An In-Depth Exploration of Anaerobic Digestion Feedstocks in the Quest to Maximize Methane with Innovative Mixes
In the realm of anaerobic digestion, the choice of feedstock plays a pivotal role in determining the efficiency and output of the digestion process.
This segment delves into the various conventional and emerging feedstocks, examining their energy value, digestion efficiency, and biochemical methane potential. Let’s embark on a comprehensive journey through the world of anaerobic digestion feedstocks.
Historically, manure, other livestock waste, and wastewater sludge were the primary feedstocks for anaerobic digestion.
However, recent times have seen a shift towards more diverse sources. Food processing waste, household organic waste, agricultural residues, yard waste, and even Municipal Solid Waste (MSW) in the form of the organic fraction of municipal sold waste (OFMSW) have emerged as promising feedstocks.
At the same time “SSO” (source separated organic) waste especially that available from household waste kerbside collection, has been rising globally as governments see the imperative of reducing the high impact of food waste as a source of carbon emissions.
In the UK, for example, the government has (in the autumn of 2023) announced that weekly collection, not only of household food waste but that generated in all other sectors will be mandatory by 2026. Meaning that the quantity of SSO in the UK will rise substantially and furthermore, most of that should be destined for biogas production given that anaerobic digestion, not composting, is the deemed sustainable method.
These alternatives not only diversify the range of usable materials but also enhance gas production, thereby offering a more sustainable approach to energy recovery.
A pile of whole crop maize silage awaiting its use as an anaerobic digestion feedstock. By following good silaging methods farm digesters may run all year round on the feedstock from one crop.
Feedstock Categories Based on Source
Feedstocks for anaerobic digestion can be broadly categorized based on their origin:
Agricultural and Farm Side: Includes harvest remains, energy crops (e.g., corn silage, sorghum, wheat, clover), algae biomass, and manure from livestock such as cattle and poultry.
Municipal Sources: Comprises separated organics like fruit and vegetable waste, food waste, coffee and tea filters, yard or garden waste (leaves, plants, branches), and wastewater sludge.
Industrial Waste: Encompasses waste from the food and beverage processing industry, dairy industry, starch and sugar industries, as well as slaughterhouse waste.
Before digester operators can accept wastes according to these categories will depend on the local regulatory requirements. For example, in the UK before an agricultural biogas plant operator decides to accept anything other than agricultural farm waste it may be necessary to apply for and obtain a more comprehensive PPC permit, with all the restrictions that may apply.
Similarly, accepting new feedstock types may entail more stringent application of the Animal By-Products Regulations and the addition of a pasteurization stage.
All biogas plant operators will need to balance the advantages of accepting new feedstocks against additional processing requirements required by the local waste regulations.
Anaerobic Digestion Feedstock Energy Value and Methane Production Potential
Different feedstocks have varying methane production potentials or energy recovery values. High organic content that is easily degraded by anaerobic bacteria is considered ideal for anaerobic digestion.
Food waste, for instance, has a significantly higher methane production potential and biodegradability compared to biosolids.
Table 1: Energy Value Profile of Anaerobic Digestion Feedstocks
This table illustrates the relative methane production potential of various anaerobic digestion feedstocks. It's important to note that these values are indicative and can vary based on specific conditions and treatment processes.
An example of curbside-collected source-separated food waste anaerobic digestion feedstock, collected from a Scottish town, before depackaging. Before it is used as an anaerobic digestion feedstock efficient removal of the plastic bags, and other plastics is essential to avoid it entering the digester where it may accumulate.
Enhancing Gas Production through Co-Digestion
While manures and wastewater sludge have traditionally been used in anaerobic digesters, they are not the most energy-rich feedstocks.
To maximize gas production and the overall efficiency of anaerobic digestion (AD) processes, the focus in recent years has shifted towards the utilization of high-energy feedstocks. These high-energy feedstocks are rich in organic matter and have the potential to significantly enhance the biogas production capacity of AD plants. One innovative approach that has gained prominence in this regard is the practice of co-digestion, which involves blending these high-energy feedstocks with traditional substrates like manure or sludge.
