Nitrogen balance and cycling in an ecologically engineered septage treatment system

Septage disposal presents technical difficulties due to its high concentrat ions of solids and nutrients. Many existing treatment options create excess ive 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 S ystem (TM) [SAS]) that combines aeration and activated solids recycling wit h aquatic and constructed wetlands treatment components. A septage-treating SAS in coastal Massachusetts, USA, produced an effluent of 113 mg/l chemic al 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 soli ds production was low at 0.81 g/g influent TSS, including harvested plant b iomass. 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, follow ed by secondary gravity clarification. Nitrogen removal in the aquatic tank s was limited by denitrification, which was limited by the availability of anaerobic microsites. Mineralization of particulate organic nitrogen, follo wed by nitrification and denitrification, occurred simultaneously during pr imary treatment and in the aquatic tanks. Final solids polishing and denitr ification occurred in a constructed wetland. Although 88% of the influent n itrogen was removed during primary treatment, further treatment in the aqua tic tanks and wetlands was required to reduce effluent nitrogen to tertiary standards. Nitrogen removal in the whole system was primarily by sedimenta tion of waste solids, accounting for 57.6% of the influent nitrogen, follow ed by denitrification, at 40.9%. Direct uptake of nitrogen by plants was in significant, 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 tert iary standards with low solids production in small, decentralized facilitie s. (C) 2001 Elsevier Science B.V. All rights reserved.