Disinfection

Disinfection is the final barrier against waterborne disease: it kills or inactivates bacteria, viruses and protozoa that survive earlier treatment. Chlorine is the most common disinfectant because it is effective, cheap, easy to dose and leaves a residual that keeps protecting water in the distribution network. It is applied as chlorine gas, sodium hypochlorite solution or bleaching powder.

When chlorine is added, part of it is consumed by reactions with organic matter, iron, manganese, ammonia and other substances; this is chlorine demand. What remains is the residual. With ammonia present, chlorine first forms chloramines (combined residual); beyond the breakpoint, free chlorine residual appears, which is a stronger disinfectant. The breakpoint chlorination curve explains this. Effectiveness depends on concentration and contact time, captured in the CT concept, as well as pH (free chlorine works better at lower pH as hypochlorous acid), temperature and turbidity, which can shield organisms. A typical target is a residual of at least about 0.2 mg/L at the farthest tap.

Alternatives include ozone, a strong oxidant that also removes taste and colour but leaves no residual; ultraviolet light, effective against protozoa like Cryptosporidium and Giardia that resist chlorine; and chloramines for long networks. Chlorination can form disinfection by-products such as trihalomethanes when organic matter is high, controlled by removing organics before chlorination rather than by reducing disinfection.

Safety matters at the plant. Chlorine gas is toxic, so tonners and cylinders are kept in ventilated rooms with leak detectors, absorption systems and breathing apparatus, and staff are trained in emergency response. Small rural schemes often use hypochlorite solution or tablets for simpler handling.