Bree Ferguson presents Technical Report Final Oral Defense on 7/22

Many drinking water utilities that apply chloramines for secondary disinfection temporarily convert to free chlorine. The purpose of free chlorination is to inactivate bacteria attached to pipe walls within the distribution system (DS); in particular, nitrifying bacteria that are commonly found in chloraminating systems and responsible for nitrification. However, negative effects can occur. A transient and minimal total chlorine residual is predicted because the free chlorine must first oxidize the combined chlorine before forming free chlorine residual. The switch back to chloramine from free chlorine will produce the same transient in total chlorine residual. Free chlorination also will increase the chlorinated disinfection byproduct concentrations within the DS. Both of these effects were documented as important through measurements in the DS of Durham, North Carolina, Carthage, North Carolina and Greenville, South Carolina. In addition, ammonia oxidizing bacteria were shown to survive in biofilms exposed to chloramines (~3 mg/L-Cl2) and free chlorine (~1.5 mg/L-Cl2) for extended periods within the DS. Switching to free chlorine was effective at terminating nitrification episodes, as measured by chemical parameters, but not effective at long-term prevention or control. Disadvantages associated with disinfectant switching, such as increased DBPs and reduced chlorine residuals may not out-weigh the associated benefits.

Committee Members:
Dr. Francis A. DiGiano
Dr. Philip C. Singer
Dr. Frederic K. Pfaender

For further information please contact Rebecca Riggsbee Lloyd by email at Rebecca_Lloyd@unc.edu