Critical review of effluent dissolved organic nitrogen removal by soil/aquifer-based treatment systems.

Soil/aquifer-based treatment systems improve wastewater effluent quality by removing trace contaminants in the soil and/or aquifer during groundwater recharge. This paper critically reviews these systems with a focus on removing nitrogen, particularly low levels of dissolved organic nitrogen (DON) present in the wastewater effluent. DON in wastewater effluent is a concern because of its contribution to nitrogen concentration in surface or groundwater and its role as a precursor of nitrogenous disinfection by-products, which are harmful to human health. Biodegradation and sorption are the main DON removal mechanisms in the subsurface environment where most of the removal happens in the vadose zone. Different factors such as temperature, redox conditions, retention time, indigenous microbial community, and soil type affect DON removal in soil/aquifer-based treatment systems. Lack of sufficient current knowledge underlines the need for designing lab/field scale systems for further determination of the relative contribution of biodegradation and sorption, optimal hydraulic loading rate, and the relationship between DON characteristics such as functional groups and physiochemical processes and its removal. Future research needs for DON removal in soil/aquifer-based treatment systems are identified.