Co-digestion represents a strategic and sustainable solution that leverages the strengths of various feedstocks to create a synergistic effect within the AD system. Here's how it works and why it's so promising:
Diverse Feedstock Mix: Co-digestion allows AD operators to diversify their feedstock mix by incorporating high-energy materials such as food waste, agricultural residues, energy crops, or organic industrial byproducts. These feedstocks typically have a higher energy content compared to manure or sludge alone.
Enhanced Biogas Yield: When these high-energy feedstocks are blended with traditional substrates, they introduce a richer and more balanced nutrient profile into the digester. This promotes optimal microbial activity, leading to a higher biogas yield. The synergistic effect of co-digestion results in greater methane production and increased energy generation.
Improved Process Stability: Co-digestion can enhance the stability of the AD process by mitigating potential challenges associated with the use of certain high-energy feedstocks. For instance, food waste may have variable compositions, but when co-digested with manure or sludge, it can help maintain a stable pH level and prevent process upsets.
Waste Diversion: By incorporating organic waste materials like food scraps or industrial residuals, co-digestion contributes to waste diversion from landfills. This aligns with environmental sustainability goals and reduces the emission of methane, a potent greenhouse gas, from decomposing organic matter in landfills.
Economic Viability: Co-digestion can also be economically attractive. Some high-energy feedstocks may have value-added benefits, such as the generation of valuable byproducts like nutrient-rich digestate or excess heat for on-site use or sale. This can offset operational costs and create additional revenue streams.
Flexibility and Adaptability: The flexibility of co-digestion allows AD facilities to adapt to changing feedstock availability or regulations. Operators can adjust the blend of feedstocks based on seasonal variations or market conditions, ensuring a reliable and continuous supply.
In conclusion, the integration of high-energy feedstocks through co-digestion represents a significant advancement in AD technology. It offers a sustainable means of increasing biogas production, improving process stability, reducing waste, and potentially enhancing the economic viability of AD operations. As the world seeks cleaner and more efficient energy solutions, co-digestion emerges as a promising strategy to harness the full potential of organic waste in the transition towards a greener future.
Common Co-Digestion Practices
Co-digestion often involves mixing a base feedstock like sludge or manure with a high-strength organic material. Examples include fats, oils, grease (FOG), food waste, and slaughterhouse waste. The addition of these high-energy materials to the digester results in a significantly increased biogas yield.
Benefits of Co-Digestion
Renewable Energy Generation: Increased biogas production can be
utilized for on-site energy needs or sold to the grid, enhancing the overall sustainability of the process.
Greenhouse Gas Reduction: Diverting organic waste from landfills to anaerobic digesters prevents methane emissions in landfills and instead harnesses it as a renewable energy source.
Waste to Energy Utilization: Co-digesting food waste and organic fractions of municipal solid waste not only minimizes waste streams but also recovers valuable energy and resources.
Cost Recovery: Enhanced biogas production and on-site power generation can help anaerobic digestion facilities recover operational costs.
Assessing Biogas Potential of Feedstocks
There are two ways in which the biogas potential of feedstocks can be assessed:
biochemical sampling and analysis such as the FOS/TAC Method
BMP Assay.
Although the biogas potential or Biochemical Methane Potential (BMP) of feedstocks can be estimated through lab-based methods many biogas plant operators like to back up chemical theory with by practical experimentation.
The BMP assay involves combining feedstock samples with an inoculum in a controlled environment to measure the amount of biogas produced.
Note: Results differ widely for each feedstock. This table is a guide to typical values only. Analyse your own feed stock. Do not rely on the values in this table.
Procedure of BMP Assay
Feedstock samples are mixed with an inoculum and nutrient solution in a closed container.
Temperature control is maintained, and the solution is agitated to ensure uniform mixing.
The assay typically lasts 30 to 60 days, during which biogas production is measured periodically or continuously.
Control samples containing only inoculum and nutrient solution are used to determine baseline gas production.
This may also be referred to as a lab-scale or “pilot process” study.
Key Parameters Analyzed in BMP Assay
Feedstock Characteristics Parameters such as pH, chemical oxygen demand (COD), total solids, and volatile solids are assessed to determine the appropriate quantity of feedstock needed for the assay.
Organic Material Degradation: The assay reveals the extent of organic material breakdown and the biodegradability of the feedstock.
Total Biogas Production and Composition: Measurement of the total volume of biogas produced, along with its composition (e.g., methane, carbon dioxide), provides insights into the efficiency of the digestion process.
