EVOLVING APPLICATIONS OF SWMM FOR MULTI-PHASED ANALYSIS OF CSO ABATEMENT ALTERNATIVES

The analysis of combined sewer overflow (CSO) abatement alternatives often proceeds in an evolving manner. Landside models are generally used to determine which outfalls need CSO abatement, sometimes in conjunction with receiving water quality models. Once CSO outfalls in need of abatement are identified, an optimal level of abatement needed to achieve water quality standards can be determined. With the required abatement level determined, various methods of achieving that level can be explored. One way of abating CSO is with CSO storage and subsequent treatment of stored CSO at a wastewater treatment plant. Storage or retention of CSO is either offline, in the form of a tank, or it is inline using fixed or inflatable weirs or a combination of the two. Once the level and type of storage/abatement have been determined and a preliminary design completed, the proposed abatement scenario can be simulated to assist in designing the abatement facilities for proper hydraulic performance. As the project proceeds through planning and design of CSO abatement, the model is refined to test alternatives and set the detailed specifications required.This approach was applied to plan the abatement of CSOs within the Hunts Point Wastewater Pollution Control Plant (WPCP) service area in the Bronx, NY. Initially EPA's Stormwater Management Model (SWMM) RUNOFF, TRANSPORT, and a simpler model, Rainfall Runoff Modeling System (RRMS), were applied to evaluate CSO discharge and pollutant loading to three tributaries of the East River. The Bronx River, Westchester Creek and the Hutchinson River receive CSO discharges from outfalls as shown in Figure1. Water quality modeling of these tributaries assessed the effects of CSO on dissolved oxygen and total coliform concentrations. Major CSO outfalls in the upstream reaches of these tributaries were targeted for abatement, which was preliminarily, set in terms of offline storage volumes.Two CSO outfalls (HP-23 and HP-24) located on the Bronx River were targeted for a “knee of the curve” level of abatement equivalent to 4 MG of offline storage. The kneeof- the-curve, which is defined in EPA's demonstration approach for a long term control plan (LTCP) under the national policy, is the point where incremental water quality improvement diminishes compared to increased cost for additional CSO control. Further analysis of the combined sewers using SWMM EXTRAN indicated that in-line storage dams could provide the equivalent CSO abatement. A series of inline storage dams were proposed for these two outfalls. Two in-line storage dams providing approximately 0.42 MG of storage were proposed for HP-24 and seven in-line dams providing approximately 4.96 MG of storage were proposed for HP-23. Recent plans for HP-23 include extending its outfall to provide the equivalent CSO load reduction and thereby reduce, or possibly eliminate the number of inline storage dams for both HP-23 and HP-24.Two other CSO outfalls (HP-14 and HP-15) located on the Hutchinson River were recommended for abatement by a 7 MG offline storage tank. As the siting of a tank proved to be difficult, storage conduits in city streets were later adopted as an alternative to the tank. The hydraulics of the Hutchinson River CSO abatement facility were modeled using EXTRAN to assist in the design of diversion weirs (regulators) and outfalls.An EXTRAN model of the Hunts Point collection system is being used to simulate the retention of CSO in the offline and inline storage facilities for the four outfalls above, as well as a 12 MG offline storage tank for HP-25 at the head of Westchester Creek, and the pumping of CSO to the WPCP. These continuous simulations will be used to develop the controls for operating the CSO abatement facilities.