Faced with developing a city-wide LTCP covering 14 combined sewer overflow (CSO) service areas and over 450 permitted outfalls, the New York City Department of Environmental Protection (DEP) needed to identify cost-effective collection system modifications that could reduce CSO without
compromising the level of service provided to its customers. Given the unique configuration of many of the larger control structures in the City's collection systems, systematic regulator improvements that had been performed elsewhere in the City on smaller regulators would not be possible.
Thus, the City-Wide Long-Term Control Plan (LTCP) team evaluated regulator structures with large discharge volumes to identify those that were physically capable of accommodating retrofits such as bending weirs, inflatable dams, and weir geometry modifications. This paper presents one cost-effective
collection system retrofit proposed under the New York City LTCP that is expected to provide water quality benefits equivalent to a 12 million gallon (MG) CSO retention facility at a fraction of the cost. The two regulator structures that discharge to a single outfall are particularly suited
to take advantage of low-cost weir modifications due to their comparatively high fraction of sanitary flow, low weir crest elevations, and side-flow geometry. The two regulators collectively generate large CSO events with high pollutant loads, accounting for about half of the 670 MG of CSO
discharged to the dead-ended waterbody in a typical year. Water quality has been predictably impaired, a condition that is exacerbated by poor mixing dynamics, but modeling and observed data indicated that not all CSO discharges resulted in contraventions of the water quality standards for
dissolved oxygen and bacteria. Comparing complete CSO removal with partial CSO reduction alternatives suggested very little incremental water quality benefit, and a proposed 12 MG retention facility had already been approved by the state permitting authority. With these benchmarks, the LTCP
team focused on improving conveyance through this portion of the collection system to minimize the severe CSO events that appeared to be the source of the water quality impairments. Two side overflow weirs in the collection system were each raised by two feet and lengthened by a factor of
over 2.3, reducing CSO by 115 MG annually. This approach is expected to yield similar water quality benefits to the previously approved tank while saving the City 280 million.
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