Pillar 4:
Digestate Policy

Executive Summary

Pillar 4: Digestate Policy offers a guide to inculcate sustainable farming practices including reduction of chemical fertiliser use and improvement of soil fertility through using the co-product of biogas. Digestate is a valuable biofertiliser that can be used as a renewable source of critical fertiliser resources, such as nitrogen and phosphorus, and can provide many benefits to agriculture and other land applications, especially where soil improvement is needed.

Management and quality of digestate is a key area of focus for the biogas plant operators and digestate producers and thus, it is important to establish minimum standards for the quality, storage and application of digestate to land to maximise its benefit and ensure it is safe for use in agriculture, forestry, horticulture, land restoration and landscaping.  There is also the impetus to a introduce biofertiliser obligation among other avenues to stimulate the market for digestate to transition the conventional farming business. These elements among others are explored in the recommendations.

Conclusion

By adopting the recommendations made in this pillar, digestate managers and end users are empowered to employ best management practices when handling and applying digestate to land in order to realise its value as effectively as possible.

 

PILLAR 4: Digestate Policy

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Introduction 

The two main outputs from the anaerobic digestion (AD) of organic material are biogas and digestate. The digestate consists of water, microbial biomass and the undigested fraction of the input materials. It contains the nitrogen (N), phosphorus (P), potassium (K) and other macro- and micronutrients that are present in the input materials and retained through the AD process.  

Digestate is a valuable biofertiliser that can be used as a renewable source of critical fertiliser resources, such as nitrogen and phosphorus, and can provide many benefits to agriculture and other land applications, especially where soil improvement is needed.

Producing good quality digestate is essential to delivering these benefits and should be a key area of focus for the AD operator and digestate producer.  Similarly, digestate managers and end users must employ best management practices when handling and applying digestate to land in order to realise its value as effectively as possible.

Digestate is a valuable product; however, its value is often not reflected in its price. In most parts of the world it is currently given for free to farmers for use in agriculture, transported at the cost of the operator, and sometimes even applied at the cost of the operator. Some government intervention is required to stimulate and support the digestate market.

 

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4.1. Set targets for sustainable farming

To encourage sustainable farming practices, governments must start with setting targets for the percentage of land under sustainable farming, the reduction of fertiliser use and the improvement of soil fertility. Being a renewable and biological fertiliser, these policies will also support and encourage the use of digestate in country.

 

EXAMPLE

The EU Green Deal aims to reduce nutrient losses by at least 50% while ensuring no deterioration in soil fertility. This is expected to reduce the use of fertilisers by at least 20% by 2030.¹

 

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4.2. Take a holistic approach to digestate to maximise its benefits

Digestate is a valuable product of the AD of organic feedstock. While developing policies, it must be remembered that the drivers for the use of digestate might vary at a national, regional and even farm level. While mitigation of climate change or food security may be the driver at the national level, at the farm level it could be the quality and structure of soil or improving profit margins from organic farming. A holistic and multifaceted approach is, therefore, required to maximise the benefits from the use of digestate so that it satisfies both national interest and local needs.

4.2.1. Use for soil improvement

Digestate is a valuable source of critical fertiliser resources such as nitrogen and phosphorus. It can provide many benefits to agriculture and other land applications, especially where soil improvement is needed.

Minimal carbon remains in the digestate after the AD process, as it has been extracted in the form of biogas. Nevertheless, what carbon does remain can help improve soil structure and its water retention capacity, as well as reducing dependence on synthetic fertilisers.²

4.2.2. Completes nutrient recycling

Nitrogen, phosphorus and potassium are considered key elements essential for plant growth and are the constituents of fertilisers applied to crops to increase yields. Digestate promotes sustainable farming by closing nutrient loops – nitrogen, phosphorus and potassium are returned to the soil from the organic wastes that have been fermented inside the digestor to make biogas. ³ Nutrients in the input materials or feedstock are mostly conserved through the AD process and retained within the digestate. 

Only small amounts of nutrients are lost to the gas phase as trace gas in the biogas – ammonia gas (NH₃) and hydrogen sulphide (H₂S) – or as precipitates such as iron sulphide (FeS). 

4.2.3. Role in climate change mitigation

Manufacturing fertilisers is a highly energy-intensive process, so digestate can play a role in climate change mitigation by reducing the use of chemical fertilisers. ⁴ It can also support the short-term storage of carbon in the soil.

Diverting manures and slurries from being directly applied to soil to mechanisms that will control emissions, such as the AD process, will significantly reduce emissions from agriculture.

4.2.4. Increased crop yields

AD makes nutrients available through the biological process of digestion. The chemical forms of the nutrients transform from complex molecular forms into a simpler form as the organic matter is digested, making them more readily available for absorption by the crops they are applied to, leading to higher crop yields. ⁵

4.2.5. Improves food security

As a locally produced renewable fertiliser, digestate can reduce the dependence of agriculture and the economy on chemical fertilisers, as well as energy prices.  This is particularly important both economically and strategically, as synthetic fertiliser production is heavily concentrated in few countries. 

