Biogas from Wastewater: A Sustainable Energy Solution

Biogas, a renewable energy source derived from organic waste, holds immense potential as a sustainable alternative to fossil fuels. Within the wastewater treatment industry, it is produced through the anaerobic digestion of sewage sludge, a process that also stabilises the sludge, reducing odours and eliminating pathogens. Biogas exemplifies circular economy principles with applications ranging from electricity generation to vehicle fuel by transforming waste into value. As the world seeks cleaner energy solutions, biogas from wastewater treatment offers a scalable and cost-effective pathway to sustainability for urban municipalities.

Biogas as an Energy Source

Biogas is a combustible gas composed mainly of methane (CH₄) and carbon dioxide (CO₂). It is predominantly produced through anaerobic digestion (AD) — a process in which microorganisms break down organic materials in oxygen-free conditions. Common feedstocks for biogas production include agricultural residues, food waste, landfill waste and sewage sludge from centralised wastewater treatment facilities.

The benefits of biogas are both significant and multifaceted. If harvested from a sustainable source, it offers a cost-effective and scalable renewable energy solution, particularly in regions with organised agricultural practices and waste management systems. By converting organic waste into a valuable energy resource, biogas supports a circular economy—reducing waste, lowering greenhouse gas emissions, and contributing to sustainable energy systems. If the material is also used to replace more carbon-intensive sources, then it further reduces the greenhouse gas emissions of the process.

Understanding Biogas within the Wastewater Treatment Industry

The primary aim of wastewater treatment facilities is to treat domestic wastewater, industrial wastewater and stormwater to meet environmental and public health standards, ensuring that the treated effluent can be safely reused or discharged into natural water bodies. A significant by-product of this process is sludge, the solid portion of wastewater left over after the liquid portion is separated for treatment. Managing sludge is a crucial and often costly process, and for facilities serving populations of approximately 5,000 or more, anaerobic digestion is one of the most effective options.

Anaerobic digestion involves placing sludge in large, enclosed tanks known as digesters, where microorganisms break it down in the absence of oxygen. This process produces biogas while stabilising the sludge, reducing its odour and pathogen content. The dual benefit of energy generation and waste stabilisation has made AD an essential component of sustainable wastewater management for many municipalities the world over.

Applications of Biogas

Once biogas is extracted from sludge, it can be utilised as an energy source or repurposed into other valuable materials in various ways:

Combined Heat and Power (CHP) Systems

One of the most common and efficient biogas applications is via CHP systems or engines. This process, also called cogeneration, transforms biogas into electricity and heat. The electricity can power plant operations, reducing reliance on external energy sources, while utilities can use the heat to maintain digester temperatures or even heat buildings.

Certain plants, typically sludge centres, produce excess electricity that can be sent to a local power grid. The Hengelo Energy Factory in the Netherlands, owned by utility Waterschap Vechtstromen, is a prime example of an energy-positive sludge centre using cogeneration.

Biofuel Production

Raw or minimally treated biogas can be used directly in boilers or furnaces. However, it can also be upgraded by removing impurities such as carbon dioxide, hydrogen sulfide, and water vapor to produce biomethane—a purified form of biogas with a 95% or higher methane content, commonly referred to as renewable natural gas (RNG).

Biomethane can be injected into natural gas grids or used as compressed natural gas or liquefied natural gas for vehicles, supporting decarbonisation efforts in the transport sector. The Piscataway Bioenergy Facility, owned by the Washington Suburban Sanitary Commission in the United States powers the municipality’s bus fleet using biogas.

Emerging Applications

Biogas can be reformed to produce hydrogen, an emerging energy source with significant potential in low-carbon energy systems. Additionally, carbon dioxide (CO₂) from biogas can be captured for commercial use in industries such as beverage carbonation or as a feedstock for chemical production.

When biogas cannot be utilised immediately, it is often flared for safe disposal. While flaring prevents methane—a potent greenhouse gas—from being released into the atmosphere, it wastes valuable energy, underscoring the importance of effective biogas management.

Improving Biogas Capture and Production

Technological advancements profoundly influence the future of biogas in wastewater treatment and in the management of other waste streams.

Technologies to monitor fugitive emissions and ensure efficient biogas capture are increasingly critical as methane emissions from biogas systems have come under scrutiny. Methane is a far more potent greenhouse gas than carbon dioxide. Recent studies highlight that inefficient biogas systems in the wastewater sector are a significant source of these emissions.

Equally important are advanced anaerobic digestion technologies, which are revolutionising traditional digestion systems in wastewater and sludge facilities. These innovations include pre-treatment methods and modifications to digestion systems that boost biogas production and improve biosolids handling. Technologies such as thermal hydrolysis and staged digestion are some examples. Scandinavian countries and the United Kingdom are notable adopters in this space, with some of their facilities now leveraging advanced digestion to achieve energy neutrality—or even positivity.

Sludge-derived Biogas: What’s in Store?

The production of biogas from wastewater sludge can help address global energy, economic, and environmental challenges. As the world grapples with rising energy costs, biogas offers a scalable and sustainable alternative to fossil fuels. Biogas from wastewater sludge is an easily accessible source. Wastewater treatment in most populous cities in developed countries is already centralised, and many utilities already use anaerobic digestion to harvest the material. With cities' populations expected to grow, sludge as a resource is also secured for the future. As more municipalities centralise wastewater treatment, the amount of sludge will also increase.

Municipal wastewater utilities can unlock the potential of biogas by re-evaluating their sludge treatment processes. For facilities still reliant on landfilling sewage sludge, transitioning to advanced anaerobic digestion (AD) can deliver significant cost savings and environmental benefits. Meanwhile, utilities already employing AD can explore innovative technologies to enhance biogas production and capture, ensuring maximum energy recovery.

Supportive policy frameworks, such as the US Renewable Fuel Standard, pave the way for a shift towards biofuels and other renewable energy sources, reducing dependence on imported energy. In the European Union, the revised Wastewater Treatment Directive sets an ambitious target of energy neutrality for wastewater facilities by 2045. This directive is expected to drive investment in advanced biogas optimisation technologies and infrastructure upgrades.

By implementing forward-thinking strategies and leveraging emerging technologies, wastewater utilities can harness biogas and position wastewater sludge as the valuable resource it is, contributing significantly to the global transition toward renewable energy.

Want to learn how several facilities have increased biogas production using advanced digestion based on thermal hydrolysis? Go to our customer stories.