Agricultural straw has surged as one typical by-product of crops with the ever-increasing grain demand. These straws have the great potential to be one of the most important feedstocks for biogas and hydrogen production through anaerobic digestion in the future. The effective treatment of straw can also be realized to produce biofuel and composite organic fertilizer. Among them, the main components of straw are cellulose, hemicellulose, and lignin. However, the cellulose and hemicellulose in the straw are fused to form the complex polymers, whereas, the lignin can form the complex lignocellulosic carbohydrates in the outermost layers of cellulose and hemicellulose. Both cellulose and hemicellulose can be encapsulated in the inner layers under the strong interaction forces between the molecules and between hydrogen and covalent bonds within the molecular structure. As such, the energy conversion rate of straw can decrease during anaerobic digestion. Therefore, the straw must be pretreated to break the lignin structure before anaerobic digestion. Fortunately, freeze-thaw pretreatment can be expected to simply operate in this case. However, low-temperature refrigeration has high energy consumption, while the commonly used acid-alkali soaking solution is easy to cause the potential risk of environmental pollution. In this study, the soaking solution was taken as the biogas slurry rich in ammonia nitrogen, one of the by-products during anaerobic digestion. The natural average temperature (−20 ℃) was used to simulate northern China in winter. A systematic investigation was made to clarify the effects of biogas slurry soaking and freeze-thaw pretreatment on the physicochemical properties of rice straw, in terms of soaking temperature, soaking liquid-solid ratio, soaking time, and the number of freeze-thaw cycles. The results showed that there was a damaged lignin structure and hydrogen bond of lignocellulose molecules after the pretreated rice straw. A weakening trend was also found in the absorption peaks of O-H, C-H, C=C, C=N, N=N, and N=O groups. The crystal structure of rice straw was almost unchanged, but the crystallinity of rice straw decreased significantly, reaching 29.82% and decreasing by 21.38%, compared with the untreated straw. The C/N ratio of rice straw was significantly reduced to below 30 during biogas slurry immersion. By contrast, there was a relatively small effect of freeze-thaw treatment on the C/N ratio of the immersed rice straw, which was reduced to the minimal 28.19 suitable for anaerobic digestion. The removal rates of lignin, hemicellulose, and cellulose, and the volatile fatty acid (VFA) concentrations increased first and then decreased, with the increase of soaking temperature. The liquid-solid ratio showed a continuous increase trend with the increase of soaking time and freeze-thaw cycles. Soaking time and the number of freeze-thaw cycles had significant interactive effects on the lignin removal, while soaking temperature, soaking time, and liquid-solid ratio had significant interactive effects on the VFA concentrations. The maximal lignin removal rate and VFA concentration reached 40.06% and 4 140 mg/L, respectively, in the pretreatment process of biogas slurry immersion combed with freeze-thaw. Therefore, the practically feasible performance was achieved in the combined freeze-thaw pretreatment of rice straw with biogas slurry soaking. The finding can lay a theoretical foundation for the wide application of low-cost and harmless freeze-thaw pretreatment of straw in northern cold regions.

Aerobic pretreatment is one of the important heat-producing steps by aerobic microorganisms at high solids loading. The energy consumption can be effectively reduced to warm the feedstocks for the subsequent anaerobic digestion. Among them, aerobic pretreatment can usually employ forced continuous ventilation or intermittent ventilation. The cold air can often tend to impact the microorganisms and the pretreatment at the low ambient temperature. Excessive heat loss can also occur, leading to serious challenges during methane (CH₄) production. In this study, an integrated aerobic-anaerobic fermentation was adopted to regulate the dynamic aeration in the aerobic pretreatment, according to the oxygen concentration during the reaction. A systematic investigation was made on the temperature rise during aerobic pretreatment, the methanogenic characteristics during dry anaerobic fermentation, as well as the energy consumption and production capacity under different ambient temperatures. The inoculation and leachate reflux were also combined for the integrated operation. The results showed that the aerobic-anaerobic process was integrated to realize the stable aerobic pretreatment. Dry anaerobic digestion was used to realize the rapid conversion and seamless connection of the aerobic to the anaerobic stage. Furthermore, the aerobic pretreatment with dynamic aeration significantly increased the temperature of the feedstocks. The fluctuations impacted the warming of the aerobic pretreatment under the ambient temperature. The temperature also elevated in the first 72 h under the ambient temperature of 11.7-17.3 ℃. The highest temperature reached 57.9 ℃ under the conditions where the aeration was not preheated. Meanwhile, a large amount of the volatile fatty acids (VFAs) was produced during aerobic pretreatment. The rapid startup of the dry anaerobic digestion promoted significantly the CH₄ production. There was less energy consumption for the dynamic aeration during aerobic pretreatment. There was only accounted for 7.64% to 12.69% of the total energy consumption of the integrated operation. While the warming energy consumption during the anaerobic fermentation accounted for 87.31% to 92.36%. The particle size of the corn straw dominated the CH₄ production. The cumulative CH₄ production under 3-5 mm particle size increased by 113.93% and 81.95%, respectively, compared with that under ≤1 mm and 7-8 mm particle sizes. There was no significant difference in the effects of the different aeration temperatures on the CH4 production and the VFA concentrations. The maximum threshold of the total solids (TS) was 20% for dry anaerobic digestion. The cumulative CH₄ production increased by 24.24% and 78.98%, respectively, under 20% TS, compared with the 15% and 25% TS. The cumulative CH₄ production decreased from 183.92 to 156.76 L and 158.04 L, as the inoculum concentration increased from 40% to 60%. There was no significant difference in the CH4 production when the leachate was refluxed 3 to 4 times per day. The optimal combination of the aerobic pretreatment was obtained with a straw particle size of 3-5 mm, dynamic aeration without heating, and dry anaerobic digestion with a total solids load of 20%, an inoculum concentration of 40%, and leachate refluxing four times per day. The CH₄ yield, CH₄ production per unit of electricity consumption, and CH₄ production capacity reached 218.99 L/kg, 6.352 L/kWh, and 7.840 MJ/kg, respectively. The bacterial communities in the liquid and solid phases also differed outstandingly from the two phases of hydrolytic acidification and CH₄ production. There was a great variation in the relative abundance of the bacterial community in the liquid phase during anaerobic digestion. The relative abundance of the Bacteroidota decreased from 62% to 34%. Both the relative abundances of the Firmicutes and Proteobacteria increased from 22% and 9% to 25%. There was a more stable relative abundance of the bacterial community in the solid phase. The relative abundances of the Firmicutes and Bacteroidota increased from 52.0% and 12.3% to 52.7% and 15.3%, respectively. While the relative abundance of the Proteobacteria decreased from 15.3% to 8%. This finding can also provide a strong reference to reduce the consumption loss of dry anaerobic digestion in the northern cold area.