Composting Digestate, A Guide for Anaerobic Digester Operators

Solid digestate can be composted, and composting is one of the most common ways to turn it into a stable, saleable soil product. The digester has already removed much of the easily degraded carbon, so the material that comes out is wet, rich in ammonium and short on energy and structure. A working digestate compost line therefore depends on three things: dewatering, a dry carbon-rich bulking agent, and controlled aeration.

Key takeaways

  • Digestate is the residual material left after digestion. The U.S. EPA notes that its solid and liquid portions are usually separated and that each needs some post processing before its value is realized.
  • Digestion converts organic nitrogen to ammonium and removes part of the volatile solids, so digestate behaves differently from the raw feedstock: more ammonia risk, less heat.
  • Cornell’s general composting targets still apply: a starting carbon to nitrogen ratio near 30:1 and 50 to 60% moisture. Digestate rarely meets either without amendment.
  • Mesophilic or thermophilic digestion is not the same as a pathogen kill step. Under the federal biosolids rule, anaerobic digestion is listed as a process to significantly reduce pathogens, and composting at 55 degrees Celsius is listed as a process to further reduce them.
  • Rules for digestate handling vary by state. California, for example, gives uncontained digestate 24 hours to be stored in a sealed structure, composted or removed.
  • Run trial batches before you fix a recipe. Digestate from two digesters is rarely the same material.

What digestate is, and why it is not a finished product

The U.S. EPA AgSTAR program describes digestate as the residual material left after the digestion process, composed of liquid and solid portions that are often separated and handled independently. EPA lists the possible uses of the solids as animal bedding, fertilizer, compost and soil amendment, each “with appropriate treatment.” That qualifier is the reason this guide exists.

Sustainable Generation (SG) makes the same point from the operating side. Its digestate page says that, in SG’s experience, digestate needs further treatment to create a marketable product, and that composting reduces its odors and finishes the biological process. An SG article on mobile organics processing adds that digestate frequently needs additional stabilization, moisture management and curing before it becomes a consistent soil product.

This guide covers the solid fraction. Liquid digestate is a fertilizer and wastewater question and is managed separately.

How digestate differs from the feedstock that went in

A digester changes the material in ways that matter at the compost pad. The figures below come from a review published by the eXtension Farm Energy community and describe digested dairy manure. Food waste and mixed-waste digestate follow the same pattern, but you need your own lab data.

Property What the digester does What it means for composting
Nitrogen form Organic nitrogen is mineralized to ammonium while total nitrogen is conserved. Washington State University data cited in the review show ammonium rising from about 40% of nitrogen entering the digester to about 60% leaving it Ammonia is released as soon as the material is aerated and heated. Carbon has to be added to hold it
Energy One study cited in the review found total and volatile solids reduced by 30 to 40% Less readily degradable carbon is left to generate heat, so a pile can be slow to reach pathogen reduction temperatures
pH The review says pH stays fairly neutral inside the digester and cites one report of higher pH in digested manure Higher pH favors ammonia gas over dissolved ammonium
Moisture and structure Solids leave a separator or dry digester wet and fine Low free air space. The pile slumps and goes anaerobic without a coarse bulking agent
Phosphorus and potassium Conserved through digestion The compost carries the nutrients forward, so product analysis matters for buyers

Two consequences follow. First, the carbon to nitrogen ratio of digestate is lower than that of the original feedstock, because carbon left as biogas while nitrogen stayed. Cornell Waste Management Institute explains in its compost chemistry notes that at ratios below about 30:1 nitrogen is supplied in excess and is lost as ammonia gas. Second, the material is biologically quieter than raw food waste or manure. It needs help to compost, not restraint.

Dewatering: get the moisture down before the pad

Cornell’s compost physics notes put the optimum moisture for composting at 50 to 60%, with decomposition slowing and anaerobic pockets forming above about 65%. Digestate solids usually start above that range, and every point of water you leave in has to be absorbed by amendment you must buy, haul and screen out later.

