Spatial heterogeneity of organic matter, organic acids and biogas production during solid-state anaerobic digestion process

Studies on the spatial heterogeneity of organic matter, organic acids, and biogas production play a key role in improving the efficiency of biogas production during solid-state anaerobic digestion. In this paper, based on the mixed materials of pig manure and straw inoculated with activated sludge, mesophilic digestion (37°C) in the laboratory was designed and conducted over a period of forty-six days in order to understand the spatial heterogeneity dynamics of organic matter, organic acid, and biogas production rates along with the depth of the fermentation materials. In this experiment, organic material, organic acids, and gas production rates from different reactor depths were monitored during a solid-state anaerobic digestion process, and the vertical dynamics of the heterogeneous environment were analyzed. The following results were obtained. 1) During a solid anaerobic fermentation process, both TS and VS have obvious longitudinal variation; that is, the mass fractions of total solid (TS) and volatile solid (VS) increase with increasing height of the fermentation material (HFM) from the reactor bottom. Closer to the top of the fermentation material, longitudinal heterogeneity increased, whereas the concentration of TS and VS decreased more quickly. TS and VS of fermentation material in the top layer were highest and reached 17.41% and 12.16%, respectively. However, TS and VS in the E layer were lowest, reaching 6.72% and 4.30%, which differend little from the TS and VS of fermentation materials in the F layer. Furthermore, the TS and VS contents in the A layer were 2.59-fold and 2.83-fold greater than those in the E layer. 2) During the solid anaerobic fermentation process, low weight molecule organic acids also have significant longitudinal variation; that is, the mass fractions of acetic acid, propionic acid, butyric acid, and valeric acid increase with increasing HFM from reactor bottom. Closer to the top of the fermentation material, longitudinal heterogeneity increased, whereas the concentration of these short-chain organic acids decreased more quickly. The mass fraction of acetic acid, propionic acid, butyric acid, and valeric acid of the fermentation material in the A layer increased 17.08, 30.07, 62.79, and 54.54 times, respectively, those of F layer, and increased 1.11, 1.28, 1.29 and 1.03 times those of E layer. 3) The gas-production rate and cumulative gas production per unit of VS increased with the HFM from reactor bottom; the bottom had the lowest values but top had the highest values. The cumulative gas production per unit of VS from the A layer (1.36 L/g) was the maximum within the column, as compared to the minimum cumulative gas production per unit of VS from the E layer (0.36 L/g), a reduction of 74%. 4) The correlation was markedly positive among main parameters, such as the cumulative gas production, the cumulative gas production per unit of VS, HFM, TS, VS, and organic acid content. Furthermore, a power function curve was fit between cumulative gas production and HFM, and the adjusted R square (0.9754) was statistically significant. All results showed that a core anaerobic zone existed in the reactor during the solid-state anaerobic digestion process, and a significant power function relationship existed between the cumulative gas production and HFM. Therefore, it suggested that locating the core anaerobic zone and monitoring it in the actual project can improve the efficiency of the entire anaerobic digestion system.