Disinfection in the final effluent of wastewater, lead to high Ecoli

Good day,   Am having a problem with the final effluent of one the biggest water treatment works of 16,5ml/d. The WWTWs disinfect with chlorine gas. The free chlorine and Total chlorine also monitored on shift basis, and it shows that chlorine is present in the effluent.  And both free and total are above 2.20 mg/l, on shift basis. My questions was:  Why when we analysed E coli, we detect is with a value of >2419600 per 100ml? Does the chlorine gas effective or not? What is the best way we can used to reduce E coli?   I hope i will receive good responds from the forum.   Thank you,   Mr Mangena P