Biogas digestate is a nutrient-rich byproduct of biogas digesters that can be used as a soil amendment or fertilizer to improve crop production. It contains essential elements like nitrogen, phosphorus, and potassium, as well as amino acids, vitamins, and microorganisms. Digestate can also help improve soil health, reduce greenhouse gas emissions, and promote better plant growth.

https://www.researchgate.net/publication/289317071_Agricultural_Use_of_Biogas_Digestate_as_a_Replacement_Fertilizers

Thankyou Dr Prem Baboo, your detailed explanation says it all and thanks once again for sharing the insights and the JPG which itself is a self explanatory JPG.

I think the answer to this question can vary based on the input feedstock. In most biogas plants, co-digestion is performed, where different types of feedstock are fed to the plant. For optimal usage of digestate remaining after the anaerobic digestion (AD) process, the elemental nutrients of the input feedstock, especially nitrogen (N), phosphorus (P), and potassium (K), should be specified. This can be used as a metric to compare the conventional type of fertilizer used on land to that of biofertilizer (i.e., digestate). Sometimes, digestate needs post-treatment processes to make it applicable to land. Biogas plants that are planned to use the input feedstock for both bioenergy and biofertilizer conduct this analysis and provide farmers with biofertilizer specifications.

Here is an example:

https://www.attero.nl/en/locations/wijster/

In some small-scale agricultural biogas plants, where the inputs are agricultural residues and manure, farmers provide feedstock to the plants free of cost. In return, they take the output feedstock as biofertilizer for free and use it on their land. This makes sense since the feedstock doesn’t require specific pasteurization processes.

However, in some industrial biogas plants where the input feedstock includes slaughterhouse waste, the usage of digestate requires specific post-treatment processes and analysis of the elemental and pollutant content of the output digestate.