The Carbon Footprint of Wastewater Treatment Plants

Wastewater treatment plants serve vital public health and environmental purposes, but they are also notable sources of greenhouse gas emissions. These facilities consume significant energy and release potent gases such as methane and nitrous oxide during treatment processes.

Reducing the carbon footprint of wastewater treatment is, therefore, a crucial component of climate action, complementing the sector’s primary goal of safeguarding water quality. Lowering WWTP emissions contributes to climate mitigation and often delivers energy and cost savings for utilities, making the effort a win-win for sustainability and operational efficiency.

Wastewater Treatment Emissions in a Nutshell

Although wastewater treatment contributes a relatively small share to global GHG emissions, estimated at around 1–5%, the volume is far from negligible. Recent studies estimate that sewered wastewater and sludge management emit approximately 257 million tonnes of CO₂-equivalent per year. When emissions from unsewered sanitation systems, such as pit latrines and septic tanks, are included, the total more than doubles.

But what exactly comprises the total impact or carbon footprint of the industry?

A carbon footprint is the total amount of greenhouse gases (GHGs) emitted by an activity or entity, expressed as carbon dioxide equivalent (CO₂e). In the context of a wastewater treatment plant (WWTP), it encompasses the following:

Wastewater Industry’s Carbon Footprint (CO₂e) =

+ Direct process emissions: methane CH₄, N₂O from biological treatment

+ Indirect emissions from electricity and heat use

+ Emissions from chemical production and transport, sludge handling and disposal, maintenance and construction

– Avoided emissions from energy recovery, nutrient recycling, or biogas utilisation

These various emissions are produced through multiple pathways:

      Biological Treatment Processes

Though essential for breaking down organic matter and removing nutrients, these processes can generate greenhouse gases like nitrous oxide (N₂O) and methane (CH₄), which contribute significantly to a plant's carbon footprint.

N₂O is a byproduct of the microbial conversion of nitrogen during nitrification (ammonia oxidation) and denitrification (nitrate reduction under low oxygen conditions). Though emitted in small volumes, N₂O has a global warming potential 265-300 times greater than CO₂ on a 100-year basis, making even small quantities impactful. Nitrous oxide can account for a large share of the GHG emissions in WWTPs, particularly in facilities designed for high nitrogen removal.

Anaerobic Processes and Fugitive Emissions

Anaerobic processes, which occur in the absence of oxygen, can also produce methane. Many modern WWTPs use anaerobic digesters to treat sludge and capture methane for energy production. However, if these digesters or reactors are poorly sealed or operated, methane can escape into the atmosphere.

These unintentional leaks of methane and N₂O, termed fugitive emissions, can also come from biogas tanks, stripping in aeration tanks, and other areas in the facility.

Methane has a lower long-term warming impact than N₂O, but it is still approximately 28 times more potent than CO₂ over 100 years, and over 80 times more potent over a 20-year period.

Methane emissions from poorly managed or non-sewered wastewater systems pose a serious environmental concern, especially when wastewater is treated in open anaerobic lagoons or left in sewers. These emissions contribute up to 4.7% of global human-caused methane emissions.

Energy Consumption (Aeration and Pumping)

Aeration systems supply oxygen to support aerobic bacteria, and pumps move large volumes of water. These processes are energy-intensive, accounting for the largest source of emissions in many plants, especially in regions reliant on carbon-intensive power grids.

Globally, the water sector accounts for about 4% of total electricity consumption, with wastewater treatment alone making up around 1%. The high energy demand not only results in CO₂ emissions off-site, but also strains power infrastructure and utility budgets. Aeration systems specifically consume 50-60% of a WWTP's total energy.

Sludge Management

Wastewater treatment processes generate sludge, or biosolids, that must be properly handled. If sludge is stabilised by anaerobic digestion, much of its organic carbon is converted to biogas (CH₄ + CO₂), which ideally is captured for energy use. However, the remaining biosolids or solid material requires disposal or reuse. Transporting these large volumes towards their final endpoint typically causes considerable carbon emissions and is also a significant cost driver for wastewater utilities.

The common endpoints for biosolids include land application, incineration, or landfilling, each with its own emissions profile:

In sum, sludge management is a major part of a WWTP’s life-cycle GHG footprint. A China study found that the emissions from sludge processing (including final disposal) could account for roughly 40% of a WWTP’s total GHG emissions.

Technologies that valorise sludge, such as producing energy or useful products from it, can turn a liability into an asset, as will be discussed in the best practices section. Minimising final sludge volume and avoiding landfilling not only cuts emissions but aligns with a circular economy approach.

Regulatory Pressures and Emissions Reporting

Increasing awareness of wastewater’s climate impact has led to growing regulatory and voluntary pressures on utilities to measure and reduce their carbon footprints. Many countries now include wastewater emissions in their national GHG inventories. The Intergovernmental Panel on Climate Change (IPCC) provides guidelines for estimating these emissions, and utilities are gradually moving towards more accurate, site-specific reporting.

At the corporate and utility level, Scope 1, 2, and 3 emissions reporting has become the standard. Within the industry, these cover the following:

Scope 1: Direct process and fugitive emissions

Scope 2: Indirect emissions from purchased energy

Scope 3: Indirect emissions across the value chain (e.g. chemicals, transport). While Scope 3 accounting is still developing in the wastewater sector, leading utilities are working to assess these emissions for a full picture of their environmental impact.

