Establishment of nitrification and denitrification in an existing conventional, step‐feed activated sludge plant designed for suspended solids and carbonaceous biochemical oxygen demand removal was evaluated in a full‐scale study at the Tallman Island Water Pollution Control Plant in New York City. One of four biological treatment batteries consisting of an aeration tank and two rectangular secondary clarifiers was isolated from the rest of the plant and was used for the study. Nitrification rates and their temperature dependency were determined using NH3‐N and NO3‐N profiles developed along the aeration tank during the summer and winter months. The data suggested the occurrence of simultaneous nitrification and denitrification in the aeration tank. Virtually complete denitrification was observed within the sludge blanket of the secondary clarifier.

New York City DEP in collaboration with the City College of New York has developed a step-feed BNR process that has demonstrated significant nitrogen removal. The process is being considered for implementation at several water pollution control plants. Several bench scale and pilot studies have been initiated to develop design criteria for step-feed BNR. In this paper the methods used to measure maximum specific nitrifier growth rate, µ A and nitrifier decay rate, b A are presented. Sequencing batch reactors, SBRs, operating on primary settling tank effluent have been selected to measure µ A . The SBRs are operated for a minimum of three SRTs to allow for acclimation of the nitrifiers and achieve pseudo steadystate conditions. Subsequently, nitrification rates are measured and using the BioWin model to simulate SBR operation, µ A is calculated. Biomass from the SBRs is also subjected to starvation operating conditions in order to measure b A . Data from one of the studies is used to illustrate the method and also underline the importance of measuring both µ A and b A for a given wastewater.

As the need to retrofit secondary treatment plants for biological nutrient removal (BNR) has become widespread, process variations such as step-feed BNR have been developed to adapt existing treatment facilities to meet effluent requirements. Since there are relatively few fullscale installations of step-feed BNR, the process and retrofit design concepts are still under development. Although step-feed BNR has a relatively small footprint for a suspended growth process, it also is more complex than plug-flow or complete-mix regimes, and presents unique challenges to the designer. This paper discusses major design components including primary effluent flow splitting, chemical feed, dissolved oxygen carryover, nitrate recycle, and foam control; illustrates their impact on performance through process modeling examples; and shows how they are being resolved in New York, NY; Cumberland, MD; and North River, VA.

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