Digestate as Fertilizer

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Digestate upgrading techniques6 Digestate upgrading techniquesFurther upgrading and conditioning are important ways of savingstorage, transport, and application costs. Upgrading can be the way tonon-agricultural marketing, particularly in regions where the digestatecannot be fully applied in liquid form.When additional storage capacities have to be createdor digestate has to be transported over long distances,many operators of biogas plants consider investing indigestate upgrading. In addition, the idea of being ableto establish new marketing channels, e.g. for privategardeners, garden and landscape companies, soil worksand fertilizer manufacturers, plays an ever-greater role.But before the choice of treatment method can bemade, the exact target should be defined. For example,is it generally possible to use digestate as fertilizer oris a complete treatment up to the discharge into waterbodies necessary? Or is it only important to reduce thequantity and thus the water content in the digestate?Is excess heat from the biogas plant available for thistreatment? Or should appearance and handling be optimised,because sales to other farmers have declineddue to the competitive situation with other organic fertilizers?Is phosphorus or the nitrogen content a limitingfactor for the application due to legal requirements?The decision on the best process very much dependson the individual plant concept and regional conditionsand cannot be made across the board. Furthermore,the individual processes are evaluated according togoal, technology, energy and equipment requirements,which can vary greatly from manufacturer to manufacturerand sometimes also differ from the informationprovided. The upgrading technologies presented in thispublication are merely intended to provide an initial assessmentand an overview of the various options.The first treatment step is usually separation into a liquid,pumpable, and a solid, stackable fraction. Two differentproducts are already created here, which can beused in very different ways. The solid fraction is lower innitrogen and K 2O but enriched with P 2O 5. It can be usedto build up humus mainly because of its higher carboncontent. The transport worthiness also increases withincreased DM content. Since phosphate also often limitsthe use of digestate, the liquid separated fractionwith a relatively lower P 2O 5content can be used betterin relation to the nitrogen requirements of the crop tobe fertilized.The evaporation and the membrane filtration (mostlyultrafiltration and reverse osmosis) are available fordewatering and thickening the liquid fraction. There isalso increasing interest in processes such as strippingand precipitation or in the recovery of ammonium sulphatesolution from liquid digestate. Ammonium sulphatesolution can also be obtained in an acid scrubberafter drying or vacuum evaporation. By removingwater, the necessary storage capacity can be reduced,which must be maintained in cold countries if fertilizationis no longer required in late autumn and winter.The drying of the solid fraction and, if necessary, thegranulation or pelletisation of the dried digestate arewell established techniques. The focus here is on improvingthe appearance, handling, storage suitability,and marketability. During pelletising, the individual nutrientsor other mineral and organic additives obtainedfrom the liquid fraction can be added in order to be ableto offer fertilizers as required by the customer. This isan interesting perspective for a future market for fertilizersthat can be produced from digestate.Especially with the digestation of municipal biowaste,composting is carried out downstream in many countries.Marketing composts is established in many countriesand the application leads to a very high humussupply. In some cases, the liquid digestate from theseplants is introduced into waste water treatment plant(WWTP) together with municipal waste water by convertingthe nitrogen contained into atmospheric nitrogenN 2(see Chapter 6.4: “Biological treatment”) andthus is no longer available as nutrients.In any case, the goal should be responsible optimisationof nutrient management, which can certainly be associatedwith a reduction in costs or increase in addedvalue. A sustainable decision should, therefore, also bemade with regard to NH 3emissions from the individualprocesses, which can be implemented by appropriatetechnologies, such as acid scrubbers. The ammoniumsulphate solution produced can then be used agriculturallyas mineral fertilizer or marketed externally.14
Digestate upgrading techniques — Separation6.1 SeparationScrew pressDigestate feedingScreening drumScrew conveyorSolidfractionOutlet flapLiquid fractionThe goal of separation is to mechanicallyseparate the digestate into a liquid and solid fraction.There is no reduction in volume; only the need forstorage tanks for liquid digestate is reduced by around10-20% by separating the solid fraction, dependingon the composition of the starting materials and separationtechnology. Separation is usually the first stepbefore further processing.The screw press is the most commonly used technique.This is when a rotating screw conveyor whichsits in a screening drum presses the digestate againstan outlet flap from which the solid fraction can exit.The liquid fraction is separated by the screening drumwith a defined hole width of 0.5-1mm. Contact pressure,hole width, and back pressure of the outlet flapdetermine the degree of separation. The technologyis mature, robust, and simple. The power consumptionis between 0.2-0.6 electrical kilowatt hours percubic metre (kWh el/m³) input, depending on versionand size.The stackable solid fraction with a DM content of 20-40% is more cost-efficient in terms of transportability.Above all, carbon and phosphate are enriched, whichmeans that solid digestate is suitable as good phosphorusand humus fertilizer for catch crops and maincrops with long growth cycles.The liquid fraction with a DM content of 1-8% hashigh flowability and can, therefore, easily drip off theplant and enter the soil. Due to the enrichment of NH 4,this fraction is a fast-acting nitrogen fertilizer that isimmediately available to plants (e.g. maize, cereals,rape seed, grassland).Screw press15
Page 1 and 2: Digestateas FertilizerBIOGAS Know-h
Page 3 and 4: ContentStatements .................
Page 5 and 6: PreamblePreambleAgriculture is one
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Page 23 and 24: Marketing strategies7 Marketing str
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Page 29 and 30: Significance of digestate in develo
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Page 39 and 40: Reference plantsMaier GbRPresented
Page 41 and 42: Reference plantsRegeb BersenbrückP
Page 43 and 44: Reference plantsTotal digestate tre
Page 45 and 46: Reference plantsTully AD PlantPrese
Page 47 and 48: Matrix overview of the company dire
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Page 55 and 56: Application technologiesVogelsang S
Page 57 and 58: Project developer Digestate marketi
Page 59 and 60: Indian Biogas AssociationEuropean C
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Digestate as Fertilizer

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