CFD simulation of the mixing process and performance evaluation of a two-way flow Chinese dome digester

This is the authors' abstract. We don't add key points for this paper.

The Chinese dome digester (CDD) is a cost-effective, self-mixed biodigester that is widely used in developing countries to treat biodegradable waste such as food waste. However, the scum formation owing to inadequate mixing remains a persistent issue in CDD operations. To address this issue, a newly designed two-way flow CDD (TWF-CDD) was proposed to improve mixing, prevent scum, and enhance methane yield. Computational fluid dynamics (CFD) simulations, scum-breaking tests, and semi-continuous anaerobic digestion (AD) experiments were employed to assess the performance of the TWF-CDD and compared it with the conventional CDD (C-CDD). In the CFD simulation, a 3D volume of fluid model was applied. The mixing conditions of the semi-continuous AD were 0 (control) and 8 times per day. The regions inside the digester with slurry velocity <0.02 m s−1 were considered dead zones. The C-CDD and TWF-CDD exhibited dead zone volumes of 46.7% and 25.3%, respectively. The scum-breaking test aligned with the CFD observations. Scum persisted even after 5 mixing cycles in C-CDD, whereas TWF-CDD required only 3 mixing cycles to completely break scum, indicating its superior mixing performance. In the AD experiments, mixing 8 times per day effectively prevented scum formation in TWF-CDD, whereas C-CDD still showed an average scum thickness of 14 mm. The average methane yield was 477 mL g-VS−1 and 648 mL g-VS−1, respectively for TWF-CDD and C-CDD. These results effectively demonstrated the efficacy of the TWF-CDD design in improving mixing, eradicating scum, and maximizing methane yield with almost no additional investment cost.