risk-based guidance for bsi pas110 digestates in gb ...€¦ · 17 th european biosolids and...

17th

European Biosolids and Organic Resources Conference

www.european-biosolids.com

Organised by Aqua Enviro Technology Transfer

RISK-BASED GUIDANCE FOR BSI PAS110 DIGESTATES IN GB AGRICULTURE

Taylor M1*

, Chambers B1, Litterick A

2, Longhurst P

3, Tyrrel S

3, Gale P

4, Tompkins D

5.

1ADAS UK Ltd,

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

2Earthcare Technical

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

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

5WRAP, The Old Academy,

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

Abstract

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.

Keywords

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

Introduction

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

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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

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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

http://www.sepa.org.uk/waste/waste_regulation/guidance__position_statements.aspx.

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 70oC, with a particle size <12 mm) either before or after digestion is legally

required. Pasteurisation is also a key requirement of the PAS110 specification, even where animal

by-products are not processed.

There are three main types of digestate (whole, liquid and fibre), with whole digestate being the

most commonly available. Some anaerobic digestion plant operators opt to separate the digestate

into liquid and fibre fractions for operational reasons. The fibre fraction typically has a dry matter

content of between 20 and 40% and the liquid fraction between 1 and 6%, although these

proportions will vary depending upon the separation process or processes employed.

Where relevant, digestate applications to agricultural land must also comply with the Animal By-

Products Regulations, whereby, pasture land cannot be used for grazing within 3 weeks (or 2 months

for pigs) of applying digestate; see http://www.environment-agency.gov.uk/netregs/63499.aspx.

‘Typical’ nutrient content of digestate

Digestate varies in its nutrient content, depending on the input materials (i.e. feedstocks), nature of

the AD process and post-digestion processing. The ‘typical’ nitrogen, phosphate and potash content

of food-based digestate is illustrated in comparison with livestock slurries in Figure 1. However, as

the nutrient content of digestate will vary between AD plants, it is advisable to obtain recent

digestate analyses for use in nutrient management planning.

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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)

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Digestate also contains agronomically useful amounts of phosphate and potash, which should be

used in nutrient management planning. Additionally, digestate contains small quantities of trace

elements, with the amounts present dependent on concentrations in the feedstocks used.

Availability of major nutrients from digestate

Most of the nitrogen in digestate will become available to the crop in the year of application, as it is

mainly present as RAN (i.e. ammonium-N). However, the RAN content of digestate can be lost by

two main routes: ammonia volatilisation to air and, following the conversion of ammonium-N to

nitrate-N in the soil, though nitrate leaching to surface and ground waters.

As a general rule, around 50% of the phosphate and around 90% of potash in digestate will be

available to the crop in the year of application. These values should be used in calculating crop

nutrient requirements where a response to phosphate/potash is expected (e.g. ADAS soil P/K Index

0/1; SAC very low/ low status) or where responsive crops are grown. Where the soil is at or above

the target soil P/K status (i.e. ADAS Index 2 or above; SAC moderate status or above) the total

phosphate/potash content of the digestate should be used in nutrient balance sheet calculations. A

typical digestate application (during the crop growing season), based on recent fertiliser prices (N

and P2O5 = 80 p/kg; K2O = 60 p/kg), is worth well over £100/ha in saved fertiliser costs.

When and how to apply digestate

To make optimum use of the N content of digestate, it should be applied at times of maximum crop

growth - generally during the late winter to summer period. To take into account N losses when

incorporating digestate into a nutrient management plan, use of the MANNER-NPK (MANure

Nutrient Evaluation Routine) or PLANET/PLANET Scotland decision support systems is recommended

(www.planet4farmers.co.uk). MANNER-NPK/PLANET predict the fertiliser N value of field applied

organic materials, taking into account manure analysis data (total N, ammonium-N and nitrate-N),

soil type, application timing and technique, ammonia, nitrate and denitrification losses, and the

mineralisation of organic-N. Advice at present, until current research studies are complete, is to use

pig slurry as a proxy for digestate. For further information on making best use of the nutrient

content of organic materials please refer to “The Fertiliser Manual (RB209)” (Defra, 2010), “SAC

Technical Note TN622” (SAC, 2010) or WRAP Guidance (WRAP, 2012b).

To make best use of whole/separated digestate RAN, the ADQP recommends that ‘low emission

application equipment’ is used, namely a bandspreader (trailing hose/trailing shoe, see Plate 1) or

shallow injector (see Plate 2), which will reduce ammonia losses (and odour nuisance) compared

with surface broadcast application. Bandspreading equipment is now available that allows accurate

topdressing across full tramline widths, without causing crop damage and contamination, and

increases the number of spreading days (Chambers et al., 2007; Defra, 2010). On uncropped land,

rapid soil incorporation following application (i.e. within 24hrs) will also reduce ammonia losses and

odour nuisance.

