DOWNSTREAM EFFECTS ON DENITRIFICATION AND NITROUS OXIDE FROM AN ADVANCED WASTEWATER TREATMENT PLANT UPGRADE

Abstract

All humans excrete waste. In developed countries, this waste is often treated at a wastewater treatment plant (WWTP). Eventually, the nutrient-rich, treated wastewater—effluent—enters a water body to be diluted or naturally processed. However, in the case of the Regina (Canada) WWTP, this dilution does not immediately occur as the effluent is released into the small, effluent-dominated system of Wascana Creek. This study capitalized on a novel opportunity to determine the effects of WWTP upgrade on: in-stream water quality, nitrogen (N) cycling measured as denitrification rates, and nitrous oxide (N2O) concentrations and emissions. Using a before-after-control-impact (BACI) design, nutrient, sediment, and gas samples were obtained before and after the upgrade at both upstream (control) and downstream (impact) sites on both Wascana Creek and the larger, downstream Qu’Appelle River. Although nitrate (NO3–) concentrations did not significantly change post-upgrade, I found that the upgrade significantly reduced concentrations of ammonium (NH4+) and toxic un-ionized ammonia (NH3), which declined by ~35 times from pre-upgrade values, ultimately mitigating potential toxicity (Environment Canada 1999). The WWTP significantly impacted denitrification rates at downstream sites. Denitrification rates at sites downstream of the WWTP were >1200 times the rates at the upstream site. While impacts were lesser, denitrification rates at the larger Qu’Appelle River downstream site were still >20 times the rates at the upstream site. Denitrification rates were unaffected by the upgrade. Moreover, NO3– saturation, a negative indicator of ecosystem health, existed both before and after the upgrade at impacted sites. To the best of my knowledge, aquatic N2O concentrations immediately downstream of the WWTP are the highest known values for a natural system. Concentrations reached as high as 114,000 percent saturation pre-upgrade and 110,000 percent saturation post-upgrade; no significant change was observed pre- vs. post-upgrade across all impacted sites. It was determined that not only did N2O concentrations from the WWTP effluent span an impact zone of ~5 km in Wascana Creek, but also that the origin of these extremely high concentrations came directly from the effluent. Predictors of both denitrification rates and N2O concentrations were identified. Nitrate was a predictor of denitrification, while NO3– and denitrification rates were significant predictors of N2O concentrations outside the effluent impact zone. This study showed that enhanced effluent N removal can help mitigate risk of NH3 toxicity; however, decreases in NH3 and NH4+ concentrations did not significantly impact downstream N2O emissions or denitrification rates.