Sharing Resources: New York City's Alternative Strategy for Combined Water and Wastewater Treatment Solids Handling

New York City (the City) water is derived from the Croton, Catskill, and Delaware systems. The Croton system (Croton) is the oldest and smallest of the three systems and provides overall system flexibility and essential redundancy to the Catskill and Delaware systems. Under a Consent Decree, the City is required to implement filtration and disinfection for Croton to comply with current and anticipated drinking water standards. Therefore, the City is designing a 290 mgd water treatment plant (WTP) that reduces operation complexity and plant footprint. The design includes an off-site combined solids processing strategy at the Hunts Point wastewater treatment plant (Hunts Point). This strategy involves solids conveyance from the Croton WTP through a seven mile force main to Hunts Point. Hunts Point dewaters indigenous and visitor digested sludge and is capable of dewatering additional solids loads.Conventional approaches for combined processing of water and wastewater treatment solids typically involve routing the water treatment solids to the head of the wastewater treatment plant. This strategy can result in additional pumping, thickening and possibly digestion costs. Additionally, significant amounts of chemical sludge in the primary solids may render them of limited use for adaptation in resource recovery strategies such as primary solids fermentation for biodegradable carbon production. Further, un-settlable colloidal inert matter present in the water treatment residuals can accumulate in the wastewater treatment activated sludge unit and impair performance. Given future operation in biological nitrogen removal mode at Hunts Point, inert solids accumulation is even less desirable. In a significant improvement over the conventional strategy, the proposed design introduces the water treatment solids at the point of wastewater treatment solids processing, thereby minimally increasing the contribution of water treatment solids to the activated sludge unit.To evaluate the potential effects of combined solids processing on dewatering operations at Hunts Point, a comprehensive experimental study has been formulated with the following objectives:determine advantages and disadvantages of blending solids versus separate solids handlingdetermine the impact of blending solids on solids-liquids separation and dewaterabilitydetermine optimal centrifuge operating conditions for maximal solids-liquids separation and dewaterabilitydetermine optimal polymer dose for maximal solids-liquids separation and dewaterabilitydetermine compliance of cake solids with USEPA CFR 503 standards for land applicationdetermine the impact of the centrate emanating from combined solids processing on nitrification kineticsPreliminary bench-scale centrifugation studies have indicated a higher degree of solids-liquids separation (lower supernatant solids concentration) but lower dewaterability for samples containing a higher proportion of water treatment solids.