Convert biowaste into renewable carbon It's no longer just a laboratory idea, but a reality that is changing the way we manage our organic waste, produce energy, and reduce emissions. From sewage sludge treatment to the digestion of the organic fraction of municipal waste, an entire technological and economic ecosystem is being built around this new way of utilizing organic matter.
In this context, projects such as BIOKAR in the Basque Country, advanced biogas plants like the one in Nieheim, Germany, local initiatives for the valorization of biowaste in Spanish municipalities, and the promotion of hydrogen derived from biomass The use of biogenic CO₂ as a resource paints a comprehensive picture of where the circular bioeconomy is headed. Below, all these aspects are explored in detail, integrating both technological advancements and their environmental, economic, and social impacts.
From organic waste to high-value renewable carbon
Traditional organic waste management has, for decades, involved sending large quantities of biowaste to landfill or its almost exclusive use for basic energy recovery, thus wasting its potential as a material resource. In a community like the Basque Country, for example, more than 500.000 tons of sludge from wastewater treatment plants (WWTPs), digestate, agricultural and forestry waste, and pruning debris are generated annually, much of which is disposed of inefficiently.
This linear model assumes a loss of resources and a source of emissions of greenhouse gases, in addition to increasing management costs. In response, projects have emerged that address biowaste as a raw material to generate biochar, biomethane, renewable hydrogen, and other products with direct applications in industry, agriculture, and construction.
In this paradigm shift, the key lies in combining thermochemical technologies (such as pyrolysis or hydrothermal carbonization), advanced biological processes (optimized anaerobic digestion), and capture and utilization systems. Biogenic CO₂ that is generated during conversion, thus closing the carbon cycle in short time horizons.
BIOKAR Project: transforming bio-waste into functional biochar
The BIOKAR project is conceived as a structural response to the problem of underutilized organic waste in the Basque Country, proposing to convert up to 500.000 tons of biowaste annually in high-value-added biochar for multiple industrial applications. The initiative focuses on wastewater treatment plant sludge, digestate, and agroforestry byproducts that today mostly end up in landfills or are burned to produce energy.
To achieve this, the BIOKAR consortium is focusing on two families of thermochemical technologies: the hydrothermal carbonization (HTC)This method is suitable for waste streams with high moisture content, while pyrolysis is more appropriate for dry fractions. The main objective is to convert more than 80% of the initial organic waste into stable biochar, minimizing the final volume that requires further management.
In addition to optimizing conversion, work is being done on the functionalization of biochar produced. This involves modifying its physical and chemical properties—for example, by increasing its carbon content above 70% and expanding its specific surface area above 500 m²/g—so that it can effectively and efficiently replace fossil coal in various industrial processes.
The biochar obtained will be validated in several lines of use: as an adsorbent material in the water treatment contaminated with emerging compounds, as a component of carbon aerogels intended for advanced gas filtration, and as an additive and soil stabilizer in construction materials, also contributing to long-term carbon sequestration.
This entire approach allows biochar to be positioned not only as a byproduct, but as a strategic resource capable of displacing fossil-based materials, reducing CO₂ emissions associated with their production and use.
Environmental, economic and circular economy impact
Estimates made within the BIOKAR framework indicate that advanced valorization of the 500.000 tons of biowaste currently underutilized annually could prevent around 13.000 tons of CO₂ equivalent per yearThis reduction comes both from the smaller amount of waste sent to landfill and from the replacement of fossil coal with renewable biochar.
At the level of the circular economy, the project anticipates a significant increase in Material productivity and the circularity rateIt is estimated that material productivity could increase by more than 90%, while resource circularity would increase by around 50% thanks to the integration of biochar into existing value chains.
From an economic point of view, BIOKAR projects an approximate added value of 5 million euros per year For participating companies, this value will be realized once the model is deployed at an industrial scale. This value derives from the sale of functionalized biochar, as well as from associated environmental services and reduced waste management costs.
Boosting this value chain also has a clear effect on employment, by promoting the creation of skilled jobs in areas such as process engineering, materials characterization, advanced plant operation, and sustainability consulting. Overall, the Basque eco-industry is strengthening its position as a leader in bioeconomy and climate neutrality.
This approach aligns directly with the Basque Country's Circular Economy Strategy 2030 and the Waste Prevention and Management Plan 2030, which identify the biowaste as a strategic priority to move towards a low-carbon, competitive production model based on the efficient use of resources.
A consortium that covers the entire value chain
BIOKAR's robustness is based on a consortium that integrates agents from the collection and management of biowaste From the industrial application of biochar to technological R&D, the project is led by Cadagua, a company that contributes its experience in the engineering, construction, and operation of water treatment plants.
