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The Chinese dome digester (CDD) is a low-cost and the most popular anaerobic digester that is used for the treatment of organic waste such as food waste and cow dung. However, the main challenge of CDD is scum formation due to inadequate mixing intensity. This study explores computational fluid dynamics (CFD) to characterize mixing in CDD and the effects of mixing frequency (0, 4, 6, and 8 times per day) on the performance of semicontinuous anaerobic digestion to break scum and enhance methane yield. The flow field simulation on a lab-scale CDD by Ansys Fluent (v.19.2), a finite volume solver, estimated that 45% of CDD working volume was occupied by dead zones which could nurture scum. The simulation results elicited the optimization of mixing frequency. Four CDDs were operated to investigate the optimum mixing frequency. The average scum thickness for the non-mixed digester was 2 ± 0.1 cm compared to 0.2 ± 0.1, 0.8 ± 0.1, and 1.3 ± 0.2 cm for the mixed digesters (4, 6, and 8 times per day, respectively). The average methane yields for 0, 4, 6, and 8 times per day were 206 ± 191, 602 ± 87, 555 ± 59, and 492 ± 109 mL g-VS−1, respectively. Four times per day was the optimum mixing frequency and the energy required to break scum was 6.1 ± 0.3 Joules per mixing cycle. This study proves that by optimizing the mixing frequency in CDD, scum formation can be controlled without additional investment cost.

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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.

In this study, semi-continuous anaerobic digestion of lignin-rich steam-exploded Ludwigia grandiflora (Lignin = 25.22% ± 4.6% total solids) was performed to understand better the effect of steam explosion on the substrate solubilisation and inhibitors formation during the process. Steam explosion pretreatment was performed at 180 °C for 30 min at a severity factor of 3.8 to enhance the biogas yield of the lignocellulosic biomass. The semi-continuous anaerobic digestion was performed in a continuously stirred tank reactor for 98 days at an initial hydraulic retention time of 30 days and an organic loading rate of 0.9 g-VS L−1day−1. The performed steam explosion pretreatment caused biomass solubilisation, resulting in enhanced biogas production during the process. During the anaerobic digestion process, the average biogas yield was 265 mL g-VS−1, and the pH throughout the operation was in the optimum range of 6.5–8.2. Due to fluctuations in the biogas yield, the hydraulic retention time and organic loading rate were changed on day 42 (50 days and 0.5 g-VS L−1day−1) and on day 49 (40 days and 0.7 g-VS L−1day−1), and 1 M of NaOH was added to the liquid fraction of the steam-exploded L. grandiflora during the latter part of the operation to maintain the stability in the reactor. Therefore, the steam explosion pretreatment helped in the degradation of L. grandiflora by breaking the lignocellulose structure. In addition, changes in the operating conditions of the anaerobic digestion led to an increase in the biogas production towards the end of the process, leading to the stability in the CSTR.