Practical points for the dewatering step:

  • Measure the cake, daily at first. Separator performance drifts, and a recipe built on last month’s moisture will be wrong.
  • Decide where the liquid goes. The pressate carries dissolved ammonium and belongs in the digester’s liquid management system, not on the compost pad.
  • Keep storage short and contained. California’s in-vessel digestion rules, covered below, give uncontained digestate 24 hours.

Bulking and recipe: adding carbon, structure and air space

The bulking agent does three jobs for digestate: it raises the carbon to nitrogen ratio, it soaks up water, and it holds open the pores that let air move through the pile. Cornell’s table lists wood chips or sawdust at 100 to 500:1, bark at 100 to 130:1, straw at 40 to 100:1 and autumn leaves at 30 to 80:1. Our list of compost feedstocks covers each amendment in more detail.

A workable sequence for setting the recipe:

  1. Sample and test the digestate for moisture, carbon, nitrogen, ammonium, pH, bulk density, and any metals or physical contaminants your permit names.
  2. Test the candidate amendments for the same basic values.
  3. Calculate a blend that lands near 30:1 and inside the moisture range. Cornell publishes the mixture equations, and SG’s How It Works page describes 55 to 65% as the initial moisture it aims for in a covered pile.
  4. Check the blend for structure, not only chemistry. A mix can hit both numbers and still be too dense to aerate. Squeeze it, stack it, and measure bulk density.
  5. Build trial batches with two or three ratios and log temperature and oxygen. Santa Barbara County did this at its digester site: SG’s Goleta, California, profile describes multiple batch tests run to determine the proper mix recipe before the full-scale system was built.
  6. Mix thoroughly and build the pile the same day.
  7. Lock the recipe, then recheck it whenever the digester’s feedstock, retention time or dewatering equipment changes.

Aeration, temperature and pathogen reduction

Digestion is not a sanitation guarantee. The federal biosolids rule makes the distinction clear: in Appendix B to 40 CFR Part 503, anaerobic digestion (15 days at 35 to 55 degrees Celsius, or 60 days at 20 degrees) is listed among the processes to significantly reduce pathogens, while composting is listed among the processes to further reduce pathogens (PFRP). For an aerated static pile or in-vessel system, PFRP means holding 55 degrees Celsius or higher for three days. For a windrow, it means 55 degrees or higher for 15 days with at least five turnings. Part 503 applies directly only where the digester took in sewage sludge, but the same numbers appear elsewhere, for example in the FDA produce safety rule for composted soil amendments of animal origin, so check which standard your permit and your buyers use.

Because digestate has less energy left in it, reaching and holding 55 degrees is the part of the process most worth proving in a pilot. Three things help:

  • Enough degradable carbon in the blend. Fresh ground green material brings energy as well as structure. Old, weathered overs bring structure only.
  • Aeration matched to demand. Too much air strips heat and ammonia from a low-energy mix. Feedback control that responds to pile oxygen and temperature avoids both over-aeration and oxygen starvation. Our guide to compost process control explains the ranges.
  • Insulation and containment. A cover or a layer of finished compost keeps the outer shell of the pile at temperature and limits ammonia loss.

SG’s approach is covered aerated static pile composting under a GORE® Cover, with positive aeration controlled by oxygen and temperature feedback and no building or biofilter. SG states that the process is designed to meet the EPA Part 503 PFRP time and temperature requirements, and its Goleta profile reports that PFRP temperatures (3 days at 131 degrees Fahrenheit) were met in the digestate pilot. EPA does not certify compost systems, so ask any vendor for batch temperature records from a comparable feedstock.

On emissions, an SG article on air quality rules notes that nitrogen-rich inputs and odorous sludges, including digestate, create more challenging emission profiles than yard waste alone.

Rules that apply to digestate handling

Digestate sits between two permit worlds, the digester’s and the compost facility’s. Three examples show what to look for in your own state.