Alongside Cadagua, several specialized companies participate, ensuring the comprehensive management of the different waste streams: a firm focused on industrial filtration solutions and atmospheric emissions control, another dedicated to the maintenance of green areas, forestry work and public roads, an earthmoving and waste management company using recycled aggregates, and a key player in the Basque forestry sector involved in the sustainable management of forest resources.
Added to this is a international consultancy Specialized in sustainability, carbon markets and climate change, supporting the measurement, monitoring and valuation of the climate and environmental benefits generated by the project, as well as its fit within regulatory and green finance frameworks.
From a scientific and technological perspective, a leading research center is being incorporated in thermochemical processes (pyrolysis and hydrothermal carbonization), advanced characterization of materials, and solutions for the valorization of biogenic waste. Alongside it, an environmental cluster that brings together companies and entities in the sector acts as a platform for disseminating, transferring, and scaling up the results.
This public-private framework demonstrates a commitment to a carbon-neutral production model and the willingness to move from pilot projects to real implementation in the territory, with a tangible social, economic and environmental impact.
Institutional support and funding for innovation
For these types of initiatives to progress from the laboratory phase to commercial deployment, it is essential to have public financing instruments that share the technological risk. In the case of BIOKAR, the project benefits from the support of the Basque Government's HAZITEK 2025 program, which focuses on supporting business R&D projects aligned with competitiveness, inter-sector cooperation, and sustainability.
The aid comes from the budget of the Department of Industry, Energy Transition and Sustainability, as well as from the European Regional Development Fund (ERDF), reinforcing the European dimension of the transition towards a low-carbon economyThis type of support makes it easier for companies and technology centers to test and optimize complex technologies such as HTC or advanced pyrolysis.
By connecting these projects with regional and state circular economy and waste management strategies, it is ensured that the results are not isolated, but rather integrated into broader industrial transformation plans, contribute to climate goals, and generate regulatory and economic synergies.
Advanced biogas plants: the Nieheim example
In addition to the thermochemical pathway for biochar, the anaerobic digestion of urban and agro-industrial biowaste is another major lever for converting organic matter into renewable carbon in the form of biogas, biomethane, and usable biogenic CO₂. A notable example is the Nieheim plant in Germany, operated by the Eggersmann group.
This facility, which has been operating since 2007 using batch dry fermentation, is being transformed to adopt a process of continuous dry fermentationThe aim is to substantially increase biogas production from the organic fraction of municipal waste. The modernization will allow the processing of approximately 54.000 tons of biowaste per year.
Technological change is accompanied by a change in the destination of biogas: instead of using it mainly for generate electricity, they are betting on their upgrading to biomethane with natural gas quality, which can be injected into the gas pipeline network and used in thermal and industrial uses with higher energy value.
The plant also integrates a wind turbine and a large photovoltaic deploymentThus, a very significant part of the electrical energy required for the upgrading process is produced renewablely on the premises, reducing the overall carbon footprint.
This combination places Nieheim as an example of hybrid power plantwhere bio-waste digestion is integrated with renewable electricity generation and smart energy management systems to maximize efficiency and minimize associated emissions.
Smart energy management and negative carbon footprint
One of the most innovative aspects of the Nieheim plant is its energy management based on Artificial IntelligenceThe system controls when biogas is converted into biomethane based on the availability of renewable electricity produced at the plant itself (wind and solar). If at any given time there is insufficient on-site electricity generation, the biogas is temporarily stored in large tanks.
This avoids consuming energy from the grid during periods when the electricity mix may have a higher carbon intensity, adjusting operations to prioritize periods with greater renewable energy penetration. This approach helps reduce the carbon footprint associated with the upgrading process and improve the global climate balance installation.
On the other hand, the CO₂ separated from the biogas during upgrading is used for high-value purposes. Part of it is transformed into biogenic dry ice, which is used in industrial processes such as shot blasting for surface treatment or in specialized cooling applications.
Another fraction of the captured CO₂ is permanently stored in building materials, such as recycled concrete, where it remains fixed for the product's entire lifespan. This strategy of using and storing biogenic CO₂ allows the Nieheim plant not only to produce renewable energy and climate-neutral biogas, but also to aspire to have a even negative carbon footprint.
By integrating renewable generation, bio-waste digestion, biomethane upgrading, and CO₂ capture and utilization, Nieheim becomes a benchmark for how an organic waste treatment plant can evolve into a true renewable carbon biorefinery.
Compost, fertilizers and agricultural use
Anaerobic digestion processes generate not only biogas but also a digestate that remains a resource of high agronomic interest. At Nieheim, the management of this digestate has been designed to maintain and improve the compost quality produced, complying with strict certification standards.
The digestate from the plug flow fermenter usually has too high a moisture content for direct composting. Therefore, it undergoes a separation process into solid and liquid fractions. The solid fraction is used for the production of high-quality compost, while the liquid fraction is sold as liquid fertilizerespecially in nearby agricultural areas.
