Dissolved organic nitrogen removal and its mechanisms during simulated soil aquifer treatment

Abstract

Soil aquifer treatment systems are known to further remove contaminants in wastewater effluent when applied through infiltration into the ground. Dissolved organic nitrogen (DON) in the effluent, a precursor for nitrogenous disinfection by-products (DBPs) such as N-nitrosodimethylamine (NDMA), is of great concern upon subsequent use of the groundwater infiltered into the aquifer. In this study, the vadose zone of the soil aquifer treatment system was simulated using 1 m laboratory soil columns under unsaturated conditions representing the vadose zone. The final effluent of a water reclamation facility (WRF) was applied to these columns to investigate the removal of N species with a focus on DON, as well as NDMA precursors. DON removal achieved was up to 99 % with an average of 68 % and was accompanied by a 52 % nitrate increase suggesting the occurrence of ammonification and nitrification through the soil columns. Around 62 % of total DON removal was seen at <10 cm travel distance, which was in accordance with higher adenosine triphosphate (ATP) concentrations at the top of the column due to more oxygen and organic matter availability. Total Dissolved N removal was drastically lowered to 4.5 % in the same column without microbial growth, which highlights the importance of biodegradation. The columns were capable of removing 56 % of the fluorescent dissolved organic matter (FDOM). Soil columns could remove NDMA precursors up to 92 % through the column with the initial concentration of 89.5 ng/L, possibly due to the removal of DON fractions. The results demonstrate the capability of the vadose zone in further treatment of DON and other organic matter before reaching the groundwater through infiltration or indirect discharge to surface water. Differences in applied water quality and the site-specific oxic conditions in SAT systems could lead to variable removal efficiencies.