Anaerobic digestion (AD) is a powerful technology for creating biogas and digestate from organic waste. While biogas is of considerable value, the liquid and solid digestate typically present little or negative economic value. In some states, such as Texas, it is illegal to land-apply digestate without expensive pre-treatment processes to remove volatile compounds and pathogens. This is likely to become the norm across the developed world, based on established best practices in Europe. In many cases in N. America, the digestate is sent by truck or pipeline to sewage treatment plants, with the nutrients of these materials ultimately ending up in a waterway via treated sewage discharge.聽
In cases where land application is allowed, the cost to transport and apply digestate, (which is heavy and wet) on agricultural fields creates a low value or negative economic factor for the AD facility operator. Land-applied digestate also continues to emit methane, even if the material was treated in an inorganic sterilization process.
The most viable way to stabilize digestate outputs without significant greenhouse gas emissions, is to fully compost the digestate in (the aerobic composting process eliminates methanogenic microbes from existing in the material). Furthermore, the only economically viable way to convert low value digestate materials into high value soil amendments that farmers will pay a premium for, is to compost the digestate.
Composting offers a high-value, environmentally responsible, and economically beneficial solution to manage digestate from AD facilities. With the right and , digestate can be transformed into a pathogen-free, structurally stable high value compost that鈥檚 ready for market.
鈥�… enhanced collaboration between AD operators, composters, and packaging manufacturers, along with supportive policy changes favoring hybrid models and promoting source separation, are crucial for successful implementation.鈥�-Anaerobic Digestion and Compostable Packaging鈥� (2024).
What is Digestate?
Digestate is the semi-solid and liquid residual material left after the anaerobic digestion of organic feedstocks such as food waste, manure, and agricultural residues. It typically contains both solid fibers rich in lignin and cellulose, and a liquid fraction high in ammonium and other potentially unstable nutrients.
While in certain cases digestate can be land-applied directly, composting significantly enhances its stability, safety, and economic and environmental value as a soil amendment. (Preprints.org, 2021).
Why Compost Digestate?聽
Composting digestate is an effective strategy for creating a high-value end product from a low value material, while improving environmental and operational factors of an AD facility. Finished compost is widely recognized and easily marketed, whether sold in bulk, bagged, or pelletized formats. The composting process relies on aerobic microbial activity, which generates sufficient heat for pathogen destruction with minimal external energy input. In fact, this microbial heat from the composting process can be captured and repurposed on-site, reducing energy costs further.
鈥淢arkets like the European Union (EU) are leading the way in processing compostable packaging, particularly liner bags used in source-separated food waste collection. This success is attributed to their well-developed infrastructure for integrating anaerobic digestion (AD) and composting, allowing them to effectively manage the compostable packaging stream on a large scale. While large-scale anaerobic digestion holds promise for specific organic waste streams, research by the Composting Consortium suggests it may not be a viable solution for processing compostable packaging in the United States, at least not on its own.鈥�-Anaerobic Digestion and Compostable Packaging鈥� (2024).
Composting facilities can be designed to be highly adaptable, which makes them capable of processing diverse organic materials, including those unsuitable for anaerobic digestion such as compostable plastics. As highlighted in a , AD facilities are generally not suitable for dealing with compostable packaging and compostable flatware as temperatures are not high enough for prolonged periods of time to achieve degradation.
鈥淲hile large-scale anaerobic digestion holds promise for specific organic waste streams, research by the Composting Consortium suggests it may not be a viable solution for processing compostable packaging in the United States, at least not on its own. Several factors contribute to this limitation鈥�- Anaerobic Digestion and Compostable Packaging鈥� (2024).
Aerobic composting also achieves significant mass and moisture reduction鈥攖ypically up to 40%鈥攖hereby lowering the volume of material requiring transport off site.聽From a facility operational perspective, composting systems are mechanically simple and low maintenance, especially when compared to thermal or chemical alternatives of digestate treatment. Environmentally, composting the digestate mitigates greenhouse gas emissions by converting anaerobic material into an aerobic system, preventing methane off-gassing and reducing ammonia volatilization (Altereko, 2020; MDPI, 2023).
AD facilities typically don鈥檛 become cost effective until they are processing at least 10,000 tons of material per year. AD facilities must also be fed daily at consistent rates, maintaining operations at roughly 80% capacity all the time, otherwise the ROI starts falling dramatically.
Having composting infrastructure co-located with an AD facility also gives flexibility to handle seasonal variations as these waste stream volumes are highly variable. Digesters need a steady continuous stream to be economical to operate, so having composting infrastructure on site gives the overall facility the flexibility it needs to be more profitable.