4.2.6. Reduced pathogens and invasive weeds

Digestate produced by AD is a safer biofertiliser than raw organic materials such as slurries and manures. Animal and plant pathogens are significantly reduced by AD, and in most cases are eradicated due to the microbial conditions inside the digester and the technical and thermal treatment of the input materials. AD greatly reduces the spread of invasive weeds by neutralising seeds that may be present in the feedstock. ⁶

4.2.7. Financial stability

Production of fertilisers is an energy-intensive process and the price of fertilisers is, therefore, closely linked to the price of energy. It follows that a fluctuation in energy prices can affect the food security of countries; the use of digestate can cushion some of that impact.

 

EXAMPLES

The BiogasDoneRight concept, published by Consorzio Italiano Biogas e Gassificazione (CIB), has provided evidence of a number of benefits of integrating digestate use into sustainable farming practices, including increased yields, higher profits, improved soil fertility and carbon content and mitigation of emissions^{. 7}

The IEA Bioenergy publication provides evidence of the benefits of using digestate, including reduced and inactivated pathogens and weeds. ⁸ 

 

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4.3. Introduce a renewable fertiliser obligation

A renewable fertiliser obligation should be placed on fertiliser distributors and suppliers. This would mean that a proportion of the fertiliser that they supply should be a renewable fertiliser such as compost, digestate or a derived product. A similar approach has been successful in the energy industry, where an obligation on energy suppliers to procure a minimum fraction of the energy from renewable sources has been implemented by many countries. This has been discussed in Pillar 3: Biogas Use. 

A renewable fertiliser obligation on fertiliser suppliers could be key in stimulating the sector by bringing in investment for developing a digestate supply chain. By introducing this policy, fertiliser suppliers would be obligated to buy digestate from biogas plant operators to recycle nutrients and sell it alongside chemical fertilisers. This policy would, therefore, turn the cost into a revenue stream to improve the financial viability of biogas plants and also make agriculture more sustainable. Fertiliser manufacturers would also be incentivised to invest in developing digestate-based products.

At this time there are no known examples of this policy being implemented.

 

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4.4. Include digestate in an existing fertiliser regulatory framework

Most countries regulate or provide guidance for the use of fertilisers in farming. Incorporating digestate-specific regulations within the existing regulatory framework for fertilisers is essential to recognise digestate as a mainstream product and a way to maximise its benefits for the farming industry. It will also give clarity to farmers or other users on the management of the environmental risks associated with the application of digestate, such as run-off, pathogens and heavy metals.

 

EXAMPLES

In California, digestate is regulated under 14 CCR section 17896.2(a). It imposes quality controls such as maximum acceptable metal concentrations and frequency of application. ⁹

Digestate is fully integrated into the EU Fertiliser Regulation (EU) 2019/1009. ¹⁰

 

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4.5. Implement policy to procure digestate for use on public land

Local authorities can lead their residents and businesses by example. Where local governments (councils and municipalities) collect and digest food waste separately, or use AD in their wastewater treatment facilities, policies should be in place for digestate to be used on public land or sold with endorsement. 

The use of digestate in public-sector projects, e.g. parks or landscaping, can reduce the economic burden of digestate disposal on biogas plant operators, reduce council/municipality expenditure on chemical fertilisers, and contribute to the local authority’s net-zero emissions and sustainability goals. It will also increase awareness and acceptance of digestate among residents.

 

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4.6. Offer support for production of digestate

For most biogas plant operators, digestate disposal is a cost, as they often give it away to farmers for free, have to pay for its transport to farms, and sometimes pay for it to be applied. While the digestate market is being developed, governments can support biogas operators by providing direct monetary support. This could be a transitional financial instrument to improve the feasibility of biogas plants before the digestate is market is fully mature.

 

EXAMPLE

The Indian government provides direct financial payment to biogas operators or fertiliser marketing companies for every tonne of digestate produced to encourage market development. ¹¹ 

 

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4.7. Implement policies to support renewable farming

Governments encourage renewable and sustainable farming through a number of policy and financial incentives. By including digestate within the remit of these existing and new support policies, governments can stimulate a market for digestate and encourage its use. These initiatives could include those detailed in sections 4.7.1 and 4.7.2.

4.7.1. Direct financial support

Farming is a low-margin, high-risk business. Changing from conventional to renewable farming increases this risk further during the transition and stabilisation period. To provide some financial security and a buffer to ensure a smooth transition for farmers, governments can provide direct financial assistance to those farmers using renewable fertilisers. This scheme should be extended to include those farmers transitioning to the use of digestate, even though it may not be certified organic, but organic and renewable in its origin.

 

EXAMPLE

The EU’s “Farm to Fork Strategy” sets out a number of initiatives to support organic farming, including eco-schemes and direct payments for organic farmers. ¹² Similar support can be provided to farmers in the initial years of transitioning to renewable fertiliser.

 

4.7.2. Public procurement of sustainably grown produce

Governments have the power to influence and inspire. To create awareness and acceptance of digestate, governments should implement procurement policies that require sourcing sustainably grown produce to be served in public institutions such as school meals, office canteens and events. 

 

EXAMPLE

The EU’s “Farm to Fork Strategy” sets out a number of initiatives to support farming, including public authorities sourcing sustainable food for schools, hospitals and public institutions. ¹³

 

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4.8. Set quality standards for digestate

Depending on the type and quality of feedstock, the digestion process, and pre- and post-digestion treatments, the quality of digestate can vary significantly. It is, therefore, important to establish minimum standards for the quality, storage and application of digestate to land, to maximise its benefits and ensure it is safe for use in agriculture, forestry, horticulture, land restoration and landscaping. 

