Updating wastewater treatment pathways for the Seventh Carbon Budget

Wastewater treatment emissions are a small proportion of total UK emissions, but this proportion increases by 2050 in our Sixth Carbon Budget scenarios, as other sectors largely decarbonise while wastewater emissions only fall by a small amount. Assumptions on emission reductions from wastewater in our Sixth Carbon Budget scenarios were based on academic papers and discussions with academics and stakeholders in the water industry. This resulted in emission reductions of 20% by 2030 in three of our scenarios, including the Balanced Pathway. This increased to a 50% reduction by 2050 in two of our scenarios (Widespread Innovation and Tailwinds). The main solutions assumed to deliver this abatement were: • Across all scenarios - conversion of wastewater treatment plants to advanced anaerobic digestion systems (increasing the amount of biogas extracted and reducing methane emissions) and process optimisation improvements and leak identification using on-site emissions monitoring of CH4 and N2O. • More ambitious scenarios - more innovative options such as membrane-aerated biofilm reactors or partial nitrification-Anammox processes. Water companies in England and Wales submitted their draft business plans to Ofwat in October 2023. These plans are going through an ongoing verification process, so are subject to change, but include plans for wastewater treatment out to 2034, with associated emissions pathways. Aggregate wastewater emissions in draft plans are relatively steady over the period in question - differing significantly from the 20% reduction assumed in our Balanced Pathway. There is significant variation between individual water companies, with changes in emissions over the period ranging from a 60% reduction to an 50% increase by 2034, relative to 2021/22 levels. **** See Specification for more details ****