Hydrothermal carbonization, or HTC for short , is a process that turns organic waste – such as sewage sludge, plant residues, bio-waste, and other wet organic materials – into a useful, solid carbon product, called hydrochar.
HTC involves heating moist biomass in a closed pressure reactor, typically at temperatures ranging from 180 to 250°C and pressures ranging from several to a dozen or so bars. The entire process occurs without oxygen, distinguishing it from combustion. Under these conditions, organic matter undergoes chemical and physical reactions that lead to the dehydration and condensation of carbon compounds. The result is a dark, solid material resembling coal—hydrochar—as well as liquid and a small amount of gas.
According to Schwander Polska, one of the most promising applications of HTC is the processing of sewage sludge. In traditional treatment plants, this sludge is problematic – it takes up a lot of space, is difficult to transport, and often requires drying or incineration. HTC can transform it into a more compact, energy-rich, and manageable form. Additionally, the process can recover valuable components, such as phosphorus.
HTC also reduces greenhouse gas emissions, including methane, which is produced during traditional anaerobic digestion. This makes this technology potentially a key player in the green transformation and decarbonization of the municipal sector.
To sum it up, HTC not only allows for the reduction of difficult-to-manage waste, such as sewage sludge and biomass residues, but also for the recovery of value from them in the form of clean energy, nutrients, and even persistent forms of carbon that can serve as CO₂ storage. Thanks to technological possibilities and growing environmental awareness, HTC has the potential to become a key link in the creation of more sustainable, self-sufficient urban and industrial systems.
It's worth emphasizing that this technology is scalable – it can be implemented in both large municipal installations and smaller waste treatment plants. Its energy efficiency, low environmental impact, and ability to integrate with other treatment and recovery systems give HTC the potential to become the future standard for modern, green waste treatment.
In times of climate crisis, rising energy and raw material costs, and the need for better resource management, technologies such as hydrothermal carbonization are not just a scientific curiosity – they are a real response to contemporary civilizational challenges.
Hydrochar, HTC's main product, is widely used in various sectors. As a biofuel, its high calorific value allows it to power local renewable energy sources. In agriculture, as a fertilizer, it improves soil properties, increasing their water capacity and promoting the growth of microorganisms. Due to its porous structure, it acts as an effective sorbent, binding heavy metals and other pollutants in water, air, and sewage. It can also act as a material for long-term carbon sequestration, reducing its presence in the atmosphere.
The liquid produced during the process can be used to produce biogas, which increases energy efficiency and supports the self-sufficiency of local biogas plants.
In 2023, Schwander Polska commissioned Mikrobiotech sp. z o.o. to analyze the use of hydrothermal carbonization (HTC) for processing municipal sewage sludge (KOŚ) into fertilizers or solid biofuel.
The test results obtained in 2024 clearly showed that hydrochar " meets the requirements for fertilizer products and can also be used as a solid fuel; (2) the effluent produced in the HTC process meets the requirements for fertilizer products as a liquid fertilizer ."
Schwander Polska – the first manufacturer of MBR membrane modules in Poland
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