How to Produce High-Quality Organic Fertilizer from Biogas Digestate: Complete Process & Equipment Guide

Biogas plants and large-scale anaerobic digestion (AD) facilities generate immense volumes of digestate daily. While raw solid biogas digestate is rich in organic matter, humic substances, and residual N-P-K nutrients, applying it directly to farmland presents severe challenges—such as high water content, odor, fermentation heat burning root systems, and secondary weed/pathogen hazards.

Transforming biogas digestate into commercial, standardized powder or granular organic fertilizer solves the environmental disposal bottleneck and creates a high-margin revenue stream.

This comprehensive technical guide breaks down the industrial production process, equipment requirements, operational parameters, and plant design considerations.

1. Physical & Chemical Properties of Biogas Digestate

Before designing a fertilizer production line, it is essential to analyze the feedstock characteristics:

2. Complete Step-by-Step Production Process

Converting wet anaerobic digestate into marketable organic fertilizer involves two main phases: Front-end Fermentation & Maturation and Back-end Granulation & Packaging.

Step 1: Solid-Liquid Dewatering (Pre-treatment)

Raw anaerobic biogas digestate discharged from the digester contains 88%–95% water.

• Key Equipment: Heavy-duty Screw Press Solid-Liquid Separator or Decanter Centrifuge.
• Target: Separate the liquid effluent from the fiber residue, bringing the solid cake moisture down to 60%–65%.
• Output: Solid fibers are conveyed to the composting workshop; the liquid fraction is routed to wastewater treatment or formulated into liquid bio-fertilizer.

Step 2: Feedstock Formulation & C/N Adjustment

Solid biogas digestate has a low C/N ratio (typically 12:1 to 18:1) and a dense, compacted texture with poor air permeability. Aerobic composting requires a C/N ratio of 25:1 to 30:1.

• Auxiliary Materials: Add dry carbonaceous agricultural residues such as chopped straw, rice husks, wood chips, sawdust, or peanut shells (10%–25% by volume).
• Bio-Inoculants: Add specialized aerobic microbial decomposition inoculants (Bacillus subtilis, Trichoderma, actinomycetes) at 0.1%–0.2% dosage to accelerate humification and odor control.
• Mixing Equipment: Horizontal Double-Shaft Ribbon Mixer or Batching Feeder.

Step 3: High-Temperature Aerobic Composting

Aerobic fermentation is the core step to eliminate pathogens, weed seeds, and parasite eggs while decomposing unstable organic compounds into stable humic acids.

• Composting Systems:
  1. Windrow Composting (Crawler / Wheel-type Compost Turner): Ideal for medium-scale outdoor or ventilated shed operations; piles are 1.5–2.5m wide and 0.8–1.5m high.
  2. In-Vessel / Concrete Trough Composting (Groove Type Hydraulic Turner): High space efficiency, centralized exhaust air collection, and automated turning.
• Process Parameters:
  • Temperature Control: Maintain heap core temperature between 55°C and 65°C for at least 7–10 days (destroys phytotoxins, Salmonella, and weed seeds).
  • Moisture Management: Moisture naturally decreases from 60% down to 30%–35% through self-generated biological heat evaporation.
  • Turning Frequency: Turn every 24–48 hours during the thermophilic phase to ensure oxygen levels remain above 10%.
• Duration: 12 to 18 days active fermentation, followed by 7 to 10 days secondary aging/curing.

Step 4: Crushing, De-lumping & Fine Screening

Composted material contains lumps, fiber clusters, and oversized particles that must be homogenized.

• Semi-Wet Material Crusher: High-speed vertical chain crusher or cage crusher designed specifically for high-moisture organic material without clogging.
• Rotary Drum Screen / Vibrating Screen: Separates materials through a 3–5 mm mesh. The qualified fine powder can either be bagged directly as Refined Powder Organic Fertilizer or transferred to the granulation line.

Step 5: Granulation (Pelletizing)

Granular fertilizer offers superior storage stability, zero dusting, compatibility with mechanical seed drills, and higher market commercial value compared to powder.

• Recommended Granulator Technologies:
  • New Type Organic Fertilizer Wet Granulator: Uses high-speed mechanical agitation and aerodynamic forces to create dense, spherical pellets directly from fibrous digestate.
  • Disc Pan Granulator / Rotary Drum Granulator: Suitable for high-capacity production lines (5–20 TPH), producing smooth, round pellets with 90%+ balling rates.
  • Double Roller Extrusion Granulator: Dry/semi-dry compaction without requiring high-heat drying systems—ideal for small-to-medium operations.

Step 6: Rotary Drying, Cooling & Particle Grading

Granules formed in the wet state contain approximately 20%–25% moisture and require drying to prevent mold growth and caking.

1. Rotary Drum Dryer: Utilizes indirect or direct low-temperature hot air (gas, biomass, or waste heat from biogas generator sets) to reduce moisture below 15%. (Air temperature is maintained under 120°C to preserve heat-sensitive beneficial microbes and organic carbon).
2. Rotary Cooler: Quickly cools the hot pellets down to ambient temperature (+5°C) to prevent thermal degradation during storage.
3. Secondary Rotary Screener: Separates finished pellets (typically 2.0 mm – 4.5 mm diameter). Oversized granules are recycled to the crusher; fines are returned to the granulator.

Step 7: Film Coating & Automated Packaging

• Rotary Coating Machine: Applies anti-caking agent or functional biological bacteria sprays onto granule surfaces.
• Automatic Weighing & Bagging Machine: Fast, pneumatic valve bag or open-mouth bagging (25 kg / 50 kg / 1000 kg jumbo bags) with automatic heat-sealing or stitching.

3. Core Equipment List for Biogas Digestate Fertilizer Plant