Challenges in carbon footprint evaluations of state-of-the-art municipal wastewater resource recovery facilities

Wastewater treatment is an important source of direct and indirect greenhouse gas (GHG) emissions, which some wastewater operators report and account for CO₂-eq impacts through carbon footprint evaluations. We investigated the challenges with GHG emissions' accounting of three state-of-the-art energy-efficient wastewater resource recovery facilities (WRRFs) and reviewed their CO₂ accounting reports. Our study aimed to highlight the major contributors and factors to estimate emissions, including direct N₂O and CH₄ emissions and propose recommendations for public reporting of CO₂ accounting of WRRFs. We categorised emissions as direct (scope 1), background (scope 2), downstream and avoided emissions (scope 3A and 3B) and evaluated how a change in emission factor may affect how close the WRRFs are to reaching CO₂ neutrality. The results show that electricity consumption and direct emissions constitute between 20 and 70% of actual CO₂-eq emissions and therefore need careful consideration. All three plants have increasingly offset scope 2 emissions over 2014-2019, resulting in a total reduction of approximately 3211 tons CO₂-eq, corresponding to 72% of their needed cuts by 2030 set by the Danish government. No standard factors are used across the plants to estimate emissions. We propose some general recommendations that wastewater operators can apply to correctly report and account for CO₂-eq emissions. We also recommend that operators move their long-term focus from CO₂ neutrality to CO₂-eq reduction and make an effort to measure and quantify scope 1 direct emissions properly. A tax on N₂O emissions should be introduced in future policies.