Disinfection and industrial microbial control use two quite different families of chemistry, and choosing between them is decided by the water or surface being treated rather than by potency tables. Oxidising biocides – the chlorine donors, peracetic acid, ozone, chlorine dioxide – kill fast, leave no residue that resists, and are consumed by organic load. Non-oxidising biocides – isothiazolinones, DBNPA, glutaraldehyde, quaternary ammonium compounds, THPS – persist, can be dosed continuously, and are the basis of most cooling-water and closed-loop programmes. This article is a selection guide across both families for water, surface and process disinfection. It deliberately excludes preservation of a product inside its container, which is a different problem solved by different actives and is covered in our industrial preservative selection guide.
1. Two families, two jobs
| Oxidising biocides | Non-oxidising biocides | |
|---|---|---|
| Examples | NaDCC, TCCA, calcium hypochlorite, sodium hypochlorite, chlorine dioxide, peracetic acid, ozone | CMIT/MIT, OIT, DBNPA, glutaraldehyde, DDAC, THPS, isothiazolinone blends |
| Kill speed | very fast | slower, dose- and contact-time dependent |
| Residual | short-lived; consumed by organic matter | persistent; measurable over days |
| Interference | strongly reduced by organic load, ammonia, reducing agents | less affected by organic load, but consumed by nucleophiles (thiols, sulfites, amines) |
| Biofilm | poor penetration unless dosed to high residual | some actives penetrate; still limited by diffusion |
| Typical use | drinking water, pools, shock dosing, surface and spill disinfection, final effluent | cooling towers, closed loops, process water, metalworking fluids, oilfield |
| Main risk | by-products, corrosion, gas release on misuse | resistance development, environmental persistence, sensitisation |
The practical distinction is that oxidising biocides are consumed by the demand of the water, while non-oxidising biocides are consumed by the demand of the system’s chemistry. Both need a measurement to confirm that the intended concentration is actually present – which is why a dosing programme without residual or assay monitoring is not a programme.
2. The chlorine donors, compared on the numbers that matter
Every chlorine donor specification is expressed as available chlorine, which is an oxidising equivalent, not a purity. The conversion that makes the numbers comparable: one N–Cl bond is equivalent to one mole of Cl2, or 70.906 g of available chlorine.
| Product | CAS | Available chlorine | Solution pH | Physical form | Cyanuric acid per kg of chlorine | Best fit |
|---|---|---|---|---|---|---|
| NaDCC | 2893-78-9 | 56% (60% grade available); ceiling 64.48% | near neutral, 5.5 – 7.0 | granule, powder, tablet | about 1.05 kg | general disinfection, tablet dosing, safer handling |
| TCCA | 87-90-1 | up to 91.5% | acidic | granule, tablet | about 0.61 kg | maximum chlorine per tonne; continuous pool feeders |
| Calcium hypochlorite | 7778-54-3 | 65 – 70% | alkaline | granule, briquette | none | non-stabilised chlorination where hardness is acceptable |
| Sodium hypochlorite | 7681-52-9 | 10 – 15% | alkaline | solution | none | continuous dosing where on-site generation or bulk delivery exists |
| Chloramine T | 127-65-1 | about 25% | near neutral | solid | none | milder, slower release, less chlorinous odour |
| Bleaching powder | mixtures | 20 – 35% | alkaline | powder | none | only where nothing else is available |
Three comparisons are commonly misunderstood. First, higher percentage does not mean cheaper: compare delivered cost per kilogram of available chlorine, and then account for the fact that 56% and 91.5% chlorine products have different hazard classifications and different handling requirements. Second, the cyanuric acid column matters in any system that is not fully drained – a stabilised donor accumulates cyanuric acid, and per unit of chlorine TCCA adds about 40% less than NaDCC. Third, solution pH decides how much of the chlorine is present as the effective species; above pH 7.5 an increasing share is the much weaker hypochlorite ion.
