (CoC) = Makeup Volume ÷ Blowdown Volume — or — CoC = Tower Water Conductivity ÷ Makeup Water Conductivity
When water evaporates from a cooling tower, the dissolved minerals stay behind. If you started with makeup water containing 200 ppm total dissolved solids (TDS) and your tower water measures 1,000 ppm TDS, you're running at 5 cycles of concentration — the minerals have concentrated 5x.
Higher cycles = less water waste (you're getting more evaporative cooling per gallon of makeup) but higher mineral concentration (more aggressive treatment needed). Lower cycles = more water consumption and chemical waste, but easier to manage.
Most commercial towers run between 3 and 6 cycles. The optimal target depends on your makeup water quality — specifically hardness, alkalinity, silica, and chlorides.
💰 Cycles of Concentration Economics
| Cycles | Water Savings vs. Once-Through | Chemical Intensity | Scale Risk | Best For |
|---|---|---|---|---|
| 2 | 50% | Low | Low | Very hard water, minimal treatment |
| 3 | 67% | Moderate | Moderate | Hard water, basic programs |
| 4 | 75% | Moderate | Moderate | Average water quality |
| 5 | 80% | Higher | Higher | Good water, strong inhibitor program |
| 6 | 83% | Higher | High | Soft water, advanced program |
| 8+ | 87%+ | High | Very High | Expert program, excellent water |
5 Building a Treatment Program from Scratch
Analyze Your Makeup Water
Get a comprehensive water analysis from a certified lab: pH, total dissolved solids, total hardness (as CaCO₃), calcium hardness, M-alkalinity, chloride, sulfate, silica, iron, and copper. This single analysis determines your entire treatment approach.
Your local water utility publishes annual Consumer Confidence Reports with average water quality data, but a lab test of your actual supply gives more accurate, site-specific numbers.
Calculate Your Langelier Saturation Index
The LSI uses pH, temperature, calcium hardness, alkalinity, and TDS to predict scaling or corrosion tendency. Online LSI calculators are available from most treatment chemical vendors.
Target an LSI between –0.5 and +0.5 at your expected operating conditions. The LSI tells you how much acid you need to feed and what your target pH should be.
Set Target Cycles of Concentration
Based on makeup water quality and your treatment program's capability. Start conservative (3–4 cycles) and increase once you've demonstrated stable control.
Higher hardness water requires either lower cycles or a stronger (more expensive) inhibitor program.
Select and Size Chemical Feed Equipment
The minimum equipment for a self-managed program:
- pH controller + acid metering pump (feeds sulfuric acid)
- Timer or ORP controller + biocide pump (feeds sodium hypochlorite)
- Proportional inhibitor feed — often tied to a makeup water meter
- Conductivity controller + motorized blowdown valve
- Chemical storage: day tanks or drums with containment
Establish Your Monitoring Schedule
- Daily: Visual inspection, pH, conductivity, free chlorine residual, chemical feed pump operation check
- Weekly: Inhibitor residual testing, bacteria dip slides (total aerobic count)
- Monthly: Full lab water analysis, basin cleaning if sediment is visible
- Quarterly: Legionella sampling (strongly recommended), Water Management Plan review
Document Everything
ASHRAE Standard 188 requires a written Water Management Plan for buildings with cooling towers. Even if your jurisdiction doesn't legally mandate compliance, maintaining documentation protects you in a liability situation.
Your plan should include:
- System descriptions and flow diagrams
- Control measures with limits
- Monitoring logs
- Corrective action procedures
- Team responsibilities
A well-documented treatment program isn't just a regulatory checkbox. When something goes wrong — and in water treatment, something always eventually goes wrong — your logs tell you exactly when conditions changed and what corrective actions were taken. That trail protects your equipment and your liability exposure.
6. Legionella & ASHRAE 188 Compliance
This is the section that matters most — and the one that keeps facility managers up at night. Legionella pneumophila, the bacterium that causes Legionnaires' disease, thrives in cooling tower water. The CDC estimates 10,000–18,000 cases of Legionnaires' disease occur in the U.S. each year, and cooling towers are the most commonly identified source of community outbreaks. The disease kills approximately 1 in 10 people who contract it.
