A cooling tower’s entire job is to get rid of heat by evaporating water, and that’s exactly where the trouble starts. Every litre that evaporates leaves its dissolved minerals behind, so the water still circulating gets a little more concentrated with every hour the tower runs. Left alone, that concentration climbs until it starts depositing scale, eating into metal surfaces, or feeding algae and bacteria in a warm, wet, sunlit basin. None of this happens overnight. It builds up quietly, which is why most plants only notice something is wrong once heat transfer has already dropped, a pump has failed, or a shutdown for cleaning shows up on the maintenance calendar.
This guide walks through what’s actually happening inside a cooling tower, the problems that show up when water chemistry isn’t controlled, and what a proper treatment program looks like beyond just adding chemical to a dosing tank.
Why Cooling Tower Water Quality Matters
Poor water chemistry doesn’t stay contained to the water. It works its way into everything the water touches.
Scale acts like insulation on a heat exchanger surface. Even a thin layer, something you’d barely notice by touch, forces the system to run hotter or longer to move the same amount of heat, which pushes energy consumption up. Corrosion does the opposite kind of damage: it thins pipework and heat exchanger tubes from the inside until a leak or a failure forces an unplanned shutdown. Fouled cooling tower fill loses airflow and loses its ability to cool water efficiently, and once biological growth gets into it, cleaning becomes a bigger job than anyone budgeted for.
The pattern is always the same: poor water chemistry leads to deposits, corrosion, or biological growth, which reduces heat transfer, which raises both operating costs and maintenance costs. Everything downstream of water quality — energy bills, equipment life, how often you’re calling in a cleaning crew — traces back to how well that water is controlled.
What Happens to Water Inside a Cooling Tower?
A cooling tower runs on a simple loop: make-up water enters the system, some of it evaporates as it does its job of removing heat, and the minerals that were dissolved in that evaporated water stay behind in the basin. Run this cycle long enough without intervening and the remaining water becomes a concentrated soup of calcium, magnesium, silica, chlorides and whatever else was in the make-up supply.
This is where cycles of concentration, usually just written as COC, comes in. COC is a simple ratio: how much more concentrated the circulating water is compared to the fresh make-up water going in. A tower running at 5 cycles has water that’s roughly five times more concentrated in dissolved solids than what’s coming in from the main. The only way to keep that number from climbing indefinitely is blowdown — deliberately draining off some of the concentrated water and replacing it with fresh make-up.
Conductivity is the practical handle operators use to manage this. Since dissolved solids conduct electricity, conductivity rises in a fairly predictable way alongside concentration. Most modern systems use a conductivity controller to trigger blowdown automatically once the water crosses a set threshold, rather than relying on someone remembering to open a valve on a schedule.
Common Cooling Tower Water Quality Problems
Scale Formation
As water concentrates, dissolved minerals like calcium carbonate, calcium sulphate, magnesium salts and silica eventually exceed what the water can hold in solution, and they start precipitating out onto the warmest surfaces in the system — usually the heat exchanger. Once scale starts forming, it tends to build on itself, because a rough scaled surface gives new deposits more places to grab onto than a clean one does.
Corrosion
Water chemistry that’s too aggressive, whether from low pH, high dissolved oxygen, or an imbalance in dissolved salts, attacks bare metal directly. This shows up as pitting, thinning pipe walls, and eventually leaks. Corrosion and scale are often treated as opposite problems, and in a sense they are: water aggressive enough to corrode metal usually isn’t concentrated enough to scale it, and vice versa. A treatment program has to hold that balance rather than solving one problem by making the other worse.
Microbial Growth and Biofouling
A cooling tower basin is close to ideal conditions for bacteria and algae: warm water, constant aeration, sunlight on the deck, and a steady supply of nutrients. Left unchecked, microbial growth forms biofilm on tower fill and heat exchanger surfaces, which insulates against heat transfer in much the same way scale does, and it can trap corrosive byproducts against the metal underneath. Legionella is one microorganism worth naming specifically here, since it’s the reason cooling towers get regulatory attention in many regions, though a full discussion of Legionella risk management is really its own topic.
