Variation of Oxidation-Reduction Potential Along the Breakpoint Curves in Low-Ammonia Effluents

This study was part of an integrated series of investigations conducted at the 6.6 m³/s (150 mgd) City of Phoenix, Arizona, 91st Avenue Wastewater Treatment Plant to improve the chlorination system and optimize chlorine use after the plant was converted to a nitrification-denitrification (NdeN) plant. The objective of this study was to evaluate chlorine breakpoint behavior and assess oxidation-reduction potential (ORP) with different regimes of the breakpoint curve to reflect ORP behavior in a mixed oxidant and reductant environment typical of a chlorine disinfection system in an NdeN plant. The information was also useful for confirming the results of earlier studies on the fate of nitrite-nitrogen (NO^-₂-N) in chlorine reactions, the phenomenon of microbreakpoint formation, and the relative competition for chlorine between ammonia and organics and between ammonia and other inorganics including NO^-₂-N. Laboratory analysis included measurement of the chlorine species (free chlorine and mono-, di-, and trichloramines) and NO^-₂-N, nitrate-nitrogen, ammonia, pH, dissolved oxygen, and ORP in low-ammonia effluents (effluents spiked with various amounts of ammonia) after addition of various chlorine doses. The study indicated that the overall shape of ORP curves follows the pattern of the breakpoint curves. However, the ORP curves were relatively flat at the monochloramine hump region of the curve. Also, ORP increased sharply at the beginning of the monochloramine region and at the beginning of the free chlorine region. The ORP curves eventually flattened again after the initial steep rise. Thus, at the flat portions of the curve, a marginal advantage could be derived with further increases in chlorine dose, and contact time becomes a critical factor in disinfection efficiency. A dip in the ORP curve was invariably observed at the breakpoint even though the dip was of a lesser degree than that of the residual curve. Greater concentrations of dichloramine (which have a greater ORP than monochloramine) expected at the downswing of the breakpoint curve could not sustain the maximum ORP levels observed at the monochloramine region. This suggested that organochloramines may also be a factor in the ORP dip at the breakpoint. Finally, ORP did not vary in direct proportion to total chlorine residual or in direct proportion to the concentration of individual chlorine residual species. Implications of these results are comparatively discussed with reference to the two methods of chlorination control: chlorine-residual-based control and ORP-based control.