---
title: "Cycles of concentration explained: cooling tower water chemistry, bleed-off and why it matters for Legionella"
source_url: https://legionella.io/articles/cooling-tower-cycles-of-concentration-water-chemistry/
canonical_url: https://legionella.io/articles/cooling-tower-cycles-of-concentration-water-chemistry/
pillar: "Building Types & Use Cases"
summary: "What cycles of concentration, conductivity, bleed-off and inhibitors actually mean on a cooling tower report, and how the chemistry feeds your Legionella risk."
primary_keyword: "cycles of concentration"
date_published: 2026-06-27
date_reviewed: 2026-06-27
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Quote, adapt or republish with attribution to REMOTE TECH LTD and a link to source_url."
license_url: https://creativecommons.org/licenses/by/4.0/
---

# Cycles of concentration explained: cooling tower water chemistry, bleed-off and why it matters for Legionella

Every time water evaporates from your cooling tower. It leaves its dissolved minerals behind in the water that stays. Run the tower long enough and those minerals build up, getting more concentrated with each pass. "Cycles of concentration" is simply the number that tells you how concentrated the circulating water has become compared with the fresh water you feed in. Get it wrong and you grow scale, corrosion and fouling, the exact conditions Legionella thrives in.

If you are the responsible person for an evaporative tower and your contractor's monthly report reads like a chemistry exam, this is the chapter nobody handed you. The microbiological control sits on top of the water chemistry, not beside it.

## The plain-English chain

Start with what a cooling tower physically does. It rejects heat by evaporating a portion of the circulating water. The minerals dissolved in that water, calcium, magnesium, chlorides, silica and the rest, do not evaporate. They stay behind and concentrate.

To stop them concentrating forever, two things happen. You deliberately throw away a little of the salty circulating water (the **bleed**, also called blowdown), and you top the system up with fresh **make-up water** to replace both what evaporated and what you bled off. The make-up dilutes; the bleed removes. The balance between them sets how concentrated the water settles.

That concentration ratio is the **cycles of concentration**. Roughly, it is how many times more concentrated the circulating water is than the make-up you put in. Two cycles means twice as concentrated; five cycles means five times. The figure is commonly approximated as the ratio of the circulating water's conductivity to the make-up water's conductivity, though your treatment programme will confirm the exact method for your site.

### Conductivity: the meter that stands in for "how salty"

You cannot easily measure every dissolved salt in real time, so the industry measures **conductivity** instead, how well the water carries an electric current, which rises as dissolved solids rise. More salts, higher conductivity. It is the practical proxy for concentration.

This is why a conductivity probe usually sits on the tower wired to a **controller**. When conductivity climbs past its set point, the controller opens the bleed valve and dumps concentrated water until conductivity drops back; fresh make-up follows automatically. That little loop is what holds your cycles steady. When you see "conductivity" all over the service sheet, this is what it is policing.

## Why more cycles is tempting, and where it bites

Running more cycles means bleeding less and using less make-up water and less chemical. It saves money and water. So the natural pull is always towards higher cycles.

The catch is that everything dissolved in the water concentrates too. Push the cycles too high for your local water and you cross the point where:

- **Scale forms.** Hardness salts come out of solution as scale on hot surfaces and the fill. Scale insulates, wrecks heat transfer, and, the part that matters here, gives biofilm and bacteria a rough, sheltered surface to colonise.
- **Corrosion accelerates.** Concentrated chlorides and aggressive water chemistry eat at metal, and corrosion products add to the debris and sludge in the system.
- **Fouling builds.** Suspended solids, biomass and corrosion debris settle into sludge in the pond and low-flow areas.

Scale, corrosion and fouling are not just engineering nuisances. HSG274 Part 1 treats a clean, well-controlled system as a precondition for microbiological control: deposits shield organisms from biocide and feed the biofilm that Legionella lives in [1]. A fouled tower is a harder tower to disinfect, whatever the biocide programme says on paper. If you need the wider control-document context, [a worked cooling tower written scheme of control](https://legionella.io/articles/cooling-tower-control-scheme-worked-example/) shows where the chemistry limits, bleed settings and responsibilities should sit.

### Inhibitors: the chemistry that lets you run safely

To run useful cycles without the damage, the treatment programme doses **inhibitors**, a **scale inhibitor** to keep hardness salts in solution, and a **corrosion inhibitor** to protect the metalwork. There is often a dispersant to keep solids suspended so the bleed can carry them away rather than letting them settle. These are the "inhibitor" and "treatment product" lines on your report, dosed in proportion to the make-up so concentration stays within range.

So the picture is one balanced system: cycles set by bleed and make-up, conductivity policing the set point, inhibitors holding back the side effects, and biocide controlling the microbiology on top of a system kept clean enough for the biocide to work [1].

## What nobody tells you

Here is the connection the operational overviews skip: **the water-saving instinct and the Legionella-control instinct pull in opposite directions, and the chemistry is where they collide.**

Chasing higher cycles to cut your water bill or hit a sustainability target is not a neutral efficiency move. Every extra cycle concentrates the salts, raises scale and corrosion potential, and increases the demand on your inhibitors and your biocide. A tower quietly run a notch too high to save water can be fouling up months before a Legionella sample ever flags it, and the dip slide and the biocide reading can both look acceptable while a scale and biofilm habitat is forming underneath them.

