Forms of chlorination and affecting factors

Forms (aspects) of chlorination

1. Plain chlorination: chlorination is the only treatment given to the water. Dose about 0.5 to 1 mg/l, with at least 30 min contact. Used for fairly clear water of good quality, such as from protected springs, lakes or deep wells.
2. Pre-chlorination: chlorine is added before the filters (or before sedimentation) to reduce the load of organisms and algae, control taste and odour, oxidise iron, and help coagulation.
3. Post-chlorination: the normal disinfection step, applied after filtration, just before the water goes to the clear water tank. The dose is set to leave a residual of 0.2 to 0.5 mg/l.
4. Double chlorination: pre- and post-chlorination used together; also called two-stage chlorination.
5. Break-point chlorination: chlorine is added past the break point so that free residual is present (see the curve).
6. Super-chlorination: the dose is much higher than the demand (3 to 15 mg/l or more), followed by de-chlorination (using sulphur dioxide, sodium thiosulphate, sodium sulphite or activated carbon). Used during epidemics, after repair of mains, when the contact time is short, or for very poor quality water.
7. De-chlorination: removal of the excess chlorine after super-chlorination.
8. Chloramine process: ammonia is added with the chlorine to form chloramines, which give a more lasting, but weaker, residual, used in long distribution systems.

Applications: pre-chlorination in turbid or algae-rich water, post-chlorination in every plant, super-chlorination in emergencies and for new mains, and break-point chlorination for water that contains ammonia or has taste and odour problems.

Factors affecting chlorination

1. Chlorine dose and demand: dose must exceed the demand (organic matter, ammonia, iron) to leave a residual.
2. Contact time: the longer the contact, the better the kill ($C^n t$ = constant); usually at least 30 min.
3. pH: hypochlorous acid (HOCl) is a much stronger disinfectant than OCl$^-$; at low pH more HOCl is present, so chlorination is best at pH 6 to 7.
4. Temperature: higher temperature speeds up the reaction and increases the kill rate; cold water needs a longer contact time or a higher dose.
5. Turbidity and suspended matter: particles shield bacteria and consume chlorine; clear water is disinfected better.
6. Kind and number of organisms: viruses, cysts and spores are more resistant than bacteria.
7. Nature of chlorine compound: free chlorine is a faster disinfectant than combined (chloramines).
8. Mixing: proper, quick mixing ensures uniform contact.
9. Ammonia, organic nitrogen and reducing substances in the water.