Suspended Solids and Fouling
Cooling towers pull air across a large surface area to evaporate water, and that air brings dust, pollen and airborne debris with it. Combined with corrosion byproducts and biological debris already in the system, this settles out as sludge in low-flow areas, plugs strainers, and reduces flow through the tightest passages in the heat exchanger.
How Cooling Tower Water Treatment Controls These Problems
Each of these problems has a fairly specific chemical answer, though the right combination and dose always depends on the water and the system it’s running in.
| Problem | Treatment Approach |
| Scale | Scale inhibitors / antiscalants |
| Corrosion | Corrosion inhibitors |
| Microbial growth | Oxidising and non-oxidising biocides |
| Biofilm / fouling | Biodispersants |
| High alkalinity | De-alkalisation where required |
| Existing deposits | Descaling and cleaning |
Aries Chemtech manufactures products across each of these categories, but which one applies — and at what concentration — is a question a water analysis answers, not a generic product list.
Why Chemical Dosing Alone Is Not Enough
Buying the right chemical solves maybe half the problem. A working treatment program also needs:
• Chemical dosing set to actual system demand, not a fixed rate
• Conductivity monitoring to track concentration in real time
• pH monitoring, since most inhibitors only perform within a specific pH window
• Blowdown control tied to that conductivity reading
• Regular water-quality testing — hardness, alkalinity, chlorides, silica
• Microbiological monitoring, not just biocide dosing on a calendar
• Periodic cleaning of fill, basins and heat exchangers
• Adjustment of the whole program as operating conditions change — new season, different make-up water source, higher load
The correct chemical, dosed at the wrong rate or into water conditions nobody is tracking, will not deliver the result on the data sheet. Dosing without monitoring is guessing with better tools.
Understanding Cooling Tower Blowdown and Cycles of Concentration
Blowdown is the deliberate discharge of a portion of the concentrated circulating water, replaced by fresh make-up water. It exists purely to keep dissolved solids from climbing past the point where scale, corrosion or fouling become unavoidable.
Cycles of concentration, as covered above, is the ratio between circulating water concentration and make-up water concentration. Running at higher cycles means less water consumption and less blowdown — which is attractive from a water-cost and sustainability standpoint — but it also means the circulating water is carrying a heavier mineral load. Push cycles too high for the water chemistry involved and scale or corrosion risk increases sharply, sometimes past the point any antiscalant can hold back.
Conductivity is what makes blowdown practical to automate. Rather than guessing when to blow down, a controller reads conductivity continuously and opens the blowdown valve once it crosses a setpoint tied to a target COC. Get that setpoint wrong — either from bad water analysis data or from a system that’s changed since it was last calibrated — and you either waste water blowing down too early or let concentration run past a safe limit.
There’s no universal answer for how many cycles a given tower should run at. It’s a balance between water savings and scaling or corrosion risk, and that balance is set by the specific make-up water quality, the treatment chemicals in use, and the materials the system is built from.
How to Choose the Right Cooling Tower Treatment Program
There’s no single “best” cooling tower chemical. What works depends on a list of specifics that only a water analysis can answer properly:
• Make-up water quality and source
• Hardness
• Alkalinity
• Silica content
• pH
• Conductivity / total dissolved solids
• Cooling tower design and fill type
• Recirculation rate
• Operating temperature
• Materials of construction — mild steel, copper, galvanised, stainless
• Target cycles of concentration
• Existing scale or corrosion condition in the system
This is exactly why a program built around a proper water analysis outperforms one built around a generic product recommendation. Two towers running the same load in the same city can need different treatment simply because their make-up water comes from different sources.
Cooling Tower Water Quality Monitoring Checklist
Daily or regular checks:
• pH
• Conductivity
• Chemical residual (inhibitor or biocide level)
• Blowdown operation — confirming the valve is actually cycling as expected
• Water appearance — clarity, colour, any visible growth
Periodic checks:
• Hardness
• Alkalinity
• Chloride
• Silica
• Microbiological condition
• Corrosion indicators — coupon results or corrosion rate data
• Scale or deposit condition on heat exchanger surfaces
None of this comes with a fixed dosage table you can apply everywhere. Dosage always depends on the specific water and system conditions in front of you, which is exactly how a proper treatment program should be approached.
