Explain Chlorine Residuals

Free Chlorine Residuals: Cl₂, HOCl, OCl-  

Combined Chlorine Residuals: Chloramines (mono, di and tri chloramines)

The disinfecting ability of chloramines is much lower than that of free chlorine.

Satisfactory results can be obtained if a free chlorine residual of 0.2 to 0.5 mg/L can be maintained in the distribution system. Also if approved by health authorities  combined chlorine residuals in the range of 1 to 2 mg/L should maintained at distant points in the distribution system.    

 Trihalomethanes (the following) which are formed as a result of chlorination are harmful and this must be avoided.

CHCl₃ trichloromethane (chloroform)

CHBrCl₂  bromodichloromethane

CHBr₂Cl dibromochloromethane

CHBr₃   tribromomethane (bromoform)