Combining fermentation and vermicomposting for sustainable pet food waste management: Effects of waste profile and inoculum source

Ingredient production is the main driver of pet-food GHGs and land use. Indeed, the pet food industry uses nearly 495,000 km² of land annually. Yet, waste streams from production and declassified products remain underexplored as resources to produce high quality soil amendment. This study proposes an integrated approach to valorise such organic wastes through microbial fermentation prior to vermicomposting. Two experiments were conducted. (1) the effects of kibble incorporation ratio (15 % vs. 30 %), fibre source (spruce flakes, SF vs. beech sawdust, BS), and container size (50 L vs. 500 L) on fermentation dynamics, worm preference, and vermicompost properties were assessed. (2) the functional equivalence of commercial inoculum (EM®) and locally sourced indigenous microorganisms (IMO) was evaluated. Results showed that fibre type significantly influenced fermentation outcomes at low kibble incorporation, with BS lowering pH (from 9 in SF30% to 5.2 in BS15%, P < 0.001) and increasing redox potential (+213 %, P < 0.001). Electroconductivity was strongly affected by both kibble ratio (+110 %, P < 0.001) and fibre source (+130 %, P < 0.001). Worms displayed strong preference for BS-based substrates (+493 % density in 7 days, P < 0.001). Vermicompost derived from SF exhibited slower nitrogen and carbon mineralisation than BS. Container size had no effect on fermentation. EM® and IMO performed equivalently in fermentation efficiency, worm attraction, and functional microbial activity (metagenomic/metaproteomic analyses). This work demonstrates that combining microbial pre-digestion with vermicomposting can upcycle pet food waste into valuable soil amendments, while supporting local circular bioeconomy and sustainable waste management.