Biogas Plants and Digestate Processing

During the processing of digested slurry in biogas plants, the liquid digested slurry is separated after the fermentation process into a phosphorus-rich solid fraction and a liquid phase with high nitrogen and potassium content. This separation technology is crucial for the efficient recovery of nutrients, compliance with country-specific legal application limits, and the reduction of transportation and storage costs. Precise dewatering and optimized solids management form the foundation for the economical and sustainable operation of any biogas plant.

Biogas production from renewable raw materials, liquid manure, solid manure, and organic waste (e.g., bio-waste, food waste, and production residues) has established itself as a cornerstone of the renewable energy supply: Biogas is a gas derived from biomass and consists of 55–70% methane (CH₄). Today, it is primarily used in the processing of biogas into biomethane and its subsequent injection into the natural gas grid, as well as through the capture of CO2 and its commercialization. In addition, the digested residue is processed into high-quality fertilizer products. Thousands of biogas plants worldwide produce large quantities of digested residue daily, which must be processed efficiently. The resulting digestate contains valuable nutrients such as nitrogen, phosphorus, and potassium—but its high water content of approximately 90–97% (dry matter content of approximately 3–10%) makes transportation and application costly and logistically challenging.

Added to this is the increasing pressure from fertilizer regulations and environmental requirements: Especially in regions with high livestock densities, nitrogen surpluses require targeted processing and redistribution of nutrients. The separation of fermentation residues plays a central role in this process. The solids recovered through decanting can be transported to nutrient-poor areas as a concentrated, transportable fertilizer, while the liquid phase can be used on-site as liquid fertilizer or further processed into a fertilizer concentrate. At the same time, biogas production offers environmental benefits: When used for energy, only the carbon dioxide previously sequestered by plants is released. This makes biogas significantly more climate-friendly than fossil fuels.

With its decanter centrifuges, Flottweg offers customized solutions for the complete treatment of digested slurry in biogas plants and industrial applications. Our machines reliably meet the demands of modern biogas plants—whether with or without the use of polymers. In addition, we achieve maximum solids recovery, an exceptionally low solids content in the liquid phase, and ensure round-the-clock availability. We draw on decades of experience to optimize the treatment process through the selection of the right decanter, effective wear protection, and intelligent process automation.

Decanter centrifuges for Biogas Production

For maximum dry matter content and high throughput in biogas plants—designed for continuous 24/7 operation with a wide variety of substrates and minimal to no use of polymers

The Fermentation Residue Treatment Process

After anaerobic digestion for biogas production, the digested residue still contains valuable organic substances and plant nutrients. Through targeted processing, these nutrients can be optimally utilized and the digested residue can be recycled as a high-quality fertilizer. First, coarse solids such as straw, corn cobs, or plastic debris are removed by screening. The pretreated substrate, with a solids content of up to 35% DM, is then conveyed directly into the decanter centrifuge. Polymer dosing is optional and is customized and optimized by Flottweg for each specific substrate mix.

The process produces acentrate (liquid fraction) that contains the majority of the nitrogen in a plant-available, ammonium-bound form. The dewatered solids are stackable, storable, and ideally suited for use as a phosphorus fertilizer for transport by truck.

Overall, the treatment process comprises several steps: from the receipt of the raw digested residue to its separation into recoverable solids and the liquid phase. In biogas production at biogas plants, the biological process is preceded by the chemical stages of hydrolysis, acidogenesis, acetogenesis, and methanogenesis.

Environmental Responsibility Meets Economic Efficiency

Flottweg decaners enable efficient separation of digested sludge and lay the foundation for targeted nutrient recovery. While phosphorus is predominantly concentrated in the solid fraction, nitrogen and potassium remain in the liquid phase, with nitrogen largely present in the form of ammonium, which is readily available to plants. This nutrient distribution facilitates application tailored to specific needs and supports compliance with state-specific fertilizer regulations (DüV) as well as the EU Nitrates Directive.

Biogas is produced from renewable raw materials, such as energy crops (e.g., silage corn, rapeseed), manure, and organic waste, which are available in virtually unlimited quantities. Therefore, Flottweg’s technology further promotes the circular economy and supports the efficient use of resources.

At the same time, mechanical dewatering significantly reduces the volume that needs to be stored or transported: 100 m³ of digested residue typically yields 15–20 m³ of solids and 80–85 m³ of liquid phase. The significantly reduced volume of solids brings relief to storage capacity and noticeably lowers transportation costs.

What Role Will Biogas Play in the Future?

Biogas, also known as biomethane, has a promising future as a key component of renewable energy. It is not dependent on weather conditions and can be flexibly converted into electricity, heat, or fuel. Its use contributes significantly to reducing greenhouse gas emissions, secures regional and national energy supplies, and reduces dependence on energy imports. Thanks to technological advancements, more efficient production processes, and its integration into circular economy concepts, the importance of biogas will continue to grow in the future.