Methane production performance of semi-continuous alcoholization/acidification type two-phase anaerobic digestion

Abstract: A novel semi-continuous two-phase anaerobic digestion(AD) system characterized by methanogenesis from alcoholization was constructed, and the organic loading rate of the system was gradually increased from 2.0 to 6.0 g/ (L·d). According to the previous research results, the HRT(hydrodynamic retention time) of the first phase of the alcoholization/acidification type semi-continuous two-phase AD system was set for 2 d. Compared to the first phase of the traditional two-phase anaerobic digestion, the HRT was set for 2, 5, and 8 days, respectively. After 25 days of continuous operation, the change tendencies of acetic acid, propionic acid, butyric acid, etc. in the first phase hydrolysis products were analyzed under a certain OLR (organic loading rate). The results showed that when the acidification phase residence time was 5 and 8 days, the hydrolysis acidification effect of the system was better than that at 2 days, which may be due to the stable state of the acidified bacteria microbial community in the system. Therefore, in this experiment, 5 days was selected as the hydraulic retention time of the acidified phase. The methanogenic phase volume of the two systems was 8 L and the hydraulic retention time was 25 d. The results showed that 5.0 g/(L·d) was the maximum OLR for stable operation of the two systems in the study. By comparing the alcoholization/acidification type system with the traditional two-phase anaerobic digestion system, when the maximum OLR of the system reached 5.0 g/(L·d), the methane yield of the alcoholization/acidification type system reached 421.52 mL/g, which was 6.8% higher than that of the traditional system (394.48 mL/g). In this study, the TVFA (total volatile fatty acid)/alkalinity ratio was a more sensitive indicator than the pH value to detect the operating state of the methanogenic phase. The ethanol concentration in the first phase of the alcoholization/acidification type system accounted for 33.4%, which was beneficial to maintain the pH value and the stable operation of anaerobic digestion after the materials in the first phase entered to the second phase. Compared with the traditional two-phase anaerobic digestion system, the alcoholization/acidification type system had a shorter first phase hydraulic retention time of 2 days. Moreover, comparing with the traditional two-phase system, the volume methanogenesis rate of alcoholization/acidification type system was increased by 18.3%. All of the results showed that the alcoholization/acidification type system had obvious advantages in the methanogenic performance compared with the traditional system. And it also had the potential to improve system stability and reduce the scale of the plant and save the energy required for hydrolysis acidification in industrial applications. By comparing the first phase of the two systems, it could be found that the total amount of hydrolyzed products in the two groups was close when the OLR was 3.0 and 4.0 g/(L·d). When the OLR increased to 5.0 g/(L·d), the ethanol proportion in the first phase of the alcoholization/acidification type system was higher than that in the traditional two-phase system. Meanwhile, the propionic acid concentration was lower than that of the traditional two-phase system. Propionic acid was a substance which was more difficult to be degraded by methanogens in all volatile fatty acids, and its accumulation in the methanogenic phase inhibited the activity of methanogens. These results showed that it was beneficial to keep a stable pH value and alkalinity environment for the methanogenic phase, which ensured the advantage to improve the stability of the anaerobic system.