Based on the potential risks and levels of treatment required, feedstocks may be classified as waste/non-waste, fresh/non-fresh crop, or animal by-products (ABPs). ABPs should be further categorised into:

• Category 1: high-risk carcasses and body parts that may be infected with diseases. These must not be processed in an AD facility. 
• Category 2: carcasses, unhatched poultry and digestive tract content. These are high risk but can be processed in an AD facility.
• Category 3: carcasses and body parts passed fit for human consumption, domestic catering waste, eggs and hatchery by-products. These are considered low risk and can be processed in an AD facility.

Based on these classifications, different levels of treatment and storage may be needed for feedstocks or wastes to become stable and safe for use.

As a minimum, the standards on the parametres and levels in sections 4.8.1–4.8.8 must be included.

4.8.1. Source segregation of classified wastes

Different feedstocks or wastes require different levels of treatment to become a safe and sustainable biofertiliser. It must, therefore, be required that different classifications of waste are collected separately or source segregated. These must be handled separately and must not cross-contaminate. 

 

EXAMPLES

PAS110 provides guidance on the source segregation of feedstock and the handling and application of feedstock and digestate. ¹⁴

EU REGULATION (EC) No 1069/2009 lays down the health rules for animal by-products processed in biogas plants. ¹⁵

 

4.8.2. Duration and temperature of AD

AD can take place at different temperatures ranges, such as mesophilic digestion at 37–40°C and thermophilic digestion at 55°C. At these temperatures, rates of digestion vary for different feedstocks. To ensure an adequate level of decomposition, sanitation and homogeneity, guidelines on the minimum duration and temperature of AD must be developed.

 

EXAMPLE

EU fertiliser regulations require that for digestate to become a product, the feedstock must be digested in a mesophilic or thermophilic process for an adequate retention time and/or followed up by a pasteurisation or composting process. ¹⁶

 

4.8.3. Oxygen uptake rate and residual biogas potential

To ensure that the digestate applied to land will not adversely impact the oxygen levels of the soil or cause methane emissions, it is important to ensure that the AD process has been completed and the digestate has stabilised. This can be measured by the maximum oxygen uptake rate or maximum residual biogas potential. The limits for these must be set and compliance enforced through environmental regulations.

 

EXAMPLE

EU fertiliser regulations have set the maximum allowable oxygen uptake rate at 25mmol O₂/kg organic matter/h. The maximum residual biogas potential is set by the EU at 0.25l biogas/g volatile solids, ¹⁷ while PAS 110:2014 in the UK sets it at 0.45l biogas/g volatile solids. ¹⁸

 

4.8.4. Particulate contamination

Particulate contamination, such as plastics, glass or metals, may be present in food waste collected from households, businesses or industrial waste, but there is a low likelihood of these being present in crop residues or animal slurries. When present, these particulate contaminants may damage AD equipment such as macerators, pumps and motors. Since these contaminants cannot be digested, they may be broken and passed into the digestate, causing irreversible pollution of soils that it is applied to. 

It should be noted that contamination from plastic is primarily of concern when domestic and commercial food waste is digested. Appropriate de-packaging technologies must be installed at the front end to remove as much plastic as possible before the digestion process. Upstream policy on separate food waste collections should seek to remove all such contaminants from the organic fraction. This prevents plastic being broken down into micro-plastics, which cannot be removed with the currently available technologies.

Maximum limits of macro- and micro-contamination should be set for digestate.

 

EXAMPLE

EU fertiliser regulations put a maximum limit of 3g/kg dry matter of macroscopic impurities above 2mm for glass, metal or plastics, and no more than 5g/kg dry matter of the sum of the macroscopic impurities. These limits are scheduled to decrease over a period of time.

 

4.8.5. Additives

Additives may be added to facilitate the digestion process to improve performance. These may be macro-nutrient supplements like P, N and sulphur (S), micro-nutrient supplements like iron (Fe), nickel (Ni), molybdenum (Mo), cobalt (Co), tungsten (W) or biological additives such as microbial inocula, enzymes or compounds to inhibit ammonia emissions. The additives must be registered and regulated with a maximum limit set as a fraction of the feedstock.

 

EXAMPLE

EU fertiliser regulations limit the use of additives to a maximum of 5% of total input material weight.

 

4.8.6. Polymers

Polymers may be added during the digestion process as nutrients, coating agents, binding agents, or to increase the water-retention capacity of the digestate. These polymers must be biodegradable, non-toxic to plants and earthworms, and cause minimal nitrification inhibition.

 

EXAMPLE

EU fertiliser regulations have specified expectations on the biodegradability of polymers used, toxicity tests to be conducted, solubility in phosphate buffer solution, and the concentration of free formaldehyde. ¹⁹

 

4.8.7. Pathogens

AD is a biological process, and in a biological process micro-organisms are not just expected but needed. However, care must be taken to limit and minimise pathogens, as their presence in feedstocks for AD may be passed on to crops and humans via digestate. In order to prevent this from happening, pasteurisation requirements must be defined. While the requirements of individual species may be defined by countries from time to time, indicator species such as E. Coli and Salmonella must be controlled.

 

EXAMPLE

The PAS 110:2014 standard in the UK ²⁰ and EU Fertiliser regulation ²¹ both put the upper limit for E. coli in digestate at 1,000CFU/g fresh matter and requires that no Salmonella be present in any 25g sample of digestate. 

