What Is Digestate and What Is It Used For?
Digestate is the stabilized effluent that leaves a biodigester: an NPK-rich biofertilizer. Composition, handling and agronomic value explained.
Digestate is the stabilized liquid or semi-solid material that leaves a biodigester after the anaerobic digestion of biomass (manure, agro-industrial effluents, organic waste). It is what remains once bacteria have converted the degradable organic matter into biogas. Far from being waste, it is a biofertilizer rich in nitrogen, phosphorus and potassium (NPK), ready to be applied to the soil.
In other words: for every tonne of substrate that enters the digester, part of the carbon leaves as biogas (methane and CO2) and the rest leaves as digestate, retaining virtually all the mineral nutrients of the original material.
How it forms and what it contains
During anaerobic digestion, microorganisms break down carbohydrates, proteins and fats in the absence of oxygen. Carbon is released as biogas, but nitrogen, phosphorus, potassium and micronutrients stay in the liquid phase. The result is a product with already-mineralized organic matter and greatly reduced pathogens and weed seeds.
Digestate is usually separated into two fractions with a screw press or centrifuge:
- Liquid fraction: high in ammonium nitrogen (NH4-N), rapidly available to the plant. Applied via fertigation or tanker.
- Solid fraction: rich in phosphorus and organic matter, useful as a soil amendment or compost.
Typical composition (indicative values)
Composition varies widely with the substrate and retention time, but these ranges are representative of digestate from manure and agro-industrial effluents:
| Parameter | Typical range | Note |
|---|---|---|
| Dry matter (TS) | 2–8 % | Lower after phase separation |
| Total nitrogen (N) | 2–6 kg/m³ | 50–70 % as ammonium N |
| Phosphorus (P2O5) | 0.5–3 kg/m³ | Concentrates in the solid fraction |
| Potassium (K2O) | 2–6 kg/m³ | Highly soluble, in liquid phase |
| pH | 7.5–8.5 | Slightly alkaline |
Why it matters
Digestate replaces synthetic mineral fertilizers and closes the nutrient loop of agribusiness. Compared with raw manure, it has clear advantages: less odor, nitrogen that is more available to the crop, and a markedly lower pathogen and seed load thanks to biological stabilization. For the farmer, this means an on-site input that lowers fertilization costs and brings order to effluent management before the environmental authority.
Advantages over untreated manure
- Lower odor emissions during storage and application.
- Fast-uptake ammonium nitrogen for the crop.
- Significant reduction of pathogens and viable weed seeds.
- Homogeneity: more predictable dosing and pumping.
Where it fits in Tech Tank projects
In a turnkey biogas plant, digestate is not a separate problem to solve: it is part of the design. Sizing the industrial biodigester determines how much digestate is generated and at what quality, which in turn sizes the storage lagoons or tanks, the phase separation and the field-application logistics. In dairy-farm biodigester projects, the liquid fraction returns to the pastures as fertilizer, and storage infrastructure must cover the months when application is not possible due to rain or crop stage.
Tech Tank integrates the biodigester, storage and covers within its biogas and biodigesters line, so that digestate management is resolved from the engineering stage. If you are evaluating a project and want to know how much digestate your operation would generate, contact us for a technical assessment.
Frequently Asked Questions
Is digestate the same as biodigester sludge?
Not exactly. Digestate is the full stream leaving the reactor. "Sludge" usually refers to the more solid, concentrated fraction separated afterwards. Both come from anaerobic digestion but are handled differently.
Can digestate be applied directly to crops?
The liquid fraction is applied via fertigation or tanker, respecting doses based on available nitrogen and crop stage. It is advisable to verify current environmental regulations and the permit with MADES before defining the application plan.
How much digestate does a biogas plant produce?
As a rule of thumb, the mass of digestate is close to the mass of substrate that enters minus the biogas released. In practice, much of the liquid volume is retained, which is why storage is sized carefully.
Does digestate smell?
Far less than raw manure. Anaerobic stabilization reduces volatile compounds and organic acids, though some ammonia odor may remain during application.
Can it be used as the only fertilizer?
It provides NPK and organic matter, but the nutrient balance depends on the substrate. It is usually complemented with soil analysis and, if needed, corrective mineral fertilization.