A Subsurface Upflow Wetland System for Removal of Nutrients and Pathogens in On-Site Sewage Treatment and Disposal Systems

Abstract

On-site sewage treatment and disposal systems, commonly referred to as a septic systems, consist basically of a septic tank and soil adsorption field or drainfield. It may represent a large fraction of nutrient loads and pathogen impacts in vadose zone and groundwater systems. It includes not only nitrogen (N) and phosphorus (P), but also pathogen indicators such as fecal coliform and Escherichia coli, which indicate the presence of other disease-causing bacteria flowing into the aquatic system and potentially adversely affecting public health. Constructed wetlands, an effective small-scale wastewater treatment system with low energy and maintenance requirements and operational costs, will cover current needs for nutrient and pathogen removal. In our study, a next-generation subsurface upflow wetland system that is filled with green sorption media (e.g., mixes of recycled and natural materials) along with selected plant species was tested as a substitute for the conventional drainfield in septic tank systems. Four parallel subsurface upflow wetlands (i.e., three planted versus one unplanted) were built to handle 454 L/day (120 gallons/day) of septic wastewater flow. It proved effective in removing both nutrients and pathogens. During the test run in 2009, the planted wetlands achieved a removal efficiency of 84.2%, 97.3%, 98.93%, and 99.92% in total nitrogen, total phosphorus, fecal coli, and E. coli, respectively. A stress test conducted in winter 2010 successfully verified the reliability of this treatment process. Denitrification and precipitation were shown to be the dominant pathways for removing N and P, as evidenced by mass balance and real-time polymerase chain reaction analyses.