Nitrous oxide emissions from wastewater treatment - Revisiting the IPCC 2019 refinement guidelines

Many entities (such as water utilities and government agencies) that operate or manage wastewater treatment plants (WWTPs) are seeking to estimate and reduce greenhouse gas (GHG) emissions. Nitrous oxide (N2O) is a major GHG emitted directly from WWTPs with nitrification/ nitrogen removal processes. These N2O emissions are fugitive, highly variable and relatively difficult to measure. In the absence of direct emissions measurements, using typical GHG accounting guidelines (e.g., those published by the Intergovernmental Panel on Climate Change, IPCC), the calculation methods for estimating emissions apply one or more emission factor(s) (EF). The EFs reference parameters in wastewater treatment that can be measured or estimated reasonably reliably (e.g., influent total nitrogen (TN) load). For GHG estimation and reporting purposes, most operating entities are reliant on default EF values for wastewater N2O listed in GHG accounting guidelines (e.g., IPCC). Such default factors have historically been derived from expert opinion, using limited literature references, and based on actual measurements from wastewater treatment plants. When compared to the older IPCC (2006) Guidelines, the latest default EF for wastewater treatment N2O in the IPCC 2019 Refinement represents an increase of approximately 32-fold from 0.05% to 1.6% of influent TN load. This increase was derived from a literature review, as described in Volume 5 (Chapter 6, Annex 6A) of the IPCC (2019) Refinement to the 2006 Guidelines. In this study, the literature data cited by IPCC (2019), were checked and the same linear regression approach used to reproduce the derivation of an ‘average’ (default) EF for N2O from full-scale WWTPs. Several apparent literature citation errors were identified in the relevant annex that appear to have resulted in an over-estimation of the ‘average’ (default) EF in the IPCC (2019) Guidelines. After correction of these errors to align with the source literature data, the outcome of this study is a suggested revised default EF of 1.1% (±0.16%, α = 0.05) of WWTP influent TN load emitted as N2O. This revised default value is supported by inclusion of datapoints from newer literature references (2018-2021), in addition to those cited by IPCC (2019). It also aligns more closely with the weighted average N2O emission factors published by the Danish Environmental Protection Agency in 2020.