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The Long Island Sound Study (LISS), a partnership between the United States Environmental Protection Agency, New York, and Connecticut, was formed by Congress to address water quality concerns related to eutrophication and hypoxia. Nitrogen was identified as the causal agent for the observed water quality problems, and a set of phased Nitrogen removal targets were implemented to reduce discharges to the Sound by more than 50 % over a 15-year period of time. Over the course of those 15 years, the nitrogen reductions will “step-down” so that a phased adaptive management approach is taken to achieve the long-term nitrogen removal goals, with a 23% reduction from 1990 levels after 5 years (16 mg/L TN based on 2005 flow), a 44% reduction after 10 years (12 mg/L TN based on 2005 flow) and the full 50% reduction in 15 years (8.5 mg/L TN based on 2005 flow).

The New York City Department of Environmental Protection (NYCDEP) has four wastewater pollution control plants (WPCPs) that discharge to the Upper East River, and ultimately, Long Island Sound: Hunts Point, Tallman Island, Bowery Bay, and Wards Island, with a combined secondary treatment capacity of over 700 mgd. Step-Feed Biological Nutrient Removal (BNR) was identified as the most feasible technology for implementation at the large scale WPCPs to achieve the necessary Nitrogen removal. To assist in the development of BNR designs and the management of these simultaneous upgrades, the Department formed the Advanced Wastewater Treatment (AWT) Team, to provide design and program management expertise. The AWT Team has assisted NYCDEP in meeting Consent Order milestones, development of BNR facility plans, production of site specific BNR design guidelines, review of BNR design drawings and specifications, and the development of programmatic guidance for design engineers at each of the four WPCPs.

BNR guidance was developed by the NYCDEP and AWT Team (NYC/AWT) to ensure that a common approach was followed by the large number of consulting firms, as well as the Department itself, over the duration of the project. Major elements of the BNR upgrades detailed in the guidance include:

Aeration System Upgrades – Provides new blowers and an enhanced process air distribution system to ensure better nitrification

Upgrades to Aeration Tanks – Baffle walls to separate oxic and anoxic zones to allow nitrification/denitrification to occur with flexibility of zone, 1/6, 1/3, etc. down each pass

Froth Control Systems – Reduces the population of foam-producing bacteria

Alkalinity Addition Systems – Provides alkalinity required for nitrification and pH maintenance

Return Activated Sludge Upgrades – Allows Aeration Tanks to carry a higher solids inventory

Separate Centrate Treatment – Provides a dedicated aeration volume to nitrify ammonia-rich centrate

Improved Flow Splitting and Control – To optimize Nitrogen removal

Carbon Addition – Provides additional carbon in the form of methanol to assist in denitrification

Guidance provided specific details on many other aspects of this program and allowed for seamless coordination between the NYCDEP and its consultant teams.

The NYCDEP faced challenges in scheduling and coordinating multiple major capital construction projects, meeting Consent Order construction completion mandates, and simultaneously meeting interim Nitrogen discharge limits. As a tool to assist NYCDEP, the AWT/DEP Team calculated the flows and loads to each WPCP based on population and employment projections and predicted treatment performance before, during, and after construction using the BioWin process model. The BioWin modeling included treatment limitations, such as reactors taken offline for construction, as well as treatment enhancement as the Nitrogen upgrades were placed online and ultimately included carbon addition. The specifics of the modeling results will be presented. The BioWin results provided projected effluent nitrogen concentrations that were used to create a management tool, nicknamed the “Bulge.” The Bulge is a graphical representation of the combined effluent nitrogen predictions over the course of the entire construction window for the BNR upgrades.

As construction began on the Upper East River Plants, the AWT/DEP Team has used the Nitrogen Bulge to track actual East River performance versus AWT predictions over the same timeframe. The accuracy of the Bulge predictions conducted using the BioWin process model were confirmed; the AWT Bulge is within 1.1% of actual plant performance between October 2003 and April 2006.

In order to meet future water quality objectives, NYCDEP has committed to a second phase of upgrades at the four Upper East River WPCPs. The second phase will reduce Nitrogen discharges to approximately 5 mg/L and will provide the necessary infrastructure to reliably meet Nitrogen discharge limits year-round. The AWT Team will continue to provide design guidance and perform a program management overview through this second phase of the program.

Document Type: Research Article


Publication date: 2007-01-01

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