 

8.8.8. Potential toxic elements

Potentially toxic elements (PTEs), such as cadmium, chromium, mercury, nickel, lead, copper and zinc – also known as heavy or transition metals – have toxic effects on humans, flora or fauna. The concentrations of these in digestate must, therefore, be controlled. Additionally, concentrations of inorganic arsenic and biuret must be managed.

 

EXAMPLE

Both EU fertiliser regulations ²² and PAS 110: 2014 ²³ specify the maximum allowable concentrations of PTEs in digestate.

 

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4.9. Require nutrient profiling of digestate

While digestate is a valuable asset, its composition and quality can vary significantly based on the feedstocks used to produce it, the AD process it has been through and how it was handled post-digestion. How digestate is used will depend on what soils and crops it will be applied to, what weather conditions it will be applied under, the machinery used to apply it and number of other criteria. 

While chemical fertilisers have a clearly defined nutrient profile, the variable profile of digestate can pose a challenge for its users. It is, therefore, essential that a clear nutrient profile is provided for every batch of digestate. This will allow farmers and other users to provide their crops with the right nutrition at the right time to maximise their yields. This information is also key for the protection of surface-water bodies in areas where the level of nitrogen or other nutrients that can be applied to the soil is restricted.

4.9.1. Labelling requirements

The nutrient profile of a batch of digestate will vary based on the feedstock used to produce it. It is important that every digestate batch or packet is clearly labelled and accompanied by a nutrient profile to allow farmers to make the best use of it. 

4.9.2. Parametres to be tested and declared

For digestate to be effectively used as an organic fertiliser, the following parametres must be tested and declared in an information sheet.

• Primary nutrients: nitrogen (N, total nitrogen, organic nitrogen, ammoniacal nitrogen), phosphorus (P, P₂O₅) and potassium (K, K₂O)
• Secondary nutrients: calcium (Ca, CaO), magnesium (Mg, MgO), sodium (Na, Na₂O) and sulphur (S, SO₃)
• PTE concentrations: cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn)
• pH value
• Organic carbon (absolute, ratio to total nitrogen)
• Dry matter (also referred to as “total solids”)
• Loss on ignition (also referred to as “volatile solids” and a measure of organic matter)
• Production date 
• Form of the physical product.

 

4.9.3. Directions for use

Essential information on the best use of the digestate should be included, such as:

• the product’s claimed function
• instructions for intended use, such as application rates, timing and frequency, and target plants or crops
• recommended storage conditions
• the impact on nutrient release, water retention and binding should be mentioned if the product contains polymers, in addition to any restrictions and guidelines on its use
• measures to manage risks to human, animal and plant health, to safety or to the environment, if applicable.

 

EXAMPLE

EU Fertiliser regulations ²⁴ and UK PAS 100 ²⁵ require a combination of the parametres in 4.9.2 to be included.

 

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4.10. Develop digestate certification through independent verification

To monitor, verify and implement the digestate quality and labelling standards set, it is recommended that an independently audited certification scheme is developed. This will provide safety and quality assurance for digestate as a product, which in turn will build user confidence and a market for digestate. It will also safeguard environmental and human health.

 

EXAMPLE

In Sweden, Avfall Sverige has been running a voluntary digestate and compost certification scheme since 1999. ²⁶ The audit and certification of digestate (SPCR 120) is done by the Rise Research Institute of Sweden, an independent certification body. ²⁷ The technical requirements for certification are overseen by a steering committee and reviewed annually. A list of certified plants is also made available. ²⁸

In Germany, Bundesgütegemeinschaft Kompost e.V. (BGK) provides digestate certification that has been recognised by RAL (German Institute for Quality Assurance and Labelling). It provides certification for digestion products under RAL GZ 245 and for digestion products produced from renewable energy crops under RAL GZ 246., ^{29 30} A total of 189 biogas plants are certified for RAL Quality Assurance for Fermentation Products. ³¹

The American Biogas Council Digestate Standard Testing and Certification Program is a voluntary, industry-led, third-party verified standard method of quantifying, characterising and communicating the physical and chemical qualities of digestate. ³²

 

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4.11. Require implementation of a quality management system

A quality management system (QMS) is recommended to plan, achieve and demonstrate effective control of all operations and quality management activities necessary to produce digestate that is fit for purpose. It covers the entire production process from inputs to final product. By implementing a robust QMS, digestate producers can ensure consistent quality, meet regulatory requirements and build customer confidence in their product.

4.11.1. Key components

 The key components of a digestate QMS are:

• quality policy and objectives: defines the organisation’s overall quality intentions and direction
• process management: identifies and manages key processes involved in digestate production
• document control: ensures all relevant procedures, work instructions and records are maintained and up to date
• resource management: ensures adequate resources (human, infrastructure, work environment) are available
• product realisation: covers planning, customer-related processes, design, purchasing, production and measurement.
• measurement, analysis and improvement: includes monitoring, internal audits, control of non-conforming product and continual improvement.

 

4.11.2. Specific elements for digestate

• Hazard analysis and critical control points: identifies and controls potential hazards in the production process
• Input material control: strict management of feedstocks used in AD
• Process monitoring: continuous checks on process conditions throughout every stage of processing
• Quality control: frequent or continuous verification of product quality, including digestate sample testing
• Corrective actions: defined procedures for addressing any quality issues or non-conformances
• Validation: obtaining evidence that the QMS elements are effective in producing digestate of the required quality.

