Nitrogen balance and cycling in an ecologically engineered septage treatment system

Abstract Septage disposal presents technical difficulties due to its high concentrations of solids and nutrients. Many existing treatment options create excessive waste solids or fail to remove nutrients, which, upon discharge, impact groundwater and aquatic systems. This study evaluated nitrogen removal in an ecologically engineered wastewater treatment system (the Solar Aquatic System™ [SAS]) that combines aeration and activated solids recycling with aquatic and constructed wetlands treatment components. A septage-treating SAS in coastal Massachusetts, USA, produced an effluent of 113 mg/l chemical oxygen demand, 16 mg/l total suspended solids (TSS), 0.56 mg/l ammonium nitrogen, 6.1 mg/l total nitrogen and 1.5 mg/l total phosphorus. Waste solids production was low at 0.81 g/g influent TSS, including harvested plant biomass. Primary treatment was by biological oxidation with activated solids, followed by gravity clarification. Further oxidation and ammonium removal occurred in planted aerated aquatic tanks enclosed in a greenhouse, followed by secondary gravity clarification. Nitrogen removal in the aquatic tanks was limited by denitrification, which was limited by the availability of anaerobic microsites. Mineralization of particulate organic nitrogen, followed by nitrification and denitrification, occurred simultaneously during primary treatment and in the aquatic tanks. Final solids polishing and denitrification occurred in a constructed wetland. Although 88% of the influent nitrogen was removed during primary treatment, further treatment in the aquatic tanks and wetlands was required to reduce effluent nitrogen to tertiary standards. Nitrogen removal in the whole system was primarily by sedimentation of waste solids, accounting for 57.6% of the influent nitrogen, followed by denitrification, at 40.9%. Direct uptake of nitrogen by plants was insignificant, at 0.5%. 1.0% of the total influent nitrogen left the facility in the effluent water. The SAS has the capability to treat septage to tertiary standards with low solids production in small, decentralized facilities.