A cooling tower biocide programme is rarely one chemical. It is an oxidising biocide held at a continuous low residual under halogen or ORP control to keep the bulk water knocked down, backed by non-oxidising biocides slug-dosed on rotation to reach what the oxidiser cannot, usually with a biodispersant to lift the biofilm so the dose gets to it. Everything in it is set, and proven, by dip-slide and Legionella results.
That layered design is the part the operational overviews give a single line and move past. If you are a responsible person reading a water-treatment contractor’s regime, the question is not whether they dose a biocide — almost everyone does — but whether the programme is doing the two different jobs a recirculating tower needs, and whether anyone can prove it from the records.
Two jobs, two families of biocide
A cooling tower water treatment programme splits its microbial control across two chemistries that fail in opposite ways. Oxidising biocides — chlorine, bromine (often from BCDMH), chlorine dioxide, ozone — work by oxidising cell material indiscriminately. They are fast, broad-spectrum and cheap. They are also consumed by organic load, lose strength as pH and temperature rise, corrode metal, and barely penetrate an established biofilm. So they are the continuous base of the programme: a low free-halogen residual held all the time, with periodic higher shock doses.
Non-oxidising biocides — DBNPA, isothiazolinone, glutaraldehyde, THPS, quaternary ammonium compounds — poison specific cell functions rather than burning everything in reach. They act more slowly, more selectively, and several of them persist in the water and penetrate biofilm far better than a halogen does. They are slug-dosed on a schedule rather than held continuously, and they are the part of the regime that does the deeper kill the oxidiser misses.
That division, and the rest of the evaporative-cooling control scheme it sits inside, is set out in HSG274 Part 1 explained: controlling Legionella in cooling towers and evaporative condensers [1].
Oxidising vs non-oxidising biocide, side by side
| Decision axis | Oxidising biocides | Non-oxidising biocides |
|---|---|---|
| Typical examples | Chlorine, bromine (BCDMH), chlorine dioxide, ozone | DBNPA, isothiazolinone, glutaraldehyde, THPS, quats |
| Role in the programme | Continuous “base” control of the bulk water | Periodic deeper kill, rotated between products |
| How it’s dosed | Low residual held continuously plus shock doses, controlled by ORP / residual | Slug-dosed on a schedule, held for a contact period |
| Speed and persistence | Fast-acting, short-lived, quickly consumed | Slower, more persistent in the water |
| Biofilm penetration | Poor on its own | Better; some target biofilm organisms directly |
| Main weaknesses | Corrosive, pH/temperature sensitive, used up by organics | Cost, COSHH handling, resistance if not rotated |
| What proves it’s working | Oxidant residual / ORP trend | Dip-slide and Legionella results after dosing |
Why you rotate the non-oxidising biocides
Microbial populations adapt. Dose the same non-oxidiser indefinitely and you select for the organisms that tolerate it. Alternating two chemistries with different modes of action keeps that from happening, which is why a credible programme names at least two non-oxidisers and rotates them rather than leaning on a single product [1][2].
Rotation only helps if the dose actually stays in the system long enough to act. A cooling tower continuously bleeds water to control dissolved solids, so a slug of non-oxidising biocide dosed into a freely bleeding system drains away before it has done its work. The bleed — the blowdown — should be suspended or cut back for a defined holding period after dosing, then resumed. If the records show biocide additions but no holding-time control, you are paying for chemistry that is going down the drain rather than killing anything.
Biodispersants: getting the biocide to the biofilm
Biofilm is where the real problem lives. Legionella shelters inside the slime layer on the pack, pond and pipework, where a bulk-water residual barely reaches it. A cooling tower biodispersant is a surfactant dosed to penetrate and lift that biofilm, breaking it up so the biocide gets to the bacteria underneath and the dead material flushes out on the next bleed.
The dispersant is usually dosed ahead of or alongside a non-oxidising biocide, because the pair does what neither manages alone: the dispersant opens the biofilm, the biocide kills what is now exposed. A regime that doses biocide but never a dispersant is treating the water and ignoring the reservoir that keeps reseeding it.
ORP and halogen control: how the oxidiser is actually held
The oxidising side is usually run automatically. ORP — oxidation-reduction potential, read in millivolts — is a proxy for the oxidising power of the water, and a controller doses halogen to keep ORP inside a target band, topping up as organic load consumes the residual. It is the same closed-loop principle covered in Automated dosing systems for Legionella control: measure, compare to setpoint, dose.
