Evaluation of Constructed Wetland Performance in New Mexico, 2007

Wastewater treatment wetlands, also referred to as constructed wetland systems (CWS), were installed in New Mexico and other states during the 1990s to provide low cost and low maintenance wastewater treatment options to small communities served by on-site treatment systems. An analysis completed in 1995 examined the design and performance of 18 subsurface flow constructed wetlands. At that time, most of the systems were relatively new and little information was obtained about their long term performance. This project examined the performance of 11 constructed wetlands built between 1990 and 1996, five of which were included in the 1995 study. The systems were analyzed to assess their overall performance. The analysis consisted of site visits, sampling of some systems, and evaluation of monitoring data submitted as required by ground water discharge permits where available. Four systems do not meet NMED permit requirements for total nitrogen, three systems consistently meet these requirements and four systems exhibit variable compliance. It was found that systems with some level of pretreatment beyond that provided by a septic tank were able to obtain sufficient nitrogen removal, whereas a wetland cell alone achieved poor nitrogen removal. The principal limiting factor appears to be the lack of aerobic zones which prevents the systems from achieving adequate levels of nitrification. Performance may be improved by incorporating components such as aeration within the cells, a trickling filter within the system, or nitrification tanks. Maintenance of all electrical, mechanical and plumbing equipment in a wetland system is critical as treatment rates were seen to drop drastically with the failure of electrical components such as pumps or aerators.