This double use allows return organic nutrients to the soil, improving its structure and fertility, while simultaneously closing the organic matter cycle. The experience accumulated since the mid-nineties by the Eggersmann Group's composting division has contributed to perfecting control panels, maturation times, and material mixtures.
In practice, farmers in the region benefit from a stable supply of organic amendments and liquid fertilizers derived from municipal and agro-industrial waste, creating a virtuous circle between cities and countryside that reduces dependence on fossil-based fertilizers.
This model demonstrates that the valorization of biowaste is not limited to energy production, but encompasses a whole range of material products based on renewable carbon that keep the captured carbon in the soil or in long-lasting products.
Hydrogen derived from biomass as an energy carrier
Another key aspect of the transition to renewable carbon is the production of hydrogen from biomass (Bio-H₂). Recent research from Yale University has analyzed in detail the viability of this energy vector as a tool to reduce emissions, especially in sectors where decarbonization is complicated, such as steel, certain chemical processes or heavy transport.
Hydrogen is considered a clean fuel during use, as the energy conversion does not generate CO₂, but the associated emissions depend heavily on the production method. Currently, much of the hydrogen is obtained by reforming natural gas, with a high carbon footprintIn contrast, Bio-H₂ appears as an alternative that, while not always as low in emissions as hydrogen produced by electrolysis with renewables, does offer very significant reductions compared to fossil hydrogen.
The Yale study combined tools from life cycle assessment (LCA) with the GCAM global change analysis model, integrating aspects of supply, demand, incentive policies, and resource availability. The developed framework allows for the assessment not only of direct emissions but also of long-term effects in different sectors and regions.
Various production methods were analyzed, including the electrolysis powered by renewable energy and the gasification or reforming of biomass and agricultural and forestry waste. Consideration was also given to how incentives will change, taking into account, for example, the planned elimination of certain tax credits for clean hydrogen in the United States starting in 2027.
The results indicate that the incorporation of hydrogen derived from biomass Adding hydrogen to the energy mix can multiply emissions reductions by 1,6 to 2 in the period 2025-2050 compared to scenarios where this type of hydrogen is not used, especially if there is no broad and uniform carbon price.
Biomass, forest residues and policies to support Bio-H₂
Biomass suitable for conversion into Bio-H₂ includes both energy crops Specific species (such as miscanthus or switchgrass) can be used, as well as a wide range of agricultural and forestry residues. The use of forestry residues is particularly interesting, as it helps reduce fuel accumulation in forests, decreasing the risk of fires and generating economic value in rural areas.
In the absence of a national carbon price, which researchers consider unlikely in the short term in some countries, sectoral incentives play a significant role. Measures such as subsidies targeted at steel mills or other industries that adopt hydrogen-based processes They could accelerate the implementation of Bio-H₂ and significantly improve emissions reduction.
The study suggests that, under certain circumstances, the specific subsidies Measures aimed at lowering the adoption costs of hydrogen in industry may be even more effective than a general carbon price in driving the transition to low-carbon energy carriers.
It is also noted that, although renewable-powered water electrolysis offers the potential for virtually emission-free hydrogen, it faces significant limitations, such as high capital costs, the availability of land for renewables, and intensive water use. In this context, Bio-H₂ emerges as a complementary solution, especially useful in the short and medium term.
Taken together, these findings reinforce the idea that converting biowaste and biomass into vectors such as renewable hydrogen not only helps close carbon cycles, but also opens up new opportunities for the circular bioeconomy in territories with abundant organic resources.
Municipal bio-waste plants and public-private agreements
At a more local level, the implementation of biowaste treatment plants that produce biogas and biomethane is generating collaboration agreements between municipalities and private companies. An illustrative example is the agreement being considered in a municipality like Colmenar Viejo, where a treatment and recovery plant of organic matter from selective collection.
In this case, developers specializing in waste management and renewable energy will be responsible for the design, construction, operation, and maintenance of the facility, which will transform organic matter into biogas. After purification, the biogas will be converted into biomethane suitable for direct injection into the basic gas pipeline network, in addition to generating by-products for agricultural use.
The plant will have a maximum treatment capacity of 75.000 tons of biowaste per year and will be designed with strict environmental criteria: no slurry or animal remains will be accepted, and work will be carried out with closed circuits and sealed enclosures and there will be no open ponds, thus reducing odor emissions and potential impacts on the environment.
A key demand of the municipal government has been the replacement of the old open leachate pond through a closed and covered system that recirculates the contents, avoiding any risk of infiltration into the soil or aquifers, and improving the social acceptance of the plant.