 

 4.11.3. Implementation and maintenance

• The QMS should be integrated into daily work activities
• Specific controls must be monitored and recorded, and their efficacy evaluated during and after process validation
• The system should be regularly reviewed and updated to ensure continuous improvement.

 

4.11.4. Standards and certification

The QMS is a tool to ensure and demonstrate compliance with relevant standards and certification for digestate production. It should, therefore, be developed in accordance with applicable standards and regulations and tied into auditing and certification processes. 

 

EXAMPLE

ISO 9001 provides a global standard for QMSs. ³³

In the UK, PAS110 standard requires the implementation of QMS with an annual audit. ³⁴

 

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4.12. Require a nutrient management plan for digestate end-user

As digestate has high percentages of nutrients available for plant growth, like all fertilisers it must be managed to ensure soil health and air quality, and to avoid leakage into the wider environment, especially water courses. Digestate managers, farmers and other end users must employ best management practices when handling and applying digestate to land in order to realise its value as effectively as possible.

A nutrient management plan (NMP) is a detailed strategy on how nutrients in the form of digestate will be applied on-farm to maximise their use and minimise their environmental impact. Regulations must stipulate that all fertiliser application, including digestate, is accompanied by such a management plan. The requirement to have a plan, along with training, can improve farm profitability, enhance soil quality and reduce emissions and water pollution.

Key elements of an NMP are detailed in sections 4.12.1–4.12.5.

 4.12.1. Soil testing

When applying digestate to soils, farmers and operators must take into account the capacity of soils to absorb the nutrients. Comprehensive soil testing should be required to create the current nutrient and structure profile of the soil. Alongside nutrients like nitrogen, phosphorus, potassium and magnesium, a good understanding of pH, salinity, soil structure and moisture-holding capacity is needed to assess digestate application needs. 

 4.12.2. Assessment of crop needs

The amount of nutrients needed will depend heavily on the type and stage of growth of the crops. Nutrients must be properly managed to maximise their efficient use. Levels that are too low will affect yields, and those too high may result in environmental pollution. Farmers and operators must take into account past, current and future crops in their NMP. 

 4.12.3. Nutrient-loss risk assessment

A site-specific nutrient-loss risk assessment considering soil type, topography, weather patterns, nutrient application methods, crop type and growth stage should be undertaken for losses from run-off, leaching, volatilisation, denitrification and erosion. Measures to control and manage these risks should be implemented.

4.12.4. Product analysis   

A detailed analysis of digestate for nutrients must be conducted (if not already available) and matched with the needs of the crop and soil. The parametres that must be considered as a minimum are discussed in Section 4.9. 

4.12.5. Identification of the right source, method, rate and timing of digestate  product application

Based on the above capacity, needs and risks, the right method, rate and timing of digestate product application should be determined for the crop.

 

EXAMPLE

In the UK, the agricultural industry with support from the government has created detailed guidance and template for creating an NMP. ³⁵

In the US, all concentrated animal feeding operations must develop an NMP. ³⁶ Both the US Environmental Protection Agency and Department of Agriculture provide training, guidance and assistance in SMART nutrient management planning. ³⁷

 

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4.13. Actively manage nitrogen applied to soil

While nitrogen is essential for plant growth, in high concentrations it is harmful to people and nature. High levels of nitrogen make water unsuitable for drinking and can also cause eutrophication of marine water. ³⁸

To minimise the environmental risks from excessive nitrogen application to land, policies are needed at both regional and farm levels. These may include:

• monitoring nitrate concentrations of water bodies
• identifying polluted water bodies and those at risk of being polluted
• designating Nitrate-Vulnerable Zones (NVZs)
• limiting fertiliser (organic or mineral) application rates and timing in NVZs, taking into account crop needs, soil nitrogen and all nitrogen inputs
• establishing codes of good agricultural practice and NMPs
• requiring minimum storage capacity for manure, digestate and compost
• regularly monitoring and reporting on nitrate concentrations in water bodies and groundwater, assessments of NVZs, the impact of NMPs and forecast of future trends in water quality.

 

EXAMPLE

The EU has legislated the Nitrates Directive (91/676/EEC) with the aim of preventing pollution of water bodies by nitrates. ³⁹

 

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4.14. Develop programmes that specifically support phosphorus recycling

Like nitrogen, phosphorus is a key nutrient for plant growth. However, there are specific environmental and supply-chain risks pertaining to the availability and use of phosphorus that must be managed. These include:

• Role in eutrophication: phosphate overload can lead to eutrophication of coastal and inland waters. In areas where there are already higher concentrations of phosphates in the water (such as in Republic of Ireland, ⁴⁰ the Gulf of Mexico, USA, ⁴¹ and Denmark ⁴² ), the application of phosphorus to land must be closely controlled. 
• Geographically concentrated sources: phosphorus is essential for plant growth. However, unlike nitrogen and potassium, it cannot be chemically manufactured. It can only be mined from mineral phosphorite or rock phosphate reserves that are heavily concentrated in a few countries: Morocco 70%, China 5%, Syria 3%, Algeria 3% and Russia, South Africa, the US, Egypt and Jordan with 2% each). ⁴³ A robust and diversified supply chain for phosphorus is, therefore, essential for food security.
•  Peak phosphorus: similar to the concept of peak oil, phosphorus is forecast to reach its peak as soon as 2033. This is based on the assumption that demand will surpass the economically feasible supply of this non-renewable and limited resource, leading to a decline in production or an increase in its price. Due to the heavy dependence of food security on the supply of phosphorus as a fertiliser, it is important that it is used efficiently and recycled. 

