Revisiting greenhouse gas accounting protocols: a case study on wastewater treatment in Canada

Greenhouse gas (GHG) accounting protocols are increasingly used by governments, cities and organizations to quantify their GHG emissions impacts. However, methodological differences between protocols can lead to significant variations in reported emissions. This study presents a comparative analysis of assumptions, computational approaches and emission factors for six widely recognized protocols: (1) the 2019 Refinement to the 2006 Intergovernmental Panel on Climate Change (IPCC) Guidelines, (2) the International Council for Local Environmental Initiatives Community Protocol, (3) Canada’s GHG Reporting Program, (4) Canada’s National Inventory Report methodology, (5) the GHG Protocol Corporate and Corporate Value Chain (Scope 3) Standards, and (6) the GHG Protocol for Cities. A revised lifecycle GHG accounting approach maximizing the usage of plant data and accounting for scope 2 and select sources of scope 3 emissions is presented. The GHG inventories for four wastewater treatment plants in Canada for the year 2019 are presented using the protocols 1–4 and the revised lifecycle approach. The results highlight significant divergences in GHG inventories due to methodological differences across the cited accounting protocols, particularly in the accounting of methane and nitrous oxide process emissions. While no single protocol consistently reported the lowest emissions, the 2019 IPCC Guidelines consistently produced the highest estimates among the four protocols considered. Further, the revised GHG inventory results show that scope 3 emissions, which are typically excluded for regulatory reporting to avoid double-counting, averaged at 15% across the four WWTPs. Harmonizing accounting methodologies with the latest scientific literature and national datasets, while incorporating scope 3 emissions, leads to more consistent and accurate GHG inventories. This alignment is essential for formulating effective strategies to mitigate GHG emissions.