Managing Sustainable Nitrogen Removal from Wastewater Carbon Sources for Denitrification

Abstract

External carbon sources are essential for effective nitrogen removal in biological post denitrification at wastewater treatment plants (WWTPs). However, the commonly used fossil-based methanol contributes significantly to the climate impact of WWTPs. This study examined both technical and sustainability aspects of transitioning to more sustainable carbon sources, focusing on methanol and ethanol. Denitrification performance during carbon source transitions was evaluated in laboratory-scale moving bed biofilm reactors (MBBRs), designed to replicate full-scale conditions. Methanol, ethanol, and a methanol/ethanol mixture were tested in a system acclimated to methanol. Results showed comparable performance between the mixture and methanol alone. No major differences were observed when switching from methanol to ethanol. Switching back to methanol resulted in a temporary decrease in performance, suggesting a short acclimation period may be needed. In parallel, a sustainability assessment using a multi-criteria analysis framework was conducted to develop a basis for future evaluation of fossil-free methanol and ethanol alternatives. Relevant options and criteria across ecological, social, and economic dimensions were defined, and both qualitative and quantitative data were collected. Based on the findings, three of the 11 criteria are considered ready for direct application in future assessments. One criterion was excluded due to a confirmed lack of impact, while the remaining seven require additional data or further investigation. All alternatives are recommended for continued evaluation, except recycled ethanol, which requires further testing due to uncertainties related to impurities.