De-chlorination means removing the chlorine from the water. This is generally required when super-chlorination has been practiced. The de-chlorination process may either be carried out to such an extent that sufficient residual chlorine of 0.1 to 0.2 mg/L only remains in water after de-chlorination. The common de-chlorinating agents are sulphur dioxide gas, acivated carbon, sodium thiosulphiate, sodium metasulphate, and ammonia.