Free Bromine in Cooling Towers: Why Total Bromine Isn't Enough
In a cooling tower, a total bromine reading tells you how much bromine-based oxidant is present, not how much of it is actively killing bacteria. Free bromine, and in particular the share present as hypobromous acid at the system's pH, is what does the work, so a "compliant" total bromine result can sit alongside poor microbial control.
This is an under-discussed problem. On sites with hard, alkaline make-up water, such as much of London and the South East, it is common to find weekly biocide readings comfortably inside the target range while dip slides creep upwards and, occasionally, Legionella is detected. The chemistry explains why, and the fix is consistent with what HSE's technical guidance already says.
What HSG274 Part 1 Says About Bromine Reserves
The Approved Code of Practice (ACOP) L8 requires a written control scheme for evaporative cooling systems, and the detail sits in HSE's technical guidance, HSG274 Part 1, which was revised as a second edition in 2024. For halogen biocides it states that a measurable reserve is typically maintained using DPD No 1 testing, in the range of:
Chlorine: 0.5–1.0 mg/l as Cl2
Bromine: 1.0–2.0 mg/l as Br2
The 2024 edition went further than many people realise. It explains that hypochlorous and hypobromous acids are far more biocidally active than the hypochlorite and hypobromite ions they turn into as pH rises, and that DPD No 1 results should be pH-adjusted to account for this. It also expects the target reserve to be adjusted where weekly dip slides and periodic Legionella analysis show that control is not effective. In other words, the published range is a starting point, not a guarantee.
Free, Combined and Total Bromine Explained
Three terms are often used loosely, and the difference matters:
Free bromine: hypobromous acid (HOBr) plus the hypobromite ion (OBr-). Of the two, HOBr is the effective biocide; OBr- is much weaker.
Combined bromine: bromamines, formed when bromine reacts with ammonia and organic nitrogen compounds from process leaks, airborne contamination or biological activity. Bromamines retain more biocidal activity than chloramines do, but they are generally less effective and less stable than hypobromous acid.
Total bromine: free plus combined.
The DPD method for bromine responds to hypobromous acid, hypobromite and bromamines alike, so a routine DPD No 1 bromine result is, in practice, close to a total bromine figure. It cannot tell you how much of that reading is active acid, how much is weak hypobromite and how much is combined bromine. A reading of 1.5 mg/l looks the same on the record sheet whether the water is at pH 7.8 or pH 9.3, but biologically those are very different systems.
How pH Changes Free Bromine in Cooling Towers
Hypobromous acid is a weak acid. As pH rises, more of it dissociates into hypobromite. The balance point (the pKa) is about 8.6–8.7 at 25°C, compared with about 7.5 for hypochlorous acid. That is why bromine is preferred for alkaline cooling water, and why HSG274 Part 1 notes that bromine compounds retain their activity at higher pH but are still affected.
Using a pKa of about 8.65, the approximate share of free bromine present as active hypobromous acid is:
pH 8.0: about 80%
pH 8.5: about 60%
pH 9.0: about 30%
pH 9.5: about 12%
For comparison, hypochlorous acid is only around 1% of free chlorine at pH 9.5. These figures are approximate and shift with temperature and dissolved solids, but the trend is clear. A system running at pH 9.3 with a DPD No 1 reading of 1.5 mg/l may have well under 0.5 mg/l of active hypobromous acid, before any allowance for bromamines.
Hard, alkaline make-up water makes this worse. As water evaporates and cycles of concentration rise, alkalinity concentrates and the system pH climbs, often towards 9 or above unless it is controlled. Some sites run very close to pH 9.5. Every reading on those sites can be "in range" while the biocide is doing a fraction of the job the range assumes.
Warning Signs That Total Bromine Is Masking Poor Control
In practice, the pattern tends to look like this:
Bromine readings consistently within 1.0–2.0 mg/l, yet dip slide or TVC counts drifting upward week on week.
Legionella detected in quarterly samples with no obvious failure in the biocide log.