ASHRAE Standard 188 (Legionellosis: Risk Management for Building Water Systems) requires buildings with cooling towers to develop and implement a Water Management Plan (WMP). The standard calls for:
- Establish a Water Management Team with defined roles and authority
- Create system flow diagrams identifying all water systems
- Conduct a hazard analysis to identify where Legionella risk exists
- Define control measures with measurable control limits (pH, biocide residual, temperature)
- Establish monitoring procedures and frequencies for each control point
- Define corrective actions when control limits are exceeded
- Maintain documentation and records of all monitoring and corrective actions
- Verify the plan is effective through periodic validation (e.g., Legionella testing)
The CDC recommends that ALL buildings with cooling towers develop a Legionella Water Management Program. In New York City, it's the law — Local Law 77 requires cooling tower registration, quarterly Legionella sampling, and a certified WMP. Chicago, Maryland, and other jurisdictions are implementing similar requirements. The regulatory trend is clear: mandatory Legionella management programs for cooling towers are coming everywhere.
Key Control Measures for Legionella Prevention
- Maintain oxidizing biocide residual at all times — free chlorine ≥1 ppm or equivalent ORP reading
- Keep pH below 8.0 for optimal biocide efficacy — hypochlorous acid is 80-100x more effective than hypochlorite ion
- Eliminate stagnation — no dead legs in piping, drain idle systems or maintain treatment during shutdown
- Shock-treat with 5-10 ppm free chlorine for 4-6 hours after any shutdown period before restart
- Clean basin sediment regularly — sediment harbors and protects Legionella from biocides
- Test for Legionella quarterly using culture method (CDC ELITE-certified lab) or validated qPCR
- Maintain water temperature awareness — Legionella grows most aggressively between 95-115°F, which is precisely the range most cooling towers operate in
Legionella testing costs $150-300 per sample from a certified lab. Quarterly testing for a single tower runs $600-1,200 per year. Compare that to the average cost of a Legionnaires' disease outbreak response: $500,000 to over $5 million in remediation, legal fees, and business interruption. Testing is not optional — it's the cheapest insurance you can buy.
7. Monitoring & Testing Protocols
- Visual inspection: look for leaks, overflow, excessive drift, foam, algae on fill or basin, unusual color or odor
- pH measurement: use a calibrated portable meter or test strip (target: your calculated setpoint, typically 7.0-8.5)
- Conductivity reading: check the blowdown controller display or use a portable meter (verify you're at target cycles)
- Free chlorine test: use DPD test strips or a colorimetric test kit (target: 1-3 ppm)
- Chemical feed check: verify all metering pumps are operating, chemical tanks have adequate product level
- Inhibitor residual: test for phosphonate or molybdate residual to confirm inhibitor is feeding properly
- Bacteria dip slides: submerge a dip slide in tower water, incubate per instructions, read total aerobic count (TAC). Target: <10⁴ CFU/mL
- Cycles verification: calculate actual cycles from conductivity readings vs. makeup conductivity. Compare to target.
- Blowdown volume check: verify blowdown valve is operating and flow rate is reasonable
- Full laboratory water analysis: send a sample to a certified lab for complete analysis (pH, TDS, hardness, alkalinity, chlorides, sulfate, silica, iron, copper, bacteria)
- Basin inspection and cleaning: remove sediment, debris, and biological accumulation from the cold water basin
- Fill and drift eliminator inspection: look for fouling, damage, or mineral deposits
- Distribution deck/nozzle check: verify even water distribution across the fill
- Legionella sampling: collect sample per CDC/ASHRAE guidelines and submit to a certified laboratory
- Water Management Plan review: update the WMP based on any system changes, monitoring trends, or corrective actions taken
- Performance trending: review all monitoring data for trends (rising conductivity = blowdown issue, declining inhibitor = feed issue, increasing bacteria = biocide issue)
- Equipment calibration: verify pH and conductivity controllers against a lab-calibrated portable meter
A single data point tells you almost nothing. Track your results over time in a spreadsheet or logbook. A rising conductivity trend means your blowdown valve may be failing. A dropping inhibitor residual means your feed pump is losing prime or the tank is empty. Rising bacteria counts despite adequate chlorine may indicate biofilm establishment. Trending catches problems before they become emergencies.
