Enzymatic hydrolysis of organic waste is gaining relevance as a complementary technology to conventional biological treatments. Moreover, biorefineries are emerging as a sustainable scenario to integrate waste valorization and high-value bioproducts production. However, their application on municipal solid waste is still limited. This study systematically evaluates the techno-economic feasibility of the conversion of the organic fraction of municipal solid waste (OFMSW) into high-value bioproducts through enzymatic hydrolysis. Two key variables are examined: (a) the source of the enzymes: commercial or on-site produced using OFMSW, and (b) the treatment of the solid hydrolyzate fraction: solid-state fermentation (SSF) for the production of biopesticides or anaerobic digestion for the production of energy. As a result, four different biorefinery scenarios are generated and compared in terms of profitability. Results showed that the most profitable scenario was to produce enzymes on-site and valorize the solid fraction via SSF, with an internal rate of return of 13%. This scenario led to higher profit margins (74%) and a reduced payback time (6 years), in contrast with commercial enzymes that led to an unprofitable biorefinery. Also, the simultaneous production of higher-value bioproducts and energy reduced the economic dependence of OFMSW treatment on policy instruments while remaining energetically self-sufficient. The profitability of the biorefinery scenarios evaluated was heavily dependent on the enzyme price and the efficiency of the anaerobic digestion process, highlighting the importance of cost-efficient enzyme production alternatives and high-quality OFMSW. This paper contributes to understanding the potential role of enzymes in future OFMSW biorefineries and offers economical insights on different configurations.

This paper aims to explore an alternative pathway to valorize the organic fraction of municipal solid waste. It is based on the use of enzymatic hydrolysis to obtain a sugar-rich fraction with the potential for liquid fermentative processes and a partially hydrolyzed solid that is evaluated as a substrate for solid-state fermentation. Different strategies to modify the pH of the solid substrate to reach a neutral pH, suitable for the growth of biopesticide producer Bacillus thuringiensis, have been explored. The use of alkaline cosubstrates was evaluated on two different scales and temperature was assessed as a preliminary indicator of the scale-up viability of the process strategy. By ensuring a proper pH throughout the process, the growth and sporulation of Bacillus thuringiensis were achieved. The best cosubstrates and mixing ratios were 50% of digested sewage sludge and 25% of digested organic fraction of municipal solid waste, which led to a spore concentration of 1.1 × 10⁹ spores g⁻¹ of dry matter and 6.4 × 10⁸ spores g⁻¹ of dry matter, respectively. Overall, a reproducible and flexible solid-state fermentation process has been achieved for hydrolyzed organic municipal waste based on the use of alkaline urban wastes as cosubstrates. This valorization pathway fits with the concept of urban biorefineries. Graphical Abstract

Organic waste treatment, including its many strategies and objectives, is one of the most rapidly changing sectors of environmental technology. It is closely related to sustainability and other critical issues, such as global warming. The first years of this century were the time of a transition from traditional technologies for the “disposal of” waste, such as incineration (with poor energy recovery) or landfill (more or less controlled), to biotechnologies that are more profitable, such as composting and anaerobic digestion. However, recent developments are focused on advanced technologies in the framework of a circular bioeconomy, maximizing the production of biomaterials and renewable energy using raw organic waste or digested materials. This perspective paper delves into the second transition in the field of technologies for treating and valorizing organic waste, highlighting emerging technologies such as anaerobic digestion enhanced with nanomaterials or biochar to substitute fossil natural gas, solid-state fermentation to obtain bioproducts that have a “chemical twin” with a high environmental impact, and pyrolysis as a predominant thermal treatment due to the production of biochar, probably the most promising biomaterial in today’s research. All these technologies exploit the potential of organic waste for bioenergy production and material utilization, in line with circular principles.

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.