Factors affecting chlorination

The efficiency of chlorination (kill of micro-organisms) depends on the following factors.

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.

The overall relationship used in design is $C^n\,t = \text{constant}$ for a given percentage kill, where $C$ is the concentration of the disinfectant, $t$ the contact time and $n$ a constant (about 1 for chlorine).