Innovative Onsite Wastewater Treatment Systems for Nitrogen Removal: A Recirculating Gravel Filter with a Preanoxic Zone and a Recirculating Gravel Filter with a Postanoxic Woodchip Bed

Abstract

Nitrate released from septic system effluents can percolate through the soil, potentially contaminating ground water and exacerbating surface water eutrophication. Alternatives to conventional septic-drainfield systems can remove nitrogen from this waste-stream, thereby protecting water quality. This study compares the nitrogen removal efficiency of two onsite treatment systems designed to maximize nitrogen removal via sequential nitrification-denitrification. The key requirements for denitrification (including oxic conditions for nitrification and anoxic conditions for denitrification and bioavailable carbon), were provided in both systems, however several key design features differed. The first system was a vegetated recirculating gravel filter (Vegetated RGF) and was designed as a single step system, incorporating nitrification and denitrification conditions in tandem and utilizing the septic tank effluent as the carbon source. The second system separated the nitrification and denitrification steps, using an RGF for nitrification and a vegetated woodchip bed for denitrification. Both systems were fed residential wastewater influent for one year. The two-stage Woodchip bed system provided far greater nitrogen and fecal coliform removal than the single-stage system, however, the two stage system was highly sensitive to temperature and removed less nitrogen during cold months. The Vegetated RGF average effluent total nitrogen (TN) was 15.1 ± 1.9 mg/L, which equates to an average of 69% TN removal. The Woodchip bed system produced an average effluent of 1.7 ± 1.0 and 6.4 ± 4.2 mg-TN/L, during warm and cold months, respectively, resulting in an average TN removal of 92%. BOD, TSS and TP removal were similar in both systems. The Woodchip bed system exhibited exemplary nitrogen and fecal coliform removal and therefore has the potential to vastly improve residential wastewater treatment.