Dissolved inorganic nitrogen cycling in a river receiving wastewater: the response to changes in wastewater treatment effluent

摘要

Nitrogen is an essential element for all life forms, but when in excess in an aquatic ecosystem, it can cause an imbalance in the trophic status. A significant amount of nitrogen is released into the environment by wastewater treatment plants, representing a major point source of reactive nitrogen in urban environments. This nitrogen source presents a potential threat to the ecological integrity of the aquatic ecosystem if poorly managed and monitored. Wastewater discharges can increase the primary production in surface waters, frequently degrading the integrity of the receiving aquatic ecosystem through the addition of high organic matter loads and the associated oxygen consumption during oxidation of the organic matter. The Grand River (south western Ontario) is a river highly impacted by human activities; the watershed is home to approximately 925,000 inhabitants, has approximately 34 water control structures and receives discharge from agricultural fields and 30 wastewater treatment plants. The Central Grand River is particularly influenced by wastewater discharges from five large wastewater treatment plants (WWTPs) in the Region of Waterloo. Due to the impacts of wastewater effluent on the ecological integrity of the Grand River, the Region of Waterloo embarked on a series of upgrades to its two largest WWTPs: the Waterloo and Kitchener plants. This research presents a before-and-after approach used to study and understand the effects of the changes in the operation of the Kitchener wastewater treatment plant. The research documents changes in dissolved inorganic nitrogen dynamics downstream of the Kitchener wastewater treatment plant. Ammonium and nitrate concentrations and nitrogen isotopic composition (δ15N) was monitored in the Central Grand River, with a special focus on the 5700 m reach downstream of the Kitchener wastewater treatment plant effluent over a period of four years (2010 to 2013). The over-riding change in the quality of the wastewater effluent was a decrease in ammonium concentration, resulting in a reduction in the period of oxygen depletion during summer, low flow conditions. After the upgrades, most of the ammonium was oxidized by submerged aeration inside the wastewater treatment plant. However, the concentration of nitrate in the effluent increased as a result of the upgrade to a nitrifying system. The observed rate of ammonium decrease before in the Central Grand River (adjusted by travel time with a flow velocity of 0.3 m/s) varied between 0.7 and 2.47 mgN-NH4+h-1before the upgrades.Together with the changes in concentrations, the observed differences in the isotopic composition of ammonium (δ15NNH4+) and nitrate (δ15NNO3) suggest that ammonia volatilization, assimilation and nitrification occurred in the Central Grand River downstream of the Kitchener wastewater treatment plant. Before upgrades, ammonium concentrations in the effluent discharged to the Grand River were higher than 20 mgN-NH4+/L and the δ15NNH4+ varied between +4 and +10‰. The nitrate concentration in the effluent was frequently between 2 and 4 mgN-NO3-/L and the δ15NNO3- from -6 to +1‰. In the 5700 m reach of the river downstream of the Kitchener wastewater treatment plant, the ammonium concentration decreased to between 2 to 0.5 mgN-NH4+/L and the δ15NNH4+ increased from +5‰ to +30‰. After upgrades (2013), the ammonium concentration in the effluent was low (≤ 6 mgN-NH4+/L) due to more efficient ammonium oxidation (and possibly volatilization and assimilation) and the δ15NNH4+ was ≈23‰, increasing to +30‰. After upgrades, the nitrate concentration in the effluent was 22 (±5) mgN-NO3-/L, and the δ15NNO3- downstream of the Kitchener wastewater treatment plant was between +11 to +14‰. After the upgrades, the nitrate concentration downstream of the outfall from the Kitchener wastewater treatment plant varied likely influenced by intra-annual variations (seasonal variation in temperature) and inter-annual variations (variable river discharge). Downstream of the Kitchener wastewater treatment plant effluent, the dissolved inorganic nitrogen did not return to previously observed background level. The observed differences in the dissolved inorganic nitrogen concentrations among seasons and years were not only attributed to changes in the quality of the WTP’s effluent, but also a result of upstream nitrate inputs from agricultural sources. Ammonium assimilation by epilithon was measured in experimental conditions by blocking bacterial oxidation with a chemical inhibitor (acetylene). Ammonium assimilation was observed at velocities above 1 μm N-NH4+ h-1, with a calculated ammonium assimilation rates from 377 to 519 um N m-2 h-1. Nitrate assimilation rates were calculated to be 58 to 65 um N m-2 h-1. Thus, epilithon assimilation contributed from 26% to 100% of the ammonium loss in each experimental unit. Ammonia volatilization in the Grand River downstream of the effluent before upgrades was estimated to range between 0.61 and 0.13 ugN-NH3/L per metre, or 0.18 to 0.04 ugN-NH3/L per second; representing a decrease of approximately 50% of the ammonia discharged from the Kitchener wastewater treatment plant. This is the first time that ammonia volatilization is estimated for a river receiving wastewater treatment plant effluent. Additionally, the ammonia isotopic fractionation factor due to volatilization (αvolatilization) was calculated experimentally as 1.019 (±0.0024) at pH 8.5, and the kinetic and equilibrium isotopic fractionation factors were calculated as αequlibrium=1.036 (±0.0024) and αkinetic=1.050 (±0.0024).A box model that uses ammonium and nitrate concentrations and isotopes of both ammonium and nitrate for estimating the rates of these processes in rivers shows that, before upgrades, the rate constant for gas exchange and ammonium assimilation were similar, however, the change in concentration was larger for ammonium assimilation. After upgrades, the rate constant for nitrification was one order of magnitude higher than before upgrades. The rates estimated by the model for each process , likely changed after upgrades due to the reduced mass of ammonium available for volatilization, oxidation and assimilation. The box model provided constant rates simultaneously estimated for the three processes; thus, the differences between the observed data in this research and the box model are assumed to be the result of: i) overestimated volatilization, ii) the experimentally measured assimilation on epilithon only and iii) the propagation of the error. Due to the separation between the ammonium (δ15NNH4+) and the macrophytes isotopic composition (δ15NTN), it is proposed that some macrophytes and possibly periphyton can be used as an environmental archive that allows one to observe the effects of the wastewater treatment plant effluent discharged into the Grand River. This archive can be used as tool to complement water quality monitoring for assessing changes in water chemistry of rivers and streams receiving wastewater treatment plant effluents. The most important contribution of this thesis is that it provides a well-documented before-and-after case study of the effects of WWTPs upgrades on the dissolved inorganic nitrogen cycling in an anthropogenically-impacted river. As such, the research provides valuable information that allows regulatory agencies and water managers (i.e., the Region of Waterloo, the Grand River Conservation Authority) to evaluate the impacts and effectiveness of potential upgrades at wastewater treatment plants in order to understand the changes in nitrogen concentrations and loads in receiving waters. This case study can be useful where regional municipalities or regulatory agencies plan to upgrade WTP’s in areas with similar geographic and climatic conditions as the observed at in the Central Grand River; however, sampling and monitoring protocols must be designed on a site by site basis taking into consideration baseline conditions and the actual objectives of the final users.