Study On Coupling Technics Of Solid-state Biogas Fermentation And Aerobic Composting Based On Rice Straw Mixed With Pig Manure

In recent years,the comprehensive utilization of agricultural straws and livestock manure has attracted wide attention in many countries.Compared with wet anaerobic fermentation,solid-state anaerobic fermentation possesses the many merits such as low water consumption,high gas production rate and low cost of digested residue treatment.However,the organic matter during solid-state fermentation cann’t be completely degraded.Although the content of residual organic matter in residue is high,the content of humic acid is low.Aerobic composting can increase the content of humic acid,thus increasing the fertilizer efficiency of products and improving soil quality.In addition,the mixture of straws and manure can balance the C/N ratio of fermentation substrate and promote the growth of microorganisms.And the combination of anaerobic biogas fermentation and aerobic composting of digested residue can promote carbon cycling and carbon neutralization.Therefore,in this work,the solid-state biogas fermentation performance was optimized using mixture of pig manure and straw as fermentation substrate at mesophilic condition（35±1°C）.The composting characteristics of digested residue were also investigated.Finally,combined with the analysis of microbial community structure,the mechanisms of biogas fermentation,humic acid formation and nitrogen transformation were discussed,respectively.The results showed that the C/N ratio,straw particle size and TS%（total solid content）had significant effects on solid-state anaerobic fermentation.The obtained optimal fermentation conditions included C/N ratio of 27.5:1,straw particle size of 0.85mm and TS%of 25%.Under these conditions,microbial metabolism was active,which leads to faster degradation of lignocellulose substrate and superior gas production performance,with daily gas production amount of reaching 2200 m L,the cumulative biogas production amount of 26304.50 m L,the gas production rate of 0.43 m³/（m³·d）,the methane concentration of 64.88%,and the removal rates of lignocellulose compositions were up to 65.98%,62.59%and 16.99%,respectively.During aerobic composting,the physicochemical properties were improved at the aeration rate of 0.75 L/min,which was most suitable for microbial growth and metabolism,and accelerated the humification process.The VS degradation rate of the compost was 19.16%.The humic acid content was 100.89 mg/g.In addition,the GI was103.07%,the C/N ratio was 13.40 and the T value was 0.52.The results also showed that the inoculation of Bacillus subtilis during composting was conducive to the mineralization,humification and maturity of digested residue.When Bacillus subtilis was inoculated at 3%,the removal rates of lignocellulose substrate was the highest,reaching 49%,16.13%and 19.35%respectively.The best maturity of the fermented substrate was achieved.When Bacillus subtilis was inoculated at 0.5%,the maximal humic acid content of 129.36 mg/g was obtained,indicating that carbon was well fixed.In order to further improve the operation stability and treatment capacity of solid-state fermentation reactor,biochar was added into the fermentation substrate.In the solid-state anaerobic biogas fermentation,the biochar concentration of 10 g/L significantly promoted the growth and reproduction of microorganisms.The daily gas production amount and the cumulative gas production were up to 2655 ml and 38943.50ml,respectively,and the gas production rate reached 0.52 m³/（m³·d）.Furthermoer,the removal rates of lignocellulosic compostions were 76.69%,85.03%and 14.01%respectively,and the degradation rates of acetic acid and propionic acid,which were the main intermediates during biogas fermentation,were 41.86%and 45.75%respectively.These results indicated that biochar could promote microbial growth,accelerate the degradation of lignocellulose and intermediates,and improve the gas production performance of the reactor.During the composting of the digested residue,the optimal composting effect was achieved when the biochar concentration was 15 g/L.After fermentation,the TN（total nitrogen）content reached 11.04 mg/g,which was 23.35%higher than CK.The NH₄⁺-N content was low（0.23 mg/g）,and the NO₃^--N content reached 0.41 mg/g,which was 23.35%higher than CK,indicating that the performance of nitrogen fixation was improved and the nutrient loss was reduced.The humic acid content was up to 199.3 mg/g,which indicated that the humification process was accelerated.Furthermore,GI was up to 146.95%,and C/N ratio was down to 10.8.T value and NH₄⁺-N/NO₃^--N ratio was 0.41 and 0.56,respectively.In order to further reveal the succession of microbial community structure in anaerobic biogas fermentation and aerobic composting of digested residue,microbial diversity analysis was engaged.When the biochar concentration was 10 g/L,the bacteria with strong hydrolysis were the dominant phylum.Synergistata,which was related to acidification,was the dominant phylum in the middle and late stage of fermentation,indicating that the organic acids produced in the early stage of fermentation were gradually degraded and utilized.At the genus level,the abundance of lignocellulose degrading microorganisms gradually increased with prolonging the fermentation,indicating that lignocellulose was well degraded.In addition,the abundance of Syntrophobacter,which could degrade propionic,increased significantly over time of fermentation,indicating that more propionic acid was degraded as adding biochar.Geobacter was detected in the middle and late stage of fermentation,indicating that the microorganisms related to DIET was enriched in the reactor with biochar.At species level of methanogens,the abundance of Methanosarcina and Methanosaeta,which could accept electrons from Geobacter and reduce CO₂to methane,increased significantly over time,which confirmed the possibility of biochar promoting DIET.Furthermore,during composting,when the biochar concentration was 15 g/L,at the phylum level,the bacteria community with the ability to degrade macromolecular organic matter was the dominant phylum.The abundance of Actinobacteriata related to the formation of humus increased significantly at the late stage of fermentation,indicating that the humification process was relatively improved at the later stage of composting.At the genus level,the abundance of Azoarus and Pesudofulvimonas increased gradually,indicating that the effect of nitrogen fixation was better,and more NH₄⁺-N was transformed into NO₃^--N.