All of the below answers are ...

All of the below answers are useful.  Organic material prevents chlorine from killing bacteria.  It is unusual to find E coli in chlorinated clean water providing there is sufficient contact time.  There are certain gram positive sporing rods Bacillus species, which grow on the specialist media providing a false positive.  They are normally in the spore form and not actively growing in the presence of chlorine.  As soon as they come across artificial media they germinate showing up as viable growth.