Anaerobic Digestion Feedstock Optimization: Balancing Nutrients and Energy for Sustainable Biogas Generation

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.

Conventional vs. Emerging 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.

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.

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:

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.

An example of a depackaging system that poses none of these problems is the Drycake Twister Depackager.

Costs of Biogas Feedstocks

Agricultural Feedstocks

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:

The pdf file is available here.

Waste Management Feedstocks

The cost of a number of organic waste feedstocks is available on the Let’s Recycle website here.

Potential Feed Anaerobic Digestion Plant Feed Materials

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).