These are greenhouse gases that are released into the atmosphere as a direct result of the organisation’s actions.

Most of the water sector’s direct emissions come from treating sewage and wastewater. The bacteria that break down organic material and remove excess nutrients produce greenhouse gases while they work.

These gases can be released into the atmosphere at several stages during the wastewater treatment process, as pictured in the diagram below.

Transport fuels used by the water sector’s vehicles are the second largest source of direct emissions. These emissions can be reduced by switching to renewable fuels and zero emissions vehicles.

Diagram illustrating wastewater treatment process flow from collection through screening, primary, secondary, tertiary, and lagoon treatments, highlighting biogas handling, anaerobic digestion, and biosolids management. Key components use color-coded lines for liquid (blue), solid (orange), and gas (green), with methane (CH4) and nitrous oxide (N2O) emissions indicated at various stages, emphasizing treatment steps and gas capture.

This infographic is generic and aims to demonstrate where methane (CH4 in blue) and nitrous oxide (N2O in red) emissions can typically arise during wastewater treatment. It does not represent the different scale of emissions during different stages of treatment. This is because the amount of emissions at each stage is dependent on many factors and the individual setup of a wastewater treatment plant. While carbon dioxide is produced in multiple stages of the wastewater treatment process, it often does not result in a net increase in atmospheric carbon dioxide and as such has been excluded from this graphic.

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Page last updated: 07/05/26