Water Quality effluent

I have question here regarding the effluent standard at one of the Wastewater Treatment Works, the chlorine levels are as follows: Free chlorine = 5mg/l and Total chlorine = 8mg/l, although E coli = >2044 per 100ml. The suspended solids = 20mg/l.  We use chlorine gas as disinfection, the capacity of Treatment works is 12.5 md and it treat domestic influent only. And is Biological process.  My question is: 1. Why E coli is so high although chlorine is too high? 2. Can anyone assist proper treatment of E coli. NB: I will appreciated any responds.   Thanks