Chlorine, chlorine dioxide and peracetic acid each have a specific niche in water and wastewater disinfection. This page covers the chemistry, CT value targets, disinfection byproduct (DBP) trade-offs, residual measurement and dosing-system design for each.
Disinfection is a product of concentration and time, not just dose.
All chemical disinfectants achieve pathogen inactivation as a function of disinfectant concentration C (mg/L) multiplied by contact time t (minutes) at a given temperature and pH. The product C·t (in mg·min/L) is called the CT value. Each disinfectant has published CT values for various pathogens at 99% (2-log) and 99.9% (3-log) inactivation; design engineers must size the contact tank, target residual concentration and operating pH to deliver the required CT in worst-case conditions (low temperature, high pH, low concentration). Drinking-water regulators specify minimum CT values for Giardia, Cryptosporidium and viruses; wastewater consent typically targets E. coli and faecal coliform counts. CT calculations are central to disinfection compliance.
The most widely-deployed disinfectant, supplied as 10–15% w/v solution. Hypochlorite is convenient and relatively safe vs chlorine gas, but degrades over weeks of storage — especially in heat and sunlight.
Best for: drinking-water primary disinfection, secondary disinfection (residual), wastewater effluent before discharge, water reuse.
NaOCl decomposes to chlorate (ClO₃⁻) and chloride (Cl⁻) over time. Chlorate is regulated (UK: 0.7 mg/L drinking-water limit). Hot, sunlit storage accelerates decomposition.
For sites with high consumption or remote location, electrolytic generation of 0.8% NaOCl from brine eliminates bulk delivery, chlorate accumulation and storage risks. Capital expenditure higher; Operating expenditure lower at large scale.
Cool, dark storage extends shelf life. Tank volume typically sized for 7–14 days’ consumption to ensure freshness.
Chlorine dioxide is a strong selective oxidant generated on site from sodium chlorite (NaClO₂) + acid, or NaClO₂ + sodium hypochlorite. It cannot be stored in bulk and must be produced as needed.
Best for: high-pH source water, biofilm control in distribution networks, hospital water systems, food processing, refinery cooling water.
| Method | Reagents | Yield | Use case |
|---|---|---|---|
| Chlorine-chlorite | NaClO₂ + Cl₂ gas | 90–95% | Large plants with Cl₂ gas |
| Acid-chlorite | NaClO₂ + HCl | 80–85% | Mid-size; safer than gas Cl₂ |
| Electrolytic | NaCl + electricity | 30–60% (mixed oxidant) | Small / remote |
| Two-precursor (NaClO₂ + NaOCl) | Both as liquids | 80–90% | Most common for industrial |
Peracetic acid (CH₃COOOH) is a strong oxidising disinfectant supplied as 5–15% w/w solution stabilised with acetic acid and hydrogen peroxide. It is widely used in food, dairy and pharmaceutical industries for surface and CIP disinfection and is gaining ground in wastewater effluent disinfection.
Best for: food/dairy/brewery CIP, wastewater effluent before discharge to sensitive waters, hospital effluent, low-DBP applications.
Wastewater treatment plants increasingly choose PAA over chlorine because: no THM/HAA formation, no need for downstream dechlorination, food-safe degradation products, ambient pH operation.
PAA is ~3–5x more expensive per kg active disinfectant vs hypochlorite. Storage stable for ~6–12 months refrigerated; less in warm warehouses. Vapours irritate eyes & lungs; ventilation required.
| Criterion | NaOCl | ClO₂ | PAA | UV | O₃ |
|---|---|---|---|---|---|
| Capital cost | Low | Med | Low | Med-High | High |
| Operating requirement | Low | Med | Med-High | Med | High |
| Crypto inactivation | Poor | Good | Moderate | Excellent | Excellent |
| Distribution residual | Yes | Yes | Short | No | No |
| DBP risk | THM, HAA | ClO₂⁻ ClO₃⁻ | Negligible | None | Bromate (if Br present) |
| Effective pH | 6.5–8.0 | 4–10 | 3–8 | n/a | 5–9 |
| Best for | Drinking water, general | Biofilm, high pH | Food, CIP, wastewater | Wastewater, reuse | Tertiary, ozone+BAC |
For information on UV and ozone systems, see the UV disinfection and ozonation pages.
HDPE or FRP for NaOCl/PAA; PE or PP for chlorite. Vented to scrubber; bunded to 110%; UV-shielded for hypochlorite.
Hypochlorite degradation doubles per 10°C rise. Insulated tanks & pipework essential in summer; consider chilled storage for high-consumption sites.
Solenoid or motorised diaphragm; PVDF or PTFE wetted parts. Pulsation dampener on discharge. Auto-prime feature for solenoid pumps.
Sized for required CT at minimum-temperature, maximum-flow scenario. Baffled to approach plug-flow (T₁₀/T > 0.5). Outlet sample tap for residual measurement.
Continuous online amperometric or DPD-colorimetric analyser. Trends back to SCADA. Loop-back to dosing pump speed control via PID.
Eyewash + emergency shower within 10 m. Gas detection for chlorine gas systems. PPE storage. Bund leak detection. Spill kit. Cl₂ gas: scrubber + bypass alarm.
Match dose to demand: too little fails disinfection; too much costs money and creates DBPs.
Disinfection demand varies with temperature, organics, ammonia and flow. Fixed-dose operation either over-doses on cold/clear days or under-doses during peak. Modern installations use closed-loop residual control:
Modern plants commonly save 15–30% chemical requirement vs fixed-dose, plus material DBP reductions.
Read more on dosing control strategy| Drinking-water (entry to network) | 0.5–1.0 mg/L FC |
| Drinking-water (network end) | 0.1–0.3 mg/L FC |
| Hospital water | 0.5–1.5 mg/L FC or ClO₂ |
| Wastewater effluent (consent) | typically <0.5 mg/L FC (or PAA-based) |
| Cooling tower (biofilm control) | 0.5–2.0 mg/L FC continuous, shock at 3–5 |
| Food & dairy CIP | 50–200 mg/L FC during cycle |
Every chemical disinfectant produces some by-products by reaction with natural organic matter (NOM). Some are regulated; all should be minimised.
Mitigation strategies: remove NOM upstream (coagulation + GAC), switch to lower-DBP disinfectant (PAA, UV, ClO₂), reduce contact time at high chlorine concentration, replace breakpoint chlorination with monochloramine.
Chemical-free disinfection — ideal companion or alternative to chlorination.
Read MoreStrong oxidant for taste/odour, DBP precursor and emerging contaminant removal.
Read MoreMaterial compatibility & pump type for each disinfectant chemistry.
Read MoreClosed-loop residual control for optimum dosing and minimum DBP.
Read MoreSend us your water flow, organic loading, pH range, target CT and regulatory consent. We will recommend the disinfectant, contact tank sizing, dosing infrastructure and residual control loop.
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