Solid digestate use

Solid digestation is formed when the digestion residue from the biogas plant is separated with a screw separator. Unlike, for example, wastewater treatment plants, separating livestock farms does not require chemical additions. The fibres of the grass enable a good separation result in the so-called separation process by direct separation. The advantage of separation is the transfer of nutrients to different fractions. At Demeca’s customer farms, the phosphorus content of dry fraction has been 1.4–4.2 times higher than slurry and 2.4–14.5 times higher than the liquid fraction of digestate.

Solid digestation is remarkably homogeneous, so the application result is also uniform. The spreadability of the liquid part is also improved because its viscosity is lower than slurry and unseparated digestate.

Phosphorus regulation (64/2023) section 8 enables phosphorus equalisation in five-year periods in Finland. The maximum annual phosphorus fertilisation may be deviated from by fertilisation using less phosphorus in other years. It is therefore possible, for example, to increase phosphorus levels with solid digestation when basing grass and to use liquid fractions of digestate or auxiliary manure for fertilisation in subsequent years. The arrangement provides more opportunities for fertilisation and fertilizer logistics planning.

In Finland nitrate regulation (1250/2014) accordance of the section 10.4 dry manure and organic fertiliser products with a dry matter content at least 30 per cent may be stored in the field for a maximum of four weeks during the application period pending application. In Finland spredding period is from April to the end of October. This possibles to transfer the solid digestation to the fields to wait for spredding when the fields cannot yet be modificate and the spring rush has not begun. It is good to note that four weeks of storage during the application period does not mean official plough the field. This kind of storage doesnt have to do plough reporting and is not concern the structural requirements.

Solid digestation suites well for cow bedding. According to Natural Resources Institute Finland research ”Recycled manure solids as a bedding material: Udder health, cleanliness and integument alterations of dairy cows in mattress stalls” cows had less abrasions and udders were cleaner compared to bedding peat. The research the dry matter content of separated bedding was on average 25 per cent. The research found no differences in cell counts between peat or separated slurry, although a small effect on mastitis could not be completely excluded.

Separated digestate is even better suited as cow bedding than separated slurry. The biogas plant significantly reduces most bacteria, such as coliforms and listeria (Nag et al.). At Demeca’s customer farms, the dry matter content of separated digestate has varied between 35 and 42 per cent. Solid digestation has been clearly drier than the dry fraction used in the Natural Resources Institute Finland’s research. It is justifiable to judge that drier bedding is better for animals. When using Demeca’s Renki bedding device, the cow bedding is always fresh and smoothly spread litter still dries on the floor. Separated digestate advantage is that it doesnt dust when it is dried.

Sources:

Frondelius, L., Lindeberg, H., & Pastell, M. (2020). Recycled manure solids as a bedding material: Udder health, cleanliness and integument alterations of dairy cows in mattress stalls. Agricultural and Food Science, 29(5), 420–431. https://doi.org/10.23986/afsci.95603

Nag r. et al. Evaluation of pathogen concentration in anaerobic digestate using a predictive modelling approach (ADRISK). Science of Total Environment 800 (2021) 149574 https://doi.org/10.1016/j.scitotenv.2021.149574