Optimizing Anaerobic Digestion through Feedstock Selection and Co-Digestion
Optimizing the anaerobic digestion process involves careful selection of feedstocks and potentially utilizing co-digestion strategies based on the results from biochemical sampling and analysis such as the FOS/TAC Method and will also often include the results from a BMP Assay to assess optimum retention time etc.
By combining anaerobic digestion feedstocks with complementary characteristics, and testing for the results the optimum can be arrived at and the overall efficiency of the digestion process can be enhanced.
This optimization not only increases biogas yield but also improves the stability and resilience of the digestion process. Some operators have reported that incidental costs of trace nutrient and enzyme dosing have also been reduced.
Criteria for Selecting Co-Digestion Feedstocks
Biodegradability: Feedstocks should be easily broken down by anaerobic bacteria, ensuring efficient gas production.
Methane Production Potential: High-energy feedstocks with significant methane production potential are preferred.
Non-Inhibitory: The feedstock should not contain substances that inhibit the anaerobic digestion process.
Nutrient Balance: Co-digestion feedstocks should complement the nutrient profile of the primary feedstock, enhancing the overall process.
Availability and Cost: The feedstock should be readily available at a reasonable cost and in sufficient quantities for sustainable operation.
Handling and Storage: Easy reception, handling, and storage in the anaerobic digestion facility are crucial for operational efficiency.
Emerging Trends and Future Prospects for Yield Optimization from Anaerobic Digestion
As the field of anaerobic digestion evolves, new trends and technologies are emerging. Innovations in digester design, feedstock pre-treatment, while process optimization is paving the way for more efficient and sustainable biogas production.
The integration of anaerobic digestion with other renewable energy technologies, such as solar and wind, is also being explored to create more resilient and versatile energy systems.
For example, the temperature control of a reactor is another important aspect of biogas yield and quality and innovations are already applied in which solar arrays are used in conjunction with biogas reactors providing benefits such as:
24/7 energy output from the combination of solar with biogas
winter heating for the digester tanks in cold climates
availability of non-sacrificial energy in summer in hot climates to cool the anaerobic digestion process from excessive heat build-up.
The anaerobic digestion feedstock, and its reliable supply, is everything to an Anaerobic Digestion Process. The biggest successes come from close attention to the volume, nature, composition and variability in composition, of the available feedstock, or range of feedstocks to be accepted. These must be planned for from the outset when designing an AD Plant.
Feedstock and Anaerobic Digestion Processes
The crop fed biofuel AD plant designer starts with a huge advantage over other AD developers due to the fact that their feedstock supply is under their control, not subject to purchase in the organic waste market.
There will be seasonal variations for the available crops of course, unless year-round storage can be provided for total feedstock inputs, but these can be predicted.
Silage use as a digester feedstock year-round is, for example, an established practice.
Sewage sludge AD and agricultural waste fed AD processes are possibly the next best for security of feedstock provision.
Also where anaerobic digestion plants are built to process industrial wastes, from a reliable known waste stream, these are also reasonably secure in the knowledge of the characteristics of their feedstock materials.
Image: Feedstock Conveyor at the Greenfinch Plant, Ludlow, UK
However, things can get really difficult for AD Plants being built to process primarily food waste from householders within council household and commercial SSO collection services.
When these plants are built it is at present an act of faith to know to what extent the public will segregate their wastes to send them to the AD Plant.
The solution is to install depackaging equipment, but not just any depackaging technology.
Depackaging systems that macerate, screen organic waste by size reduction, hammer, and mill the waste are not sustainable because they create microplastic and add to plastic pollution.
The cost of any biogas feedstock is an important consideration during feedstock selection. We found the following table, available to download on Scotland’s Farm Advisory Service website:
At Anaerobic-Digestion.com we are often asked whether a particular organic waste is suitable feedstock for AD treatment. So, we thought that we would devote a portion of this web page to providing a list of some of those we perceive to be the most common.
