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The Grass Island wastewater treatment plant in Greenwich, Connecticut is one of the first Long Island Sound plants that was modified to achieve an interim target nitrogen level of 10 mg/L with minimal plant changes and costs. The plant was designed as a step-feed aeration system
in the early 1960's and was upgraded in 1991 to increase its design capacity to 12.5 million gallons per day (MGD). Facility alterations included the addition of two step-feed aeration tanks bringing the total to five, and three new 115-ft diameter secondary clarifiers, which replaced
three rectangular clarifiers. Each step-feed aeration tank consists of four aerated zones. The sequencing aeration control (SAC) in the first zone (Zone 1) in each aeration tank was a unique feature of the 1991 modifications. The SAC strategy sequentially induces oxic/anoxic regimes in
the zone to improve activated sludge settling. Nitrogen removal was not required at the time. Interim modifications to meet the nitrogen target level of 10 mg/L were done in 1997 when the plant was modified to the plug-flow Modified Ludzack-Ettinger (MLE) process. Even though the SAC
strategy was not designed originally for nitrogen removal, its operation was effective in maintaining anoxic conditions in Zone 1. Pumps installed in the end of Zone 4 recycle nitrified effluent to Zone 1. The MLE process currently operates at a 200 percent internal recycle rate, resulting
in an annual average nitrogen removal rate of 70%, and achieving an average total nitrogen concentration of 6.5 mg/L. The Grass Island plant operates at an average flow of 10 MGD, a hydraulic retention time of 8 hours, and solids retention times between 12 and 21 days. The dissolved
oxygen in the aeration zones is kept below 1.5 mg/L, and in the anoxic zones below 0.2 mg/L. The improvements at the Grass Island plant show that a simple process, such as the MLE, is very effective in removing nitrogen from municipal wastewater. The process is easy to install,
easy to operate, and inexpensive. Nitrogen levels of less than 7 mg/L can be easily achieved with minor plant modifications and without addition of alkalinity and carbon source.
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