risk-based guidance for bsi pas110 digestates in gb ... · pdf file 17 th european biosolids...

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  • 17 th

    European Biosolids and Organic Resources Conference


    Organised by Aqua Enviro Technology Transfer


    Taylor M 1*

    , Chambers B 1 , Litterick A

    2 , Longhurst P

    3 , Tyrrel S

    3 , Gale P

    4 , Tompkins D

    5 .

    1 ADAS UK Ltd,

    ADAS Gleadthorpe, Meden Vale, Mansfield, Notts, NG20 9PD.

    2 Earthcare Technical

    Ltd, Manor Farm, Chalton, Waterlooville, Hampshire, PO8 0BG. 3 Cranfield University, Bedford, MK43

    0AL. 4 AHVLA, Woodham Lane, New Haw, Addlestone, Surrey, KT15 3NB.

    5 WRAP, The Old Academy,

    21 Horse Fair, Banbury, OX16 0AH. * Corresponding Author Tel. 01623 844331 Email [email protected]


    This paper reports on the development of guidance for the application of BSI PAS110 digestates

    (biofertilisers) in agriculture. Evidence drawn from a comprehensive quantitative risk assessment of

    the potential for harm to crops, humans, animals and the wider environment was used to develop

    practical guidelines on the use of digestate. A matrix of differing crop types, aligned with existing

    agricultural guidance is presented, which takes account of the opportunities for hazard prevention

    from the feedstock source, pathway of application and point of exposure to sensitive receptors.

    Scenarios that considered the highest plausible combination of hazards arising from anaerobic

    digestion feedstocks, processing, land application rates and differing crop categories were used to

    prepare the guidance. The work provides practical, evidence-based guidance for the beneficial

    application and use of digestate as a sustainable nutrient source in agriculture.


    Digestate, biofertiliser, best practice, recycling organic materials to land.


    The recycling of organic materials to land is regarded as the best practicable environmental option in

    most circumstances, completing both natural nutrient and carbon cycles. Organic materials are

    valuable sources of major plant nutrients (i.e. nitrogen – N, phosphate – P2O5, potash – K2O and

    sulphur – SO3), which are essential for plant growth and therefore sustainable crop production.

    Organic materials also provide a valuable source of organic matter, which improves soil water

    holding capacity, workability and structural stability etc.

    Anaerobic digestion (AD) involves the breakdown of biodegradable materials (such as household

    food waste, livestock slurry and waste from food processing plants) in the absence of oxygen.

    During the AD process methane is released that can be used to provide heat and power, and a

    digestate is produced. In the UK, the quantity of digestate (from source-segregated biodegradable

    materials) currently recycled to agricultural land is relatively small (around 1 million tonnes fresh-

    weight; WRAP, 2012a), when compared with livestock manures (around 90 million tonnes), biosolids

    (3-4 million tonnes) or compost (around 2 million tonnes). It is predicted that there could be up to 5

    million tonnes of food-based digestate produced by 2020 (DECC/Defra, 2011), as the drive to

    remove organic materials from landfill increases and the need to generate gas/electricity from

  • 17 th

    European Biosolids and Organic Resources Conference


    Organised by Aqua Enviro Technology Transfer

    renewable sources grows. The EC Landfill Directive sets strict limits on the amount of biodegradable

    municipal waste that can be disposed of via landfill; the amounts must be reduced by 65% in 2020

    compared with 1995 levels (EC, 1999).

    In addition to the legislative drivers, there is increasing agricultural demand for organic materials

    (e.g. digestate, compost, etc.) as sources of crop available nutrients. This is largely due to recent

    volatility in the price and also availability of manufactured fertilisers, as a consequence of the

    growing demand for food as the world’s population continues to grow. As a result, farmers and

    growers are becoming increasingly interested in and reliant on organic materials to supply crop

    nutrient requirements. Additionally, as the importance placed on carbon footprinting increases,

    farmers and land managers will be encouraged to reduce the carbon footprint of their products, for

    example, through using organic materials to offset the use of manufactured fertilisers.