Anaerobic Digestion Feedstock List
Dairy Farm and piggery etc waste slurries
Dairy Product and other Food Processing wastes
Household Food Waste with or without Green Waste
Brewing Industry effluents and wastes
Supermarket Waste*
Municipal Solid Waste from Kerbside collections of Green Waste
Municipal Solid Waste from Kerbside collections of Green Waste and Kitchen Waste*
Catering Industry Wastes*
Organic Market Wastes*
Sewage Treatment Works sludges
Silage/ley crops (may be included and kept in storage to provide a feedstock for use to augment feed during winter seasons. Stored biomass is needed for times when the availability of other feed materials will be reduced)
Other organic wastes available locally from other process industries.
* – When considering acceptance of some wastes, early thought should be given to the effects on the plant of Animal By-Products Regulations compliance, and necessary provision for handling and processing ABPR waste. ABPR applies to the EU states, but equivalent regulations should also be in force elsewhere. This is to avoid cross contamination health risks (eg Foot and Mouth disease).
Extended Anaerobic Digestion Feedstock List
We held a session at which people suggested the following list of anaerobic digestion feedstocks which can be used for fuelling biogas plants:
Anaerobic Digestion Plants can receive and treat the following types of anaerobic digestion feedstock:
– Fish oil
– Fish waste
– Dairy products
– Milk containing antibiotic residues
– Whey and concentrated whey
– Sewage sludge from wastewater treatment plants
– Grease sludge from foul water chambers, sewage sumps, pumping stations, grease traps etc
– Seaweed (but check salinity first)
– Animal feed waste/residues
– Flotation sludge
– Graded organic domestic waste
– Gastrointestinal contents from slaughterhouses
– Waste from slaughterhouses. Subject to regulatory requirements in-force: All category III types and category II types that have been sterilised at 133 degrees Celsius for 20 minutes.
– Pig/ livestock slaughterhouse blood
– Poultry – chopped – category III after sterilization
– Chicken waste/ feathers (Note: Chicken Litter can be troublesome due to its high ammonia content.)
– Eggs and egg waste products
– Sawdust from mink pelt processing
– Mink feed waste/residues
– Potatoes and potato pulp
– Coffee grinds, tea leaves etc
– Fruit, including residues from fruit processing
– Lemon peel, orange peel etc
– All cereal products
– Bakery waste – flour, bread and finished products
– Grass, maize – energy crops
– Ochre-rich water/ochre from wastewater treatment as long as there is a high organic content
– Waste from jam production
– Ices, ice-cream, all forms of ice-cream products
– Margarine, out of date butter, suet
– Waste from the production of soya oil and margarine
– Distillery waste
– Soft drinks and beer
– Brewer’s yeast
– Bleaching earth with high organic content
– Glycerine, including de-icing fluids from airports
– Coolants and antifreezes. For example, monoethylene glycol (blue coolant), organic acid coolant (green and red coolant)
– Sugar, molasses, etc.
– Drug residues from pharmaceutical companies
– Soap residues from soap manufacturers
– Past sell-by date food waste
– Household collection food waste
– Food products that have been bacteriologically or chemically contaminated
– Non-food organic/ vegetable/ putrescible products
– Catering and food processing factory waste
– Organic fraction of residudal wastes after processing of municipal solid waste in Mechanical Biological Treatment (MBT) Plants
– Manure and farming slurries
* Categories indicate requirements for prevention of the spread of disease. The requirements vary. Check these requirements with your local regulatory authority before use as an anaerobic digestion feedstock.
Article is not good. All food waste that it still edible should go to food banks. It is wrong to suggest food waste should go to a digester, except when it is mouldy.
James Riddley – Yes. We agree. We support the “Waste Hierarchy” which is also supported by the UK government, which requires that if possible all waste is first of all avoided, but if waste can’t be avoided it should next be re-used (i.e. for food waste it should sent to a food bank).
If it is unsuitable to be sent to a food-bank it should be recycled, so for food waste that means that only then should the food waste be sent to an anaerobic digestion plant.
We do not intend to encourage waste to be sent straight to a food waste digestion plant, which is still edible. We apologise if that was your interpretation of this page.
Confer with your experienced biogas expert when adding new feed which will change substrate conditions. Know about about issues and risks. Be sure to comprehend every point made. This is the way to avoid problems with digester yields drop after new material in substrate, and trying to keep the biogas production manageable.
Take note taht BV Dairy are reducing their carbon footprint (volume of carbon dioxide and other greenhouse gases emitted in manufacturing and distribution operations). Emissions can be reduced by cutting fossil fuel consumption or generating renewable energy. They are doing both – generating heat and electricity.
