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 CO-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 CO accounting reports. Our study aimed to highlight the major contributors and factors to estimate emissions, including direct NO and CH emissions and propose recommendations for public reporting of CO 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 CO neutrality. The results show that electricity consumption and direct emissions constitute between 20 and 70% of actual CO-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 CO-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 CO-eq emissions. We also recommend that operators move their long-term focus from CO neutrality to CO-eq reduction and make an effort to measure and quantify scope 1 direct emissions properly. A tax on NO emissions should be introduced in future policies.