Effect of impeller design and operating conditions on food waste hydrolysis: Insights from experiments and...

Efficient hydrolysis of food waste (FW) is essential for advancing sustainable biorefinery and promoting the circular economy. This study systematically investigated the starch-rich FW hydrolysis using experiments and computational fluid dynamics (CFD) simulations, focusing on the solid-to-liquid (S/L) ratio, enzyme dosage, impeller type, and impeller position. Four types of impellers-two double Rushton impellers with small and large blades, a helical ribbon impeller, and a screw mixing impeller-were designed and fabricated to compare their mixing performance and FW hydrolysis efficiency under various conditions. The experimental results were then used as inputs into the CFD simulation model to ensure accuracy. The results showed that an S/L ratio of 40/60 provided the optimal balance between nutrient release and mixing efficiency. At this ratio, increasing the enzyme dosage improved hydrolysis yields, but with diminishing returns beyond 0.01%. Among the four impellers, the helical ribbon impeller at the lowest position achieved the most uniform mixing and the fewest dead zones, as confirmed by both experimental observations and CFD results. Under a 40/60 S/L ratio and 0.005% glucoamylase concentration, the helical ribbon impeller outperformed the double Rushton impeller (large blades), achieving a 48.9% increase in glucose yield over 6 hours and a 28.4% reduction in total energy consumption over 24 h. Raising the impeller height did not further improve mixing but instead increased total energy consumption. These findings provide practical guidelines for optimising bioreactor design and operation, facilitating efficient starch-rich FW valorisation within a circular economy.