8. DIY vs. Service Contract — The Real Math
Let's talk about what you're actually paying for when you sign a water treatment service contract — and what it looks like to run your own program.
Service Contract (Typical Mid-Size Tower, 200-500 Ton)
| Cost Component | Annual Cost |
|---|---|
| Monthly service fee ($800-1,500/mo) | $9,600-18,000 |
| Chemicals (included, 2-4x markup) | Included in fee |
| Quarterly lab analysis | Included |
| Annual equipment calibration | Included |
| Typical Total | $12,000-26,000/year |
What you're buying is convenience and accountability. The service company provides a trained technician who visits monthly, maintains feed equipment, adjusts dosing, pulls samples, and takes responsibility for the program. That has real value. But here's the breakdown of what that cost actually represents:
| What You Get | Actual Value |
|---|---|
| Chemicals (at their markup) | $3,000-8,000/year |
| Monthly technician visit (1-2 hours) | $1,800-3,600/year |
| Quarterly lab testing | $800-1,600/year |
| Annual Legionella testing | $600-1,200/year |
| Program design and oversight | $1,000-2,000/year |
| Actual cost of services | $7,200-16,400/year |
| Service company margin | $4,800-9,600/year |
DIY Program
| Cost Component | Year 1 | Year 2+ |
|---|---|---|
| Chemicals at direct pricing (Alliance) | $1,500-4,000 | $1,500-4,000 |
| Feed equipment (one-time purchase) | $2,000-5,000 | $0 |
| Lab testing (outsourced) | $1,500-2,000 | $1,500-2,000 |
| Your technician's time (2-4 hrs/month) | Internal cost | Internal cost |
| Total | $5,000-11,000 | $3,000-6,000 |
Savings potential: 40-60% annually after Year 1
If you don't have a maintenance person who can commit to consistent daily monitoring, if your facility is a hospital or large public building with heightened Legionella liability, or if you operate under strict regulatory oversight, a service contract may provide accountability and expertise that's worth the premium. The point of this section isn't to eliminate service companies — it's to help you understand what you're paying for so you can make an informed decision. Some facilities run a hybrid approach: buy chemicals directly from Alliance at wholesale pricing and hire a consultant for quarterly program reviews.
9. Safety & Handling
A Safety Data Sheet (SDS) is provided with every Alliance Chemical order. Certificate of Analysis (COA) documentation is available per lot upon request. Always review SDS documents for all treatment chemicals before handling.
10. Alliance Chemical — Your Direct Chemical Source
Sulfuric Acid 93% — Industrial Grade
The workhorse of cooling tower pH control programs. Available in quarts, gallons, 5-gallon pails, 55-gallon drums, and 275-gallon totes. COA documentation included with every shipment.
Made in the USA
Owned and operated since 1998
COA Per Lot
Certificate of Analysis provided with every shipment
SDS Included
Safety Data Sheet with every order
Government Trusted
Trusted by DOD, DLA, NASA, SOCOM, Space Force (CAGE Code: 1LT50)
Bulk Pricing
Drums, totes, and bulk quantities with volume discounts
Purchase Orders
Net terms and PO processing for business accounts
Frequently Asked Questions
Daily: pH, conductivity, and free chlorine (takes 5 minutes with portable test equipment). Weekly: bacteria dip slides and inhibitor residual. Monthly: full laboratory analysis sent to a certified lab. Quarterly: Legionella sampling is strongly recommended for all cooling towers per ASHRAE 188 guidelines.
Most cooling towers operate best between pH 7.0 and 8.5. Your specific target depends on your Langelier Saturation Index (LSI) calculation, which accounts for water chemistry, temperature, and TDS. The goal is to keep LSI near zero to balance scale and corrosion tendencies. Your pH target is the most important variable — work with a water treatment professional or use an LSI calculator to determine it for your specific water.
