Effects of biochar/attapulgite on the anaerobic digestion performance of stover under different organic loading rates

Maize stover can be one of the most valuable waste resources in modern agriculture. Particularly, there is the a significant potential to for the waste-to-energy conversion and carbon emission reduction via anaerobic digestion. However, conventional anaerobic digestion can be limited to the scalability and practical application, such as the low tolerance for high organic loading, accumulation of volatile fatty acids, and instable unstable biogas production. Alternatively, biochar and attapulgite have emerged as the promising functional additives, in order to enhance the performance of the anaerobic digestion. Acidification can be mitigated to promote the microbial colonization and buffering capacity. The synergistic effects can be produced to provide a feasible strategy for the efficient anaerobic digestion. Nevertheless, the systematic studies still remain scarce on the dynamic regulation of the biochar-attapulgite composites under varying organic loading rates, as well as their overall effects on the energy efficiency and economic viability. In this study, a systematic investigation was made to explore the effects of the biochar and attapulgite (applied both individually and in combination) on the semi-continuous anaerobic digestion of ensiled maize stover under varying organic loading rates. Ensiled maize stover was taken as the substrate. A semi-continuous anaerobic digestion was operated at five organic loading rates: 0.5, 1.0, 1.5, 2.5, and 3.5 g/(L·d). Methane production and anaerobic digestion were compared on in the stability. The treatments were set as a control group (CK), a biochar group (BC), an attapulgite group (AT), and three composite additive groups with biochar-to-attapulgite mass ratios of 7:3, 5:5, and 3:7. The key parameters were monitored, including the daily gas production, methane content, volatile fatty acids (VFAs) concentration, pH, and intermediate alkalinity to partial alkalinity ratio (IA/PA). Energy conversion efficiency was then calculated after 43 days of continuous operation. The results showed that both methane concentration and methane production efficiency were gradually enhanced with the increasing loading rates, when the organic loading rate (OLR) fell within the range of 0.5-1.5 g/(L·d). The methane volume fraction of the BC/AT 7:3 group reached 54.97%, resulting in a methane yield of 367.46 mL/g, under an organic loading rate of 2.5 g/(L·d). This yield was 15.09%, 9.20% and 3.02% higher than those of the CK (319.27 mL/g), the BC (338.08 mL/g), and the AT group (359.82 mL/g). Meanwhile, the IA/PA ratio in the BC/AT 7:3 group was remained stable, ranging from 0.26 to 0.39. The pH was maintained between 7.1 and 7.3. While the concentration of VFAs was fluctuated between 15.83 and 42.23 mg/L. All parameters were remained below inhibitory thresholds, indicating the robust operational stability of the anaerobic digestion. In addition, the combined addition of biochar and attapulgite increased the soluble chemical oxygen demand (_SCOD) and conductivity during the anaerobic digestion of ensiled maize stover. The methane efficiency was enhanced by the substrate hydrolysis, acidification, and direct interspecific electron transfer (DIET) between microorganisms. The BC/AT 7:3 group also demonstrated an energy conversion efficiency as high as 73.21%, thus marking a 9.38 percentage points increase, compared with the CK group. There were the enhancements of 4.46 and 4.49 percentage points over the BC and AT group, respectively. The high methane production was primarily attributed to high energy conversion efficiency. In summary, the anaerobic digestion performance of ensiled maize stover was significantly enhanced under the organic loading rate of 2.5 g/(L·d) with the BC/AT 7:3 ratio. This strategy was achieved the simultaneous optimization of methane production and system stability. Engineering feasibility was also demonstrated to enhance the stover consumption and energy conversion efficiency. Thereby, the finding can also provide a scientific basis approach to developing the an efficient energy conversion of agricultural wastes.