Regulatory pressures are intensifying, especially in the EU, where the newly revised Urban Wastewater Treatment Directive is indirectly driving decarbonisation by requiring energy neutrality for WWTPs by 2045. In the UK, water companies have committed to net-zero emissions by 2030, and regulators are holding companies accountable for their progress. Many other countries, including the US, Canada, and Australia, have set GHG targets for their water utilities, with some large utilities working towards carbon neutrality.

International organisations like the International Water Association (IWA) and the Water Environment Federation (WEF) are providing tools to guide GHG accounting. Carbon pricing and trading schemes, while still emerging, offer financial incentives for emissions reductions, with some wastewater projects earning carbon credits for methane destruction. These developments indicate that GHG emissions reporting and reduction are becoming standard components of wastewater utility management.

Best Practices for Reducing Carbon Emissions in WWTPs

Reducing the carbon footprint of WWTPs requires both technological upgrades and operational changes, as well as supportive policies and financing. Below are some of the best practices and strategies:

Renewable Energy Integration

Even with maximal efficiency, WWTPs will always require substantial energy. Integrating renewable energy sources into wastewater facilities is a best practice that can dramatically reduce net emissions. Biogas from anaerobic digestion is a major opportunity, while modern CHP (combined heat and power) systems can efficiently convert this biogas into electricity and heat, reducing reliance on grid power.

Other renewable energy initiatives gaining traction in the industry include:

• Solar photovoltaics that can be installed on rooftops, land, or reservoirs. Some utilities are also exploring wind power and small-scale hydro.
• Heat recovery from treated wastewater via heat exchangers and heat pumps, contributing to space heating or digestion.
• In hot climates, cool effluent can be used for space cooling.
• Algae grown on wastewater can be harvested for biofuels, or microbial fuel cells that directly generate electricity from wastewater are being researched

Advanced Process Optimisation and Innovation

Rethinking treatment processes can significantly reduce emissions. Traditional nitrogen removal methods, such as nitrification-denitrification, for example, are both energy-intensive and major sources of N₂O emissions. In contrast, shortcut processes like anammox and its variants bypass large parts of the conventional cycle, offering substantial energy savings for utilities.

Advanced technologies such as membrane bioreactors (MBRs) and aerobic granular sludge systems enable compact treatment with high effluent quality. Although MBRs typically require more energy, they support water reuse, helping to offset emissions associated with alternative water sources. Some facilities are also piloting anaerobic treatment for mainstream sewage, which generates biogas and reduces the need for aeration.

Chemical usage presents another opportunity for optimisation. Minimising excessive chemical dosing and prioritising biological nutrient removal can lower Scope 3 emissions. Altogether, modern treatment technologies provide a pathway to low-carbon performance without sacrificing quality.

Sludge Valorisation and Circular Economy

Sludge has long been viewed as a waste byproduct, but in a circular economy approach, it is a valuable resource stream. Anaerobic digestion for biogas production reduces emissions and can be boosted with complementary technologies.

Some facilities have significantly reduced final sludge volumes while producing high-value biosolids and achieving energy neutrality. This has been accomplished in part through centralised sludge treatment and the use of advanced anaerobic digestion technologies, including thermal hydrolysis, which enhances biogas production and digestion efficiency.

This diverts organics from landfill and enhances biogas output, potentially turning WWTPs into net energy producers.

Nutrient recovery is another path. Phosphorus can be recovered as struvite, a marketable fertiliser. Some systems also recover nitrogen as ammonium sulphate. These efforts reduce the need for industrial fertiliser production and its associated emissions.

Biosolids can be land-applied or converted to biochar via pyrolysis, which locks carbon and yields energy-rich byproducts. Future opportunities include bioplastics and algae-based fuels, aligning with the vision of wastewater plants as resource recovery facilities.

Apart from the technologies and initiatives listed above, utilities can also greatly benefit from upgrading to energy-efficient equipment, accurate monitoring using emissions sensors, as well as automation. Low-cost actions, such as fine-tuning control systems, cleaning blower filters, and implementing AI for load forecasting, further enhance efficiency. Energy audits, staff training, and supportive policies with clear incentives or performance targets have been shown to be effective methods for emission reduction as well.

A Climate Opportunity Not to Waste

Wastewater treatment, though often overlooked in discussions on climate change, presents both an urgent challenge and a significant opportunity for decarbonisation. The sector is responsible for hundreds of millions of tonnes of CO₂e emissions annually, and as countries expand sanitation services (building new plants or upgrading systems), energy use will rise unless new plants are designed with efficiency and renewable energy in mind.

The technologies to transition WWTPs into low-carbon, energy-positive systems already exist. Through biogas capture, advanced treatment processes, nutrient recycling, and renewable integration, utilities can reduce emissions while improving service resilience and cost-effectiveness.

By embedding emissions considerations into planning, operations, and policy, the sector can evolve into a cornerstone of climate-resilient infrastructure, benefitting people, the planet, and the bottom line.

Want to better understand the carbon impact of different sludge treatment processes and make climate-smart decisions in sludge management? Read this paper by Cambi technical expert Bill Barber, which analyses the carbon footprint of key biosolids strategies—including liming, composting, digestion, pasteurisation, drying, incineration, and their various configurations.