3. The non-oxidising biocides, compared on the same basis
| Active | CAS | Spectrum | pH window | Persistence | Typical role in water systems |
|---|---|---|---|---|---|
| CMIT/MIT | 26172-55-4 / 2682-20-4 | broad, fast | acidic to neutral | moderate | in-can and process preservation, not open cooling water |
| DBNPA | 10222-01-2 | very fast, broad | acidic to neutral | very short – hydrolyses | shock dosing, biofilm knockdown, final effluent |
| Glutaraldehyde | 111-30-8 | broad, active against sulphate-reducing bacteria | broad | moderate | cooling water, oilfield, metalworking |
| DDAC | 7173-51-5 | bacteria, algae, fungi | broad | high, surface-active | algae and biofilm control, pool algicide, odour control |
| THPS | 55566-30-8 | broad, especially sulphide systems | broad | moderate | oilfield water, cooling water, sulphide scavenging |
| OIT | 26530-20-1 | fungi, yeasts | acidic to neutral | high in films | not a water treatment active; film protection |
The important structural point: most non-oxidising biocides are dosed continuously at low concentration with periodic shock doses, and the programme usually alternates actives to limit resistance. A single non-oxidising active dosed for a year without rotation is a predictable route to reduced effectiveness, because biofilm populations adapt.
4. Selecting by application
| Application | First choice | Alternative | Key constraint |
|---|---|---|---|
| Drinking water, municipal or small supply | chlorine donor approved for drinking water (NaDCC, hypochlorite, chlorine gas) | chlorine dioxide, ozone where approved | regulatory approval is product- and market-specific; by-product limits apply |
| Emergency and point-of-use water | NaDCC tablets | chlorine solution with test kit | turbidity must be low; contact time must be respected |
| Outdoor swimming pool | TCCA tablets for maintenance | NaDCC for shock | cyanuric acid accumulation; pH control |
| Indoor pool, spa | non-stabilised chlorine or chlorine dioxide | NaDCC with dilution discipline | chloramine and by-product control, ventilation |
| Open recirculating cooling tower | oxidising shock plus a non-oxidising programme (glutaraldehyde, THPS, DDAC, DBNPA) | chlorine dioxide | biofilm penetration, corrosion control, discharge consent |
| Closed loop and chilled water | non-oxidising (isothiazolinone or THPS) | glutaraldehyde | persistence matters; oxidisers can attack inhibitors |
| Municipal wastewater disinfection | hypochlorite or NaDCC | peracetic acid, UV | chlorine demand and by-product formation; dechlorination before discharge |
| Food and beverage surface sanitation | chlorine donor approved for food contact | peracetic acid, quaternary ammonium | organic soil consumes chlorine; rinse requirements |
| Livestock drinking lines | NaDCC or hypochlorite | peroxide products | dosage must be safe for stock; organic load in the line |
| Membrane systems (RO, UF) | non-oxidising where membranes are polyamide | chlorine with dechlorination upstream | free chlorine degrades polyamide membranes |
| Oilfield produced and injection water | THPS, glutaraldehyde, DBNPA | chlorine dioxide | sulphide load, corrosion, SRB control |
5. The five-step selection sequence
- Fix the objective. Drinking water, surface disinfection, biofilm removal, algae control and preservation are five different problems with different actives. Write down what must be achieved and how it will be measured.
- Characterise the water. pH, alkalinity, ammonia nitrogen, organic load (COD/TOC), sulphide, iron and manganese, temperature. Ammonia and organic load consume oxidisers before any residual appears, and sulphide consumes both oxidising and some non-oxidising actives.
- Filter by regulation. Drinking water, food contact, pool and biocidal uses each have their own permitted lists and dose limits, and they differ by market. This step usually removes most of the technically suitable options.
- Choose the dosing mode. Continuous low dose, intermittent shock, tablet release or batch treatment. The mode determines whether a short-lived active such as DBNPA or a persistent one such as DDAC is appropriate.
- Prove it with measurement. Residual chlorine by DPD, active concentration by assay or test kit, and microbial counts (heterotrophic plate count, ATP, or specific organisms). A programme without measurement cannot be optimised and cannot be defended in an audit.
