What is digestate?
Digestate is a by-product of the anaerobic digestion (AD) process, representing the treated/digested substrate. Some characteristics of digestate are shown in the table below:
Nutrient distribution in digestate, compared with cattle and pig slurries.
Digestate is more homogenous, compared to raw pig or cattle slurry, with an improved N-P balance. It has a declared content of plant nutrients, allowing accurate dosage and integration in fertilisation plans of farms. Digestate contains more inorganic nitrogen, which is more accessible to the plants, than untreated slurry.
How can I store the digestate and in what form?
Digestate can be temporarily stored in specially built storage containers. Legislations in many European countries require up to nine months storage capacity for digestate (as well as for untreated animal manure and slurry), in order to ensure optimal and efficient utilisation as a fertiliser and to avoid application during the winter season.
Digestate can be stored in concrete tanks, covered by natural or artificial floating layers or membranes, or in lagoon ponds.
Storage tanks covered with natural floating layer. Source: Danish Biogas Association
An important way of preventing emissions and leakage is to store and handle digestate properly.
Experience in Denmark shows that placing an artificial floating cover on digestate storage tanks can reduce ammonia volatilization from 20% to less than 2%.
Floating cover on digestate storage tanks reduces ammonia volatilization. Source: DIAS
Will the liquid digestate still produce gas during storage and if so how do I deal with it?
After AD treatment, emissions of methane (and of ammonia) from digestate are possible, but many years of experience with good agricultural practice proved that notable reductions of emissions, odours and nutrient leakage are possible.
After leaving the digester, digestate can still produce certain amounts of methane and ammonia. Experience from Denmark shows that up to 20% of the total biogas production can take place outside the digester, at ambient temperature in storage tanks. The process is often called post-digestion and the amount of biogas produced depends on the AD process temperature and the retention time inside the digester.
In order to prevent methane emissions from digestate and to collect the extra biogas production, storage tanks for digestate should always be covered with a gastight membrane for gas recovery.
When digestate is transported to storage facilities out in the fields, these should also be covered with a natural floating layer, as a minimum, in order to reduce the risk of further emissions.
Membrane covered storage tanks. Source: Danish Biogas Association
Is it possible to recycle the digestate to the farmland?
The standard use of digestate is as liquid fertiliser on agricultural land.
For optimum utilisation of digestate as fertiliser, the same basic principles as those used for untreated slurry and manure, must be considered:
- Sufficient storage capacity (minimum 6 months)
- Restricted season of application as fertiliser
- Amount applied per hectare
- Application technique
Due to its higher homogeneity and flow properties, digestate penetrates into soil faster than raw slurry. Nevertheless, application of digestate as fertiliser involves the risk of nitrogen losses through ammonia emissions and nitrate leaking. N-efficiency will increase considerably and nutrient losses by leaching and evaporation will be minimised if digestate is used as fertiliser in conformity with good agricultural practice. For that, some simple rules should be respected:
- Avoid too much stirring of digestate before application
- Only apply cooled digestate, from the post storage tanks
- Application in the fields must be done with dragging pipes, dragging hoses, direct injection in soil or disk injectors
- If applied on the surface of soils, immediate incorporation in soil is necessary
- Depending on the crop, digestate should be applied at the start of the growing season or during vigorous vegetative growth ( the best time for application in European conditions)
- Application to winter crops should be started with 1/3 of the total N requirement
- Optimum weather conditions for application of digestate are: rainy, high humidity and no wind (dry, sunny and windy weather reduces the N-efficiency considerably)
Some general measures are recommended, in order to produce digestate of good quality, suitable for safe recycling:
- Consistent control of the AD process parameters (temperature, retention time etc.) to obtain a stable end product (digestate)
- Sanitation of digestate, according to your country's regulation standards, for effective pathogen reduction
- Periodical sampling, analyses and assessment of chemical composition of digestate
- Integration of digestate into the farm fertiliser plan and utilisation of “good agricultural practice” for application of digestate on farmland.
- Careful selection of AD co-substrates (other than animal slurries), based on complete declaration and description of each feedstock load: origin, composition, content of heavy metals and persistent organic compounds, pathogen contamination, other potential hazards etc.