The second thing nobody spells out: **biocide demand is a chemistry problem, not just a dosing problem.** Oxidising biocide gets consumed by the very organics, debris and scale that high cycles encourage. The dirtier the water, the more biocide is eaten satisfying that demand before any reaches the bacteria you are trying to kill. So a control failure can show up not as "the dosing pump stopped" but as "the dosing is the same and the residual keeps disappearing", because the water got dirtier. Reading conductivity, cycles and the fouling indicators alongside the biocide residual is what lets you catch that, rather than reading the biocide line alone.

None of this means run minimum cycles and waste water. It means recognise that the cycles target is a deliberate risk-and-cost trade-off your competent water-treatment provider sets against your specific make-up water, and that nudging it up "to save a bit" is a decision with a microbiological tail.

## Reading the chemistry lines on a report

Use this as a first-pass review before you call the contractor. It is not a substitute for their treatment programme, but it tells you which questions are worth asking.

| Report line | What it tells you | What to ask if it drifts |
| --- | --- | --- |
| Make-up conductivity | The baseline dissolved-solids level in the incoming water | Has the incoming water changed, or are we comparing against an old baseline? |
| Circulating conductivity | How concentrated the tower water has become | Is the bleed valve opening at the set point, and is the probe clean and calibrated? |
| Cycles of concentration | The rough ratio between circulating and make-up water | Is the actual cycle range still inside the treatment provider's target for this site? |
| Inhibitor level | Whether the scale and corrosion protection is present at the intended dose | Is low inhibitor a dosing fault, a make-up-rate issue, or a bleed/control problem? |
| Biocide residual | Whether the microbiological control chemistry is still available after demand | If the residual is falling, are fouling, organics or high cycles consuming it before it reaches the bacteria? |

## What to do first

You do not need to become a chemist. You need to be able to read the report as the person legally accountable for the tower.

On your next service sheet, find five things: the make-up conductivity, the circulating conductivity, the resulting cycles of concentration, the conductivity set point the controller bleeds at, and the inhibitor and biocide doses. Ask your contractor to tell you, in writing, the target cycles range for your site and why it was chosen for your make-up water. Then check that recent results are sitting inside that range rather than drifting up month on month.

If cycles have crept above target, or conductivity is regularly overshooting the set point, or the report notes scale, corrosion or fouling, treat those as early Legionella-risk signals and ask what is being done, not as someone else's chemistry footnote. Read those trends alongside [cooling tower Legionella sampling and action levels](https://legionella.io/articles/cooling-tower-legionella-sampling-action-levels/), not after the microbiology has already failed.

This is general explanation, not a control scheme. Your tower's cycles target, set points, inhibitor selection and biocide regime must be set and reviewed by a competent person against a site-specific risk assessment and your actual water analysis, because the right figures depend entirely on your local water and plant [2]. Nothing here is a substitute for that assessment.

## FAQ

### What cycles of concentration should my cooling tower run at?

There is no universal number. The right range depends on your make-up water's hardness and chemistry, the inhibitor programme and the plant itself, so a tower in a hard-water area may sit lower than one fed soft water. Your water-treatment provider sets the target for your site; the useful question is whether your actual results stay within whatever range they specified, not whether you match someone else's figure.

### Is bleed-off the same as blowdown?

In day-to-day cooling-tower language, yes, both describe deliberately discharging a portion of concentrated circulating water to control the dissolved-solids level, with fresh make-up replacing it. You will see the terms used interchangeably on reports. What matters is that the bleed is actually happening and is controlled, usually by a conductivity controller, rather than left to chance.

### Why does my biocide residual keep disappearing if the dosing hasn't changed?

A common cause is rising biocide demand. Organics, suspended solids, corrosion debris and scale all consume oxidising biocide before it reaches the bacteria, so a dirtier system "eats" more biocide at the same dose. That is why fouling and high cycles matter to microbial control. Read the residual alongside the conductivity, cycles and any fouling notes, and raise a falling residual with your contractor rather than assuming the dose is fine.

### Does high conductivity by itself mean a Legionella problem?

Not directly, conductivity measures dissolved solids, not bacteria. But persistently high conductivity points to high cycles, and high cycles raise the scale, corrosion and fouling that create biofilm habitat and increase biocide demand. So treat a creeping conductivity trend as a risk indicator worth investigating, alongside your dip-slide and Legionella sampling results, rather than as proof of contamination on its own.

## Related reading

- [Cooling tower drift eliminators explained](https://legionella.io/articles/cooling-tower-drift-eliminators-explained/)
- [A worked cooling tower written scheme of control](https://legionella.io/articles/cooling-tower-control-scheme-worked-example/)
- [Cooling tower Legionella sampling and action levels](https://legionella.io/articles/cooling-tower-legionella-sampling-action-levels/)
- [Cooling tower biocide programme](https://legionella.io/articles/cooling-tower-biocide-programme/)

## Sources

[1] HSE, HSG274 Part 1 (2024), “Requirements of a cooling water treatment programme”, p.17. https://www.hse.gov.uk/pubns/books/hsg274.htm
[2] HSE, ACoP L8 (2013), “Carrying out a risk assessment”, p.12. https://www.hse.gov.uk/pubns/books/l8.htm