Suitable compactness of corn stover improving characteristics of solid-state anaerobic digestion

Abstract: To study the effects of compactness on the performance of solid-state anaerobic digestion (SS-AD), the SS-AD of corn stove was conducted with different total solid (TS) contents. The digestion material with the TS content of 20% was loaded into the reactors at the compactness of 0.56, 0.71 and 0.79 kg/L, and the compactness of the digestion material with the TS content of 25% was 0.43, 0.56 and 0.70 kg/L, respectively. The results showed that with the same TS content, the compaction degree had no significant effect on the peak time of daily methane yields, but the peak value significantly decreased (P<0.05) with the compaction degree increasing. For the reactors with the initial TS of 25%, the peak value of daily methane yield of the reactors with the compactness of 0.70 kg/L decreased by 12.8% and 21.8% compared to that with the compaction degree of 0.56 and 0.43 kg/L, respectively. The variation trends of methane concentration and daily methane yield and cumulated methane yield (based on volatile solid, L/(kg·d)) of the reactors with different compactness of the digestion material had no significant difference. However, if calculating the biogas yield based on the volume of the reactor, the result would be completely different. Both of the peak values of daily methane yield and accumulated methane yield were positively related to the compactness. The effects of compactness on solid state anaerobic digestion process were enhanced with the increase of the initial TS content. With the initial TS content of 20%, the peak values of daily methane yield and accumulated methane yield of the reactors with the compactness of 0.79 kg/L were 0.83 L/(L·d) and 32.48 L/L, respectively, which were 29.2% and 37.1% higher than that of the reactors with the compactness of 0.56 kg/L. When the initial TS content was increased to 25%, the peak values of daily methane yield and accumulated methane yield of the reactors with the compactness of 0.70 kg/L were 0.84 L/(L·d) and 25.08 L/L, respectively, which were 52.7% and 60.2% higher than that of the reactors with the compactness of 0.43 kg/L. For the reactors with the initial TS content of 20%, the physical and chemical properties, such as TS, volatile solid (VS), pH value and total volatile fatty acid (TVFA) concentration of the substrates at top and bottom layer were obviously different during the digestion, while the difference gradually narrowed with the increase of compaction degree of the digestion material. However, the physical and chemical properties of the substrates at top and bottom layer in the reactors with the initial TS content of 25% were similar. This was probably due to the high solids content, which made it difficult to form continuous free flowing mobile phase in the substrate, and then the penetration and diffusion of the intermediates were blocked. The results above indicated that increasing packing density of the substrate with the initial TS content of 20% could improve the mass transfer process of the intermediates. The increase of the compactness made it possible to form a continuous liquid phase in the digestion material, which was beneficial to the degradation of the substrate and diffusion of the intermediates. While for the substrate with the initial TS content of 25%, increasing packing density had no significant effect on mass transfer. In summary, the increase of the compaction degree of the digestion material can maintain the normal process biogas yield of the solid-state anaerobic digestion of corn stover, and also can significantly enhance the volume efficiency of the reactors.