You can, but sulfuric acid is strongly preferred. Muriatic acid (hydrochloric acid) adds chloride ions to the cooling water, which accelerate corrosion — particularly pitting corrosion and stress corrosion cracking of stainless steel components. Sulfuric acid converts alkalinity to sulfate, which is far less corrosive. The cost difference is minimal; the corrosion difference is significant.
You cannot detect Legionella visually, by smell, or by general bacteria testing. The only way to confirm Legionella presence is through laboratory testing — either traditional culture methods (7-14 day turnaround from a CDC ELITE-certified lab) or validated qPCR testing (1-3 day turnaround). If your cooling tower has not been tested, you don't know your Legionella status. ASHRAE 188 recommends routine quarterly sampling.
Cycles of concentration (CoC) represent how many times the dissolved minerals in your tower water have concentrated compared to your makeup water. At 5 cycles, your tower water has 5x the mineral content of the makeup. Higher cycles save water but increase scale and corrosion risk, requiring more aggressive chemical treatment. Most towers operate between 3-6 cycles. This is the single most important operating parameter for your treatment program because it determines both your water consumption and your chemical requirements.
ASHRAE Standard 188 is a consensus standard (not a law by itself), but it has been adopted by reference into building codes and regulations in many jurisdictions. New York City, parts of Maryland, and several other localities have enacted laws requiring cooling tower registration, water management plans, and Legionella testing. Even where not legally mandated, ASHRAE 188 represents the standard of care — and in a liability situation, failure to follow it will be difficult to defend. The answer for most facilities is: treat it as mandatory.
Acid consumption depends primarily on your makeup water alkalinity and your makeup flow rate. A typical 200-ton cooling tower with moderately alkaline makeup water (150-200 ppm alkalinity) might consume 1-5 gallons of 93% sulfuric acid per week. Higher alkalinity water or higher makeup flow rates increase consumption proportionally. A jar test or trial feed period will establish your actual usage quickly.
Yes, provided you have a trained maintenance technician, proper chemical feed equipment, a testing program, and the discipline to monitor consistently. Many facilities — particularly those with on-site engineering staff — successfully run their own programs. The key requirements are: understanding the chemistry (this article helps), proper equipment, consistent monitoring, documentation, and a commitment to not skip testing when things get busy. Alliance Chemical can supply the chemicals; you supply the expertise and consistency.
Within days to weeks: pH and alkalinity rise as evaporation concentrates minerals. Biocide residual drops to zero. Bacteria populations explode. Within weeks to months: Scale begins depositing on heat transfer surfaces, reducing efficiency 10-30%. Biofilm establishes on all wetted surfaces. Corrosion accelerates under deposits. Legionella risk increases dramatically. Within a season: Fill packing clogs with biological growth and mineral deposits. Condenser approach temperatures rise. Equipment life shortens by years. You're now spending more on energy waste than you would have spent on treatment.
Alliance Chemical supplies sulfuric acid, sodium hypochlorite, sodium bisulfite, sodium hydroxide, phosphoric acid, and deionized water in sizes from quarts to 275-gallon totes. We provide COA documentation, SDS with every order, and bulk pricing for facility accounts. Contact sales@alliancechemical.com or visit alliancechemical.com for pricing on drums and totes.
Take Control of Your Cooling Tower Chemistry
Alliance Chemical supplies the same chemicals water treatment service companies use — sulfuric acid, sodium hypochlorite, sodium bisulfite, and more — without the service company markup. Drums, totes, and bulk quantities with COA documentation on every shipment.
Disclaimer: This guide is for informational purposes only and does not constitute engineering advice. Cooling tower water treatment involves regulated activities including Legionella risk management. Consult qualified water treatment professionals and comply with all applicable local regulations, ASHRAE standards, and your facility's specific requirements. Review the Safety Data Sheet (SDS) for each chemical product before handling. Alliance Chemical is not responsible for improper chemical application or non-compliance with applicable standards and regulations.
