IMPROVING NITROGEN REMOVAL BY RAS ADDITION TO THE SECONDARY ANOXIC ZONE: RESULTS OF FULL SCALE IMPLEMENTATION

In anticipation of increasingly stringent effluent nutrient limits, three wastewater treatment facilities, Derry Township Municipal Authority (DTMA) PA, Lancaster, PA, and Branford, CT, integrated a simple and unique process modification into recent plant upgrades called RAS Bleed to improve biological nitrogen removal. RAS Bleed involves bleeding a portion of the return sludge stream directly to the secondary anoxic zone, thereby increasing the biomass concentration in that zone relative to the concentration in the upstream sections of the process. Studies at the DTMA facility found that RAS addition to increase the MLSS gradient between the oxic and secondary anoxic zones by approximately 600 mg/l increased nitrate removal rates (lbs per day) by more than 30% and reduced effluent total nitrogen up to 2 mg/L. When the gradient was increased to 1,500 mg/L, nitrate removal rates in the secondary anoxic zone increased by 75%. Operating data from the Lancaster and Branford facilities show that nitrogen removal is performing well, presumably improved by RAS addition to the secondary anoxic zones. RAS addition to the secondary anoxic zones or “RAS Bleed” offers a relatively simple method for improving nitrogen removal without necessarily requiring supplemental carbon addition.