Are you asking what concentration of chlorine to dose your treated effluent for disinfection? It may depend on your permit requirements and will be affected by other factors, such as the presence of nitrites in the effluent if you do not have full nitrification/denitrification.
Generally you want a Cl2 dose between 1 and 2 mg/L of chlorine in your WWTP effluent. If you are trying to oxidize compounds in the WWTP process that is another matter and more information is required. Be specific.