Abstract The main waste generated during the thermochemical processes of biomass burning used for bioenergy production in cogeneration plants are ash and biochar which can be used as fertilizers in forestry and agriculture. However, several difficulties are attributed to possibilities of practical exploitation of ash and biochar due to the transporting, handling and mode of application. Granulation of energy generation waste has been shown in some circumstances as a cost-effective and environmentally friendly method. Production of granules (pellets) from wood ash and biochar leads to improved recycling and logistics of waste as well as helps to control and avoid undesirable environmental effects such as leaching of nutrient excess. The aim of this study was to prepare granules from wood fly ash and biochar mixed with organic-rich freshwater lake sediments (sapropel) used as a natural binder and a source of organic matter for enrichment of derived granules to be applicable as a soil improver. Characterization of raw materials and derived granules included range of physical and chemical analysis. Applied granulation technology involved homogenization of components following by extrusion and drum granulation, and the process was developed for prospective production of soil improvers. It was estimated that the most optimal mass ratio of raw materials applicable for the process of granulation is 67:100 (fly ash to sapropel) and 30:100 (biochar to sapropel) resulting in production of granules 3–8 mm in diameter. Among the characteristic parameters of granules apparent density, bulk density, compressive strength, total element content and element content by fractions was assessed.