Cooling tower biocide — oxidising vs non-oxidising
Short answer: Oxidising biocides such as chlorine act fast and are cheap, but they are consumed quickly and their effectiveness falls as pH rises. Non-oxidising biocides act more slowly but persist longer and penetrate biofilm better. Good practice is to alternate the two, so organisms do not develop resistance.
Why cooling towers grow biology
A cooling tower provides everything microorganisms need:
- Warm water — often in the ideal growth range
- Oxygen — the tower deliberately aerates the water
- Sunlight in open sections — algae growth
- Nutrients — dust and organic matter scrubbed from the air
Left untreated, growth is rapid and continuous.
What biofilm actually costs
- Reduced heat transfer — biofilm insulates even more effectively than scale, so the chiller works harder and electricity consumption rises
- Under-deposit corrosion — metal corrodes beneath the biofilm where inhibitor cannot reach
- Blocked fill and pipework
- Health risk — cooling towers can harbour organisms of public health concern
Oxidising biocides
These work by chemical oxidation — attacking cell walls directly. Chlorine and bromine based products are the common examples.
| Advantage | Limitation |
|---|---|
| Fast acting | Consumed quickly by organic load |
| Economical | Effectiveness falls sharply as pH rises |
| Broad spectrum | Corrosive at higher concentrations |
| Easy to monitor via ORP | Poor penetration of established biofilm |
See our ZeroScale oxidising biocide.
Non-oxidising biocides
These interfere with cell function rather than oxidising the cell wall.
| Advantage | Limitation |
|---|---|
| Persists longer in the system | Slower acting |
| Penetrates biofilm better | More expensive |
| Less affected by pH | Cannot be monitored by ORP |
| Less corrosive | Organisms can develop resistance if used alone |
See our ZeroScale non-oxidising biocide.
Why alternate them
Using a single biocide continuously allows resistant organisms to survive and multiply. Alternating between an oxidising and a non-oxidising product means organisms surviving one are exposed to a different mechanism next time.
A common pattern is oxidising biocide as the routine treatment, with a non-oxidising slug dose periodically.
ORP based dosing
Oxidising biocide is best controlled on ORP rather than by fixed dosing, because ORP indicates whether the biocide is actually effective, not merely present.
- Target typically 650 to 750 mV
- Dosing only occurs when ORP falls below the setpoint
- Chemical consumption drops because nothing is dosed unnecessarily
Use an ORP controller (₹13,650) with a dosing pump and sensor. Full details in our ORP guide.
pH comes first
See our pH correction guide.
The complete cooling water programme
| Objective | Treatment |
|---|---|
| Microbiological control | Oxidising and non-oxidising biocide |
| Algae control | Algaecide |
| Scale and corrosion | Scale and corrosion inhibitor |
| Concentration control | Blowdown on conductivity |
| Make-up hardness | Softening, where required |
The full range is on our cooling tower chemicals page.
Monitoring
- ORP — continuous, for oxidising biocide control
- pH — daily, since it governs biocide effectiveness
- Conductivity — for blowdown and cycles of concentration
- Visual inspection — the basin and fill, for visible growth
- Microbiological testing — periodically, using dip slides
Cleaning an already fouled system
Where biofilm is already established, routine dosing will not remove it — biofilm protects the organisms inside it. A shock dose combined with mechanical cleaning is usually needed to establish a clean baseline, after which a routine programme can maintain it.
Send us your tower details and water analysis and we will recommend the programme.
Cooling water treatment programme
Send your tower capacity, make-up water analysis and current problems, and we will recommend the full programme.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
What is the difference between oxidising and non-oxidising biocides?
Oxidising biocides such as chlorine attack cell walls directly and act fast but are consumed quickly and lose effectiveness as pH rises. Non-oxidising biocides interfere with cell function, act more slowly but persist longer and penetrate biofilm better.
Why should biocides be alternated?
Using a single biocide continuously allows resistant organisms to survive and multiply. Alternating between two different mechanisms means organisms surviving one treatment are exposed to a different mode of action next time.
Why does biofilm matter so much in a cooling tower?
Biofilm is a better insulator than scale, so even a thin layer significantly reduces heat transfer and the chiller has to work harder. It also causes under-deposit corrosion where inhibitor cannot reach the metal.
Why is ORP used to control biocide dosing?
Because ORP indicates whether the biocide is actually effective rather than merely present. Dosing only occurs when ORP falls below the setpoint, so chemical consumption drops and control is more reliable.
Why does pH matter for biocide effectiveness?
Oxidising biocide effectiveness falls sharply above pH eight. At high pH you can dose a great deal of chlorine and achieve very little, so pH must be brought under control before the biocide programme will work.