Uses of Solids and By-Products of Anaerobic Digestion

Anaerobic Digestate Pile at Scenic View Dairy in Fennville, MI

Anaerobic Digestate Pile, Scenic View Dairy, MI. Photo: M.C. Gould, MSU Extension

Anaerobic digestion generates a wide range of byproducts that farmers can use in their farming operations or sell. Beyond biogas used to generate electricity or as fuel, and liquids used for fertilizer or soil amendments, there are solid byproducts, which have a wide range of applications.

Table of Contents:

Value-added opportunities for fiber from digesters

Closeup of anaerobic digestate

Closeup of digestate. Photo: M.C. Gould, MSU Extension

Undigested biomass (referred to as digestate solids, fiber or biofiber) contained in the effluent (digestate) of anaerobic digesters provides opportunities for value-added byproducts. Organic fertilizer, livestock bedding, compost, fuel pellets, and construction material (medium density fiberboard and fiber/plastic composite materials) are a few examples of value-added byproducts that could be created from digestate solids.

Separating solids from the digestate

Solids can be extracted from the digestate using solid-liquid separation technologies such as slope screens, rotary drum thickeners and screw-press separators. Common solid-liquid equipment can produce digestate solids with a moisture content of 18 to 30%. The volume and the moisture content of the separated solids will vary depending on the technology used. Digestate solids are high in fiber, consisting mainly of fibrous undigested organic material (lignin and cellulose), microbial biomass, animal hair, and nutrients.


During the anaerobic digestion process, nutrients contained in the feedstock are mineralized. Mineralized nutrients are easily used by a crop. Digestate solids contain higher concentrations of plant-available nitrogen and phosphorus compared to as-excreted manure, according to research. The high carbon content of digestate solids adds organic matter to the soil and improves the water holding capacity of the soil. Actual nutrient content of digestate solids will vary depending on feedstocks, digester type, management, and solid-liquid separation technology. Digestate solids as a fertilizer source can be used “as separated” (wet), blended with other materials and composted or dried and pelletized.

Anaerobic Digester Fiber - Freestall Bedding

Digestate Solids used in freestall at Crave Brothers Dairy, WI Photo: M.C. Gould, MSU Extension

Livestock bedding

Bedding for livestock is another opportunity for putting digestate solids to use. Utilizing digestate solids for bedding provides a significant cost offset to dairy and livestock farms. In addition, excess solids may be sold to neighboring farms for bedding or soil amendment, creating a revenue stream and route for nutrient export. Bedding with digestate solids requires intensive management to ensure that a healthy environment, with low pathogen concentrations, is provided for the animals.

Other value added products using digestate solids

Digestate solids can also be used as substrate in compost, providing sources of carbon and nutrients. Solids can be dried and pelletized for use as fertilizer or fuel. The maximum energy content of livestock manure is 8,500 Btu per pound; however ash and moisture content reduce the energy potential. As excreted, livestock waste typically has an energy content between 1,000 and 2,000 Btu per pound.

Another developing opportunity for digestate solids is as a renewable construction material. Medium-density fiberboard and wood/plastic composite material have emerged as important engineered construction materials. These engineered materials can also be created using digestate solids without sacrificing mechanical or aesthetic properties, research indicates.

Organic potting soil made with digestate

Organic potting soil made with digestate as medium. Photo: M.C. Gould, MSU Extension

References & Additional Resources

  • Gould, M.C. and M.F. Crook. 2009. On-farm Anaerobic Digester Operator Handbook. Michigan State University. East Lansing, MI.
  • Kammel, D.W. 2004. Bedded Pack Housing for Dairy Cows. Minnesota/Wisconsin Engineering Notes.
  • Matuana, L. and M.C. Gould. 2006. Promoting the Use of Digestate from Anaerobic Digesters in Composite Materials: Final Report. Community Energy Project Grant No. PLA-06-42.
  • Zering, K. and B. Auvermann. April, 2009. Livestock and Poultry Environmental Learning Center. Value of Manure as an Energy Source.

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Peer Reviewers

  • Teodoro Espinosa-Solares

Feedstocks for Biogas

Anaerobic digestion of manure and other feedstocks produces biogas which can be burner to make energy on farms. Learn how to evaluate a feedstock and which ones to exclude for biogas production.

Anaerobic Digester in Charlotte, VT.  Photo: Caragh Fitzgerald, University of Maine.



A great variety of organic material can be used in anaerobic digesters as a feedstock for generating biogas. However, there are scientific, engineering and legal limits to what can be added successfully to a digester. In addition, the feedstock needs to be a liquid mixture with an appropriate moisture content. For example, mesophilic complete mix tank digesters (the type most commonly used today) typically operate best with a mixture of 4 to 8% solids in water. Digesters require various moisture contents, depending on the design and operation of the system.

Feedstocks for Anaerobic Digestion

Manure in dairy barn.

Most easily biodegradable biomass materials are acceptable as feedstocks for anaerobic digestion. Common feedstocks include livestock manure, food-processing waste, and sewage sludge. The energy production potential of feedstocks varies depending on the type, level of processing/pretreatment and concentration of biodegradable material. Listed below are feedstocks that can be commonly used in anaerobic digesters:

  • Livestock manures
  • Waste feed
  • Food-processing wastes
  • Slaughterhouse wastes
  • Farm mortality
  • Corn silage (energy crop)
  • Ethanol stillage
  • Glycerine as the product from biodiesel production
  • Milkhouse wash water
  • Fresh produce wastes
  • Industrial wastes
  • Food cafeteria wastes
  • Sewage sludge

Livestock manures are generally lower-energy feedstocks because they are predigested in the gastrointestinal tracts of the animals. Manure, however, is an easy choice for anaerobic digestion because it generally has a neutral pH and a high buffering capacity (the ability to resist changes in pH); contains a naturally occurring mix of microbes responsible for anaerobic degradation; provides an array of nutrients, micronutrients, and trace metals; is available in large quantities; and can be transferred by pump.

