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Beginning in the early 1980's, the waters of Western Long Island Sound showed significant degradation and low dissolved oxygen (hypoxia) caused primarily by nitrogen enrichment from runoff, atmospheric deposition, and sewage plant discharges. Located on the Western reach of long Island Sound near New York City, the Stamford, CT, Water Pollution Control Authority (WPCA) was the first municipality in CT to begin experimenting with nitrogen removal in the early 1990s and conducted a study funded by the U.S. Environmental Protection Agency (EPA) to demonstrate new nitrogen removal technology. This project was followed in the late 90's by plant improvements to further enhance nitrogen removal (50 to 60% nitrogen removal). The Stamford WPCA recently completed and commissioned a 105M nitrogen upgrade and expansion to 90,840 m3/day (24 mgd). About 50M was associated with nitrogen removal processes. Designed to achieve nitrogen removal on the order of 80 to 90% to levels of 4 mg/L Total nitrogen (TN) or less, the upgraded plant has been operating in the high level nitrogen removal mode since the spring of 2006.

Treatment consists of preliminary and primary treatment followed by suspended growth biological nitrogen removal (BNR) with biological reactors using pre- and post-anoxic zones (methanol fed to post-anoxic zone), ultra-violet (UV) disinfection, and discharge to the Sound. Solids handling includes thickening of primary and WAS, dewatering and land-filling. Construction of a sludge dryer is on-going and is expected to be completed in late 2007. Unique treatment challenges for this project included cold water (12°C), a history of severe foaming, and high levels of refractory nitrogen. Operating at 68,200 m3/day (18 mgd), the plant began achieving very low effluent TN (3–4 mg/L) in May of 2006 when final construction and equipment related issues were resolved and methanol addition to the secondary anoxic zone was begun. Although the plant achieves complete nitrification and nitrate removal to less than 1.0 mg/L, effluent refractory organic nitrogen has been high relative to other municipal POTWs (organic nitrogen of 3.8 mg/L in May, with a soluble component of 2.0 mg/L). Nocardia-foam control measures have performed successfully. Ammonia and nitrate analyzers are being added to further optimize performance. The sale of nitrogen credits can off-set nearly half of plant operating costs.
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Document Type: Research Article

Publication date: 01 January 2007

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