Oxidising vs Non-Oxidising Biocides — When Each One Wins
Direct answer
Oxidising biocides kill fast, cheaply and indiscriminately; non-oxidising biocides penetrate biofilm and survive conditions that destroy an oxidant. A working cooling programme uses both: a continuous or shock oxidant to hold the bulk water, plus a periodic non-oxidiser to strip established biofilm and break resistance. Chlorine and bromine are consumed by every organic in the system, so in high-COD, high-ammonia or high-pH water the oxidant demand becomes uneconomic — that is where bromine beats chlorine (it stays active as hypobromous acid up to about pH 8.7, where chlorine has largely dissociated) and where chlorine dioxide beats both, because it is unaffected by pH and by ammonia. Non-oxidisers are the answer when the system holds process leakage, when the oxidant residual cannot be maintained, or when biofilm has already formed.
Comparison
| Oxidising | Non-oxidising | |
|---|---|---|
| Examples | Hypochlorite, bromine (BCDMH), chlorine dioxide, peracetic acid | DBNPA, isothiazolinone, glutaraldehyde, quat |
| Speed of kill | Minutes | Hours (DBNPA fast, isothiazolinone slow) |
| Biofilm penetration | Poor once biofilm is established | Good |
| Consumed by organics | Yes, heavily | Largely no |
| pH sensitivity | High (chlorine), low (bromine, ClO₂) | Low |
| Typical use | Continuous / daily shock | Weekly or fortnightly slug dose, alternated |
| Cost per treatment | Low | Higher |
Building a real programme
Hold a continuous free residual in the bulk water (typically 0.2–0.5 mg/L free chlorine or 0.5–1.0 mg/L free bromine in cooling towers), then alternate two non-oxidisers on a rotation so the population cannot adapt to one chemistry. Dose non-oxidisers as a slug to the tower basin with the blowdown closed for the holding time of the product — a non-oxidiser diluted straight out of the system does nothing. Add a biodispersant before the biocide when biofilm is visible; it lifts the matrix so the biocide reaches the cells underneath. Judge the programme by dip-slide or ATP trend and by heat exchanger approach temperature, not by water clarity.
Compatibility and safety
DBNPA hydrolyses rapidly above pH 8 and is destroyed by bisulphite — never dose it into a dechlorinated stream. Isothiazolinone is deactivated by reducing agents and by high sulphide. Glutaraldehyde is neutralised by ammonia and amines, which rules it out where ammonia-based corrosion inhibitors are in use. Quaternary ammonium compounds foam badly in towers and are inactivated by anionic dispersants and by high hardness. Never mix an oxidiser and a non-oxidiser in the same dosing line or day tank; keep peracetic acid away from hypochlorite entirely.
Legionella duty
Legionella control is a system design problem before it is a chemical problem: dead legs, stagnation, drift eliminator condition and water temperature dominate. Chemically, a maintained oxidant residual plus a documented rotation of non-oxidisers, verified by culture or qPCR sampling on a fixed schedule, is the defensible position. Record every dose, residual and result — the log is the evidence, not the invoice.
What to send with your enquiry
System volume and holding time, water analysis (pH, ammonia, COD, hardness, chloride), current oxidant and residual achieved, process leakage risk, existing biocide and rotation, dip-slide or ATP history, and any discharge consent limits that constrain what you may release to sewer.