Transforming wastewater into renewable natural gas and advancing the circular water economy

What if the very infrastructure built to treat a city’s sewage could hold the key to cleaner air, lower energy bills and a path toward a clean energy revolution? Across the U.S., wastewater treatment plants (WWTPs) are beginning to do just that by undergoing a quiet but revolutionary transformation. Once seen as purely a sanitation system, these facilities are now emerging as an unexpected and steady source of renewable energy generation. At the heart of this transformation is renewable natural gas (RNG). By capturing methane during the wastewater treatment process and converting it into RNG, WWTPs are redefining what waste can do.

This innovation isn’t just about cleaner fuel. It represents a critical step toward a circular water economy, a model where water, energy and waste are reused and reintegrated to support long-term environmental and economic sustainability.

What is the Circular Water Economy?

The circular water economy is a systems-based approach to water management that prioritizes resource efficiency, reuse, and sustainability. Instead of viewing wastewater as waste to be discarded, this approach sees it as a valuable resource. It moves away from the traditional “take-make-dispose” model and instead looks to extract value from every stage of the water cycle. The goal is to close the loop on water, energy and nutrient flows by reintegrating outputs back into the system.

In this framework, wastewater becomes a source of energy, reusable water and recoverable nutrients. WWTPs play a central role by recovering methane produced during the breakdown of organic matter in sewage. When purified, this methane becomes RNG, a viable substitute for natural gas. This not only reduces emissions but also enhances the resilience and efficiency of urban infrastructure.

How RNG is Made from Wastewater

When organic waste breaks down in an oxygen-free (anaerobic) environment, which happens naturally in sewage systems and treatment plants, it emits biogas. This gas is composed primarily of methane and carbon dioxide, along with other trace compounds. Methane, while potent as a greenhouse gas, is also highly combustible, making it an excellent energy source if captured and refined.

The production of RNG from wastewater follows a series of biological and chemical processes designed to extract and refine methane:

Collection & Anaerobic Digestion: Organic material in wastewater is separated and placed in anaerobic digesters. These oxygen-free environments promote microbial activity that breaks down the waste and produces biogas.
Gas Capture & Purification: This raw biogas is about 60–65% methane. The remaining impurities, including water vapor, hydrogen sulfide and CO₂, are removed in a series of purification treatment steps.
Conversion to RNG: After purification, the remaining methane meets pipeline standards and becomes RNG. It is chemically similar to conventional natural gas and suitable for injection into existing distribution systems or for compression and use as a vehicle fuel.
Distribution & Use: RNG can serve various energy needs, including powering municipal fleets, long-haul trucks, industrial boilers, and even electricity generation. Unlike solar or wind, RNG is dispatchable, available 24/7, regardless of weather or time of day.

Emissions Reduction Through Methane Capture

Methane is a potent greenhouse gas, with over 80 times the warming potential of carbon dioxide over a 20-year period. When it escapes into the atmosphere from landfills, manure lagoons or WWTPs, it significantly accelerates global climate change. According to the World Economic Forum, wastewater utilities contribute to 5% of global greenhouse gas emissions, and methane from wastewater is a major culprit.

By capturing and converting methane into RNG instead of allowing it to escape, WWTPs can significantly reduce their greenhouse gas emissions and drastically lower their climate impact. Even better, when RNG is used to replace diesel in heavy-duty transportation, it can reduce lifecycle emissions by as much as 300% to 400%. This results in a negative carbon intensity, displacing fossil fuel, and prevents methane from being released, providing a powerful tool for cities aiming to meet aggressive climate targets.

Economic and Operational Benefits for Utilities

Producing RNG from wastewater isn’t just a win for the environment, it can also offer financial and operational benefits in addition to environmental gains:

Royalties & Revenue Sharing: By partnering with experienced RNG developers, municipalities can earn royalties without taking on capital costs or technical risk.
Renewable Fuel Credits: RNG used for transportation generates valuable credits under federal and state programs like the Renewable Fuel Standard (RFS) and California’s Low Carbon Fuel Standard (LCFS).
Local Job Creation: Building and operating RNG facilities creates skilled jobs in engineering, operations and maintenance — helping transition into a clean energy economy and bolster local workforces.
Energy Cost Savings: Facilities that use RNG on-site can lower their utility bills and reduce dependence on volatile energy markets. This improves operational stability and resilience.

Why More Cities Should Act Now

While the landfill sector has moved quickly to develop RNG projects, with more than 25 states currently producing landfill-based RNG, wastewater systems have been slower to adapt. Yet the opportunity is enormous. Most urban areas and major metros already have the necessary infrastructure, including anaerobic digesters and extensive sewer networks.

By expanding the number of RNG projects at WWTPs, cities can:

Maximize the value of their existing infrastructure
Enhance climate adaptation efforts, improving climate resilience
Reduce greenhouse gas emissions and help meet sustainability targets
Reduce reliance on imported energy and diversify energy sources
Reinforce local energy independence

With these advantages, wastewater-based RNG represents a missed opportunity in many regions. Increasing investment and focus in this area can yield rapid and impactful returns.

RNG as a Long-Term Clean Energy Solution

Unlike some renewable energy sources that face storage or intermittent challenges, RNG provides a steady, scalable energy supply that addresses some of the limitations of intermittent renewable energy sources like solar and wind. It offers a consistent energy stream, making it suitable for sectors that require around-the-clock energy, such as industrial processing and utility operations and is especially useful in sectors that are harder to electrify like long-haul trucking, maritime transport and heavy industry.

Because RNG is chemically identical to fossil natural gas, it can use current infrastructure for storage and transport. This reduces transition costs and makes integration into the existing energy system more feasible. For sectors that are difficult to electrify, RNG offers a practical, long-term clean energy alternative.

The Path Ahead: Scaling the Circular Water Economy

As more cities and utilities look to align with climate commitments and bolster resilience, RNG from wastewater emerges as a practical and impactful solution. By transforming what was once considered a burden into a renewable asset, WWTPs can support the broader principles of the circular water economy and help cities make the most of their existing assets.

This transformation is more than a technical upgrade; it’s a reimagining of what municipal infrastructure can achieve. However, the core infrastructure is already in place, and the benefits are clear. By scaling wastewater-based RNG projects, communities can reduce emissions, generate clean energy and create local economic opportunities. The technology exists, infrastructure is ready and the benefits are proven. What’s needed now are champions to rise and gather the team’s collective will to act and see sewage not as waste, but as the cornerstone of tomorrow’s clean energy.