Codigestion: Integrating Organic Waste into Sludge Treatment

Codigestion is an approach some wastewater treatment plants use to enhance biogas production and optimise existing anaerobic digestion infrastructure. By introducing additional organic waste streams—such as food waste, grease, or industrial by-products—alongside sewage sludge, facilities may increase energy recovery and improve resource efficiency. However, the feasibility of codigestion depends on multiple factors, including infrastructure capacity, feedstock availability, and operational considerations. This guide explores the key aspects of codigestion in wastewater plants, including common feedstocks, benefits, challenges, and real-world applications.

Codigestion Explained

Codigestion, also spelled co-digestion, refers to the anaerobic treatment of multiple organic feedstocks within a single digester. In wastewater treatment plants, this involves combining sewage sludge with smaller amounts of external organic wastes such as food waste, fats, oils, and grease (FOG), and other industrial organic by-products.

Wastewater utilities typically integrate codigestion into their wastewater infrastructure to enhance biogas generation and optimise the use of existing anaerobic digestion (AD) infrastructure. Codigestion may be part of a municipality's larger waste-to-energy or circular economy initiative, or it may just present an additional income stream for the wastewater facilities treating organic waste that would otherwise end up in landfills.

Common Feedstocks for Codigestion with Sludge

Codigestion has been shown to improve process stability and biogas yields in various studies. Successful codigestion, however, depends on selecting compatible organic wastes that complement the microbial breakdown of sewage sludge and testing the proportions at which they best undergo digestion. The most common feedstocks that are combined with sludge include:

Food Waste and Industrial Organic Waste

Food waste is one of the most widely used feedstocks in codigestion due to its highly biodegradable organic matter, particularly carbohydrates and lipids, which significantly increase methane production. This waste stream can come from households, food processing industries, restaurants, and grocery stores. Common examples include fruit and vegetable scraps, bakery and dairy residues, brewery waste, juice pulp, and meat and fish waste.

Adding food waste to sewage sludge typically boosts biogas yields by 20–40% compared to sludge-only digestion. However, different food wastes have varying compositions, which can impact the stability of the digestion process. High-fat and protein-rich materials require careful management to avoid ammonia inhibition, foaming, or digester imbalances.

Fats, Oils, and Grease (FOG)

 FOG waste, commonly collected from restaurant grease traps and food manufacturing facilities, is an energy-dense material that can significantly enhance methane production. This feedstock is energy-dense and can substantially increase methane production. However, excessive FOG can lead to scum formation, foaming, and operational challenges. Many facilities implement controlled dosing strategies to mitigate these risks and closely monitor digester conditions.

Agricultural residues

Though less common in urban wastewater plants, agricultural residues such as straw, husks, and animal manure have been successfully codigested with sludge in certain regions. These materials provide a more consistent biogas yield over time. However, agricultural waste often requires pre-processing to improve digestibility.

Advantages and Challenges of Codigestion

Codigestion presents several advantages for wastewater treatment plants, making it an attractive sustainability and energy recovery strategy.

Advantages

✔ Increased Biogas Production – Codigestion typically enhances methane yields, improving energy recovery from wastewater treatment plants.

✔ Waste Reduction – Reduces the volume of organic waste sent to landfills, mitigating greenhouse gas emissions.

✔ Provides Endpoint for Food and Other Organic Waste – Food waste from kitchens and other organic byproducts are quite difficult to deal with as waste material. They are not easy to incinerate or compost, and anaerobically digesting them as a sole feedstock presents problems for digesters due to their high protein and fat content.

✔Asset Maximization and Cost Savings - Wastewater plants with excess digestion capacity can generate extra income by accepting food waste, as utilities are paid to process it. This also boosts biogas production, adding further economic benefits. Additionally, it can be a greener solution for municipal food waste management, especially if there are limited and costly disposal outlets for food waste.

✔ Nutrient Recovery – The digestate from codigestion, if used as fertiliser or a soil product following local regulations, will recycle nutrients back into soils, promoting sustainable agriculture.

Challenges and Considerations

✖ Process Complexity – Requires careful feedstock management to maintain microbial balance in the digester.

✖ Infrastructure Costs – Retrofitting existing WWTPs for codigestion or building new facilities with codigestion capacity will incur additional capital costs. This must be assessed against other disposal methods.