    How is digestate produced? (i.e. what is anaerobic digestion)

    Digestate is a natural product which results from the controlled biological decomposition of

    biodegradable materials in the absence of oxygen. Suitable input materials include domestic and

    commercial food wastes, livestock manures and energy crops. Digestates are an alternative to

    manufactured fertilisers and by using them, farmers and growers can improve the sustainability of

    their cropping systems, whilst saving money on purchased fertiliser. Also, digestate (in particular the

    fibre fraction) can help to build soil organic matter levels, which can improve soil quality, crop health

    and yields over the longer-term.

    The UK government supports AD as one of the best ways to recover value from organic wastes –

    primarily because energy in the form of biogas is produced naturally as part of the digestion process.

    This biogas can be used as a substitute for natural gas (biogas can also be upgraded to biomethane

    for injection into the national gas grid), to produce green electricity or heat, or it can be compressed

    for use as a transport biofuel. Anaerobic digestion is a key part of the UK government’s strategy to

    increase the production of renewable energy and help combat climate change.

    Why use digestate?

    Digestate is an excellent source of readily available nitrogen (N), phosphate (P2O5), potash (K2O),

    sulphur (SO3) and trace elements. Digestate is also a source of organic matter, with fibre digestate a

    particularly valuable source. Farmers and growers can save on manufactured fertiliser costs by using

    locally available digestate.

    What is Biofertiliser?

    The BSI PAS 110 biofertiliser certification scheme (BSI, 2010) provides a baseline quality standard for

    digestate, ensuring that it is consistent, safe and reliable to use. In 2009, the Quality Protocol for

    Anaerobic Digestate (ADQP) was launched in England, Wales and Northern Ireland to provide a clear

    framework for the production and supply of quality digestate i.e. biofertiliser (WRAP/EA, 2009). It

    builds on BSI PAS 110 by clarifying which waste materials can be used in quality digestate production

  • 17 th

    European Biosolids and Organic Resources Conference


    Organised by Aqua Enviro Technology Transfer

    and by ensuring accurate record keeping when PAS 110-compliant digestates are used in agriculture,

    field horticulture, landscaping and land restoration.

    ADQP-compliant digestate is classed as a product, not a waste, and therefore does not require an

    environmental permit/exemption for transport or application to agricultural land. The ADQP does

    not apply in Scotland, where PAS110 biofertiliser can be used without further regulation in

    accordance with good agricultural practice and the SEPA Regulatory Position Statement; see


    Types of digestate and their benefits for agriculture and field-grown horticulture

    Anaerobic digestion systems vary widely in terms of their design. However, the main types are either

    continuous wet or dry systems, that are run at either mesophilic (30-40°C) or thermophilic (50-60°C)

    temperatures. Most UK operators use mesophilic anaerobic digestion (MAD) systems. Where animal

    by-product materials (category 2 and 3) are included in the feedstock, an additional pasteurisation

    phase (i.e. 1 hour at 70 o C, with a particle size

  • 17 th

    European Biosolids and Organic Resources Conference


    Organised by Aqua Enviro Technology Transfer

    Figure 1: ‘Typical’ nutrient content of food-based digestate and livestock slurries

    Digestate is an excellent source of readily available N (i.e. ammonium) which is potentially available

    for immediate crop uptake. Food-based digestate typically contains around 80% of its total N

    content as RAN, compared with around 70% for pig slurry and 45% for cattle slurry (Figure 2).

    Digestion of livestock slurry will typically increase RAN by around 10% of the total N content.

    Food-based digestate

    Pig slurry

    Cattle slurry

    % RAN % Organic N

    Figure 2: Readily available N (RAN) content of food-based digestate in comparison with

    ‘typical’ values for pig and cattle slurry (adapted from Taylor et al., 2010)

  • 17 th

    European Biosolids and Organic Resources Conference


    Organised by Aqua Enviro Technology Transfer

    Digestate also contains agronomically useful amounts of phos