AWWA WQTC62501

The objective of this study was to gain an understanding of the factors affecting the stability of monochloramine in waters treated by ozone-biologically activated carbon (BAC) processes. The kinetics of monochloramine decay in both natural and synthetic waters were determined by bench-scale batch experiments. Natural waters were sampled at various treatment stages in David L. Tippin Water Treatment Facility (DLTWTF) located at Tampa, Florida. The laboratory synthetic waters were prepared with standard Suwannee River natural organic matter (NOM). Treating ozonated waters by BAC filters at the DLTWTF resulted in substantial loss of monochloramine stability in spite of total organic carbon (TOC) and turbidity decrease and pH increase. Monochloramine was the least stable in the samples collected immediately after the filter backwashing. The kinetics of monochloramine decay were found to be related to the amount of particles generated by the BAC filters. Consequently, filtering the BAC effluents with 1.2 µm and 0.4 µm polycarbonate iso-pore filters resulted in dramatic increase in monochloramine stability. Similar phenomena have been reported in literature and some researchers have postulated that the increase in monochloramine demand in the BAC filter effluents might be caused by activated carbon fines produced from extensive abrasion of GACs during backwashing. This hypothesis was proven invalid in this study. Controlled addition of fine particles of GACs up to 1 mg/L did not show any appreciable change in monochloramine decay kinetics in organic-free synthetic waters. In contrast, addition of GAC fines to the ozone treated natural water samples obtained from the DLTWTF resulted in a decrease of the chloramine demand. Further batch kinetic experiments performed with commercial powdered activated carbon (PAC, WPL carbons, Calgon Corp., Pittsburg, PA) suggested that monochloramine demand by PAC was also negligible up to a PAC concentration of 1 mg/L. Includes 10 references, table, figures.