With composting infrastructure already in place the AD facility can bypass the least desirable material for the digester and put those materials directly into the composting process. 鈥淓urope has several models that showcase the success of collaboration between AD operators and composters, driving innovation and best practices鈥his can be achieved through integrated business models that combine anaerobic digestion (generating renewable energy) and composting on a single site 鈥� creating capacity to process compostable packaging and make compost from the digestate. Strong partnerships among AD operators, composters and manufacturers are crucial in Europe.鈥� -Anaerobic Digestion and Compostable Packaging鈥� (2024).
With the focus on biogas and RIN credits, little attention has been paid (in the USA) to the value of digestate, and U.S. digestate markets remain underutilized. The liquid fraction of digestate contains high nutrient value as fertilizer for agriculture crops, but most anaerobic digestion facilities do not sell their digestate. Conversely, the separated digested solids are not considered a finished product and require further maturation (e.g., via composting) to meet regulatory standards for use. As such, digestate currently has little monetary value in the U.S. and is only marginally composted.鈥� -Anaerobic Digestion and Compostable Packaging (2024). Composted digestate has agricultural benefits, showing 40% to 100% greater yields when compared to raw digestate or inorganic fertilizers ().
Methane emissions from digestate: Digestate materials exit the digester in an anaerobic condition. That means the digestate continues emitting some methane after it is removed from the digester. Fugitive emissions from improperly managed digestate can potentially offset the positive environmental impacts of the AD facility, (ie reduced/captured GHG), making it potentially challenging to get carbon credits and other funding sources these systems rely on.聽When raw digestate is field applied the result can be the creation of significant amounts of methane and nitrous oxide emissions (which together account for roughly 88% () of GHG emissions from agriculture). In contrast, well controlled aerobic composting reduces methane and nitrous oxide emissions during storage and after land application ().
Composting digestate has an obvious impact on reduced methane emissions (CH4), however its impact on nitrous oxide (N2O) emissions is more subtle and poorly understood. Generally speaking, composting allows for the conversion of volatile ammonia (NH3) and ammonium (NH4+) into more stable organic nitrogen and nitrate (NO3-), by promoting microbial processes that support nitrate consumption in the soil. Aeration during composting promotes aerobic conditions, which allows for the oxidation of ammonia to nitrate and then nitrate reduction, transforming N2O into N2 gas. This process makes the nitrogen more stable, reducing its loss from the soil in runoff and as N2O. ()
Composting the digestate, using controlled aeration, rapidly transitions the digestate into an aerobic form, eliminating the vast majority of the GHG emissions associated with digestate, while converting the material into a higher value end product for use as agricultural inputs.
Economics of Land-applied Digestate vs Market Value of Composted Digestate
Let鈥檚 consider a hypothetical but realistic scenario comparing an AD facility that produces 100,000 tons per year of digestate. Below is a quick summary of key financial factors if they land-apply the raw digestate, or if they compost that material and sell it for the minimum market rate for bulk compost.
Scenario 1: Land-apply the digestate
Assumptions:
- The AD facility needs to cover the cost of transportation and field application.
- The land application happens within an average of a 100 mile distance from the AD facility. (200 miles round trip).
- 20 tons of digestate per truck load.
- Trucking Cost $2.50 per mile (cost of truck, fuel, driver).
Annual Cost of Land Application to the AD facility: 5000 truckloads multiplied by $2.50 per mile and 200 miles per truck trip = $2,500,000 USD potential annual cost for land-application.
Scenario 2: AD facility installs an on-site controlled aeration composting facility to process all the digestate into marketable compost products
Assumptions:
- Capital Expense to build a controlled aeration composting facility for 100k TPY: $5 million USD
- Annual Operating Cost to run that composting facility: $500,000 USD
- Finished compost volume produced annually 100,000 cubic yards or 50,000 tons.
- Minimum net market value for the finished compost: $15 per cubic yard
- Annual compost sales revenue: $1,500,000 USD.
Annual potential net operating income from compost sales: $1,000,000 USD.
The Poland Approach to AD and Composting Synergy
In Poland, and broadly in Europe, AD facilities are often operated in combination with composting infrastructure, to enable each process to be optimized in terms of capacity factors and the highly variable volumes of different materials coming in on a seasonal basis.
As such the approach being taken in Poland for cities that are implementing organics management programs is to start with building green waste and source separated organics composting facilities. As those scale up volumes (it takes time to build collections in any municipal or private diversion program) they eventually add in an AD facility.
This allows facilities to start managing organics at a much lower capital cost with composting as the primary process. Once the volumes are high enough to justify AD facility construction, they can focus on digesting materials that make sense to digest, which is primarily food waste, while composting whatever material exceeds the AD system鈥檚 capacity or its optimized feedstock input plan.聽Once the AD facility is built, they will already have the composting infrastructure in place to manage the digestate.
