Alternative method for producing organic fertiliser from anaerobic digestion liquor and limestone powder: High Shear wet granulation

Generally, the solid and liquid fractions (digestate) from Anaerobic Digestion (AD) energy production are considered as waste. This has a negative impact on the sustainability of AD processes because of the financial outlay required to treat digestate before being discharged into municipal water treatment plants or natural water bodies. The main aim of this research was to investigate feasibility of producing an organic fertiliser using anaerobic digestate and limestone powders as the raw materials employing a high shear granulation process. Two‐level factorial experimental design was used to determine the influence of granulation process variables on, the strength, resistance to attrition and yield of the granules. It was concluded from the study that it is technically feasible to produce organic fertiliser granules of acceptable strength and product yield. Increasing the liquid-to-solid ratio during granulation leads to increased granule strength and better product yield. Although the strength of the granules produced was lower than typical strength of commercial synthetic fertiliser granules (about 5 to 7 MPa), this could be improved by mixing the digestate with a polymeric binder or coating the particles post granulation.

This study showed that is was technically feasible to produce organic fertiliser granules from high shear granulation of limestone using AD digestate as the binder. Design of experiment was employed to study the effect of process variables on mechanical properties of the product. The attrition strength, crushing strength and yield of the granules were strongly influenced by the process variables.Figure optionsDownload as PowerPoint slideHighlights
► It is feasible to produce granular organic fertiliser from AD liquor and limestone.
► Effect of granulation process variables on granule yield and strength was studied.
► Granulation time and amount of AD liquor available strongly affect the product yield.
► Granule strength is mainly influenced by granulation time and impeller speed.
► Attrition strength of the granules is sensitive to process variables.