The solid fraction of wastewater digested sludge is a material rich in nutrients and organic matter that is typically applied to the soil, composted or disposed of. To further exploit its potential, it can be used as a secondary raw material for the production of marketable products, such as biopesticides, through solid-state fermentation (SSF). In this study, a thorough characterisation in terms of pyrolysis gas chromatography-mass spectrometry analysis, physicochemical methods and biodegradability analysis was carried out on four different digested sewage sludges to understand their inherent properties. The effect of these properties on the growth and sporulation of Bacillus thuringiensis during a laboratory-scale SSF process was then evaluated. Sporulation yields ranged from a maximum of 11.3 to a minimum of 0.4 (expressed as spores per initial viable cell), highlighting the influence of digestate variability on the fermentation performance. Finally, the SSF process was scaled up to bench-scale using the two solid digestates with the highest Bacillus thuringiensis viable cell and spore production to assess the robustness and reproducibility of the process. By scaling up, both sporulation yield and viable cell yield increased compared to the laboratory-scale trials with a maximum spore production of 2.3 × 10⁸ spores g^-1 DM. The results confirmed the feasibility of growing Bacillus thuringiensis by SSF in three of the four solid digestates studied, highlighting the importance of advanced techniques to obtain reliable characterisation of the digestate as a substrate for SSF and the production of valuable bioproducts. These results help to determine the most effective valorisation pathway for the solid digestate within the circular economy paradigm.

The rapid expansion of anaerobic digestion (AD) as a key technology for producing renewable energy has led to a substantial increase in digestate generation. This has intensified the need for sustainable management strategies that align with circular economy principles. While the solid fraction of digestate (SD) is traditionally applied to land or composted, its heterogeneous composition, regulatory constraints, and handling challenges restrict its wider use. This review aims to clarify the current state of SD treatment and highlight emerging opportunities to convert this underexploited resource into value-added bioproducts. A systematic bibliographic analysis of the past decade was conducted to identify consolidated and emerging SD valorisation technologies, supported by an evaluation of EU-level regulatory frameworks and the role of mechanical solid–liquid separation in enabling downstream valorisation. In addition, a comprehensive comparative table compiling physicochemical characterisation data of SD from various feedstocks and separation methods is presented, emphasising the significant variability in composition and its implications for valorisation pathways. The results show that, while composting and thermochemical routes, particularly pyrolysis, remain predominant, novel approaches such as advanced drying, pelletisation, vermicomposting, insect bioconversion, and fermentation-based pathways (including submerged and solid-state fermentation) are rapidly gaining interest. These emerging technologies enable the production of high-value products such as biochar, pellets, enzymes, microbial biopesticides, protein sources, and fungal biomass. However, their adoption is currently limited by feedstock heterogeneity, process complexity, scalability constraints, and economic considerations. Overall, SD is a versatile feedstock whose valorisation is expanding beyond agricultural applications. However, regulatory harmonisation, quality assurance, and process optimisation are still needed to encourage industrial uptake and to fully integrate SD into circular bioeconomy frameworks.