Closing the wastewater emissions gap: From hidden source to measurable action

A recent global analysis of 38 countries found that wastewater emissions are underestimated by approximately 19–27%, with a gap of up to 52–73 million metric tons of carbon dioxide (CO₂) equivalent annually due to omitted wastewater treatment/conveyance pathways according to the nature climate change journal. These discrepancies highlight a critical challenge for the sector: without an up-to-date greenhouse gas (GHG) inventory, it is difficult to plan, prioritize and deliver meaningful emissions reductions. 

Wastewater emissions are often overlooked because they are embedded within broader categories in national GHG inventories. In Canada, for example, wastewater is grouped under broader waste reporting categories, limiting visibility of its true contribution. 

These emissions originate from biological processes. The micro-organisms and bacteria that we use within wastewater facilities break down organic matter and solid waste, produce methane and nitrous oxide, both potent greenhouse gases with significant climate and environmental impacts. 

However, accurately quantifying these emissions remains complex, driven by both variability and system scope. On one hand, CH₄ and N₂O emissions, which are in themselves difficult to measure from wastewater processes, can vary significantly from one wastewater treatment plant to another, even in stable centralized plants, due to differences in operating conditions, environmental factors and process configurations. On the other hand, the complexity of wastewater systems from pumping stations to biosolids management means that key emission sources are often difficult to capture and are frequently omitted entirely. 

The research highlights that gaps in emissions reporting are driven by two core issues: 

• Lack of standardized guidance for inventory development and system boundary definition  

• Inconsistent methodologies and emission factors, which vary widely across countries and systems 

Together, these factors undermine the comparability and credibility of wastewater GHG inventories, making it difficult for utilities and governments to establish accurate baselines or benchmark performance. 

Addressing this challenge requires a shift toward standardized, credible, transparent and consistently repeatable approaches to emissions quantification and measurement estimation, particularly at the utility level, where decisions about infrastructure, operations and investments are made. 

While there have been a few practical tools made, the wastewater sector has lacked the research depth along with the means to reliably quantify greenhouse gas emissions. To bridge this gap, in collaboration with a consortium of the US Water Alliance, Northern Tilth, Cobalt Water Global and Stantec, with contributions from organizations including Jacobs, CDM Smith, Brown and Caldwell, Eddyline Strategies and Princeton University, GHD worked to develop the Water Sector Climate Emissions Reporter (WSCER) excel tool; the first tool created by the water sector for the water sector. 

Developed under Water Research Foundation Project 5188, WSCER (pronounced whisker) calculates an estimate of the emissions produced by wastewater and translates emerging best practice guidance into a practical, utility-facing Excel tool that enables: 

• Consistent, structured GHG inventories 

• Clear boundary setting across Scope 1, Scope 2 and key Scope 3 emissions 

• Standardized calculation and reporting workflows 

• Repeatable, credible, year-over-year emissions tracking and comparison

The tool was tested and validated by GHD’s Kevin Joseph using data from Halton Region (Ontario, Canada). Kevin successfully calculated emissions for the Mid-Halton and Skyway Wastewater Treatment Plants using the WSCER tool as well as identified tool refinements, demonstrating its practical value in a utility context. 

WSCER simplifies a complex process into a structured workflow. Users input key operational and system data, such as treatment processes, flows and energy use into predefined fields within the Excel tool. The calculations within the tool utilize standardized methodologies and emission factors to estimate greenhouse gas emissions, generating consistent, comparable results that can be used for reporting, planning and decision-making. 

At its core, WSCER addresses one of the sector’s biggest barriers: WSCER provides a standardized, peer review and peer created platform for estimating emissions data. This is the first step towards translating complex emissions information into decision-ready insights to promote mitigation.  

By enabling utilities to input operational data and generate consistent emissions estimates, the tool helps: 

• Reveal previously hidden emission sources, particularly process emissions from wastewater treatment 

• Improve transparency and comparability across utilities and regions 

• Support climate planning and sustainability goals target setting 

• Identify high-impact mitigation opportunities 

• Track progress over time with greater confidence 

Importantly, WSCER is supported by a comprehensive guidance report which includes the Water Sector Climate Emissions Quantification Guidelines, providing a foundation for developing life cycle GHG inventories across water and wastewater systems. This report acts as a book of guidelines for establishing industry wide guidance for water utility life cycle GHG emission and inventories. 

The development of WSCER reflects a broader industry shift. As the importance of non-CO₂ emissions grows, wastewater is no longer treated as a secondary or “other” category. 

Better data is enabling: 

• More accurate climate baselines 

• Stronger business cases for process optimization 

• Alignment between environmental goals and operational performance 

This is especially critical for regions and utilities that are earlier in their climate journey. Standardized tools like WSCER provide a pathway to build credible inventories, enabling broader participation in emissions reduction efforts. 

The key takeaway is clear: improving how wastewater emissions are measured is fundamental to reducing them. 

By combining: 

• Global research insights 

• Industry collaboration 

• Practical, utility-focused tools 

The sector is moving toward a more complete and actionable understanding of its climate impact. WSCER represents a significant step forward, enabling any utility to develop an up-to-date inventory across the entire water boundary using modern approaches.