Assessing Microbial Nitrogen Removal in Advanced Soil Treatment Areas to Reduce Septic Tank Pollution in Cape Cod.

Onsite wastewater treatment systems (OWTS) are used in areas where centralized, large-scale treatment is limited or not available, such as rural areas and coastal regions with low population densities. These systems consist of a septic tank which disperses partially-treated wastewater to a soil treatment area (STA). Reactive forms of nitrogen (N) can enter the groundwater from the STA of conventional OWTS, which are not designed to optimize N removal through sequential nitrification and denitrification. In the Cape Cod Watershed, groundwater recharged with wastewater from STAs flows into estuaries and coastal waters, and can be a major source of N pollution. Excess N in estuaries can cause eutrophication and threaten ecosystem services, such as carbon sequestration, suitable sites for aquaculture, and habitat for many types of organisms. Alternative STA designs have been developed for use in areas where N pollution needs to be minimized. One approach involves the replacement of conventional STAs – which are not designed to remove N – with advanced STAs. These low-profile leaching systems, developed at the Massachusetts Alternative Septic System Testing Center, are designed to increase sequential nitrification (in a sand layer) and denitrification (in a sand layer mixed with sawdust) as septic tank effluent percolates through the soil. We are investigating the size, structure, and activity of the nitrifying, denitrifying, and anammox microbial communities within these “layer cake” STAs, and comparing them to the microbial communities in conventional STAs. Assessing the species and functional diversity of microbial communities responsible for N removal and their relationship to system performance will help develop strategies that enhance N removal and minimize the impact of OWTS on coastal and estuarine ecosystems.