Signs Your Cooling Tower Treatment Program Needs Attention
A treatment program that’s drifting off track usually announces itself before a failure does, if you know what to watch for:
• Rising approach temperature on the cooling tower
• Falling heat-transfer efficiency
• Visible scale on fill or heat exchanger surfaces
• Rust or corrosion products showing up in the water
• Slime or algae growth in the basin or on the deck
• Unusual water colour or turbidity
• Increasing pressure drop across the system
• More frequent cleaning shutdowns than usual
• Excessive blowdown — a sign the conductivity setpoint or COC target needs revisiting
• Chemical consumption climbing without a clear operational reason
Any one of these on its own might just be normal variation. Two or three together, especially if they’re trending in the same direction over a few weeks, is usually the point to review the program rather than wait for a shutdown to force the issue.
Best Practices for Efficient Cooling Tower Water Treatment
1. Test make-up water before designing or revising the treatment program — everything downstream depends on this.
2. Set cycles of concentration based on actual water chemistry and system materials, not a round number that sounds efficient.
3. Monitor conductivity and confirm blowdown is actually responding to it.
4. Address scale early. Removing established deposits is always harder than preventing them.
5. Keep corrosion protection consistent — inhibitor levels that drift are as much a risk as no inhibitor at all.
6. Run a microbiological control program that alternates treatment types rather than relying on one biocide indefinitely.
7. Inspect and clean fill, basins and heat exchangers on a schedule, not only when a problem is already visible.
8. Revisit dosing whenever operating conditions change — new season, higher load, a different make-up water source.
Cooling Tower Water Treatment Chemicals from Aries Chemtech
Aries Chemtech manufactures cooling tower treatment chemicals designed to address scale, corrosion, microbial growth and fouling in industrial cooling systems, including antiscalants, corrosion inhibitors, biocides and biodispersants. Treatment programs are built around a water analysis and selected according to actual water quality and operating conditions, not a one-size-fits-all product line. You can see the full range on the Cooling Tower Chemicals page, including Cooling Tower Antiscalant for scale-specific applications.
For closed-loop systems, see Chiller Chemicals; for boiler-side treatment, see Boiler Chemicals; and for reverse osmosis pretreatment, see RO Antiscalant Chemicals.
Frequently Asked Questions
How do you maintain good water quality in a cooling tower?
By combining the right chemical treatment — antiscalants, corrosion inhibitors and biocides matched to the water — with ongoing monitoring of pH, conductivity and blowdown, plus periodic testing and cleaning. Chemical alone, without monitoring, tends to drift off target.
What causes scale in cooling towers?
Scale forms when dissolved minerals such as calcium, magnesium and silica become too concentrated for the water to hold in solution, usually as cycles of concentration climb without adequate blowdown or scale inhibitor to compensate.
Why is blowdown required in cooling towers?
Because evaporation continuously concentrates dissolved solids in the circulating water. Blowdown discharges some of that concentrated water and replaces it with fresh make-up, keeping dissolved solids within a safe range.
What are cycles of concentration in cooling towers?
Cycles of concentration is the ratio between the concentration of dissolved solids in the circulating water and the concentration in the make-up water. Higher cycles mean less water use but a heavier mineral load in the system.
How can corrosion be controlled in a cooling tower?
With corrosion inhibitors matched to the metals in the system, maintained at a consistent dosage and pH range, alongside regular monitoring — corrosion coupons or online monitoring — to confirm the protection is actually holding.
How do you control microbial growth in cooling tower water?
Through a biocide program, usually alternating an oxidising biocide with a non-oxidising one to prevent resistant organisms from establishing, combined with biodispersants to break up biofilm and regular microbiological testing.
How often should cooling tower water be tested?
pH, conductivity and chemical residual are best checked daily or continuously through automated monitoring. Hardness, alkalinity, chloride, silica and microbiological condition are usually tested on a periodic schedule, often weekly to monthly depending on system risk.
How do you know if a cooling tower needs chemical treatment?
Visible scale, rust in the water, slime or algae growth, rising approach temperature, or falling heat-transfer efficiency are all signs the current program — or the absence of one — isn’t controlling the water. A water analysis is the starting point for figuring out what’s actually needed.
