BOD reduction of secondary effluent by chlorine is due to oxidation of the organic material and to the formation of chlorinated organics resistant to bacterial action. One research paper showed no removal. Another researcher show up to 2 mg/l.
I was asked by a WWTP operator if adding chlorine to effluent could help remove another ppm or 2 to meet effluent limits. I had never heard of this being done but wanted to check with other treatment gurus to see if anyone had experience with or information in doing this. It sounds like it is not an advisable practice.