Additional High Value Products from Co-located AD and Composting Facilities have successfully produced pelletized fertilizer from the composted digestate, which can be sold for 100+ USD per ton. Having an aerobic hot composting process co-located with the digester enables the pellets to be produced using heat recovered from the composting process. The composted material is bio-dryed by the natural aerobic composting process to get the material to approximately 35% moisture content. Then, this material is put through a final drying process before being pelletized. The facility can also produce specialty compost soil blends with additional ingredients such as biochar or other key nutrients such as captured nitrogen or phosphorus, for particular markets.
Challenges of Composting Digestate
Despite its benefits, composting digestate presents several unique challenges. Anaerobic digestion removes much of the feedstock鈥檚 available energy (carbon that is converted into biogas/methane), leaving the digestate material that has insufficient carbon to generate aerobic microbial heat from the composting process.
In a nutshell, with digestate, most of the 鈥渆asy-to-digest鈥� material has already been consumed. This can create 鈥渞ecipe challenges鈥� that require special handling and blending with other materials to ensure that the compost feedstock has the right range of carbon to nitrogen ratios, moisture content and bulk density, before the material is composted. In most cases significant amounts of carbon bulking agents, such as shredded yard waste, needs to be blended with the digestate before the material is put through a composting process.
To compensate, operators may bypass a portion of raw feedstocks from the digester into the composting process, and blend in fresh carbon sources such as shredded yard waste or wood chips into the digestate before it is composted. This ensures that the compost feedstocks have high nutrient value and high carbon value to feed the aerobic composting microbes.
Regular turning and/or actively controlled aeration technology are required to increase the microbial activity and thermal output of the digestate being composted, ensuring the material gets hot enough to kill pathogens and create a high value finished compost.
Another challenge in composting digestate is that this material is full of anaerobic microbial culture, in an anaerobic condition. The composting infrastructure and process used must quickly force this material into an aerobic condition, which will kill off the methane-emitting anaerobic microbial culture and enable the aerobic composting microbes to quickly take over.
Digestate is often saturated with moisture, with levels often exceeding 80%, which inhibits effective composting. Compost feedstock moisture content needs to be between 40% to 60%. Blending digestate with additional carbon bulking material, and using controlled forced aeration with frequent agitation helps manage this issue by reintroducing oxygen and maintaining structure. Additionally, digestate tends to be dense and prone to compaction, so bulking agents and properly designed aeration systems are essential to maintain porosity and airflow.
Another key challenge is the carbon-to-nitrogen (C:N) ratio. Digestate is typically nitrogen-rich and relatively low-carbon. So the composting process requires supplemental carbon to achieve a balanced C:N ratio of at least 20:1, ideal for composting microbes. High ammonia (unstable nitrogen) levels (200鈥�500 ppm) in digestate can overwhelm both compost microbes and biofilters, which function best below 100 ppm.
Blending in additional carbon material is necessary to balance the overall nitrogen level of the material before it is composted. The compost aeration system will still need to be exhausted into an acid-scrubber and a bio-filter to capture ammonia odors and emissions.
Early-stage aeration and carbon addition are necessary to balance and stabilize ammonia effectively. Covers such as geomembranes are not recommended, as they trap both moisture and ammonia, exacerbating the nitrogen problem (ResearchGate, 2021; PubMed, 2022).
The aerobic composting process involves high volumes of oxygen being moved through the material. This activates the aerobic microbes which break down the material and which also produce the high temperatures that kill off pathogens. The combination of active aeration with the natural microbial-generated heat of the composting process results in lots of moisture being evaporated from the compost feedstocks. Since the digestate material starts out virtually saturated with moisture, the aerobic composting process also has the effect of gradually drying down the material.
After a relatively short active aeration process the composting digestate materials will have achieved 鈥淧FRP鈥� status of the material being above 131F continuously for at least three consecutive days, the industry standard for pathogen destruction. Depending on the characteristics of the digestate, the composting process could be as quick as a few weeks to achieve a stable high value finished product.
of the finished compost will remove the carbon bulking agent materials that require longer timeframes to break down. These 鈥渙vers鈥� can be recycled back into the next batch of digestate to be composted.
CompoBox Tunnels: Enclosed Aeration to Control Emissions and Odor
offer an enclosed, in-vessel composting solution ideal for managing emissions and odors. These systems provide precise control over aeration, temperature, and moisture, ensuring that the odors and emissions associated with fresh anaerobic digestate are stripped and treated before entering the environment.
When paired with an acid scrubber on the exhaust system, CompoBox systems can achieve over 99% ammonia removal. The stripped ammonia is converted into ammonium sulfate鈥攁 high-nitrogen fertilizer that can be captured and sold, or reintegrated into the composting process at a later stage.
By enclosing the composting process and integrating proven air treatment technologies, CompoBox tunnels offer a scalable, efficient solution for digestate composting with minimal odor and maximum nutrient recovery (Wikipedia, 2023; MDPI, 2023).