California’s in-vessel digestion rules. Under 14 CCR 17896.57, digestate not contained in an in-vessel digester must, within 24 hours, be stored or processed on-site in a sealed container or sealed structure, incorporated in an on-site aerobic compost process, or removed from the site. On-site aerobic composting of digestate is allowed only at large volume in-vessel digestion facilities that hold an In-vessel Digestion Facility Permit, and those facilities must meet the regulation’s sampling, metals, pathogen and physical contamination requirements.

Compost product limits. California’s 14 CCR 17868.3.1 caps physical contaminants larger than 4 millimeters at 0.5% of compost by dry weight. This matters most for digestate from depackaged or mixed-waste organics, where plastic fragments pass through the digester unchanged. See our list on handling depackaged food waste.

Biosolids-derived digestate. If the digester receives sewage sludge, the compost is a biosolids product and Part 503 pollutant, pathogen and vector attraction rules apply. Our biosolids composting guide covers them.

Rules change. Treat these as of October 2026 and read the current text.

Why digestion and composting pair well

The two processes do different jobs. Digestion captures energy from the wettest, most degradable fraction. Composting stabilizes what is left, completes pathogen reduction and makes a product that can be stored, screened and sold where raw digestate cannot. The two lines can share a scale house, receiving area, staff and permits, and the woody fraction that a digester cannot use becomes the bulking agent for its digestate.

Two SG projects show the pairing. At Goleta, Santa Barbara County’s anaerobic digester takes the organic fraction recovered from municipal solid waste, and the county retrofitted an open windrow operation with an SG MOBILE® System to compost the digestate residuals. SG’s profile cites odor containment, stormwater and litter as the site constraints that drove the change. In Arthur, Ontario, Walker All Treat Farms composts digestate alongside source separated organics and yard waste in SG BUNKER® Systems.

Product quality: what buyers and regulators will test

Compost made from digestate is judged on the same measures as any other compost: stability and maturity, pathogens, metals, salts, nutrient content and physical contaminants. Our list of key metrics for compost quality explains each test. Three points are specific to digestate:

  • Stability. Digestate that smells of ammonia or reheats in a stockpile is not finished. Allow curing time and test before sale.
  • Salts and nutrients. Nutrients are conserved through digestion, so the compost can be richer than a yard trimmings product. Give buyers an analysis.
  • Contaminants. What entered the digester is still there. Front-end contamination control is cheaper than screening compost.

FAQ

Can digestate be composted without a bulking agent?

Rarely. Solid digestate is wet, fine and low in carbon relative to nitrogen, so on its own it compacts, goes anaerobic and loses nitrogen as ammonia. A coarse, dry, carbon-rich amendment such as ground wood or yard trimmings supplies air space, absorbs water and brings the mix toward the 30:1 carbon to nitrogen ratio that Cornell describes as ideal.

Does anaerobic digestion kill pathogens?

It reduces them, but it is not treated as a full sanitation step. In the federal biosolids rule, anaerobic digestion is listed as a process to significantly reduce pathogens. Composting that holds 55 degrees Celsius for three days in an aerated static pile is listed as a process to further reduce pathogens. Most compost permits require the composting standard.

Why does digestate smell of ammonia when it is composted?

Digestion converts organic nitrogen into ammonium and tends to raise pH. When the material is aerated and warms up, some of that ammonium leaves as ammonia gas. Adding enough carbon, avoiding over-aeration and keeping the pile covered or capped all reduce the loss, and they keep more nitrogen in the finished compost.

How long does digestate take to compost?

It depends on the blend, the method and the product standard, so a pilot is the reliable way to find out. For its own covered systems, SG’s How It Works page describes a 4 to 8 week treatment time versus 8 to 12 months for open windrow. Curing and screening follow the active phase in any system.

Next step

If you are planning a digestate line, read how covered aerated static pile composting works, then test your own material. SG’s pilot and demonstration program runs batches on your feedstock to set the recipe and produce temperature data for a permit.

Related guides

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