Digestate
As fermentation residue of liquid or solid residue is referred to, which during the fermentation of biomass in a biogas plant remains. Because of its high content of nutrients , it is mostly used as an agricultural fertilizer. The term biogas slurry or fermentation product is used. For the fermentation residues from the production of ethanol from grain, however, the term stillage is mostly used.
Formation of digestate
Main article biogas plant
A wide variety of substrates are used in biogas plants , such as B. liquid manure , corn silage or organic waste . These contain water and organic compounds such as carbohydrates ( sugar , starch , hemicellulose , cellulose ), proteins , fats and other compounds in different proportions. A large proportion of these organic compounds is broken down by microorganisms during the anaerobic biogas process . In biogas , methane (CH 4 ) and carbon dioxide (CO 2 ) are the most important breakdown products in terms of quantity. In addition, it initially contains small amounts such as hydrogen sulfide (H 2 S) and ammonia (NH 3 ). The decomposition processes turn the viscous to solid substrate into liquid to viscous digestate with a high water content.
Composition and fertilizer value
Digestate contains considerable amounts of nitrogen, which is easily available to plants, as well as phosphorus, potassium, sulfur and trace elements. Unpolluted digestate can therefore be viewed as high quality organic fertilizer. The nutritional composition of the digestate can vary widely, depending on the substrates used.
| parameter | Average value (plants with renewable raw materials) |
Mean value (biowaste plants) |
|---|---|---|
| Dry matter | 7.0% | 6.1% |
| PH value | 8.3 | 8.3 |
| Organic matter | 51 kg / t FM ( fresh mass ) | 42 kg / t FM5 |
| nitrogen | 4.7 kg / t FM | 4.8 kg / t FM |
| ammonium | 2.7 kg / t FM | 2.9 kg / t FM |
| phosphorus | 1.8 kg / t FM5 | 1.8 kg / t FM5 |
| potassium | 5.0 kg / t FM | 3.9 kg / t FM |
nitrogen
Since only small amounts of NH 3 escape from the substrate , most of the nitrogen (around 3.4 to 5.0 kg / m 3 ) remains in the digestate. The breakdown processes reduce the amount of nitrogen bound in the organic matter, while the amount of NH 3 increases - 45 to 76 percent is present as ammonium nitrogen. Since the pH value shifts from the neutral to the slightly basic pH range (pH 8 to 8.5) during fermentation , the equilibrium also shifts from the readily soluble ammonium NH 4 + to NH 3 . When storing and spreading the digestate there is a higher risk of ammonia outgassing. It can be positive that the nitrogen is available to plants more quickly in this form. However, if the needs of the plant are low, the nitrogen applied may not be used effectively.
More nutrients
The plant-available potassium and phosphorus contents are given as around 1.8 to 3.5 kg / m 3 . Depending on the method used to desulfurize the biogas, a large part of the sulfur can also be returned to the digestate. Other nutrients added with the substrate such as phosphorus , potassium , magnesium , calcium etc. also remain in the digestate.
Fertilizer value, humus effect and pollutants
After fertilization with digestate, the microbial activity of the soil is increased for approx. 9 to 10 weeks. Earthworms, on the other hand, avoid soil fertilized with large amounts of digestate. The low viscosity of the digestate compared to liquid manure, which means faster penetration into the soil and thus reduced emissions, is rated positively.
As fertilizer in arable farming, fermentation residues can largely replace mineral fertilizers (e.g. with spring wheat ) or supplement (e.g. maize ), depending on the crop , the fertilizing effect in terms of nitrogen - depending on the crop - is usually well over 70% of the corresponding amount a mineral fertilizer.
The supply of organic matter through fertilization with digestate is only small, the humus requirement of the soil cannot be covered by digestate fertilization. The heavy metal content of digestate is so low that it does not pose a problem for agricultural use. The salmonella infestation of fermentation products from biogas plants is at a low, hardly worrying level, the clostridial contamination is similar to that of untreated manure . The transmission of plant diseases through digestate cannot be assumed either.
An analysis from the canton of Zurich showed that half of the solid fermentation products in the 2018 reporting year did not meet the additional requirements regarding plastic content. In order not to further promote the emissions of microplastics , from 2020 Bio Suisse no longer wants to distribute fermentation residues from biogas plants in its fields, in which material packaged in plastic is fermented.
Storage and application
The digestate is first collected in a storage container. A gas-tight cover for the digestate storage facility is required for new systems in order to avoid emissions of the strong greenhouse gas methane (CH 4 ).
