Digestate
A typical anaerobic digestor
Product definition
There are three types of digested products, Whole Digestate, Separated Liquor and Separated Fibre.
Whole Digestate
This is the processed material as it is unloaded from the digester.
To distinguish one product from another, definitions regarding DM content are necessary. The German RAL standard, for instance, defines whole digestate as a pumpable material with <12% Dry Matter (DM).
Separated Liquor
This is the liquid resulting from passing whole digestate through a separator or centrifuge to remove the coarse fibres. A large number of different types of separator are utilised for digestate separation, and each of these are capable of producing separated products with different qualities.
One of the values of removing the coarse fibres from the digestate, is that it produces a thin liquid that runs easily off the leaf of the plant, leaving virtually no surface residue, thus making it highly suitable for growing crop application. This can help to minimise the loss of nitrogen through volatilisation after spreading and through other paths of nitrogen loss. Therefore it is important that both the particle size and Dry Matter (DM) are defined. Liquor containing up to 6% DM material is normally acceptable, provided that the particle size is <1.2mm, for example as can be defined by the hole size in the screen of the separator used.
Separated Fibre
This is the fibre produced by the separator.
Separated fibre can be used fresh as a soil conditioner or after further aerobic composting to stabilise it, a material suitable for making into a compost product. To enable it to be aerobically composted it is generally accepted that the DM will need to be between 25% and 35%. The German RAL standard requires solid digestate products to be >20% DM to enable the fibre to be spread evenly as a soil conditioner.
Uses of Digestate
Whole Digestate
Whole digestate generally contains coarse fibres, and is therefore most useful where the addition of organic matter is beneficial to the soil. If whole digestate is spread by surface application there will be surface residue after spreading. To reduce nitrogen loss, and to maximise the benefits to the soil, it is advised that whole digestate is either spread beneath the crop cover or incorporated as soon as possible after spreading. Otherwise, it is advised that spreading is undertaken in or before light rain/mist, late in the evening or in cloudy still conditions.
Liquor
Liquor is whole digestate that has had the coarse fibres removed. It is an ideal fertiliser for grassland or for foliar application as there is negligible surface residue after spreading; it can also be used for hydroponic feeding as most of the nitrogen content is in forms of ammonium. This is the form of Nitrogen plants can use directly, however it also means the Nitrogen can be easily lost through volatilisation. It is advised that Liquor is applied directly on the ground surface or by shallow injection. To get the best effect it is advised not to spread in strong sunlight or with drying wind.
Fibre
Fibre is the coarse fibre separated out of the whole digestate when producing Liquor. The Fibre contains significant quantities of nutrients and has excellent water retention properties. It can be used fresh as a soil conditioner, or further composted by aerobic means to produce a stabilised material that can be used as a soil conditioner or as a mulch with slow release of nutrients to the plants. It is also suitable to be used as a base for other compost products. The fibre can be spread on land by standard design solid manure spreaders. It is advised to incorporate after application on ploughed land or to be spread on grassland in Autumn or early spring to get most effect.
If the fibre is used fresh it may have a detrimental effect on seed germination rates, and may temporarily reduce nitrogen availability in soil. If the composted fibre is used as a growing medium, it generally will require dilution as the nitrogen content is usually too high for good growth of young plants.
Storage of processed products
- The layout of the site, process management, packaging and transportation must be designed and operated to ensure that processed products do not become contaminated by other material that is not fully processed
- All stores must comply with local agriculture and environmental best practice
- Liquid stores must be covered to minimise volatilisation
- Fibre may be temporarily stockpiled, without cover, before spreading, however all other digestate storage should be covered and any leachate produced should be contained. Storage and must comply with local agriculture and environmental best practice
Application rates to land of processed products
Rates of application
- A nutrient management plan must be drawn up and each farmer must agree to comply with the NMP, for all application of the digested products to agricultural land
- The soil should be sampled at least every 5 years. The methods to be used for sampling must in accordance to National guidelines
- The digested products should form the basis of the NMP with other fertilisers being used to top up to annual requirements
- The rate of application must comply with local agricultural best practice codes
- Rate must not exceed crop requirements, and must consider the soil analysis and cropping history of the field
- High trajectory spreading by tanker or irrigator will not be permitted
In most situations the spreading rate will be determined by the phosphate content of the digestate product and by soil phosphate index.