The measures outlined in sections 4.14.1 and 4.14.2 are recommended for better phosphorus efficiency.

 4.14.1. Phosphorus-limited fertilisation plan

When the nutrient profile of available digestate is such that the phosphate concentration is higher than the need of the crops, the recommended practice is to apply the digestate to meet the phosphorus needs of the crop, and to supply any shortfall of nitrogen and other nutrients with mineral fertiliser.

4.14.2. Mandatory phosphorus recovery from high-phosphorus wastewater

Sewage sludge and wastewater can contain high levels of phosphorus, and the technology to extract it is well established. Given its limited supply and the environmental risk it poses, phosphorus should be recovered from wastewater when its concentration is higher than a predetermined threshold limit.

 

EXAMPLES

In Germany, starting in 2029, the recovery of phosphorus from sewage sludge with phosphorus concentration higher than 20 grams or more per kilogram of dry solids will be mandatory. ⁴⁴

In Switzerland, starting in 2026, phosphorus must be recovered from wastewater, sewage sludge or sewage sludge ash and recycled, for example as fertiliser, to replace use of imported mineral fertiliser in agriculture. ⁴⁵ The cost of this recovery will be partly recovered from the generators of the wastewater.

 

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4.15. Require odour management plan at biogas plant

Odours may be generated from an AD facility at various stages, including waste reception and storage, pre-treatment and preparation, the AD process, digestate handling and storage, biogas handling and storage, and fugitive emissions. 

For better environmental performance and acceptability of the plant, odour management plans should, therefore, be developed and implemented. These include digestate-specific measures, such as enclosed processes for digestate handling, the use of biofiltration or carbon filtration systems, covered storage for digestate, and injection of digestate into the soil. 

Odour management is discussed further in Pillar 6: Technical and Operational Quality Standards and Pillar 7: Environmental Permitting.

 

EXAMPLE

The UK provides a guide for odour management in Environmental Permits that are used by biogas facilities. ⁴⁶ Examples of odour management plans for biogas plants in the UK include the facility in Kirkburn, East Yorkshire ⁴⁷ 

 

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4.16. Monitor the impact of digestate application on soil biota

Healthy soils are important for healthy plants. Studies so far have shown that the impact of digestate on soil biota can vary significantly and can be positive, neutral or negative. It may stimulate microorganisms, bacteria and fungi, but might adversely affect surface-dwelling organisms such as earthworms. The key determining factors include the nutrient profile – available organic carbon, ammonia levels, pH and physical factors – application rate, soil type and structure. ^{48 49}

Further research and long-term field trials are needed to understand correlations between digestate application and soil biota. It is, however, important at the farm level for soil biota to be measured and monitored over time, so that any adverse impacts maybe managed.

 

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4.17. Support development and use digestate products

For every 10 tonnes of feedstock, at least 9 tonnes of digestate are produced. Giving away the digestate for free and paying for transport of these large volumes of digestate for application on farms is an expense for the biogas operators. Processing digestate into a sellable product would significantly shift the financial feasibility of biogas plants, allow expansion of the market beyond the immediate vicinity, and protect the environment in nitrogen- and phosphorus-sensitive areas.

Digestate can be 90% water when treated in a wet AD process and 50% solids when treated in a dry AD process. The solid and liquid fractions of digestate are often separated and are used very differently. These can, however, be processed further to create products such as digestate concentrates, extract ammonium sulphate fertiliser, extract phosphorus as struvite, and for pellets from the solid fraction to be used for soil improvement.

At this time, however, few products are available in the market on a one-off or trial basis. Funding and support are needed for research and development and pilot projects so that the products can be launched and sold at scale. 

 

EXAMPLES

Pelletised digestate (OrganiQ) and liquid ammonium sulphate concentrate for use as fertiliser are produced from processing of chicken manure in Latvia. ^{50 51} 

Biosolids from Blue Plains Advanced Wastewater Plant are treated and sold as a recycled slow-release fertiliser and soil amendment (Bloom). ⁵²

Technologies available for the recovery of nutrients from sewage, manure and other sources are listed here. ⁵³

The Canadian Biogas Association has developed extensive guidance on the use of digestate. ⁵⁴ A supplementary document on digestate nutrient recovery and reuse technologies, such as ammonia stripping and scrubbing, phosphorus precipitation and crystallisation, and drying and pelletisation of solid digestate, has been published to support the industry. ⁵⁵

 

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4.18. Engage with the farming community 

There is a general lack of understanding and acceptance of digestate among farmers. While a lot of work still must be done to make digestate a consistent and reliable product that can be sold in the market, it must be supported with a widespread education and awareness campaign for farmers. For those that are still unaware of the uses and benefits of digestate, training is needed on how to use it to maximise their yields while safeguarding the environment. Any concerns, especially those regarding impact on yields or biosecurity, must be addressed.

