Research On Efficient Bioconversion Mechanism And Resource Utilization Technology Of Organic Matter In Swine Manure Based On Nanobubble Enhancement

China’s pig breeding industry has developed rapidly,with large emissions of manure,but low resource utilization and serious environmental pollution.Anaerobic digestion biological treatment technology can simultaneously achieve pollutant removal and resource utilization,and has broad development prospects.However,the slow hydrolysis rate of complex organic matter in swine manure leads to long anaerobic fermentation time and low methane production,which limits the wide application of anaerobic digestion technology.Since nanobubble water（NBW）has the characteristics of high efficiency and low cost,it has attracted more and more attention as an additive to promote the performance of anaerobic digestion for methane production.However,its efficient biotransformation mechanisms have not been fully explored,particularly in terms of in microbial and methanogenic metabolic mechanisms.Moreover,anaerobic digestion can also reduce the antibiotic resistance genes（ARGs）,a new pollutant in swine manure,while the effect of NBW addition on the reduction of ARGs in swine manure has not been studied.In this study,three kinds of NBWs（Air-NBW,O₂-NBW and O₃-NBW）were prepared as additives（O₃-NBW was prepared and used for the first time）to explore their effects on methane production performance of anaerobic digestion of swine manure,reveal the mechanism of microbial transformation,and further clarify their effects on the reduction of ARGs.In addition,aiming at the problem of high ammonia nitrogen in biogas slurry after anaerobic digestion,the denitrification performance of subsequent denitrification process（shortcut nitrification-anammox）of biogas slurry was explored,realizing the deep treatment and resource utilization of manure.The main conclusions were as follows:（1）The basic properties of nanobubbles were revealed:the peak values of bubble concentration in the prepared three kinds of bubble water were dominated by nanoscale bubbles.The average particle sizes of Air-NBs,O₂-NBs and O₃-NBs were 177.5±15.1,170.0±31.3 and 171.8±30.2 nm,respectively,and the average concentrations were2.5×107,2.1×107 and 2.2×107 particles/m L,respectively.There was no significant difference in the average particle size and concentration（P>0.05）.In addition,the three nanobubbles can stably exist in aqueous solution for more than 14 days,which is conducive to the flow of water molecules and electron transport,thereby promoting the increase of methane production.（2）It was found that nanobubble water enhanced the performance of anaerobic digestion for methane production,and the metabolic mechanism of promoting methane conversion was clarified:the cumulative methane yields in groups with O₂-NBW（94.1m L/g-VS）,Air-NBW（109.0 m L/g-VS）,and O₃-NBW（143.0 m L/g-VS）addition increased by 23.4%,43.0%and 87.5%,respectively,in comparison to the control（with the same amount of deionized water addition）.Analysis of microbial community characteristics showed that NBW enhanced hydrolysis and acidification by promoting the growth of hydrolysis and acidification bacteria（Chloroflexi and Firmicutes）.And for methane metabolism,NBW promoted three major methanogenesis metabolic pathways,especially hydrogenotrophic and methylotrophic pathways.The metabolic regulation mechanism was to increase the abundance of key enzyme genes in the pathway and the abundance of genes encoding coenzyme F420（1.8-3.9 times）,enrich electron-accepting methanogens（Methanothrix）,and reduce genes related to pili formation（Pil A and Omc S）.Therefore,NBW might act as pili to promote direct interspecies electron transfer（DIET）-based methanogenic metabolism.（3）It was found that nanobubble water enhanced antibiotic resistance gene reduction in anaerobic digestion,and its degradation mechanism was preliminarily revealed:the ARGs types detected in swine manure mainly included bacitracin,tetracycline,sulfonamide,and macrolide-lincoamide-streptomycin（MLS）in this research.And the host bacteria of ARGs are mainly Proteobacteria,Bacteroides,and Firmicutes.Spearman correlation analysis showed that environmental factors such as soluble chemical oxygen demand（SCOD）,volatile fatty acids（VFAs）,time（T）and cumulative methane production（CM）were significantly correlated with most ARGs（P<0.05）.Compared with the control group,the addition of NBW could effectively improve the reduction rates of ARGs abundance and relative abundance（especially the addition of O₃-NBW）by affecting the distribution of host bacteria,and the reduction rates increased by 21.3-30.1%and 32.8-41.2%,respectively.Among them,the experimental group with O₃-NBW achieved the highest ARGs abundance and relative abundance reduction rates（64%and 56.2%）,which might be caused by the strong oxidizing effect of ozone on host bacteria.It was worth noting that the abundance and relative abundance of bacitracin ARG increased after anaerobic digestion,which was related to the abundance of potential host bacteria.（4）The advanced treatment technology of swine manure organic matter based on nanobubble strengthening was proposed,and the efficient resource utilization process was established:The swine manure treated by nanobubble-enhanced anaerobic digestion process achieved efficient carbon source recovery and ARGs reduction,but there was still high concentration of ammonia nitrogen in the biogas slurry.In this study,the shortcut nitrification-anammox combined process was used as the subsequent nitrogen removal process of anaerobic digestion.And the process achieved efficient removal of total nitrogen for simulated wastewater with high ammonia nitrogen and low carbon nitrogen ratio.The nitrite accumulation rate of the pre-process shortcut nitrification system reached 90.1%,which created good conditions for the stable operation of the post-process anammox system.The total nitrogen removal rate reached88.0%in anammox system,and nitrogen removal effect was remarkable.Therefore,the nanobubble enhanced anaerobic digestion-shortcut nitrification-anammox process could realize the advanced treatment and efficient resource utilization of swine manure organic matters,which provided theoretical basis for the development and application of livestock and poultry wastewater treatment process.