Analysis of nitrogen processes and dynamics in a sub-alpine climate constructed wetland

Abstract

Wastewater (WW) treatment is a critical process that protects public health and natural waters. Approximately 20% of United States (US) citizens rely on on-site systems for wastewater treatment; however, septic systems are often unmonitored, resulting in inadequate treatment, and the release of excess nutrient pollution. Treatment Wetlands (TWs) present a less costly and more environmentally friendly approach for centralized-rural WW treatment. Skeptics of TW technology often cite limitations for robust operation in cold climates. Therefore, research must investigate TWs in extreme climates at the field scale for the most accurate performance assessment. TWs and WW treatment plants alike rely on microbial-mediated processes for nutrient removal, which can release greenhouse gases (GHGs), including nitrous oxide (N2O), methane (CH4) and carbon dioxide. Current climate change efforts are moving toward the assessment, inventorying, and mitigation of GHGs. Enhanced nutrient removal and mitigation of GHGs can be targeted through the monitoring and analysis of emissions, as well as proper characterization of the spatial-temporal microbial dynamics that exist in TWs. This research used field-scale TW monitoring to investigate the effects of operational changes on WW treatment efficacy, GHG emission profiles, and microbial community dynamics at a TW treating concentrated ski resort WW. Typically, microorganisms exhibit reduced metabolic activity at low temperatures, resulting in decreased WW nutrient degradation. This research examined a cold-climate Montana TW over three ski seasons and observed efficient removal of influent nutrients and comparable total nitrogen removal to mechanical WW treatment plants. Emissions of CH4 and N2O were found to be heavily influenced by intermittent loading of WW and mass transport effects. In general, microbial communities showed specialization due to nutrient loading over the course of the operational season but maintained similar microbial diversity and abundances during the annual 8-month rest period. Communities additionally formed distinct niches due to redox conditions in the TW, indicating that oxygen levels had a greater influence on community structure and composition than nutrient availability. Altogether, this dissertation demonstrates that TWs are a resilient and robust technology for efficient removal of targeted WW nutrients (i.e. carbon and total nitrogen) in cold climates due to persistent microbial communities.