Exploring biorefinery alternatives for biowaste valorization: a techno-economic assessment of enzymatic hydrolysis coupled with anaerobic digestion or solid-state fermentation for high-value bioproducts

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.

Filling in the gaps in biowaste biorefineries: The use of the solid residue after enzymatic hydrolysis for the production of biopesticides through solid-state fermentation

Alternative production processes using waste are necessary to preserve non-renewable resources and prevent scarcity of materials for future generations. Biowaste, the organic fraction of municipal solid waste, is abundant and easily available. It can be fractionated into building blocks for which fermentative processes can be designed. By using solid-state fermentation, this paper proposes a method of valorizing biowaste's residual solid fraction after enzymatic hydrolysis. In a 22 L bioreactor, two digestates from anaerobic digestion processes were evaluated as cosubstrates to modify the acidic pH of the solid residue after enzymatic hydrolysis and promote the growth of the bacterial biopesticide producer Bacillus thuringiensis. Regardless of the cosubstrate used, the final microbial populations were similar indicating microbial specialization. The final product contained 4 × 10⁸ spores per gram of dry matter and also crystal proteins of Bacillus thuringiensis var israelensis, which have insecticidal activity against pests. This method allows for the sustainable use of all materials liberated during the enzymatic hydrolysis of biowaste, including residual solids.

Enzymatic hydrolysis of the organic fraction of municipal solid waste: Optimization and valorization of the solid fraction for Bacillus thuringiensis biopesticide production through solid-state fermentation

To reach a more sustainable society, the implementation of a circular economy perspective in municipal waste management becomes essential. In this work, the enzymatic hydrolysis of source-separated organic fraction of municipal solid waste (OFMSW) has been optimized as a sugar-releasing step. A liquid sugar concentrate, with a maximum reducing sugar concentration of 50.56 g L^-1, and a solid hydrolyzed fraction were obtained. The effect of the harshness of the hydrolysis conditions was evaluated on the performance of the resulting solid fraction as a substrate for Bacillus thuringiensis biopesticide production through solid-state fermentation. A production of 3.9 × 10⁸ viable cells g^-1 dry matter with a 33% sporulation ratio was achieved for milder hydrolysis conditions, highlighting the potential of the solid fraction of hydrolysis as a substrate of SSF processes. The proposed valorization pathway for the OFMSW results in a sugar concentrate with potential for fermentative processes and a fermented solid containing biopesticides from Bacillus thuringiensis.