6. Dosing arithmetic, without invented numbers
Two quantities are constantly confused. Chlorine demand is the amount consumed by the water itself – organics, ammonia, reduced iron, sulphide. Free residual is what remains after that demand has been met. Dosing to a free residual without knowing the demand is guesswork, which is why a test kit is part of the dosing system rather than an accessory.
| Calculation | Formula | Worked example |
|---|---|---|
| Chlorine mass required | target concentration (mg/L) x volume (m3) = grams of available chlorine | 4 mg/L x 25 m3 = 100 g of available chlorine |
| Product mass | chlorine mass / available chlorine fraction | 100 g / 0.56 = 179 g of a 56% NaDCC |
| Actual requirement with demand | (free residual target + demand) x volume | if demand is 3 mg/L and target is 1 mg/L free, dose for 4 mg/L |
| Cost per unit of chlorine | price per tonne / (available chlorine x 1000) | lets you compare 56% NaDCC with 90% TCCA fairly |
For non-oxidising programmes the equivalent calculation uses the active concentration and the system volume, with blowdown and losses added: the steady-state concentration in a cooling tower depends on dose rate, system volume and blowdown rate, and a tower with high blowdown needs a higher dose to hold the same concentration.
7. Five failure patterns seen in the field
| Symptom | Usual cause | Diagnostic | Fix |
|---|---|---|---|
| Chlorine residual disappears immediately | high demand: ammonia, sulphide, organics, reduced metals | measure demand by dosing to residual; check ammonia nitrogen | pre-treat or dose to demand; consider breakpoint chlorination |
| Residual is present but microbes persist | biofilm: the residual cannot penetrate the deposit | surface sampling, ATP, visual inspection of heat exchangers | shock dose plus a penetrating non-oxidising active; mechanical cleaning |
| Dose creeps up over a season | cyanuric acid accumulation, or resistance in a continuous non-oxidising programme | cyanuric acid test; rotation history | dilution or switch donor; rotate actives |
| Corrosion complaints after switching to chlorine | low pH from an acidic donor, or chloride levels rising | pH and chloride monitoring | correct pH; use a near-neutral donor; review metallurgy |
| Objectionable odour after dosing | chloramines from ammonia or amines | free versus total chlorine measurement | remove ammonia source; breakpoint chlorination; ventilation |
| Discharge consent breach | chlorinated water discharged without dechlorination | residual measurement at outfall | dechlorinate with sulphite; see our dechlorination notes in the water treatment hub |
8. FAQ
Which is better, an oxidising or a non-oxidising biocide?
Neither in general. Oxidisers are better where a fast kill and no persistent residue are wanted, and where the water is clean enough that the demand can be met. Non-oxidisers are better where persistence, biofilm penetration or compatibility with corrosion inhibitors matters. Most industrial cooling programmes use both.
Can I use NaDCC in a cooling tower?
Yes as a shock or intermittent treatment, subject to the discharge consent for chlorinated blowdown and to corrosion control. Continuous chlorination of a tower is usually a worse choice than an alternating programme with a non-oxidising active.
Why is TCCA used in pools and NaDCC in water treatment?
Because of the delivery mode and the solution pH. TCCA tablets release slowly and deliver the most chlorine per unit of freight, which suits continuous pool feeders; NaDCC gives a near-neutral solution and is easier to dose and handle for batch and general disinfection.
How do I compare prices across products?
Convert to cost per kilogram of available chlorine or per kilogram of active, then add the handling and hazard costs. A cheaper per-tonne product with a lower active content or a more demanding classification is often the more expensive option in practice.
Do I have to rotate biocides?
For continuous non-oxidising programmes, yes – rotation limits the development of reduced susceptibility in biofilm populations. Oxidising programmes do not develop resistance in the same way, but they do need demand management.
What documentation should come with a disinfectant shipment?
Batch COA with the active or available chlorine figure, SDS in the destination language, and a statement of registration or approval for the intended use category and market.
Conclusion
Start from the water, not the product. Characterise demand, pH and ammonia; filter by the regulations of the destination market; choose the dosing mode; then select the chemistry and prove it with measurements. For industrial water systems this almost always means an oxidising shock programme plus a rotating non-oxidising active, and for drinking water it means a specifically approved product. For the two chlorine donors covered on this site see the NaDCC technical article and the NaDCC product page; for dosing practice see NaDCC dosing and applications. For non-oxidising actives used in water systems see glutaraldehyde in water treatment, DDAC and chloramine T. For the overall treatment sequence see the water treatment chemicals hub.