Two things experienced operators watch. ORP halogen control is not a direct ppm reading — it shifts with pH, temperature and interfering species, so a falling ORP can mean the halogen is genuinely low or simply that pH has drifted and changed how the halogen behaves. And a controller is only as honest as its probe: a fouled or uncalibrated ORP electrode will hold a number that means very little. The setpoints themselves are not figures to copy from another site. They belong in your written scheme of control, justified by the risk assessment for that specific tower [1][2].
Proving the programme works: dip slides and Legionella
A biocide programme is a hypothesis until the microbiology confirms it. Dip slides — a field aerobic colony count — are typically read weekly as the early-warning gauge, with Legionella sampling at the less frequent interval the risk assessment sets [1][3]. The dosing is meant to close the loop with those results. A rising dip-slide trend, or a count above the action level, is the trigger to look hard at the oxidant residual, the rotation, the dispersant and the bleed — not a figure to file and forget.
This is the join the overviews skip. Dose rates and rotation are not fixed once and left alone; they are adjusted against what the dip slides and Legionella samples show, within the action levels your scheme defines. For how the biocide regime sits inside the wider treatment picture — inhibitors, conductivity control and physical cleaning as well — see Best practices in water treatment for Legionella control, and for why these systems earn this level of scrutiny in the first place, Cooling towers and evaporative condensers: high-risk systems.
What to check when you review the regime
You do not need to second-guess the chemistry to tell whether the programme is controlled. Walk the contractor’s regime against these questions:
- Is there an oxidiser under continuous residual or ORP control and at least two non-oxidisers on rotation? A tower running on a single biocide is the first red flag.
- Is a biodispersant in the programme, or only biocide?
- Do the records show holding-time or bleed control around the non-oxidising doses?
- Are action levels defined, and does an out-of-range dip slide or Legionella result actually trigger a recorded change to the dosing?
- Is the provider competent to make those calls — for instance a Legionella Control Association registered service provider working to your written scheme [4]?
This is general guidance on how cooling-tower biocide programmes are built, not a treatment specification for your tower. The chemistries, residual bands, rotation and holding times that suit a system depend on its water, load and duty, and are decisions for a competent water-treatment provider working through a site-specific risk assessment and written scheme of control. Nothing here is chemical-handling, legal or design advice — the COSHH assessment for every product is the provider’s to make.
Common questions
Is an oxidising biocide on its own enough for a cooling tower?
Rarely. A continuous halogen residual controls the bulk water but penetrates established biofilm poorly and is consumed by organic load. Most schemes pair it with rotated non-oxidising biocides and a biodispersant for exactly that reason. Whether yours needs all three is a risk-assessment call, but a tower running on one biocide alone is worth questioning.
How often should non-oxidising biocides be rotated?
There is no single national figure — it is set by your programme and risk assessment. The principle is to alternate two chemistries with different modes of action often enough to stop a tolerant population establishing. Ask your provider to show the rotation schedule and the reasoning behind it, not just a product name on an invoice.
What does ORP actually control?
ORP control adjusts the halogen dosing to hold the water’s oxidising power within a band, as a proxy for free-halogen residual. It does not read ppm directly and is affected by pH and probe condition, so it should sit alongside an actual residual test and regular probe calibration rather than being trusted on its own.
Does a low dip-slide count prove the biocide programme is working?
It is reassuring, not conclusive. A dip slide is a general aerobic count, not a Legionella result, and biofilm organisms can sit below what a bulk-water slide picks up. Treat a low, stable trend as a good sign and a rising one as a prompt to act, then confirm with the scheduled Legionella sampling.
Your next step
Take the contractor’s regime and your last quarter of tower records and line up three things: the oxidant residual or ORP trend, the dates and types of non-oxidising biocide dosed (are there at least two, alternating?), and the dip-slide results over the same period. If the dip slides drifted upward and nothing changed in the dosing, or the same single biocide appears every visit with no dispersant, raise it with your provider this week and ask them to map the programme back to the action levels in your written scheme of control.
Sources
[1] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm [4] Legionella Control Association, “Code of Conduct for Service Providers”. https://www.legionellacontrol.org.uk/