Turned Aerated Piles (TAP): Managing Structure, Moisture, and Energy
, which combine mechanical turning with controlled aeration, are robust, mechanically simple composting methods that effectively address most challenges associated with digestate composting. Regular turning enhances microbial activity and generates heat, compensating for the digestate’s low residual energy. Forced aeration maintains aerobic conditions even in wet, dense material.
TAP systems prevent compaction by regularly fluffing the pile, maintaining oxygen flow and porosity, allowing operators to compost more digestate with less supplemental bulking agent. They also facilitate easy integration of bulking agents and carbon sources to achieve the desired C:N ratio. TAP systems are cost-effective, efficient to operate, and ideal for facilities looking for flexibility and reliability in composting operations聽 (PMC, 2023;CompostingTechnology.com, 2024).
Conclusion
In summary, the synergistic benefits of co-locating advanced composting infrastructure with anaerobic digestion facilities can be summarized as:
- Composting is the highest value end-use for the digestate. Once digestate is composted you can make fertilizer pellets, compost products and soil blends that have very high market value ($50 to $500 per ton depending on how the material is marketed). Conversely digestate by itself is heavy, unstable and has low to negative market value, typically treated and/or disposed of at a loss.
- Anaerobic Digestion facilities typically require roughly ten times the capital to develop when compared with composting in terms of annual tonnage capacity factors. As such it is critical that AD infrastructure is used to process waste streams that are high value such as food waste, depack waste and manures. Green waste, woody debris, and compostable plastics don’t easily break down in digestion, leading to marginal energy generation. When mixed with digestate, these waste streams create excellent finished compost.
- Digestate can be difficult to compost because it is wet, dense, low in carbon and low in energy. As such it is advisable to incorporate some bulking agent and use composting technology that provides controlled aeration and regular agitation.
- If the AD and composting facility is in a regulated air district or has proximal neighbors we suggest putting the composting process into an enclosed 鈥溾�� for 14 days, stripping the emissions and destroying pathogens – then moving the material into a turned aerate pile system for finishing. If the facility is in a less regulated area, the composting process can skip the enclosed tunnel stage and use to process the digestate into high value compost.
Composting digestate is a high-value, sustainable, and operationally efficient strategy for managing the by-products of anaerobic digestion. The union of advanced composting infrastructure with anaerobic digestion facilities is a compelling economic opportunity that also creates improved overall environmental impact.
For decades the anaerobic digestion industry and the composting industry have operated like competitors in N. America. Both segments offer important and valuable opportunities to convert waste into value while improving the environment and leading to a more sustainable civilization. But when these two distinct approaches to organic waste recycling are combined, the net impact is greater than the sum of each process when operated independently.
is the President of Green Mountain Technologies. He brings his product development background and passion for composting to help clients successfully design, construct and operate composting facilities. Orion graduated from the University of Washington with a B.S. in Mechanical Engineering. Since joining GMT in 2019, Orion has helped clients successfully design, construct and operate dozens of facilities. He has spoken at numerous composting conferences including the , and holds several composting related patents. Prior to working at GMT Orion spent several years in new product development, where he learned the importance of empathy and communication to successful project execution. Outside of the office you can find Orion surfing, meditating, and exploring the Olympic Peninsula.
is an independent consultant leading marketing and business development for Green Mountain Technologies and has a bachelor’s degree in Journalism and Economics from Washington and Lee University.聽 In 2025 he was elected to the Board of Directors of the US Composting Council. He also has a leadership role with the Compost Capital Network. His work in composting began in 2008 when he founded the non profit Compost Power Network in Vermont in partnership with Highfields Center for Composting and several universities to develop best practices for heat-recovery from composting systems, including several successful compost-heated greenhouse projects. He has given numerous presentations about composting at conferences and taught composting workshops for Yestermorrow Design/Build School in Vermont for several years. He wrote The Compost Powered Water Heater book for WW Norton publishing company in 2013 and has authored many articles about composting and sustainability for a variety of trade publications.聽
Source:
Bibliography & References:
-
Science Direct: Intermittent aeration to reduce gaseous emission and advance humification in food waste digestate composting: Performance and mechanisms. ()
-
Altereko. (2020). Benefits of Compost and Anaerobic Digestate.
-
Composting Consortium Report: 鈥淎naerobic Digestion and Compostable Packaging鈥� (2024).
-
CompostingTechnology.com. (2024). Turned Aerated Pile Systems.
-
MDPI. (2023). Sustainability in Composting Digestate. Sustainability Journal, 16(15), 6329.
-
PMC. (2023). Greenhouse Gas Emissions During Composting.
-
Preprints.org. (2021). Composting Digestate Improves Soil Health.
-
PubMed. (2022). Biochar and Digestate Composting.
-
ResearchGate. (2021). Mitigation of NH3 Emissions in Composting.
-
Wikipedia. (2023). In-Vessel Composting.