Animal wastes containing bedding such as chicken litter with substantial quantities of wood chips or sawdust can be used successfully in anaerobic digestion. The woody material, which degrades very slowly because of its lignin structure, is essentially passed through without digestion, and retention times are based on digestion of the manure.

Blending of energy-dense feedstocks with livestock manure is a common practice to maximize biogas production by optimizing nutrient levels and providing buffering capacity. The use of manure as a base for anaerobic digestion is important because many of the energy-dense feedstocks, such as food-processing waste and ethanol stillage, are acidic, contain little if any naturally occurring microbes, and oftentimes lack the nutrients (nitrogen, trace elements, vitamins, etc.) necessary for anaerobic digestion. Potentially, farms operating anaerobic digestion systems could take on additional wastes and benefit from increased gas production as well as tipping fees.

Materials to Be Excluded from Anaerobic Digesters

Materials that should be excluded as feedstock from anaerobic digesters include those containing compounds known to be toxic to anaerobic bacteria, poorly degradable material, and biomass containing significant concentrations of inorganic material. Poorly biodegradable materials require higher retention times, meaning they must spend more time in the anaerobic digester to be broken down and converted into biogas.

Biogas equipment for electricity generation. Photo: Daniel Ciolkosz, Extension Associate, Penn State.

Inorganic materials, on the other hand, contain no carbon and cannot be converted into biogas. Materials such as sand bedding do not contribute to the biogas potentialo and may cause operational problems such as pipe clogging, premature equipment wear and volume reduction due to sludge accumulation. Also, the feedstock containing too much ammonium or sulfur should be avoided, because ammonium and sulfur inhibit anaerobic organisms.

Evaluating Feedstock Biogas Potential

The biogas potential of different feedstock materials or feedstock combinations is often difficult to predict due to differences in the source, processing, volatile solids concentration, chemical oxygen demand, moisture content, and/or inclusion of toxic compounds. The total biogas potential assay, also known as the biochemical methane potential (BMP) assay, provides an efficient and economic method for estimating biomass conversion and biogas yield of feedstocks or feedstock blends.

BMP assays are a multifaceted approach to evaluating the potential to produce biogas. BMPs are a practical, lab-based approach to identifying and evaluating potential feedstocks for anaerobic digestion. Potential anaerobic digestion feedstocks are commonly evaluated by three criteria.

  1. Feedstock characterization: Both before and after BMP assay, includes pH, chemical oxygen demand (COD), total solids (TS), and volatile solids (VS). Characterization results found prior to the experiment are used to determine the quantity of feedstock needed to maintain the BMP assay for as much as 30 days. Characterization results following the completion of the BMP assay are used to evaluate the anaerobic digestion process in terms of the destruction of the organic material.
  2. Total biogas production: Is measured throughout the BMP either through manual means or continuously by commercial software designed for tracking gas production. Biogas can be scrubbed of the carbon dioxide by running it through a potassium/sodium hydroxide solution to monitor only methane production or can be left unscrubbed to monitor the total biogas production.
  3. Biogas analysis: Biogas composition can be investigated by means of a gas chromatograph during the BMP assay. Though the capital investment is large, gas chromatographs provide accurate measurements of the constituents of the biogas produced during the BMP. Gas chromatographs can be set up to determine the concentrations of methane, carbon dioxide, nitrogen, and hydrogen sulfide gases.

The BMP assay is a combination of a single feedstock or feedstock blend, inoculum, and stock solutions in a batch system. Inoculum is used to seed the feedstock with an active anaerobic culture to initiate activity and reduce any lag time required for establishment of a culture. Stock solutions are added to assure that macronutrients, micronutrients, and vitamin deficiencies do not limit biogas production. BMP evaluations should always be completed in replication and results should be verified at pilot or full-scale, and it is strongly recommended that full-scale designs not be based on BMP results because full-scale digesters are often run at continuous mode while BMP tests are batch mode.


The biogas potential of feedstocks is an important factor when considering anaerobic digestion on your farm. But other considerations, such as economics, regulatory issues, feedstock availability on and off the farm, and end use of the biogas, should also be evaluated.


  • Chynoweth, D.P., C.E. Turick, J.M. Owens, D.E. Jerger and M.W. Peck. 1993. Biochemical Methane Potential of Biomass and Waste Feedstocks. Biomass & Bioenergy 5:95-111.
  • Liu, Y., S.I. Miller, and S.A. Safferman. 2009. Screening co-digestion of food waste water with manure for biogas production. Biofuels, Bioproducts, Biorefining 3:11–19
  • Owen, W.F., D.C. Stuckey, J.B. Healy, L.Y. Young, and P.L. McCarty. 1979. Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Research 13:485-492.
  • Steffen, R., Szolar, O., and Braun, R. 1998. Feedstocks for Anaerobic Digestion. Institute for Agrobiotechnology Tulln, University of Agricultural Sciences, Vienna.
  • Speece, R.E. 1996. Anaerobic Biotechnology for Industrial Wastewater. Archae Press, Nashville.

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