As end users of digestate, farmers must also be engaged when developing policies, regulations and standards for the industry. 

 

EXAMPLES

The Value4Farm project combines initiatives and technologies for sustainable and resilient farming. It lays emphasis on engaging with farmers and raising awareness about biogas and digestate. ⁵⁶

The BiogasDoneRight concept was aimed at raising awareness on how to integrate AD and digestate into farming activities to maximise their benefit. ⁵⁷

 

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4.19 Looking forward

Digestate is a valuable output of AD. Strong government intervention on policy, regulation and standards is required to transform it into to a product that can be sold in the market. Given the volume of digestate produced, revenue generation from the sale of digestate can significantly shift the financial feasibility of biogas plants. Policies supporting digestate use must go hand in hand with the strong policies needed to safeguard human, animal and environmental health. 

 

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Footnotes

1. Sustainable Agricultural Practices and Methods, European Commission. https://agriculture.ec.europa.eu/sustainability/environmental-sustainability/sustainable-agricultural-practices-and-methods_en. 
2. Mayerová, M., Šimon, T., Stehlík, M., Madaras, M., Koubová, M., Smatanová, M. “Long-Term Application of Biogas Digestate Improves Soil Physical Properties”. Soil and Tillage Research, 2023, 231, 105715. https://doi.org/10.1016/j.still.2023.105715. 
3. Malhotra, M., Aboudi, K., Pisharody, L., et al. “Biorefinery of Anaerobic Digestate in a Circular Bioeconomy: Opportunities, Challenges and Perspectives”. Renewable and Sustainable Energy Reviews, 2022, 166, 112642. https://doi.org/10.1016/j.rser.2022.112642.  
4. “Global Potential of Biogas”, World Biogas Association. https://www.worldbiogasassociation.org/wp-content/uploads/2019/09/WBA-globalreport-56ppa4_digital-Sept-2019.pdf. 
5. Doyeni, M.O., Stulpinaite, U., Baksinskaite, A., Suproniene, S., Tilvikiene, V. “The Effectiveness of Digestate Use for Fertilization in an Agricultural Cropping System”. Plants 2021, 10, 1734. https://doi.org/10.3390/plants10081734. 
6. “Quality Management of Digestate from Biogas Plant Used as Fertiliser”, IEA Bioenergy. https://task37.ieabioenergy.com/wp-content/uploads/sites/32/2022/02/digestate_quality_web_new.pdf. 
7. “Anaerobic Digestion and Soil Carbon Sequestration”, BioGasDoneRight. https://www.consorziobiogas.it/wp-content/uploads/2017/05/Biogasdoneright-No-VEC-Web.pdf. 
8. “Quality Management of Digestate from Biogas Plant Used as Fertiliser”, IEA Bioenergy. https://task37.ieabioenergy.com/wp-content/uploads/sites/32/2022/02/digestate_quality_web_new.pdf. 
9. Guidance: Land Application of Compostable Materials and/or Digestate,  CalRecycle. https://calrecycle.ca.gov/lea/regs/implement/landapp/. 
10. Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019 Laying Down Rules on the Making Available on the Market of EU Fertilising Products. http://data.europa.eu/eli/reg/2019/1009/oj. 
11. https://gobardhan.sbm.gov.in/whats-new/Market-Development-Assistance-Guidelines.pdf
12. “Farm to Fork Strategy”, EU Green Deal. https://food.ec.europa.eu/document/download/472acca8-7f7b-4171-98b0-ed76720d68d3_en?filename=f2f_action-plan_2020_strategy-info_en.pdf.   
13. “Farm to Fork Strategy”. 
14. PAS 110:2014 “Specification for Whole Digestate, Separated Liquor and Separated Fibre Derived from the Anaerobic Digestion of Source-Segregated Biodegradable Materials”, WRAP. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
15. Regulation (EC) No 1069/2009 of the European Parliament and of the Council of 21 October 2009 Laying Down Health Rules as Regards Animal By-Products and Derived Products Not Intended for Human Consumption. http://data.europa.eu/eli/reg/2009/1069/oj.  
16. Regulation (EU) 2019/1009 of the European Parliament and of the Council of 5 June 2019 Laying Down Rules on the Making Available on the Market of EU Fertilising Products. http://data.europa.eu/eli/reg/2019/1009/oj.  
17. Regulation (EU) 2019/1009. http://data.europa.eu/eli/reg/2019/1009/oj.  
18. PAS 110:2014. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
19. Regulation (EU) 2019/1009. http://data.europa.eu/eli/reg/2019/1009/oj.  
20. PAS 110:2014. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
21. Regulation (EU) 2019/1009. http://data.europa.eu/eli/reg/2019/1009/oj.  
22. Regulation (EU) 2019/1009. http://data.europa.eu/eli/reg/2019/1009/oj.  
23. PAS 110:2014. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
24. Regulation (EU) 2019/1009. http://data.europa.eu/eli/reg/2019/1009/oj.  