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Plate 1:Bandspread application to arable land Plate 2: Shallow injection application to

grassland

To minimise water pollution risks, digestate (and other organic materials) applications should not be

made when:

• the soil is waterlogged;

• the soil is frozen hard;

• the field is snow covered;

• the soil is cracked down to field drains or backfill;

• the field has been pipe or mole drained or subsoiled over drains in the last 12 months;

• heavy rain is forecast within the next 48 hours.

Also, applications should not be made:

• within 10 metres of any ditch, pond or surface water;

• within 50 metres of any spring, well, borehole or reservoir that supplies water for human

consumption or for farm dairies;

• on very steep slopes where run-off is a high risk throughout the year

Further information on the management of organic material applications is contained in the “Code

of Good Agricultural Practice” in England (Defra, 2009), “The Code of Good Agricultural Practice for

Wales” (WAG, 2011) or “Prevention of Environmental Pollution from Agricultural Activity” (Scottish

Executive, 2005).

Land application controls

In Nitrate Vulnerable Zones (NVZs), the total quantity of N applied in organic materials (including

digestate) must not exceed 250 kg N/ha in any 12 month period (i.e. the field N limit). In some

situations, lower application rates may be appropriate, for example, where the amount of crop

available N would exceed the crop requirement. Also, digestate (in common with other high RAN

organic materials) must not be applied prior to legume crops, as these crops have no N requirement.

As the RAN content of whole/separated digestate exceeds 30% of the total N content, digestate (like

cattle and pig slurry) applications are subject to mandatory closed spreading periods during

autumn/winter in NVZs.

© ADAS UK Ltd

© B Lewens

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Further guidance on NVZ regulations is provided in

England http://www.defra.gov.uk/food-farm/land-manage/nitrates-watercourses/nitrates/

Scotland http://www.scotland.gov.uk/Topics/farmingrural/Agriculture/Environment/NVZintro

Wales http://wales.gov.uk/topics/environmentcountryside/epq/waterflooding/nitratezones/

Also, in Scotland, General Binding Rules (GBRs) place legal restrictions on how and when organic

materials can be spread. Further guidance on GBRs and diffuse pollution regulations are available

from the Scottish Environmental Protection Agency; see

http://www.sepa.org.uk/water/diffuse_pollution.aspx.

Farmers who receive payments under the Single Payment Scheme (and certain rural development

schemes) must maintain their land in Good Agricultural and Environmental Condition (GAEC). GAEC

measures relating to soils, environmental protection and the land application of organic materials

are somewhat different in England, Wales and Scotland, but importantly in the context of digestate

recycling to land, all highlight the important of managing applications to avoid causing soil

compaction. For further information on GAEC rules; see http://www.environment-

agency.gov.uk/netregs/businesses/agriculture/61895.aspx.

Biofertiliser Matrix

Development of the risk-assessment and guidance involved a three-stage series of Sector Steering

Group (SSG) meetings at three venues in Britain. The initial meetings in Cardiff, Edinburgh and

London were held to understand and record the priority concerns for risk-assessment. Following

these first meetings, identified concerns were assessed via semi-quantitative and quantitative risk

assessment (QRA) approaches. In a number of cases, the concerns were directly relevant to best

practice guidance on digestate (biofertiliser) use.

Figure 3: Process of developing risk-based guidance

Guidance on the use of biofertiliser is summarised in the “Biofertiliser Matrix” (Table 1) and

associated crop categories (Table 2). The Biofertiliser Matrix is based on a ‘multiple-barrier’

approach to managing risk, where the effects of AD treatment processes and land spreading

practices have been considered. Where appropriate, the Biofertiliser Matrix has been developed by

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building upon other guidance documents that are already in use (e.g. FSA, 2009; CFA, 2007 and

Assured Food Standards, 2010). This will help to ensure that farmers and land managers are familiar

with the approaches taken, and should make the guidance as easy as possible to implement within

their own farming systems.

Notably, some UK farm assurance schemes contain their own rules and guidance relating to the use

of digestates, which is specific to their members. Ultimately, it is hoped that the “Biofertiliser

Matrix” will be adopted by all appropriate UK farm assurance schemes, but until that point,

digestate users are advised to consult the purchaser of their crop/produce regarding any possible

restrictions on the use of digestate.

Acknowledgements

We gratefully acknowledge Waste and Resources Action Programme funding and support

underpinning the risk-assessment and guidance. Also, we are very grateful for the significant

contribution made by stakeholder consultees who participated in the Sector Steering Group

meetings.

