Achieving 3 mg/l Total Nitrogen in a Cold Climate Without Supplemental Carbon or Filtration

Faced with a projected fifty percent increase in the sewered population and an aged existing treatment facility sandwiched between MetroNorth Railroad and Interstate 95, the Town of Westport, Connecticut needed an upgraded and expanded facility that could fit on a very small site while meeting an effluent nitrogen load limit that corresponded to a concentration of 3.1 mg/L at the design average flow condition. The Town desired an upgraded treatment plant that would be easy to operate and maintain. Due to its simple operation and forgiving nature, a fourstage Bardenpho oxidation ditch was selected as the preferred process to accomplish this goal while also achieving the very low levels of nitrogen required.The four-stage Bardenpho oxidation ditch, however, is more land intensive than other nitrogen removal processes. Extreme land constraints necessitated carefully thought out tank configurations and locations as well as a sequence of construction that would maintain full treatment to meet the plant's NPDES permit throughout construction. The final facility made use of much of the available space on the site and performed well during construction.To meet the goal of simplicity, the four-stage Bardenpho system was designed and constructed without supplemental carbon addition or effluent filters, facilities that ordinarily may be needed to remove nitrogen to the approximate limit of technology. The treatment process consists of a pre anoxic zone followed by an oxidation ditch with mechanical aerators. The mechanical aerators are controlled by an aeration control system that adjusts the speed of the aerators through VFDs. A post anoxic zone, reaeration zone and UV disinfection follow the mainreactors.Since project completion in December 2007, the 3.4 mgd facility has been operating with only one (1) of the two (2) pre-anoxic zone / oxidation ditch trains online and one (1) of two (2) post anoxic zones to treat flows that currently average approximately one half the design flow for the plant (i.e. the single train is currently being operated at its design flow).Since startup, performance has been excellent with BOD and TSS concentrations in the single digits and effluent nitrogen averaging below 3 mg/L since startup.