<i>Populus</i> Species Mitigate Nitrogen Breakthrough in Wastewater Infiltration Systems for Year-Round Treatment and Recovery

Land treatment by wastewater infiltration offers a method for coupling treatment with water reuse for biomass crop irrigation to improve crop yield while conserving water resources. It is important to understand the potential for nitrogen breakthrough in these systems. Here, we used field-scale wastewater infiltration systems to evaluate removal of nitrogen and organic matter from synthetic secondary and primary effluents. The test reactors were planted with poplar trees and compared with bare soil controls receiving the same wastewater and with poplar-planted controls receiving clean water. System properties were monitored, including physical and chemical characteristics of soil and effluent. Water was tested for total nitrogen, nitrate, ammonium, and chemical oxygen demand. Nitrogen removal in reactors with trees was always greater than unplanted reactors, especially in the dormant season, and with higher nitrogen concentrations in primary effluent. Leaf nutrient content also differed; leaves from planted wastewater reactors had significantly higher content of phosphorus, potassium, and total nitrogen than planted clean water reactors, suggesting leaf uptake as a potential fate for influent nutrients. Application of wastewater to the planted clean water controls at the end of the experiment showed removal performance that was significantly below that of treated reactors, highlighting that prior exposure to wastewater was important. This work demonstrated the potential for irrigation of planted infiltration galleries with high volumes of primary or secondary effluent, allowing simultaneous clean water reclamation, nutrient removal, and plant biomass production.