Centralised, energy positive wastewater treatment shows high potential for carbon neutrality with emissions offset through biogas and biosolids use

Wastewater treatment is an energy- and carbon emission intensive process. Aware of (inter)national targets for climate change mitigation, the wastewater sector in many countries is eager to improve its resource efficiency and has set ambitious goals to move towards net zero. This study presents a comprehensive carbon footprint of a UK municipal wastewater treatment plant (WWTP), contributing to the database necessary to understand and reduce the sector's carbon emissions. The WWTP is a net energy-producer and functions as a regional advanced anaerobic digestion (AAD) facility, with a process set-up not previously studied through carbon footprint analysis. The work a) quantifies the carbon footprint for four functional units, b) studies the impact of external liquids and solids on the footprint, c) analyses the influence of different emission factors (EF) for direct GHG emissions on the results, and d) proposes GHG mitigation measures. The net carbon footprint was between −0.06 and 0.51 kg CO 2 e/m 3 material treated, depending on the EF used. The main drivers are the direct GHG emissions methane and nitrous oxide (25–50 %), followed by natural gas (16–25 %), land-application of biosolids (10–16 %) and electricity consumption (9–15 %). Transport burdens played a minor role despite importing large amounts of external solids from other WWTPs. Both carbon burdens and credits per m 3 treated were revealed to be significantly higher than in previous studies owing to the external material treated. Promising carbon reduction measures at this and similar WWTPs should prevent fugitive CH 4 emissions from the AAD line or recover heat from the effluent. • Net carbon footprint of energy positive WWTP between −0.06 and 0.51 kg CO 2 e/m 3 • Carbon neutrality dependent on GHG emission factors for direct CH 4 and N 2 O • Credits from biomethane and biosolids outweigh 68–106 % of GHG emission burdens. • Highest credits from replacement of natural gas and mineral fertilisers • Transport emissions marginal despite centralised anaerobic digestion strategy