From an economic point of view, the agreement foresees income and returns for the city council associated with taxes such as the ICIO, the IAE or the IBI, in addition to other benefits linked to the free or discounted management on behalf of the municipal organic faction already energy services, such as self-production of renewable energy already energy services, such as providing free heating to the municipality's educational centers.
The plant will have a maximum treatment capacity of 75.000 tons of bio-waste per year and will be designed with restrictive environmental criteria: no slurry or animal remains will be received, work will be done with closed circuits and sealed enclosures and there will be no open ponds, thus reducing odor emissions and possible impacts on the environment.
A key demand of the municipal government has been the replacement of the old open leachate pond through a closed and covered system that recirculates the contents, avoiding any risk of infiltration into the soil or aquifers, and improving the social acceptance of the plant.
Environmental, social and educational benefits at the local level
The agreement for the new biowaste plant includes a set of concrete benefits for the public, beyond just waste management itself. This includes the creation of an environmental classroom where training and awareness programs on bio-waste recycling and the circular economy will be developed for residents, associations, and educational centers.
A measurement network will also be installed for air quality With at least three sensors distributed throughout the municipality, it will be possible to monitor pollutant levels and variations in real time. This information will be useful for both the administration and the public, reinforcing transparency regarding the plant's impacts.
The promoting company will also assume the costs of various training, social and environmental activities, and will cover the natural gas consumption in the schools of the municipality, generating a direct economic savings for local coffers and freeing up resources for other public services.
Another important commitment is landscape integration: trees will be planted around the perimeter and within the plot, with the aim of improving the visual integration of the facility and contributing to the carbon footprint offset associated with its activity. In addition, priority will be given to hiring local staff, promoting local employment and strengthening the link between the plant and the community.
In operational terms, organic matter collected in the municipality will have priority entry into the plant at a price of zero euros per ton up to a certain percentage of total capacity, thus incentivizing proper waste management. source separation by the residents and reduces treatment costs for the city council.
Biogenic CO₂: from gaseous waste to valuable resource
The anaerobic digestion of biowaste generates biogas consisting of approximately 60% methane and 40% carbon dioxide. Biogenic CO₂To obtain high-purity biomethane (more than 99%), it is necessary to separate both gases through upgrading processes, which produces a concentrated flow of carbon dioxide that, far from being a waste, is becoming a key resource.
Once separated, the CO₂ can undergo further purification processes and liquefactionliquefied CO₂ is transformed from a gaseous to a liquid state, eliminating impurities. This liquefied CO₂ has numerous industrial and commercial uses, and its utilization falls within the carbon capture and utilization (CCU) strategies that accompany the energy transition.
Among the most established applications of biogenic CO₂ are the manufacture of carbonated drinks, its use in greenhouses to stimulate plant growth, food preservation and certain refrigeration or freezing processes, such as that of vaccines in critical health situations.
There are also advanced industrial applications, such as metal treatment, dry ice blasting, or its use as a raw material for production synthetic fuelssynthetic methane or methanol, and even sustainable aviation fuels. In all these cases, CO₂ is integrated into products or processes that reduce dependence on fossil carbon.
Beyond its use, another option is geological storage or storage in construction materials, where the biogenic CO₂ is fixed for long periods and does not return to the atmosphere. This option allows for negative emissions, since the CO₂ originally comes from the atmosphere (captured by plants) and, after its capture, its return to the air is prevented.
Differences between fossil CO₂ and biogenic CO₂
To understand the relevance of these processes, it is essential to distinguish between fossil CO₂ and biogenic CO₂Fossil carbon dioxide is released when fuels such as oil, natural gas, or coal are burned, adding new carbon to the atmosphere, increasing its concentration and fueling climate change.
Biogenic CO₂, on the other hand, is part of short carbon cyclePlants absorb CO₂ from the atmosphere through photosynthesis and incorporate it into their biomass. When that biomass decomposes or is processed (for example, in anaerobic digesters), the CO₂ returns to the air or soil, closing a relatively quick cycle.
When we capture and utilize this biogenic CO₂ in products or store it in a stable manner, we are not increasing the total amount of CO₂ in the atmosphere, but rather managing carbon that was already part of the natural system. That is why many of these solutions are considered low or even negative carbonprovided that the entire life cycle is managed well.
Thus, converting biowaste into usable biogas, biomethane, biochar, Bio-H₂ or biogenic CO₂ requires a comprehensive strategy of renewable carbon utilizationIntegrating these technologies into public policies, industrial projects, and local agreements allows what was once a waste problem to become an asset for the energy and climate transition.
This entire network of projects, technologies, and agreements demonstrates that biowaste can become the cornerstone of a new generation of solutions based on renewable carbon, in which functional biochar, biomethane, biomass hydrogen and valued biogenic CO₂ are combined, generating at the same time emission reduction, economic opportunities, technological innovation and tangible benefits for the territory and its inhabitants.
