Impacts of Advanced Tertiary Treatment on the Nitrogen Cycling of a Hypereutrophic Lake: A Case for 303(D) Delisting

The water quality of Onondaga Lake, a 12 km2, dimictic, water body located in central New York State, has been compromised by a 100-year history of municipal and industrial pollution. The Syracuse Metropolitan Wastewater Treatment Plant (Metro), an 85 MGD tertiary treatment facility serving the City of Syracuse and surrounding area, discharges to the lake and historically accounted for 20% of its annual average inflow, and over 80% of its annual ammonia loadings. In the past, elevated ammonia loadings from the Metro treatment facility produced water column free ammonia concentrations in excess of the federal water quality criteria and state water quality standard. Moreover, nitrification of ammonia within the upper mixed layer of the lake during the summer stratified period produced nitrite concentrations in excess of the state ambient water quality standard. Contravention of state water quality standards for ammonia and nitrite, among other constituents, led to the addition of Onondaga Lake to New York State’s 303(d) list in 1996; a Phase 1 TMDL was developed in 1997 and a Phase 2 TMDL is scheduled for 2009. The Phase 1 TMDL was the primary technical basis for a Municipal Compliance Plan and Amended Consent Judgment (ACJ) that required Metro treatment plant upgrades to include the construction of an advanced tertiary, biologically aerated filter (BAF) system for ammonia removal. Construction of this system was completed in 2004. The ammonia treatment facility has reduced ammonia discharges to the lake by approximately 90% and consequently has had a pronounced impact on the nitrogen cycling within the lake. The change in ammonia loading and the subsequent response of the system pose a very strong case for removing Onondaga Lake from the list of ammonia impaired water bodies within the State of New York.

Onondaga Lake and Its Watershed

Onondaga Lake is located within Central New York State, immediately northwest of the City of Syracuse (lat. 43º06’54”, long. 76º14’34”; Figure 1). The lake is relatively small, with its main axis measuring approximately 7.6 km, a surface width ranging between 1 and 2 km, and a surface area of 11.7 km2. The lake’s depth averages 10.9 m, with a maximum of 19.5 m. Its bathymetry is characterized by two minor depressions, referred to as the northern and southern basins, separated by a shallower region near the center of its longitudinal axis (Figure 1). The littoral zone is rather narrow, with water depths of 3 m or less located typically within 100 m of the shore. However, the littoral zone widens in some areas, most notably near the lake’s southeastern end, where it extends nearly 500 m from shore at some places.

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