An introduction to GHG emission mitigation in wastewater operations

Calyx Global now rates the GHG integrity and SDG contributions of carbon credits generated by wastewater treatment projects. These projects have the potential to reduce greenhouse gas emissions while improving access to clean water, generating clean energy, advancing sustainable industrial practices and mitigating impacts on ecosystems.

What is wastewater treatment?

Wastewater treatment plants (WWTPs) are an essential component of public health and safety, as well as environmental protection. These facilities are used in many different ways, from the safe treatment of municipal waste to the management of by-products generated by industrial processes with high organic carbon content, such as starch plants or alcohol distilleries. Emissions from these facilities are a major source of greenhouse gas (GHG) emissions. In fact, 3-5% of total global anthropogenic GHG emissions are attributed to wastewater treatment plants.[1] Furthermore, WWTPs account for almost 25% of emissions from the waste sector in the United States, making it the second largest source of waste-related emissions in 2021.[2]

The emission landscape

Wastewater treatment processes can emit various greenhouse gases, notably methane (CH₄) and nitrous oxide (N₂O). Here’s a breakdown of how these gases are released:

1. Methane (CH₄) is produced during anaerobic digestion, a process that occurs in certain treatment methods where organisms break down organic matter in the absence of oxygen. Methane is about 25 times more effective than CO₂ at trapping heat in the atmosphere over a 100-year period, making it a significant contributor to climate change.
2. Nitrous Oxide (N₂O) is emitted during the nitrification and denitrification processes. These processes are essential for removing nitrogen from wastewater but can lead to significant N₂O emissions if not managed properly. A tonne of nitrous oxide is approximately 298 times more potent than a tonne of CO₂ in terms of its impact on global warming.

 Image source: Defense Visual Information Distribution Service 

Wastewater treatment typically consists of one or more anaerobic open lagoons, as seen in the above image. Wastewater is stored in the lagoon(s), and over time, the contents separate out, with the heavy sludge settling to the bottom and the oils floating to the top. The top layer acts as a seal, preventing oxygen from entering the system, which produces anaerobic (without oxygen) conditions. Anaerobic bacteria are then able to start breaking down the organic matter. This process makes the wastewater less harmful to the environment by reducing the amount of solid waste and pathogens in the wastewater, but in turn, generates GHGs, such as methane. 

GHG mitigation strategies

To reduce greenhouse gas emissions from WWTPs, several strategies can be implemented:

1. Upgrade Technologies: Investing in advanced treatment technologies, such as membrane bioreactors or enhanced biological nutrient removal systems, can enhance efficiency and reduce emissions.
2. Optimizing Operations: Regular monitoring and optimization of treatment processes can help minimize emissions. 
3. Energy Recovery: Utilizing biogas produced during anaerobic digestion for energy can lower fossil fuel consumption, reducing CO₂ emissions associated with energy use.
4. Nutrient Management: Implementing better nutrient management practices can minimize the nitrogen load entering treatment plants, subsequently reducing nitrous oxide emissions.

What technology do wastewater treatment projects use?

There are a variety of different systems that projects can install to prevent emissions during the treatment of wastewater. One of the most common is an Up-flow Anaerobic Sludge Blanket (UASB), shown in the image below. 

Image Source: Eawag: Swiss Federal Institute of Aquatic Science and Technology Technical drawings: designport, Paolo Monaco, Zurich

A UASB reactor is an anaerobic digester that captures the gas generated by bacteria. Wastewater is pumped into the bottom of the system and rises up through a layer of sludge. During this process, the bacteria aggregate into granules, which settle out. These microorganisms break down the organic matter in the wastewater, producing small bubbles of gas that rise up through the sludge blanket. Over time this gas collects at the top of the system, where it can be removed and used to generate energy in the form of electricity and/or heat.

Other types of digesters include the Anaerobic Fixed Film Reactor (AFFR), the Anaerobic Fluidized Bed (AFB) reactor and the Completely Stirred Anaerobic Digester (CSTR). While they all operate in slightly different ways, the outcome is always roughly the same (anaerobic digestion generating biogas, which is captured and can be used as fuel).

How do wastewater projects claim emission reductions?

Similarly to manure management projects, wastewater treatment projects have two pathways to reducing emissions. The main pathway is via avoided methane emissions. Most countries have regulations in place that require at least basic treatment practices. As such, the baseline scenario (also known as business as usual) typically consists of wastewater treated in open anaerobic lagoons, where the gas produced by the bacteria is allowed to escape directly into the atmosphere. However, in the project scenario, the project developer installs an anaerobic digester (such as the UASB system discussed above) that captures the gas, avoiding the emission of methane and nitrous oxide into the atmosphere. 

The second pathway for emission reductions is the utilization of the captured methane. These projects will often install generator sets and combust the gas collected from the anaerobic digester to generate electricity or heat. The energy generated can then either be sold back to the grid or used on-site by the project, displacing a portion of energy that fossil fuel-powered plants could have otherwise produced. 

Potential SDG contributions from wastewater treatment projects

Beyond reducing GHG emissions, wastewater treatment projects can contribute to a range of Sustainable Development Goals (SDGs) by improving access to clean water, generating clean energy, advancing sustainable industrial practices and mitigating impacts on ecosystems. These projects are particularly impactful in regions where wastewater treatment infrastructure is lacking or inadequate, enabling significant environmental and public health benefits. The technologies adopted in these projects, such as the UASB reactor, can enhance industrial sustainability and innovation in the waste sector. Furthermore, improved wastewater management helps reduce urban pollution, contributing to cleaner cities and healthier living environments. Another potential contribution to health is from the displacement of thermal electricity generation: when displacing energy from coal-powered plants, these projects actively reduce emissions of sulfur oxides (SOx) and nitrogen oxides (NOx).

Like other waste-based carbon projects, these projects support circular economy principles through the recycling and efficient use of nutrients and energy, including capturing biogas from waste digestion to generate electricity. With regard to the environment and ecological health, wastewater projects have the potential to help aquatic and marine life conservation by preventing untreated wastewater from entering aquatic ecosystems or contaminating soils.

Overview of wastewater treatment projects in the carbon market

At the time of this blog post, there are just over 100 wastewater projects in the carbon market, according to the Berkeley Database.[3] These projects are located all over the world, but almost half are located in Thailand, as seen in the chart below.

Wastewater projects are mostly registered under the Verified Carbon Standard and Gold Standard using a variety of methodologies and methodology combinations (e.g., ACM0014, AM0080 or AMS-I.C & AMS-I.D & AMS-III.H). However, there are one or two projects registered under the Climate Action Reserve (CAR) and ACR, using the “CAR Organic Waste Digestion Protocol” and ACR’s “Emission Reductions Through Wastewater Treatment,” respectively.

Calyx Global ratings of wastewater treatment projects

Calyx Global has developed a rating framework to assess the GHG integrity and SDG contributions of carbon credits generated by wastewater treatment projects. If you are not a subscriber but want to view our first ratings, contact us about a subscription to the Calyx Global Platform.

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[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10041530/#:~:text=The%20wastewater%20sector%20is%20a%20major%20source,CH4%20is%20produced%20in%20anaerobic%20environments%20where
[2] https://www.epa.gov/system/files/documents/2023-04/US-GHG-Inventory-2023-Chapter-7-Waste.pdf
[3] https://gspp.berkeley.edu/research-and-impact/centers/cepp/projects/berkeley-carbon-trading-project/offsets-database