Why do city water departments first remove dissolved organic substances before disinfecting with chlorine?

Dissolved organic substance present in natural water is a complex mixture compounds and contains most of the reduced carbon present in aquatic ecosytem . this can not be easily treated or disinfected by the conventional treatment . the experiments show that water sample with higher dissolved substance need higher cholrine demand  . this is the reason that before disinfecting with chlorine need to remove organic substance {{{    because it increase chlorine demand }}}

1- Because these organic substances increase the demand for chlorine
2- can cause cancer when combined with chlorine

Explanation:
This is mainly due to two important reasons:
1- The organic substances dissolved in water have very complex structure as they contain contain most of the reduced carbon found in the aquatic ecosystem and cannot be easily disinfected using the traditional methods.
These substances require very large quantities of chlorine in order to be disinfected and produce the desired results (this is proved by practical experiments).

2- When combined with large quantities of chlorine, trihalomethanes can be produced. This is a very dangerous product that causes cancer

Hope this helps :)