System pH routinely above about 8.8, or rising as cycles of concentration are increased to save water.
Ammonia, process leaks or high organic loading, which HSG274 Part 1 recognises can affect oxidising biocide efficacy.
Biocide dosing increased in response to poor counts, with little change in microbial results.
None of these proves a free bromine problem on its own. Together, they are a strong signal that the monitoring regime is measuring the wrong thing.
What To Do: Setting Control Parameters for Your System
The solution is not to abandon HSG274 Part 1's range, but to apply it the way the guidance intends.
Do not rely on total bromine alone. Ask your water treatment provider how free bromine can be measured separately from combined bromine on your system, and include it in routine monitoring where combined bromine is likely.
pH-adjust every result. Record pH at the same time and point as the biocide test, and convert the reading to an estimate of active hypobromous acid. A reading means little without the pH alongside it.
Trend the data together. Plot bromine, pH, dip slide or TVC counts and Legionella results on the same timeline. A biocide reserve that is "in range" but not delivering microbial control is not an effective reserve.
Set site-specific targets. Operational control parameters in the written control scheme should reflect the chemistry of the actual system, including its make-up water, pH and contaminants, rather than being copied from the generic range. That may mean a higher bromine target, tighter pH control, or both.
Review after any change. Revisit targets after changes to make-up water, cycles of concentration, treatment chemicals or process conditions, and after any positive Legionella result.
These decisions belong in the written control scheme, with the reasoning recorded. If you are reviewing yours, our article on keeping an accurate and accessible written control scheme covers what it should contain. The same principle of trending readings rather than simply ticking them off applies to temperature monitoring in hot and cold water systems.
Key Points
HSG274 Part 1 (2024) gives a typical reserve of 1.0–2.0 mg/l as Br2 by DPD No 1, but expects results to be pH-adjusted and targets adjusted if control is not effective.
DPD No 1 bromine results are effectively total bromine and include weak hypobromite and bromamines.
Hypobromous acid is the active biocide; at pH 9.0 only about 30% of free bromine is in this form, and at pH 9.5 about 12%.
Hard, alkaline make-up water and high cycles of concentration push pH up and erode active biocide.
Rising dip slide counts or Legionella detections despite "compliant" readings point to a free bromine problem.
Set control parameters for the real chemistry of the system and record them in the written control scheme.
Frequently Asked Questions
What is the difference between free and total bromine in a cooling tower?
Free bromine is hypobromous acid plus hypobromite ion. Total bromine also includes combined bromine (bromamines) formed with ammonia and organic nitrogen. Hypobromous acid is the most effective biocide, so total bromine can overstate how much active biocide is present.
What bromine level should a cooling tower have?
HSG274 Part 1 states that halogen biocides are typically applied to give a DPD No 1 reserve of 1.0–2.0 mg/l as Br2. It also says results should be pH-adjusted and targets changed where dip slides and Legionella results show control is not effective, so the right level depends on the system.
Does bromine work at high pH?
Bromine performs better than chlorine at alkaline pH, but it is still affected. As pH rises above about 8.5, a growing share of free bromine becomes the much weaker hypobromite ion, so the same reading delivers less active biocide.
Why do I get high dip slide counts when bromine readings are in range?
Common causes are high system pH, combined bromine from ammonia or organic contamination, and biofilm or deposits shielding bacteria. Check pH alongside each bromine result and ask your water treatment provider to assess free versus combined bromine.
References and Further Reading
World Health Organization, Bromine as a drinking-water disinfectant
A bromine reading is only as meaningful as the pH and microbial results recorded beside it. Treating the published range as a starting point, and setting targets for the water you actually have, is what turns a compliant-looking log into effective Legionella control.
Andrew Arnold
Co‑founder & Director
Andrew has over 20 years' experience in Legionella management and control. As a senior member of the Water Management Society and educated to masters level Andrew has extensive experience in legionella risk assessment of numerous systems and sectors.