Assessment of nitrous oxide production in eutrophicated rivers with inflow of treated wastewater based on investigation and statistical analysis

Accurate estimation and control of greenhouse gas emissions have been recognized as imperative in recent years. Therefore, frequent investigations under uniform environmental conditions are required to better understand this concept. Thus, six sampling sites with characteristic concentrations of nitrogen and other water quality parameters were selected to investigate the behavior of water quality parameters throughout the year and to statistically examine the correlations among the parameters. Dissolved nitrous oxide (D-N₂O) showed the highest positive correlation coefficient with NO₂-N among nitrogen species. The results of the principal component analysis suggested that river water quality could be broadly classified based on photosynthesis and contamination from treated wastewater. Photosynthesis caused an increase in pH, with concomitant decrease in D-N₂O concentration. Using the results of multiple regression analysis, D-N₂O was accurately estimated based on nitrogen concentration, pH, and concentration of organic matter in various situations. The results of a detailed survey suggested that D-N₂O was produced in the river from nitrogen sources released from the wastewater treatment plant. The main roles of the wastewater treatment plant for D-N₂O behavior in the river were the supply of the nitrogen source that was the precursor of D-N₂O, the supply of the nutrients that induced the photosynthesis, and the direct supply of D-N₂O at a low water temperature. The models based on multiple regression analysis could efficiently predict the D-N₂O concentration produced in rivers at sites downstream of the wastewater treatment plant, except for the direct supply of D-N₂O as an effluent at low water temperature.