Development and Validation of a 3-Dimensional Computational Fluid Dynamics(CFD) Model for Rectangular Settling Tanks in New York City Water Pollution Control Plants

In New York City the Wards Island WPCP was designed to provide secondary treatment for an average dry weather flow of 12.1m3/s using the step feed activated sludge process. In response to recent changes in the discharge permit limits, one additional aeration tank (AT-13) with its associated FSTs was placed in operation to demonstrate the feasibility of adapting to step-feed BNR. Recognizing the critical nature of FSTs, in the overall performance of step-feed BNR, a three dimensional computer model was developed. The model was specifically developed for the Gould II type rectangular tanks.Each FST has three bays, and the settled sludge is collected in a central cross collector using a chain and flight system. The sludge is then directed to a sludge hopper located at one end of the cross collector channel for removal. The bulk of the sludge withdrawn is RAS while a small portion is wasted to maintain the desired solids retention time (SRT) to facilitate nitrogen removal. The secondary effluent from the FSTs flows to the chlorine contact tanks for disinfection.Preliminary in-situ investigations were conducted which included tracer studies using Rhodamine WT dye, SS profiles along the depth and length of the tank, and dispersed SS profiles using the Kemmerer sampler. The data revealed the importance of settling velocities, flocculation within the FSTs, density currents, and the three dimensional nature of flow. Therefore, the approach adopted for this study was to develop a 3-dimensional CFD model which would be calibrated and validated with data collected on site. Subsequently the model would be used as a tool by NYCDEP to simulate various operational scenarios for maximum SS removal efficiency.The important mechanisms active in the SS removal efficiency of the FSTs were quantified in bench-scale and full-scale experiments and used to calibrate each of the SS removal sub models incorporated in the 3-dimensional model.