✖ Contamination Risks – Food waste and industrial by-products may introduce contaminants in increased doses to the digester, compromising digestion or biosolids quality.

Codigestion Plants Using Thermal Hydrolysis

Several wastewater treatment plants worldwide have adopted the thermal hydrolysis process  (THP) to improve the efficiency of codigestion. THP helps break down sludge and organic waste into a more digestible form, accelerating methane production and reducing sludge volumes.

As THP improves sludge biodegradability, it also frees up digestion capacity. This allows facilities using THP to treat additional sludge or organic waste and better utilise installed assets.

Some of the most notable examples of codigestion facilities utilising THP include:

Anyang Bakdal Wastewater Treatment Plant in South Korea

The Bakdal Wastewater Treatment Plant, Korea's largest underground sewage facility, was relocated beneath Saemul Park in Anyang to address odour concerns, allowing the surface area to be redeveloped into a residential complex with recreational amenities. Cambi's thermal hydrolysis process was integral to the project, enabling the co-digestion of sewage, septic sludge, and food waste leachate while reducing digester size, eliminating odours, and producing renewable energy. In 2017, the project was recognised by the International Water Association (IWA) with the Best Practices on Resource Recovery from Water Award.

Ecopro plant in Verdal, Norway

The Ecopro plant in central Norway is owned by an inter-municipal company that serves 52 municipalities. It treats 40,000 tonnes of sorted food waste and sludge from households, fish farms, and slaughterhouses. THP ensures that the company produces pathogen-free biosolids suitable for liquid and solid fertiliser production. The process also boosts biogas yields to approximately 30 GWh per year, with the compressed biogas powering public transport in the Trondheim region.

Jurong Wastewater Treatment Plant in Singapore

The Jurong Water Reclamation Plant, operated by Singapore's Public Utilities Board (PUB), treats sewage sludge from Jurong's industrial hub and surrounding residential areas while also receiving concentrated greasy waste from food establishments. To handle the growing amount of industrial wastewater, PUB started an expansion project in 2013 and decided that using Cambi's thermal hydrolysis process would offer the best flexibility and long-term value.

Kenneth W. Hotz Water Reclamation Facility in Medina, Ohio, USA

The Kenneth W. Hotz Water Reclamation Facility, the largest of three municipal plants in Medina County, Ohio, serves approximately 35,000 homes. In 2018, the plant underwent a $35 million modernisation, replacing its ageing wet air oxidation system with Cambi's thermal hydrolysis process alongside new anaerobic digesters. The upgrade reduced operating costs by 50%, generated $1.7 million in annual savings, and enabled the plant to produce Class A biosolids while meeting 30% of its energy needs through biogas. The plant processes liquid food waste from a local food manufacturer that enters the plant via dedicated piping, as well as food waste trucked in from the manufacturing facility.

The plants above use THP to treat both sewage sludge and other organic feedstock before digestion. Some facilities use or will use THP on sewage sludge before codigesting the processed sludge with the other organic feedstock. This process can be seen at the Gaoantun Water Reclamation Facility in Beijing, China, which became operational in 2017, and at Singapore's Tuas Nexus Facility – a project currently under construction and expected to be operational by 2026.

Codigestion Moving Forward

Anaerobic digestion has long played a crucial role in wastewater treatment, energy recovery, and urban sustainability. Codigestion—a process that integrates additional organic waste streams with sewage sludge—maximises the potential of anaerobic digestion at wastewater facilities with sufficient capacity while offering a sustainable solution for managing food waste and other organic material.

Countries with ample sludge treatment facilities, established food waste collection systems, industrialised agriculture, or large-scale food processing industries may be well-positioned to implement codigestion. Additionally, regions with feed-in tariffs and renewable energy incentives, such as parts of the European Union, have leveraged codigestion to support biogas production.

In contrast, many developing nations have yet to establish comprehensive frameworks for codigestion within wastewater management. However, pilot projects in developing and underdeveloped countries with high organic waste generation and limited landfill capacity show interest in this method.

Facilities and municipalities considering codigestion must assess the feasibility of utilising locally available feedstocks to determine whether the approach is economically viable, operationally efficient, and aligned with their overall goals.

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