Digestate can be used as a top-fertiliser on crops in full vegetation. This application offers little concern about loss of nitrogen as nitrate into ground water, since most of the nutrients are absorbed directly by the plants. Danish experience indicates that when digestate is applied as top-fertiliser, some of the nutrients are absorbed through the leaves.
Example of national regulations of the nutrient loading on farmland (Nordberg, 1999)
Maximum nutrient load Required storage capacity Compulsory season for spreading
Austria 170 kg N/ha/year 6 months 28/2-25/10
Denmark 170 kg N/ha /year (cattle), 140 kg N/ha/year (pig) | | | | | | | | | | |
|---|
Can I sell the digestate?
Yes, - but you have to make sure you are following your country's rules. Check this with the national agricultural advisory services and/or the national Environmental Protection Agencies (EPA’s).
It is necessary to perform chemical analysis of the digested manure. Average analyses are always best, based on good incremental samples. When you know the nutrient content in the digested manure (biogas slurry) you can make a sales calculation.
After that you need to determine the actual nutrient prices of NPK according to the world market prices or to consult an agricultural advisory centre. Then the actual sale price of the digestate based on the nutrient content can be calculated.
Which bits of legislation apply to the application of digestate to farmland?
No general answer can be given to this question. Nearly all countries have their own rules. FOr instance some EU-countries have to abide the Nitrate Directive, environmental sensitive areas, water directives etc. whereas others do not.
The best advice is to consult you country EPA authorities or their home page as well as the agricultural authority within your country.
Can digestate from biogas production really replace mineral fertilizers?
Yes, digestate can replace the use of mineral fertilisers. The fertiliser replacement value of the digestate depends upon its crop nutrient content. This in turn is a function of the quantity of crop nutrients in the material that is fed into the digester. If the feedstock, as for example cow slurry, contains 3-4 kg of total nitrogen (depending on its dry matter content) then there will be the same amount of total nitrogen present in the digestate.
When digestate is used as a fertiliser it acts like any other organic liquid such as animal slurry. Its efficiency, as in the case of any fertiliser application, will depend on a number of factors such as the method of spreading, the time of year applied, weather conditions and the dry matter concentration.
When spreading the digestate as indeed any other fertiliser, it is vital to ensure that the quantities of crop nutrients applied do not exceed crop requirements and that Codes of good Farming/Agricultural practice are followed together and that there is compliance with the EU Nitrates Directive (See below for further information) any /or other national regulations.
What are the nutritional properties of digestate?
The fertiliser value of the digestate varies from one plant to another depending upon the qualities and quantities of the individual inputs that are fed the digester. For example, an input of 100% cow slurry would go into the digester containing 3 kg of total nitrogen (see table below). It would leave the digester as digestate also with 3 kg of total nitrogen. Similarly the quantities of phosphate (P
2O
5) and potash (K
2O) in the digestate reflect those that are present in the feedstocks that go into the plant.
However a mixed feedstock with 70% dairy cow slurry, 20% grass and 10% blood would produce a digestate with a total N concentration of 5.2 kg (calculated from the table below).
Indicative nutrient content of typical feedstocks (kg/fresh tonne):
Feedstock (substrate) | kg of TS per fresh tonne | Total N | P2O5 | K2O |
Dairy cow slurry | 60 | 3.0 | 1.2 | 3.5 |
Pig slurry | 60 | 4.0 | 3.0 | 3.0 |
Blood | 110 | 13.4 | 1.3 | 1.0 |
Grass | 370 | 8.6 | 5.8 | 4.3 |
Catering waste | 320 | 8.4 | 1.1 | - |
Vegetable waste | 150 | 4.6 | 0.87 | 2.25 |
As in the case of the original inputs, not all the nutrients are available for the uptake and growth of the next crop. It is worth noting though that unlike the original slurry where 50% is usually available for the next crop when applied under optimum conditions, after digestion the proportion that is available can increase to some 70-75%. The increase arises from chemical changes that take place during the digestion process.
This illustrates how the individual feedstocks and the proportions of each that are fed to the digester can influence the nutrient content of the end product – the digestate or biofertiliser. In order to make maximum use of the digestate – in fact a biofertiliser- it needs to be applied at the correct time of the year when the crops are growing.
How much digestate can I apply to land?
There are three questions that you need to address:
- What is the crop requirement for the particular nutrient?
- How much residual nutrient is in the soil and therefore what is the difference that I need to apply to meet crop need?