25. PAS 110:2014. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
26. Certified Recycling, Avfall Sverige. https://www.avfallsverige.se/in-english/. 
27. Certification of Biofertilizer (SPCR 120), RISE. https://www.ri.se/en/what-we-do/services/certification-of-biofertilizer. 
28. Certified Biofertilizer Plants, RISE. https://publiccert.extweb.sp.se/sv/Product/List/915.   
29. Quality Assurance of Fermentation Product, BGK. https://www.kompost.de/guetesicherung/guetesicherung-gaerprodukt. 
30. Quality Assurance of NawaRo Fermentation Product, BGK. https://www.kompost.de/guetesicherung/guetesicherung-nawaro-gaerprodukt. 
31. Production, BGK. https://www.kompost.de/ueber-uns/zahlen-und-fakten/produktionsanlagen. 
32. Certifying Anaerobic Digestate Through Verified Labs, American Biogas Council. https://digestate.org/. 
33. “ISO 9001:2015 – Quality Management Systems”, ISO. https://www.iso.org/standard/62085.html. 
34. PAS 110:2014. https://www.wrap.ngo/sites/default/files/2021-03/PAS110_2014.pdf.  
35. “Nutrient Management Plan”, Tried and Tested. https://www.triedandtested.org.uk/media/h1nptq5k/nfu-nutrient-management-plans-booklet-a4-20pp_web.pdf 
36. “Understanding Nutrient Management Plans”, US EPA. https://www.epa.gov/npdes/understanding-nutrient-management-plans.  
37. “SMART Nutrient Management”, US Dept. of Agriculture. https://www.nrcs.usda.gov/sites/default/files/2024-02/SMART%20Nutrient%20Mgmt%20farmersgov%20factsheet%202022.pdf. 
38. Nutrients and Eutrophication, US Geological Survey. https://www.usgs.gov/mission-areas/water-resources/science/nutrients-and-eutrophication. 
39. Consolidated Text: Council Directive of 12 December 1991 Concerning the Protection of Waters Against Pollution Caused by Nitrates from Agricultural Sources (91/676/EEC). http://data.europa.eu/eli/dir/1991/676/2008-12-11.  
40. Water Quality and Agriculture, Environmental Protection Agency, Ireland.  https://www.epa.ie/environment-and-you/freshwater-and-marine/water-quality-and-agriculture/#:~:text=During%202023%2C%20average%20levels%20of%20river%20phosphorus%20were,the%20area%20north%20west%20of%20Dublin%20and%20Wexford. 
41. Gulf of Mexico Dead Zone, The Nature Conservancy. https://www.nature.org/en-us/about-us/where-we-work/priority-landscapes/gulf-of-mexico/stories-in-the-gulf-of-mexico/gulf-of-mexico-dead-zone/. 
42. The Potential of Regenerative Agriculture in Denmark, BCG. https://www.bcg.com/publications/2024/potential-of-regenerative-agriculture-in-denmark. 
43. “Approaching Peak Phosphorus”. Nature Plants, 2022, 8, 979. https://doi.org/10.1038/s41477-022-01247-2 
44. Sewage Sludge Ordinance, BMUV. https://www.bmuv.de/en/law/sewage-sludge-ordinance. 
45. https://www.bafu.admin.ch/bafu/en/home/topics/waste/info-specialists/waste-policy-and-measures/phosphorrecycling.html#:~:text=In%20cases%20where%20phosphorous%20is,the%20plant%20must%20be%20recovered. 
46. https://assets.publishing.service.gov.uk/media/5a7ba9a2ed915d1311060b16/geho0411btqm-e-e.pdf 
47. https://consult.environment-agency.gov.uk/psc/yo25-9dr-gwe-biogas-ltd/supporting_documents/EPRA07_Odour_Management_Plan_v13.pdf 
48. Karimi, B., Sadet-Bourgeteau, S., Cannavacciuolo, M. et al. “Impact of Biogas Digestates on Soil Microbiota in Agriculture: A Review”. Environ Chem Lett. 2022, 20, 3265–3288. https://doi.org/10.1007/s10311-022-01451-8. 
49. Van Midden, C., Harris, J., Shaw, L., Sizmur, T., Pawlett, M., Shaw, L. “The Impact of Anaerobic Digestate on Soil Life: A Review.” Applied Soil Ecology, 2023, 191, 105066. doi: https://doi.org/10.1016/j.apsoil.2023.105066. 
50. High Quality Pelleted Organic Fertilizer with a High Content of Organic Matter, OrganiQ. https://www.eggenergy.eu/en/about-product/
51. Egg Energy Ltd., OrganiQ. https://biorural-toolkit.eu/success-story/?id=782.
52. Slow Release Fertilizers, Bloom Soil. https://bloomsoil.com/
53. “Catalogue of Nutrient Recovery Technologies”, European Sustainable phosphorus Platform. https://www.phosphorusplatform.eu/activities/p-recovery-technology-inventory. 
54. Canadian Digestate Management Guide, Canadian Biogas Association. https://biogasassociation.ca/resources/page/canadian_digestate_management_guide/
55. Digestate Nutrient Reuse and Recovery Technology Summary 2024, Canadian Biogas Association. https://www.biogasassociation.ca/images/uploads/documents/2024/resources/Digestate_Nutrient_Reuse_and_Recovery_Summary_May_2024.pdf. 
56. Sustainable Renewable Energy Value Chains for Answering Farmers’ Needs, Value4Farm. https://value4farm.eu/ 
57. “Anaerobic Digestion and Soil Carbon Sequestration”, BioGasDoneRight. https://www.consorziobiogas.it/wp-content/uploads/2017/05/Biogasdoneright-No-VEC-Web.pdf.