Glossary
Residual chlorine
The chlorine left in water after disinfection, once the chlorine demand of the water has been used up; IS 10500 specifies it as free residual chlorine. Its presence at the tap shows that disinfection is still active along the pipes.
Why it matters
A residual that drops to zero between the treatment plant and the consumer can mean contamination has entered the network. Checks at taps, or chlorine analysers running continuously, help find those points.
Where it is used
Source
IS 10500:2012 (parameter list)