- Is my land in a Nitrate Vulnerable Zone (NVZ), subject to limitations on the amount of phosphate that can be applied or other limitations in the kg/ha of nutrients that I can apply under the respective national rules.
Once these questions have been answered then you will know how much of the nutrients that you need and are permitted to apply. The nutrient content of the digestate would be tested regularly for the biogas plant (and this should also apply to farmers who have their own plants) so that you will receive a status report/ quality assurance when you take delivery. There are supplementary questions that you must also address.
Are there times of the years when digestate cannot be spread?
Yes. There are limitations and each country is likely to have its own rules quite apart from any that are laid down on an international basis such as the conditions pertaining to the Nitrate Vulnerable Zones in EU member countries. The limitations are set to lessen the risk of surface run off of nutrients and percolation through under-drainage networks into ditches, streams and rivers and ultimately into lakes and coastal waters. For practical reasons, application of nutrients to land when the plants are not growing and therefore are unable to take them up is a waste of the money spent on spreading and of the monetary value of the nutrient itself. If wasted thus you will have to buy more mineral fertiliser and at to-days costs it will affect your balance sheet!
Does the use of digestate on my land bring a risk of importing pathogens onto my farm?
When the farm uses feedstocks produced on that farm there is no greater risk of importing pathogens than there was when there was no anaerobic digester. Indeed, the digestion process reduces the concentrations of pathogens very significantly relative to raw slurry or any other input. The degree of pathogen reduction depends on the number of days (hydraulic retention time) that the feedstock remains in the digester, the temperature inside the digester and also whether pasteurisation is employed. Pasteurisation is NOT required for farm-based feedstocks such as slurry and crops. Even if digestate is delivered to the farm from elsewhere the risk of importing pathogens is greatly reduced when compared with importing slurry.
If feedstocks such as cooked food, stomach contents and blood from slaughter-houses, etc form part of the feed stock mix the biogas plant MUST be compliant with the EU Animal By-Products Regulation (EC Regulation No. 1774/2002). Such material must be pasteurised either pre- or post digestion. This leads to a major reduction in faecal indictor organisms. Furthermore, the eggs of gastrointestinal worms, modular and roundworms from pigs and cattle, tapeworms from cats and dogs and the larva of lungworms in cattle for example, can all be inactivated within just 8 days residence time in a farm digester working at 35-37°C. The residence time in such digester is usually at least between 18-21 days and in many cases even longer.
How will I find out what is in the digestate?
If a farm has its own plant the owner will know what feedstocks have been used and the source of any non - farm inputs that have been received under contract from food processors, etc. and this be must pasteurised if it contains animal by-products. If you want to know the exact composition of the nutrients in your digestate (biofertiliser) you should send a sample to a laboratory for analysis
If a farmer wants to purchase or import digestate (biofertiliser) from a biogas plant elsewhere then it is likely/ desirable that he should know what feedstocks have been put into the digester the end product of which he would be applying to his land. Managers of co-digestion plants are bound by strict codes of practice set both by the EU and their respective governments. While they are not under any obligation to declare the actual feedstocks that they use to produce the biogas and digestate, they are bound by law to meet the EU and national regulations for handling animal by –products when such material has been included in the feedstock mix. Thus the digestate that is delivered to a farm should have a cover note that states that it is free from salmonella and contains less than 1,000 cells per /g of E.coli. It would also detail the nutrient content and chemical analysis of the digestate that is being delivered. Increasingly individual governments or trade associations already have developed or are developing publicly available quality assurance standards that offer a high level of biosecurity but what is more, are perceived to be acceptable standards by the supermarkets.
Will the digestate smell when it is applied?
Slightly but only for a short time. This will depend on the method of application and the weather conditions. However you and your neighbours will notice a very marked reduction in odour compared with normal slurry! The odour from the digestate is less pungent and will dissipate quickly after spreading especially if it is applied by injection, dribble bar, trailing hose or trailing shoe applicators or is incorporated quickly into the soil. The main reason for the reduced odour is that the organic fatty acids which give rise to the offensive smells associated with normally managed slurry are broken down during digestion and converted into biogas.
Source: Biogas Baromter, 2004-2008.
Biogas upgrading plant in Gothenburg, Sweden.
Filling station for upgraded biogas in Östersund, Sweden.