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Step feed biological nitrogen removal, (BNR), is the cornerstone of New York City DEP's Nitrogen Control Plan. Aeration tank thirteen, (AT-13), was designed specifically as a step feed BNR facility. AT-13 was operated at slightly above its design flow of 25 MGD and its nitrogen
removal capacity evaluated for the winter and summer months of 2000/2001. The evaluation consisted of characterization of the primary settling tank, (PST) and final settling tank (FST) effluents for COD, SS, VSS, total and soluble TKN, ammonia, nitrite and nitrate, pH, and alkalinity,
and multiple in-situ experiments to determine the nitrification and denitrification rates. The total nitrogen concentration in the PST effluent was 21.1 mg/L of which 25% was organic nitrogen, 74 % ammonia nitrogen, and 2% was combined nitrate-nitrite nitrogen. The
average concentration of total COD was 158 mg/L of which 35% was soluble and 16% was readily biodegradable COD. Process control was enhanced using an on line SS analyzer provided by Royce Instrument Corporation, New Orleans, LA (Royce 7011A Analyzer & 73A sensor) and
continuous monitoring of the RAS and WAS flow rates. The SRT averaged between 13 and 16 days during the evaluation period. PST effluent splits between the four passes and the relative volumes of the anoxic zones were varied to effect greater nitrogen removal. Total nitrogen removal for AT-13
averaged between 49% and 62% between the winter and summer periods, respectively, without addition of supplemental alkalinity or an external source of organic carbon. Approximately 8% of the nitrogen was assimilated in the waste sludge. It appeared that significant amount
of denitrification took place in the oxic zone, sludge blanket in the FSTs and RAS flow. A quantitative determination of the relative amount of denitrification in each zone was performed. Step feed BNR is a viable process that can remove significant amounts of nitrogen while taking full advantage
of the alkalinity and organic carbon available in the wastewater. For the wastewater treated the ratio of total COD to TKN was 8.1 and alkalinity to TKN was 4.5. Nitrogen removal in AT-13 was limited by the denitrification capacity rather than the nitrification rate.
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