Customer story

Circular sludge tech helps leading Dutch company convert wet waste into valuable resources

Sustainable developement goals:

Waternet teams up with REYM and C-Green to transform sludge into biofuel

Explore the innovative collaboration between C-Green, one of our portfolio start-ups, and two industry leaders in the Netherlands: Waternet, a major waste management company, and REYM, an industrial cleaning, transport and waste materials management company. Together, they are addressing the complex environmental issues associated with wet sludge generated during wastewater treatment.

The challenge

Waternet has been looking to enhance its operational efficiency and increase the sustainability of its practices. The process of cleaning wastewater to ensure its safe release into the environment results in wet sludge, a combination of wanted and unwanted pollutants and materials. Notably, sludge contains up to 90% water, making it voluminous, heavy and expensive to transport and burn.


Traditional methods of storing, composting or drying sludge come with significant environmental challenges. Storage and composting emit large amounts of methane and nitrous oxide, contributing to greenhouse gas emissions. Meanwhile, the energy-intensive nature of conventional sludge drying methods, coupled with its high pathogen content, not only poses risks to workers but can also cause environmental contamination.


Tighter regulations on the processing and handling of sludge in the EU, accompanied by increasing costs of handling the waste material, are driving the search for innovative new waste handling processes.

The solution

Waternet teamed up with REYM and Swedish start-up C-Green, to trial a process which converts wet sludge into biofuel. As part of the trial, C-Green established its mobile OxyPower HTC™ pilot plant at REYM Rotterdam in 2023, to process and analyse sludge generated by Waternet and industrial biosludge from REYM.


C-Green combines the power of hydrothermal carbonisation and wet oxidation (OxyPower HTC™) to turn the wet waste into solid hydrochar, a carbon-rich, sterile product which can be used as biofuel or as a soil enhancer. This is thanks to the ability of OxyPower HTC™ to capture the energy, nitrogen and phosphorus in the sludge. One OxyPower HTC™ plant can process up to 30,000 tons of wet sludge per year—the amount produced by a city of 300,000 – 400,000 people.

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The expected impact

Waternet already utilises a process to convert sludge into green energy in the form of natural gas. By adding this innovative process to its operations, it can reduce its emissions further: as much as 80% of greenhouse gas emissions can be avoided using C-Green’s technology compared with existing sludge management processes. At the same time, it also creates a new, valuable raw material, contributing to a more circular economy.


Waternet can also reduce its operating costs by using OxyPower HTC™, since compared to conventional sludge handling, the process is almost energy neutral— it uses the energy in the sludge to run the process, so no added heat is necessary.