Solid-state fermentation to produce biostimulant agents from green waste: A circular approach at bench-scale

• Solid-state fermentation (SSF) to produce biostimulant and biopesticide was studied. • SSF was carried out at bench scale using green waste and Trichoderma harzianum . • The production of biostimulant at bench-scale was higher than that at lab-scale. • Porosity is one of the basic indicators of a successful SSF at bench-scale. • Sequential batch experiment was conducted as a strategy to scale up the production. This study aimed to develop a bench-scale process of solid-state fermentation (SSF) to produce biostimulants (indole-3-acetic acid-IAA) by Trichoderma harzianum CECT 2929, utilizing green waste as substrate. Additionally, biopesticide activity (in form of conidial spores) was also studied. The SSF was carried out in a 22-L packed bed reactor using grass clippings as the main substrate and pruning waste or wood chips as bulking agent. When using pruning waste, IAA production reached 62 µg/g dry matter, while the fermentation with wood chips resulted in a higher IAA concentration of 120 µg/g dry matter. Higher spore counts were reached using wood chips (9.5 × 10 8 and 1.3 × 10 9 spores g −1 dry matter, respectively). Further experiments showed that decreasing tryptophan as precursor for IAA production from 0.43 % to 0.2 % (w/w) significantly decreased IAA production from 120 to 25 µg/g dry matter while it did not alter spore production. In addition, a sequential batch operational strategy was explored, consisting of four consecutive batches as a strategy to scale-up the SSF process. The highest IAA and spore productions were achieved in the first batch, resulting in 119 µg/g dry matter and 1.1 × 10 9 spores g −1 dry matter, respectively. Subsequent batches showed a decline in production, particularly in batches 3 and 4, due to contamination issues. Summarizing, the strategies proposed in this study are a considerable advance to develop SSF to produce biostimulant and biopesticide agents in the context of circular bioeconomy, using green waste as raw material.