A novel solid-state submerged fermenter (3SF) for acidogenic fermentation of food waste at high volumetric loading: Effect of inoculum to substrate ratio, design optimization, and inoculum enrichment

Abstract

This study reports the development and operation of a solid-state submerged fermenter (3SF) to produce short-chain fatty acids (SCFAs) from food waste at high volumetric loading of 55 gVS/Lreactor. To maximize food waste degradation and SCFA production in the 3SF, three factors were optimized: inoculum to substrate ratio (16%, 9%, 6%, and 4%), mesh pore size of the food waste holding chamber (4.5 mm, 2 mm, and 1 mm), and enrichment of the inoculum. The 3SF operation at higher inoculum-to-substrate ratio (ISR) of 16% resulted in 13–20% higher hydrolysis (600 ± 6.6 gCOD/kgVSadded) and SCFA yields (480 ± 8.7 gCODSCFA/kgVSadded) than those obtained at lower ISRs of 4–6%. Optimization of the mesh pore size (2 mm) further improved hydrolysis and SCFA yield to 628 ± 6.1 gCOD/kgVSadded and 517 ± 7.3 gCODSCFA/kgVSadded, respectively. Enrichment of the inoculum increased the hydrolysis and acidification rates resulting in significantly higher (P ≤ 0.05) hydrolysis and SCFA yields than the non-enriched reactor in just 8 days, implying a 43% reduction in the fermentation time. SCFA composition comprised of acetate, propionate, and butyrate for all reactors. However, an increase in ISR and the use of enriched inoculum resulted in a gradual increase in butyrate composition within the SCFA mix with a maximum butyrate composition of 46% on day 8. Microbial composition showed a high relative abundance of Enterococcus, Clostridium Sensu Stricto 1, and Bacteroides.