LAUSR

Abstract Three distinct septic systems designed for onsite removal of nitrogen (N) from residential wastewater were installed at the Massachusetts Alternative Septic System Test Center (MASSTC) and at homes across Suffolk County (SC), New York. All configurations featured nitrifying sand beds coupled with denitrifying biofilters composed of 1) a lined, saturated sand and woodchip layer, 2) a saturated box filled with woodchips, or 3) an unlined, unsaturated sand and woodchip layer. Total N (TN) in final effluent discharge from the three systems at MASSTC over more than two years were 7.1 ± 7.8, 4.3 ± 4.2, and 6.9 ± 8.4 mg N L−1, respectively representing TN reductions of 83%, 87%, and 84% from influent TN. Systems at MASSTC also removed on average 90.0–99.9% of 10 of 11 organic contaminants in pharmaceutical and personal care products, microbes indicative of pathogens, and biochemical oxygen demand. Over periods up to 16 months from start-up, effluent from three lined, one woodchip box, and three unlined systems in SC averaged 8.3 ± 9.2, 5.3 ± 3.7, and 8.7 ± 4.9 mg-TN L−1 representing removal rates of 90%, 94%, and 88%, respectively. For all systems, wastewater N was effectively nitrified year-round; N removal varied seasonally as denitrification attenuated in winter. Substantial quantities of TN were removed in the sand beds, likely due to denitrification in anoxic micro-zones. While elevated levels of carbon leached from the wood-based biofilters installed at MASSTC during the first 60 days of operation, no substantial decline in dissolved organic carbon or N removal was observed between the first 15 months of operation and the following 12 months. Collectively, the performance of these non-proprietary, passive systems suggest they may be a useful alternative septic system for protection of groundwater from elevated levels of N, organic contaminants, and pathogens.