Can you update me on SeaGas. Last year they were working on producing biomethane from seaweed through anaerobic digestion (AD). The project launched in July 2015 so we are surprised you don’t mention about this project which consists of six partners including The Crown Estate, the Centre for Environment, Fisheries and Aquaculture Science, the Scottish Association for Marine Science, Queen’s University Belfast and Newcastle University. Is it the project working?
Kevin Maguire – I understand that glycerine or glycerol are used to increase the biogas production of anaerobic digesters. It provides an easily digested food source.
In the list above, waste airport-runway de-icing fluid is said to be a possible source. In addition, it is a by-product of bio-diesel production.
I believe that 5 to 10 years ago there was an over-supply of it in the UK, from bio-diesel production. At that time it was a low cost material and paying for the transport was the biggest cost, but I imagine that with more AD plants today, there is more demand and that the cost will now be higher.
I just did some online research which led me to think that the fuel company Greenergy, might be a possible glycerine supplier.
To quote from the Greenergy website: Greenergy “manufactures its own biodiesel from wastes in order to meet its biodiesel blending obligations. The company sources waste oils and fats globally and operates two biodiesel manufacturing facilities in the UK.“.
I imagine that if there was a biodiesel manufacturing facility near an AD plant, that would be the best source for glycerine for that site.
It would be interesting to have feedback, from you. It would be good to know whether this information is correct, should you decide to follow this up.
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Article is not good. All food waste that it still edible should go to food banks. It is wrong to suggest food waste should go to a digester, except when it is mouldy.
James Riddley – Yes. We agree. We support the “Waste Hierarchy” which is also supported by the UK government, which requires that if possible all waste is first of all avoided, but if waste can’t be avoided it should next be re-used (i.e. for food waste it should sent to a food bank).
If it is unsuitable to be sent to a food-bank it should be recycled, so for food waste that means that only then should the food waste be sent to an anaerobic digestion plant.
We do not intend to encourage waste to be sent straight to a food waste digestion plant, which is still edible. We apologise if that was your interpretation of this page.
Confer with your experienced biogas expert when adding new feed which will change substrate conditions. Know about about issues and risks. Be sure to comprehend every point made. This is the way to avoid problems with digester yields drop after new material in substrate, and trying to keep the biogas production manageable.
Take note taht BV Dairy are reducing their carbon footprint (volume of carbon dioxide and other greenhouse gases emitted in manufacturing and distribution operations). Emissions can be reduced by cutting fossil fuel consumption or generating renewable energy. They are doing both – generating heat and electricity.
We think our work is an additional biomass feedstock which is not listed on this page. Please consider adding Algae to the list! Find out more at the Algal Biofuels pdf here.
Can you update me on SeaGas. Last year they were working on producing biomethane from seaweed through anaerobic digestion (AD). The project launched in July 2015 so we are surprised you don’t mention about this project which consists of six partners including The Crown Estate, the Centre for Environment, Fisheries and Aquaculture Science, the Scottish Association for Marine Science, Queen’s University Belfast and Newcastle University. Is it the project working?
HI,
I am a director of two AD plant in the north East of England,
and am interested in feed glycerine into my plant which you note as a feed stock.
Kevin Maguire – I understand that glycerine or glycerol are used to increase the biogas production of anaerobic digesters. It provides an easily digested food source.
In the list above, waste airport-runway de-icing fluid is said to be a possible source. In addition, it is a by-product of bio-diesel production.
I believe that 5 to 10 years ago there was an over-supply of it in the UK, from bio-diesel production. At that time it was a low cost material and paying for the transport was the biggest cost, but I imagine that with more AD plants today, there is more demand and that the cost will now be higher.
I just did some online research which led me to think that the fuel company Greenergy, might be a possible glycerine supplier.
To quote from the Greenergy website: Greenergy “manufactures its own biodiesel from wastes in order to meet its biodiesel blending obligations. The company sources waste oils and fats globally and operates two biodiesel manufacturing facilities in the UK.“.
I imagine that if there was a biodiesel manufacturing facility near an AD plant, that would be the best source for glycerine for that site.
It would be interesting to have feedback, from you. It would be good to know whether this information is correct, should you decide to follow this up.