The liquid digestate is spread using liquid manure tanks , while solid digestate is spread over agricultural areas with manure spreaders . Most of the fermentation residues are spread on the cultivation areas for the substrate of the biogas plant or the fodder cultivation areas of the manure-producing livestock. This return ensures a closed nutrient cycle. The positive thing about the use of liquid manure as a substrate is that the methane emissions that occur during conventional liquid manure storage are reduced. When applying liquid manure, unpleasant odors can arise, primarily from ammonia, organic sulfur compounds, hydrogen sulfide (H 2 S) and organic acids. Apart from ammonia, these compounds are broken down during fermentation. The digestate is therefore significantly less odorous than the liquid manure used.
In Germany, the fertilizer ordinance stipulates that the digestate may only be applied to agricultural land at certain times. During the so-called core lock-up periods, the spreading of digestate on arable land from November 1st to January 31st, and spreading on grassland from November 15th to January 31st is not allowed. In exceptional cases requiring approval, the time of the core blocking period, but not the length of the period, may be changed. The purpose of the ordinance, and in particular of the core blocking periods, is to protect water bodies and groundwater from excessive nutrient input.
Drying, burning
In other recycling methods, the digestate is first dried before it is spread on agricultural land. This increases the transportability compared to the highly water-containing digestate. Often fermentation residue drying is operated with heat from biogas conversion. Since the digestate can still contain a certain amount of difficult to break down carbohydrates ( cellulose , lignocellulose ), it can also be of interest as a fuel after drying. However, high mineral proportions and the sulfur and nitrogen compounds they contain lead to a relatively high accumulation of slag and cause corrosion in the boiler and relatively high emissions.
Special features of certain substrates
In other biogas plant concepts, a less watery substrate, such as. B. cereals used. The liquid phase is pressed out of the aqueous fermentation residue and used to mash the substrate. The solid phase of the separated digestate is more suitable for transport . With various dry fermentation concepts, solid digestate is also produced. One example is the fermentation of biowaste , which is usually followed by composting the digestate. Household waste can also be fermented to break down the organic content and thus reduce the total mass.
Legal classification
The legal classification of fermentation residues differs depending on the substrates used and the place of application. Only manure and renewable raw materials processed in biogas plants, the digestate is considered manure . If biowaste is also fermented, the digestate is also considered biowaste and must therefore not only comply with fertilizer law but also legal waste law, among other things with regard to safety in terms of epidemic hygiene. In this case, the digestate often has to be sanitized before it is spread. The hygienisation usually takes place before fermentation by heating to 70 ° C for one hour or by thermophilic fermentation, in which 50 ° C must be reached for the proven minimum retention time. This is prescribed by the Biowaste Ordinance , particularly for biowaste .
Individual evidence
- ↑ a b c d Page no longer available , search in web archives: Final report of an Austrian research project to reduce odor and greenhouse gas emissions from manure ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ a b Ingredients of fermentation products and possibilities for their orderly utilization in plant cultivation. ( Memento from December 12, 2013 in the Internet Archive ) Project report, Agricultural Technology Center Augustenberg , December 2008, pp. 2–7 (PDF; 43 kB)
- ↑ a b Feasibility study on the utilization of fermentation residues from agricultural biogas plants. (No longer available online.) Münster University of Applied Sciences , archived from the original on November 17, 2016 ; accessed on July 31, 2017 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Konrad Schleiss: Zurich composting and fermentation plants. (PDF; 1.1 MB) Annual report on the 2019 inspections. In: awel.zh.ch. Building Directorate Canton Zurich; Office for Waste, Water, Energy and Air, accessed January 25, 2020 .
- ↑ This is new in organic farming 2019. (PDF; 277 KB) In: shop. fibl.org . Bio Suisse , 2018, accessed on January 27, 2019 .
- ^ AID information service for consumer protection, nutrition, agriculture eV (ed.): Biogas plants in agriculture. 2007, p. 40, ISBN 978-3-8308-0682-0 .
literature
- B. Eder, H. Schulz: Biogas practice. Basics, planning, plant construction, examples and profitability of biogas plants. Ökobuch Verlag Staufen 2006, 3rd edition, ISBN 978-3-936896-13-8
- Fachagentur Nachwachsende Rohstoffe e. V. (FNR): Handout Biogas Production and Use. ISBN 3-00-014333-5 . Free 232-page brochure from the FNR.
- Text of the fertilizer ordinance
- Karen Sensel, Verena Wragge: Final report on the joint project on crop production of fermentation residues from biogas plants with special consideration of the input substrate energy crops. Agency for Renewable Resources , 2008 (PDF; 2.8 MB)