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 (NO2−–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 NO2−–N. Laboratory analysis included measurement of the chlorine species (free chlorine and mono‐, di‐, and trichloramines) and NO2−–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.
Prompted by the potential benefits of multi-phase anaerobic digestion, such as improved digester performance, increased process reliability, sludge dewaterability, and the potential to produce Class A biosolids, the City of Phoenix Water Services Department (WSD) initiated a study in January 2003 to evaluate the feasibility of converting the existing single-stage high-rate anaerobic digestion process at the 23rd Avenue Wastewater Treatment Plant (WWTP) to a multiphase digestion process. Accompanying a higher volatile suspended solids reduction, multiphase digestion can result in elevated ammonia levels in the digested sludge. Dewatered sludge centrate with elevated ammonia would be returned to the plant's liquid stream via a side stream recycle and potentially impact nitrification performance. To assess the impact of multi-phase digestion on liquid stream treatment and provide a basis for plant expansion, process modeling using Biowin32 was performed at steady-state and with diurnally varying influent loads.Plant historical records were evaluated to obtain influent wastewater characteristics, unit process performance, and plant operational configuration. 2-weeks of intensive sampling were conducted to supplement plant data for characterizing influent, establishing influent diurnal patterns, and the process response to dynamic loading, all of which was used to validate the Biowin model calibration. Due to the configuration of the plant's sampling systems, intensive sampling data were obtained almost exclusively on the primary effluent. A mass balance was performed to back-calculate the raw influent characteristics based on the plant's historical primary treatment efficiency. Both the steady state and the extended period dynamic modeling for the intensive sampling period resulted in predictions that closely match key plant measurements obtained during the same period, such as aeration basin mixed liquor suspended solids (MLSS) and volatile suspended solids (MLVSS), effluent NO 3 --N, TKN, and TSS.The calibrated model was then used to evaluate the maximum treatable flow and ammonia load, in order to determine the requirement for side stream treatment after the conversion to multiphase digestion. A set of limits was established to gauge acceptable plant operation and performance. At the winter average day maximum month (ADMM) condition, both steady state and dynamic modeling results indicate that the plant can produce satisfactory effluent while operating the existing process within the recommended limits at flow up to 63 MGD. At the rated flow, the hydraulic loadings of primary and secondary clarifiers, the BOD 5 loading on aeration basins, and the total oxygen transfer rate (OTR) at the corresponding peak day condition would exceed the recommended limits. At summer ADMM loadings with one aeration basin out of service for maintenance, the plant could treat up to 61 MGD with the hydraulic loadings of primary and secondary clarifiers, the BOD 5 loading on aeration basins, and the oxygen uptake rate (OUR) exceeding rec...
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