Chlorine dioxide and free chlorine differ in pH dependence, demand, biofilm performance and by-product profile. Chlorine dioxide forms no chloramines and generally very little THM or HAA, while chlorite and chlorate require control. Selection depends on the product label, system conditions and target organisms.
Despite similar names, chlorine dioxide (ClO₂) and chlorine (Cl₂) are fundamentally different chemicals with different disinfection mechanisms:
| Property | Chlorine Dioxide (ClO₂) | Chlorine (Cl₂) |
|---|---|---|
| Chemical Formula | ClO₂ Chlorine dioxide - a dissolved gas | Cl₂ / HOCl Chlorine gas or hypochlorous acid |
| Biofilm Penetration | Product-specific Can act on biofilm under validated label conditions | Condition-dependent Performance depends on demand, dose and contact time |
| Legionella Efficacy | Label and test-specific Check organism, matrix, concentration and contact time | Label and test-specific Check organism, matrix, concentration and contact time |
| Chloramine Formation | None Does not react with ammonia compounds | Possible Can form chloramines that require exposure control |
| THM Formation | Generally very little Control chlorite and chlorate | Yes Forms potentially harmful THMs |
| pH Sensitivity | Lower Generally less pH-dependent than free chlorine | High Free-chlorine speciation changes as pH rises |
| Taste & Odour | Condition-dependent Depends on dose and water conditions | Condition-dependent Depends on dose, demand and water conditions |
| Residual Persistence | System-dependent Verify at representative sample points | System-dependent Verify at representative sample points |
Our team can help you evaluate chlorine dioxide for your water treatment application and guide the transition from traditional chlorine.