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Table 1: Biofertiliser Matrix (PAS110/ADQP input materials) – agriculture and field horticulture

Crop type Pasteurised Non-pasteurised

Fresh produce

Group 1 �� before drilling/planting �� 12 month harvest and 6 month no

drilling interval applies

Group 2 �� before drilling/planting �� 12 month harvest and 6 month no

drilling interval applies

Group 3 ��

Grassland and forage ��

a 3 weeks no grazing period

and harvest interval applies ��

a 3 weeks no grazing period and

harvest interval applies

Combinable and animal feed crops ��

b

+Pasteurisation process compliant with Animal By-Products Regulations

a2 months no grazing or harvest period for pigs

bIf feedstocks contain maize, biofertiliser applications should be ploughed into the soil ahead of following cereal crops

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Table 2: Biofertiliser Use Matrix – Crop categories

Fresh produce Combinable and animal

feed crops Grassland and forage

Group 1 Group 2 Group 2

Soft fruit (currants, berries etc.)

Lettuce and leafy salads

Radish

Onions

Beans

Vining peas

Mangetout

Cabbage

Cauliflower

Calabrese/broccoli/kale

Courgettes

Celery

Red beet

Carrots

Herbs

Asparagus

Garlic

Shallots

Spinach

Chicory

Celeriac

Fennel

Tomatoes, cucumbers,

peppers

Etc.

Apples/pears etc.

Plums/cherries etc.

Vines

Hops

Nuts

Etc.

Potatoes

Leeks

Sweetcorn

Brussels sprouts

Parsnips

Swedes/ turnips

Marrows

Pumpkins

Squashes

Rhubarb

Artichokes

Etc.

Wheat

Barley

Oats

Rye

Triticale

Field peas

Field beans

Linseed/ flax

Oilseed rape

Sugar beet

Sunflower

Borage

Nursery stock

Bulbs

Etc.

Grass

Maize

Hay

Haylage

Swedes/ turnips

Fodder mangolds/beet/kale

Forage rye and triticale

Herbage seeds

Turf production

Etc.

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References

Assured Food Standards, (2010). Crops and Sugar Beet Standards. Website:

http://www.redtractor.org.uk/site/REDT/Templates/GeneralStandards.aspx?pageid=36&cc=GB

Accessed 1 November 2012.

BSI (2010). Publicly Available Specification No 110. Specification for Whole Digestate, Separated

Liquor and Separated Fibre Derived from the Anaerobic Digestion of Source-Segregated

Biodegradable Materials. BSI, London.

Chambers, B.J., Nicholson, N., Smith, K., Pain, B., Cumby, T. and Scotford, I. (2007). Managing

Livestock Manures Booklet 3: Spreading Systems for Slurries and Solid Manures.

CFA (2007). Microbiological Guidance for Produce Suppliers to Chilled Food Manufacturers. Second

Edition. Chilled Food Association, Kettering, UK.

DECC and Defra (2011). Anaerobic Digestion Strategy and Action Plan. Defra, London, UK.

Defra (2009). Protecting our Water, Soil and Air: A Code of Good Agricultural Practice for farmers,

growers and land managers. The Stationery Office

Defra (2010). The Fertiliser Manual (RB209) 8th

edition. The Stationery Office, Norwich. ISBN 978-0-

11-243286-9.

EC (1999). European Union Council Directive 1999/31/EC. The Landfill of Waste.

FSA (2009). Managing Farm Manures for Food Safety. Guidelines for Growers to Minimise the Risks

of Microbiological Contamination of Ready to Eat Crops.

http://www.food.gov.uk/multimedia/pdfs/manuresguidance.pdf Accessed 1 November 2012.

SAC (2010). Optimising the Application of Bulky Organic Fertilisers. Technical Note TN622. ISSN 0142-

7695 ISBN 1-85482-888-6.

Scottish Executive (2005). Prevention of Environmental Pollution from Agricultural Activity – A Code

of Good Practice.

Taylor, M.J., Rollet, A.J., Tompkins, D. and Chambers, B.J. (2010). Digestate Quality and Fertiliser

Value. Proceedings of the 15th

European Biosolids and Organic Resources Conference.

WAG (2011). The Code of Good Agricultural Practice for the Protection of Water, Soil and Air for

Wales.

WRAP and EA (2009). Quality Protocol Anaerobic Digestate. End of Waste Criteria for the Production

and Use of Quality Outputs from Anaerobic Digestion of Source-Segregated Biodegradable Waste.

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WRAP (2012a). A Survey of the UK Organics Recycling Industry in 2010. Waste and Resources Action

Programme, August 2012.

WRAP (2012b). Using Quality Anaerobic Digestate to Benefit Crops. Waste and Resources Action

Programme, summer 2012.

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