Digestate

As the digestate liquid or solid residue is designated, the remaining from the fermentation of biomass in a biogas plant. Because of its high content of nutrients it is mostly used as an agricultural fertilizer. Even the name biogas slurry or digestate is used. For the digestate production of ethanol from corn appears the label stillage is usually used.

Formation of the digestate

Main article biogas plant

In biogas plants, a wide variety of substrates are used, such as manure, corn silage or organic waste. These include 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 degraded during the anaerobic held biogas process of microorganisms. The biogas can be found with methane (CH4 ) and carbon dioxide (CO2) are quantitatively the most important degradation products. In addition, small amounts such as hydrogen sulfide (H2S ) and ammonia ( NH3) are initially listed. Due to the degradation processes is from the viscous to solid substrate, the liquid to viscous fermentation residue with a high water content.

Composition and fertilizer value

Fermentation residues contain significant amounts of readily plant-available nitrogen, also phosphorus, potassium, sulfur and trace elements. Unloaded digestate are therefore to be considered as a high quality organic fertilizer. The nutrient composition of the digestate can vary greatly, depending on the substrates used.

Nitrogen

Since escaping from the substrate, only small amounts of NH3, remains the largest portion of nitrogen (about 3.4 to 5.0 kg/m3) in the digestate. Due to the degradation processes reduce the proportion of the bound in the organic nitrogen, while the proportion increased to NH3 - 45-76 percent are present as ammonium - nitrogen ago. Since shifts in the fermentation the pH from neutral to slightly basic pH range ( pH 8 to 8.5), also the balance of readily soluble ammonium NH4 to NH3 shifts. The storage and spreading of the digestate thus there is a higher risk of outgassing of ammonia. Positive may be that the nitrogen is more readily available for plants in this form. When the demand of the plant is low, however, the nitrogen can not be expelled effectively used under certain circumstances.

Other nutrients

For plant-available potassium and phosphorus levels are about 1.8 to 3.5 kg/m3 specified. Depending on the method for the desulfurization of the biogas produced can be a major part of the sulfur can be recycled to the fermentation residue. Other nutrients supplied to the substrate, such as phosphorus, potassium, magnesium, calcium, etc. remain in the digestate.

Fertilizer value, humus effect and pollutants

After fertilization with fermentation residues, microbial activity in the soil for about 9 to 10 weeks is increased. Earthworms, however, avoid a fertilized with high amounts of digestate soil. Rated positive is the low compared to slurry viscosity of the digestate, which means a faster penetration into the soil and thus reduced emissions.

As a fertilizer in agriculture digestate mineral may vary according to culture largely replace (eg spring wheat ) or supplement ( for example maize), the fertilization effect is in terms of nitrogen - depending on the culture - usually well over 70 % of the corresponding amount a mineral fertilizer.

The supply of organic matter by the fertilization with digestate is low, the humus of the soil needs can not be met by Gärrestdüngung. The heavy metal content of fermentation products are so small that they pose no problem for the agricultural utilization. The Salmonella infection of fermentation products from cultivated biomass is at a low, hardly questionable level, the Clostridienbelastung similar to the untreated manure. From the transmission of plant diseases caused by fermentation residues is also not expected.

Storage and spreading

The digestate is first collected in a storage container. For new plants, a gas-tight cover of Gärrestlagers is prescribed to avoid emissions of the strong greenhouse gas methane ( CH4).

The application of liquid digestate is done with slurry tanks, solid digestate is spread with manure spreaders on agricultural land. In most cases the digestate are applied on the parcels for the substrate of the biogas plant and forage crops manure delivered livestock. This recycling makes for a closed nutrient cycle. Positive aspect of the use of manure as a substrate that methane emissions occurring in the common manure storage are reduced. In the application of manure odors may occur, especially with ammonia, organic sulfur compounds, hydrogen sulfide (H2S) and organic acids. Apart from ammonia, these compounds are degraded during fermentation. The digestate is therefore significantly less odor than the manure used.

In Germany, the fertilizer ordinance stipulates that the digestate may be applied only at certain times to agricultural land. Thus, in the so-called core vesting the land application of digestate on farmland from 1 November to 31 January, the application is not allowed on grassland from 15 November to 31 January. In exceptional cases, subject to approval of the vesting date of the core, but not changed the length of the period. Purpose of the regulation and in particular the core holding periods is the waters and groundwater protection from excessive nutrient input.

Drying, combustion

For other recycling routes of the digestate is first dried before it is applied to land. This increases the transport- compared with the high water content digestate. Often the digestate drying is powered by heat from the biogas into electricity. This so-called power-heat coupling, the feed-in tariff for the electricity fed increased. Since the digestate may contain a certain proportion of easily degradable carbohydrates ( cellulose, lignocellulose ), it may be interesting as a fuel after drying. However, high mineral portions and the contained sulfur and nitrogen compounds 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 is a water- poorer substrate, as used, eg cereals. Fermentation residue from the aqueous liquid phase is pressed out and used for mashing of the substrate. The solid phase of the separated digestate has a higher transport-. In various concepts of dry fermentation also creates a solid digestate. An example is the fermentation of organic waste, to which usually connects the composting of the digestate. Household waste can also be fermented to reduce the organic component and thus to reduce the total mass. The digestate is deposited in this case.

Legal classification

The legal classification of digestate is different depending on the substrates used and the Ausbringungsort. If only manure and renewable raw materials processed in biogas plants, the digestate is considered manure. Become a biowaste fermentation, so the digestate is considered biowaste and therefore must meet next fertilizer law also waste law requirements, including in relation to the epidemic hygienic safety. In this case, the fermentation residue has to be frequently sanitized before application. The sanitizing usually takes place before fermentation by heating for one hour at 70 ° C or by a thermophilic digestion, in which for at least 24 hours 55 ° C must be achieved. Especially for biowaste this is dictated by the Biowaste Ordinance.