North Carolina State University
Co-Authors: M. R. Burchell II and J. J. Kurki-Fox
Constructed wetlands provide an effective method for small communities to incorporate tertiary biological nutrient removal into their wastewater treatment train. Studies documenting the success of young (<10 years old) constructed wetlands are widely available in the literature, but, performance data on aging systems, along with techniques to extend design life, is largely unavailable. Over time, wetland plants, animals and anaerobic conditions combine to contribute to significant detritus accumulation, adverse hydraulic conditions, and reduced treatment efficiency. To recommend widespread implementation with confidence, it is crucial to understand the design life and late-stage management of these systems, while providing long-term O&M guidance that is often lacking. To provide insights into an aging wetland performance and rejuvenation techniques, the treatment efficiency and internal hydraulics of two 0.7-ha, parallel-flow wetland cells built in 1996 to provide tertiary treatment of domestic wastewater in NC are being studied. Continuous hydrology and water quality monitoring of the two cells began in November of 2018. Initial observations indicated substandard treatment performance. To improve performance and evaluate wetland rejuvenation techniques, accumulated detritus was removed from cell 1 in late April 2019 and wetland vegetation was replanted in the cell. Cell 2 was left in its initial condition to serve as a control. Tracer tests conducted since detritus removal show marked improvement of cell 1 hydraulic performance. Improved N treatment performance has also been observed in the rejuvenated cell 1. Median TN concentrations from May 2019 to January 2020 indicated a 27% concentration reduction in wetland cell 1, while wetland cell 2 had a concentration increase (-9%) over the same period. In addition to TN reductions, TP concentration reductions have been observed through the rejuvenated cell 1. Additional analyses include N load reductions and an estimation of parameter values for the k-C* model to quantify the impact of wetland rejuvenation.