Kinetic Analysis of Organic Removal in a Three-Stage Up-Flow Anaerobic Fixed-Bed (UAF) Reactor

Up-flow anaerobic fixed-bed (UAF) is a cost-effective wastewater treatment system because of its ability to retain biomass and enable effective contact, while multi-stage systems can facilitate different conditions required at different phases of anaerobic digestion. Currently, there is limited information on the process analysis, control, and design of multi-stage UAF. The objective of this study was to evaluate and develop a model to predict the performance of a three-stage UAF system using bamboo medium. Tofu wastewater (8.0–15.5 gCOD L−1) was used as substrate. The UAF reactor was tested under hydraulic retention times (HRT) of 8, 6, 4, and 2 days. The focus was on the organic removal efficiency and methane production. The highest removal efficiency was achieved during HRT 8 (96% for both total and soluble COD). Based on the Stover-Kincannon model, the maximum substrate removal rate (Umax) and saturation constant (KB) for the total COD of the overall system were 8.5 g L−1 day−1 and 7.9 g L−1 day−1, respectively. Meanwhile, the methane production rate and yield were 0.18–0.30 LCH4 L−1 day−1 and 0.06–0.19 LCH4 gCOD−1, respectively. The three-stage system could stabilize the pH and perform hydrolysis and acidogenesis in the first two stages, whereas methanogenesis predominantly occurred in the third stage. Microbial analysis indicated that methane formation was primarily driven by hydrogenotrophic methanogens, which played a significant role in overall methane production. These findings provide insights into the microbial mechanisms of methane generation and the optimization of multi-stage UAF systems.