Characterization of mixing by CFD simulation and optimization of mixing frequency to break scum and enhance methane yield in Chinese dome digester

Abstract

The Chinese dome digester (CDD) is a self-mixed, low-cost, and most popular digester that faces the challenge of scum formation due to insufficient mixing. Mixing intensity in CDD is controlled by gas valve operation during gas production and usage. This study explores computational fluid dynamics (CFD) simulation to characterize mixing in CDD and the effect of mixing frequency on the performance of semicontinuous anaerobic digestion (AD) to improve mixing intensity, break scum and enhance methane yield. ANSYS software was applied to simulate the flow fields of a lab-scale CDD and four CDDs were operated at different mixing frequencies (0, 4, 6, and 8 times per day) to investigate the optimum mixing frequency that could break scum. 45% of CDD working volume was dead zones at the top of CDD which nurtured scum. In the AD experiments, scum thickness increased progressively in the non-mixed digester (2.2 ± 0.12 cm), compared to the mixed digesters, 4, 6, and 8 times per day (0.23 ± 0.05, 0.83 ± 0.11, and 1.31 ± 0.16 cm, respectively). The optimum mixing frequency was 4 times per day and the energy required to break scum was 6.12 ± 0.25 Joules per mixing cycle. The average methane yields for 0, 4, 6, and 8 times per day were 206.39 ± 192.09, 601.45 ± 88.80, 555.83 ± 59.92 and 493.11 ± 109.76 mL g-VS− 1, respectively. This study proves that scum can be broken in CDD by an optimum mixing frequency of 4 times per day without additional investment cost.