Digestate is the nutrient-rich material left over after organic waste breaks down inside an oxygen-free tank, a process called anaerobic digestion. When bacteria consume feedstocks like food scraps, animal manure, or sewage sludge, they produce two outputs: biogas (used for energy) and digestate (used primarily as a soil amendment or fertilizer). It comes out as a mix of solid and liquid components, and its nutrient profile depends heavily on what went into the tank.
How Digestate Is Produced
Anaerobic digestion happens inside sealed reactors where complex communities of microorganisms break down organic matter without oxygen. The process works in stages. First, bacteria break large organic molecules into simpler sugars and amino acids. Then other microbes convert those into acids, which a final group of organisms transforms into methane gas and carbon dioxide. What remains after this chain of biological activity is digestate.
The whole cycle typically takes two to four weeks, depending on the temperature inside the reactor and the type of feedstock. Reactors running at higher temperatures (around 50 to 55°C) process material faster than those running at a more moderate 35°C. The feedstock can be almost any biodegradable material: crop residues, livestock manure, food processing waste, municipal food waste, or wastewater solids. Many commercial digesters blend multiple feedstocks to optimize gas production and nutrient balance.
What’s in Digestate
Digestate retains most of the nutrients that were in the original feedstock, since anaerobic digestion breaks down carbon compounds for energy but leaves behind nitrogen, phosphorus, and potassium. The pH typically falls between 6.9 and 8.9, making it suitable for most agricultural soils, though food waste digestate can occasionally be more acidic (as low as 4.4 in some cases).
Nutrient concentrations vary widely. Nitrogen content ranges from under 0.1% to around 5% of dry matter, phosphorus from trace amounts to about 1.5%, and potassium from near zero to over 1%. A digestate made from food waste and cow manure, for example, can contain up to 5% nitrogen, while one produced solely from food waste in a different system might contain just 0.01%. This variability is why testing each batch matters for anyone using digestate on cropland.
One important difference between digestate and raw manure is how quickly plants can access the nitrogen. During digestion, bacteria convert much of the organic nitrogen into ammonium, a form plant roots absorb readily. In field trials comparing anaerobically digested cattle manure to undigested manure, barley took up 41% of the nitrogen from digestate versus only 22% from raw manure. That higher availability means digestate acts more like a fast-release fertilizer, while raw manure releases nutrients slowly over months.
Solid and Liquid Fractions
After leaving the digester, whole digestate is often separated into a solid fiber fraction and a liquid fraction, each with a distinct nutrient profile. The solid portion is rich in phosphorus and organic matter, making it useful as a soil conditioner. The liquid portion carries most of the readily available nitrogen (as ammonium) and works more like a liquid fertilizer.
The two most common separation technologies are screw presses and centrifuges. Centrifuges capture about 60% more solids and phosphorus than screw presses, but they consume 4.5 times more energy per ton of digestate processed. Screw presses are the more energy-efficient choice when the goal is to recover ammonium nitrogen in the liquid stream. Adding chemical conditioners before centrifuging can lower the solids content of the liquid fraction by about 30%, producing a cleaner product for field application.
Benefits for Soil Health
Digestate improves soil in ways that synthetic fertilizers cannot. Because it contains organic matter, it builds soil structure over time, increasing the soil’s ability to hold water and reducing the need for irrigation. The EPA lists several documented benefits of land-applying digestate: increased organic matter content, reduced soil compaction, less erosion and nutrient runoff, and improved plant growth. These structural improvements compound year after year, unlike mineral fertilizers that supply nutrients without contributing to the soil’s physical condition.
For farmers, replacing a portion of synthetic fertilizer with digestate also lowers input costs and closes a nutrient loop. The nitrogen, phosphorus, and potassium in digestate originated in food or feed crops, so returning those nutrients to farmland keeps them cycling through the agricultural system rather than ending up in landfills or waterways.
Ammonia Emissions During Application
The same trait that makes digestate a good fertilizer (high ammonium content) also creates a risk: ammonia can volatilize into the air after spreading, especially when the digestate has high dry matter content. Field trials in Denmark tested multiple strategies for reducing these losses.
Separating the liquid fraction before spreading proved consistently effective, cutting ammonia emissions by 33 to 83% compared to spreading unseparated digestate. Among application methods, trailing hose and trailing shoe performed similarly, but adding a harrowing tine behind the trailing shoe to lightly bury the digestate reduced emissions by an additional 34 to 39%. Acidifying the digestate with sulfuric acid helped in some cases but proved inconsistent, likely because the effect depends on dry matter content. For anyone managing digestate, applying the liquid fraction and incorporating it into soil quickly are the most reliable ways to minimize nitrogen loss to the atmosphere.
Pathogen Destruction and Safety
Raw organic waste can harbor harmful bacteria like Salmonella and E. coli. The digestion process itself reduces pathogen levels, but how effectively depends on temperature. In a thermophilic reactor running at 53°C, 90% of Salmonella is destroyed in just 0.7 hours, and 90% of E. coli in 0.4 hours. At a more moderate 35°C, the same level of reduction takes 2.4 days for Salmonella and 1.8 days for E. coli.
For higher-risk feedstocks like animal byproducts, regulations typically require a separate pasteurization step: heating the material to 70°C for one hour before or after digestion. Equivalent sanitation can be achieved at lower temperatures held for longer periods, such as 55°C for six hours or 60°C for 2.5 hours inside a thermophilic reactor. Finished digestate that meets quality standards must contain fewer than 1,000 colony-forming units of E. coli per gram, and Salmonella must be completely absent in a 25-gram sample.
Quality Standards and Certification
In Europe and the UK, digestate can achieve “end of waste” status through certification under PAS 110, a specification developed by the British Standards Institution. Once certified, digestate is no longer classified as waste, which simplifies its use and sale as a fertilizer product. PAS 110 requires producers to control input materials, maintain a quality management system, and conduct hazard analysis at critical control points throughout production.
Heavy metal limits are a key part of the standard. Under the UK’s PAS 110, cadmium cannot exceed 1.5 mg per kilogram of dry matter, lead 200 mg, mercury 1.0 mg, nickel 50 mg, copper 200 mg, and zinc 400 mg. The EU’s fertilizing products regulation (2019/1009), which PAS 110 now reflects, sets its own thresholds: cadmium at 2 mg/kg, lead at 120 mg/kg, mercury at 1 mg/kg, and nickel at 50 mg/kg. These limits ensure that repeated application of digestate to farmland does not cause toxic metals to accumulate in the soil over decades.
Certified digestate must also contain at least 20% dry matter (for the solid fraction sold as a soil improver) and a minimum organic carbon content of 7.5% by mass. Regular batch testing verifies that every shipment meets these thresholds before it reaches a farm.