Effluent Total Residual Chlorine Decay in Outfall Pipe

Field and bench tests were conducted to determine the decay property of effluent total residual chlorine (TRC) in an outfall pipe. The pipe conveys secondary treated effluent from the Broward County North Regional Wastewater Treatment Plant to the Atlantic Ocean off the east coast of South Florida. During a field test effluent samples were taken and TRC concentrations were measured every 15 min for 6 h at three stations along the inland portion of the outfall pipe. Results indicate that the effluent TRC decay can be described by first-order kinetics. The TRC overall decay constant is assumed to be the sum of static and dynamic decay constants. The overall decay constant, determined from the field tests, is one order of magnitude greater than the static decay constant determined from the bench tests. This suggests that TRC decay in the outfall pipe is dominated by chlorine consumption by the biofilm at the pipe wall. The removal of TRC during the effluent transport in the outfall pipe is estimated as 68% at the permitted flow rate of 2.9 m³/s.