Design and experimental production of organic fertilizers from biogas digestates and secondary materials for horticultural purposes

An ever growing quantity of digestates produced from agricultural biogas facilities puts alternative use options, other than spreading those residues on agricultural land, into focus, particularly to protect the ground water from nitrogen leaching in intensively used regions. Within the framework of the FNR funded project Skarabäus, Brandenburg University of Technology (BTU) together with partners from the Institute of Agricultural and Urban Ecological Projects affiliated to Humboldt University Berlin (IASP) and the Humboldt University Berlin (HU) investigate whether biogas digestates could be converted to fertilizer products of defined composition to be used outside agricultural production particularly for gardening and landscaping. The project rationale is to agglomerate the separated digestates to produce a fertilizer which is flexible in the design of properties, easy to handle for application and  effective to plant’s growth. Basically the tumble agglomeration was considered as the main process. The properties of the fertilizer product, particularly the nutrient content with respect to nitrogen, phosphorus and potassium, was thought to be adjusted by adding nutrient rich secondary materials like meat and bone meal, replaced powders from fire extinguishers and recycled material originating from waste water treatment during the agglomeration process.

Experiments revealed that separated digestates could hardly be agglomerated due to the high amount of relatively large and inflexible fiber contained. The addition of binding agents like clay minerals strongly improved the agglomeration process. However, so far best results were achieved when separated digestates were composted prior to the agglomeration process. In this way no binding agents were necessary. Agglomerates produced from composted digestates showed a reasonable particle size distribution and nutrient- and organic matter content generally suitable for application in horticulture, given that future greenhouse and field experiments could also demonstrate the beneficial application.