Timing and areas of application
- Timing of application and the areas that receive applications should be in accordance with agricultural best practice codes
- Applications may not be made during periods when weather conditions are not suitable or when agricultural best practice codes advise spreading should not take place
- It is advised that a land management plan (as outlined in SEPA’s 4 point plan) is utilised, to ensure applications are made to land at suitable times and rates and in a manner appropriate to prevailing weather conditions, crops, land type, and surrounding features
Records
Records must be kept, for at least two years, of soil analysis, timing and rates of application and areas applied.
Nutrient analysis of digested products
All digested products must be sampled at least once per year for total-N, available-N, P, K & pH. Sampling and analysis methods must be in accordance with those specified in (TBA) When an application is being considered for award of the Digestate Standard, the applicant must show that the sampling frequency will be sufficient to ensure with reasonable reliability that the stated nutrient content of the digestate products will remain within the permitted deviation.
- NMP should be made taking into consideration 100% of available-N and a proportion of the organic-N that will depend on the time of year of application. Organic-N is considered to be Total-N minus available-N
- If materials are spread in Spring (Feb-end May) then 30% of the organic-N should be available to the crop in that year
- If materials are spread in Summer (June-Aug) then 10% of the organic-N should be available to the crop in that year
- If fibre is spread in Autumn (Sept/Oct) then 35% of the organic-N should be considered as available to the crop in the following year
Use of digested products as constituents in other fertiliser products
Digested products can be used as a basis for the production of other fertiliser products. Currently the Digestate Standard will not be applicable to these manufactured products if they contain
DIGESTATE STANDARD 
A Standard for the Application of Digestate to Farmland has been written for Scotland – here is a brief on the reasons for its creation:
Purpose of the Standard
The purpose of a Digestate Standard is to remove a major barrier to the development of anaerobic digestion, by creating a certification system that will ensure that digestate products are of a consistent and verifiable quality and suitable for growing quality food products. When this standard is achieved the digestate products are considered as fertiliser products and no longer require a waste licence or a waste licence exemption when spread on land. The Standard ensures that the digestate complies with all relevant Regulations and it is intended to be workable and affordable in practice.
It is intended to give the farmer and regulators confidence that the use of digestate (with the Standard Mark) as a fertiliser and soil amendment, will be beneficial for the land. The Standard will give reassurance to the retailer and consumer that food grown using digestate products is safe for human and animal health. The Standard will provide an effective and additional marketing tool for food, because AD facilitates sustainable principles involving recycling of biodgradeable materials, production of renewable energy and improving soil quality.
The Standard is intended to provide AD project developers with the information they need to be able to achieve best practice in plant design and operations that, as a minimum, will achieve all Regulatory requirements relating to the production of digestate products and be practical and affordable to implement.
Scope
The Standard specifies requirements for feedstock inputs, controls on the management system for the process of anaerobic digestion, quality criteria for the digestate products, the external certification scheme, guidelines for the marketing and use of the digestate, the regulatory framework and a laboratory testing methodology.
The Standard is for biodegradable materials that have not been mixed, combined or contaminated with other potentially polluting wastes, products or materials.
The requirements for the minimum quality of digestate products specifies nutrient content, potentially toxic elements, dry matter, physical contaminants, seeds, odour, plant response, particle size, stability, water retention properties, pathogens and harmful organic compounds. Digestate products take three forms, whole digestate or separated into fibre and liquor and each has distinct qualities, which are reflected both in the test requirements and in the guidelines for their use. These differences need to be recognised and evaluated so that each of the product types can be used suitably.
The Standard specifies minimum quality standards for the product but at the same time provides detailed guidelines for its application to farmland in order to ensure that the long term health of land is maintained.