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I II I I I I I I I I I I I I I I I I I ~"'il' I nC! I):'" l.,."i '.' j the District decided to investigate whether alternatives to additional nitrification/denitrification facilities existed, Camp Dresser & McKee Inc. (COM) was retained by the District to identify and evaluate alternatives to nitrification to meet Segment 15 water quality standards. This Nitrification Alternatives Study involved the development of a detailed water quality model of Segment 15, which was used in evaluating the performance of potential "non-traditional" alternatives, WATER OUALITY MODELING Interrelated physical, chemical, and biological factors influence dissolved oxygen and ammonia concentrations in any given river location, These include hydrologic characteristics, river channel geometry, oxygen producing photosynthetic activity, and oxygen consuming biological respiration and nitrification processes, Bacteria in the river water column and sediments convert organic maner to carbon dioxide and water via respiration and convert ammonia to nitrate (nitrification) while consuming large quantities of oxygen from the water. In order to simulate river response to various non-traditional alternatives, a specialized water quality computer model was constructed for Segment 15 by incorporating the Colorado Ammonia and EPA's STREAM DO IV models. The resulting model, called the Segment 15 Water Quality Model, is supported by a large data base and is considered to be the best simulation to date of water quality in Segment 15. Future additions to the data base will strengthen the reliability of this model. ..Modeling revealed that DO, not ammonia toxicity, was the limiting water quality parameter and was JIlost critical during July through September, Figure ES-2 illustrates an expected 24 hour daily average DO protile along Segment 15, with a Central Plant effluent flow of 185 mgd (North Complex ..fully nitrifying, the South Complex operating in a secondary treatment mode, and the river at its lowest flow condition), It is important to note that for this srudv, the regulatory agencies agreed that a 24-hour average DO would be used to evaluate compliance with the current water quality standards, As shown in Figure ES-2, the 5.0 mglL daily average DO standard is expected to be violated over a four mile reach extending from 88th Avenue to below the Fulton Ditch diversion and again over a six mile reach from approximately 160th Avenue (Brighton) north to Fort Lupton, The modeling results also indicated that with full nitrification of the Central Plant at its design capacity of 185 mgd, DO D-4