Development of an Integrated Environmental Impact Assessment Model for Assessing Nitrogen Emissions from Wastewater Treatment Plants

Abstract

Environmental impact assessments for wastewater treatment plants (WWTPs) have evaluated many endpoints including emissions of greenhouse gases, discharges of nutrients and discharges of toxic substances. The primary objective of this study was the development of an integrated environmental impact assessment model for wastewater treatment processes. The assessment model was based on an impact assessment methodology used in Japan for life cycle assessments. Specifically, eutrophication was taken into account in the model along with the impacts of free ammonia, because this chemical was known to have toxic effects on aquatic ecosystems. The model developed was then applied to an actual WWTP operating under two different conditions (case 1, without nitrification; case 2, with nitrification), and the best operating conditions were evaluated based on nitrogen emissions. The results showed that the main contributor to the environmental impacts of the WWTP in case 1 was ecotoxicity from discharges of NH₄-N. In case 2, the main contributor was eutrophication from discharges of total nitrogen. These results demonstrated that the overall environmental impacts of WWTPs should decrease when nitrification is employed because this will reduce the impacts associated with the ecotoxicity of NH₄-N.