In situ buffering and enzymatic regulation of acidogenesis in two-stage anaerobic digestion of composite vegetable waste

Peels of vegetable waste (VW), rich in cellulose, lignin, and hemicellulose, can impede hydrolysis due to reduced enzymatic accessibility and activity. This research comparatively evaluated the effects of composite vegetable waste (CVW) comprising of 2:3 food waste (FW):VW on biological buffering potential and enzymatic activity during acidogenesis in a two-stage (Leached Bed Reactor-airlift reactor) system. Results demonstrated that maximum buffering and enzymatic activity occurred within the first 3–5 days. The system maintained an in situ maximum buffering capacity of 0.145 mol/L (FW) and 0.089 mol/L (CVW) without chemical additives. Peak α-amylase activities reached 72.2 U/mL (FW) and 64 U/mL (CVW), while protease reached 34.94 U/mL (CVW). The ratio of 2:3 for FW:VW moderated enzymatic hydrolysis rates through its lignocellulosic content and stabilized the acidogenic environment, which in turn regulated VFA dynamics, resulting in a 1.8-fold increase in acetate yield (18.19 g/L). Conversely, FW achieved a higher biohydrogen yield of 121.3 mL/g VS (43% composition) compared to 110 mL/g VS (36.4% composition) for CVW. The FW also produced 4.2 times higher propionic acid (95.3 g/L) than CVW (22.64 g/L). Life Cycle Assessment (LCA) confirmed the sustainability of this minimized-discharge model, with H₂ production from FW showing a lower Global Warming Potential (0.056 kg CO₂ eq/L) than CVW (0.269 kg CO₂ eq/L) due to higher gas yields. The system maintained in situ buffering without chemical additives, thus advancing the development of cost-effective routes for bioresource recovery from complex organic waste. It has also been emphasized that the sustainability of process is bolstered by the recirculation of airlift reactor effluent, which mitigates the need for chemical additives.