Mitigating Transformation Of Ammonia And Fatty Acids During Anaerobic Digestion By Biochar Coupled Iron Nanoparticles

The imbalance between nutrient composition of substrates and metabolic requirements of microorganisms is likely to cause the accumulation of fatty acids and ammonia nitrogen during anaerobic digestion(AD).These phenomena would inhibit the activity of anaerobic microorganisms,thus seriously impeding the normal operation of AD.For an efficient AD process,it is of great importance to mitigate the inhibition effect caused by ammonia nitrogen and fatty acids accumulation.Application of biochar has been recognized as an effective strategy to enhance AD performance.By acting as an electron transmitter,biochar is able to engineer and accelerate the interspecies communication between syntrophism microbes,which is the potential mechanism of its merits.Basing on the influence law and mechanism of biochar mitigating ammonia nitrogen and fatty acids transformation during AD,nano iron was introduced as a coupling agent.Study of mitigating ammonia and fatty acids inhibition by biochar coupled iron nanoparticles during AD was conducted accordingly.Results of this study are expected to provide AD of agricultural waste with theoretical and practical reference.In order to deeply study the law and mechanism of biochar mediated AD,batch AD test of chicken manure was carried out at mesophilic condition.The transformation law of ammonia nitrogen and organic acid influenced by biochar supplement was explored.On that basis,continuous AD test was conducted to explore the influence of biochar supplement on methanogenesis,ammonia and acids transformation,and microbial community.In addition,the conductive material nano iron was introduced as the coupling agent for biochar.During AD with external inhibition,the syntrophy effects of biochar and nano iron on the substrate conversion efficiency,methane production,and the related microbial mechanism were studied.The research contents and main results are as follows:(1)Study on biochar mediated anaerobic digestion of chicken manure under various.The results showed that the enhanced effect of biochar mediated anaerobic digestion on methane production was more significant under high organic loading conditions.With the addition of biochar increasing in the range of 1.8%-5.2%VS,the stagnation period of methane production showed a trend of first decrease and then increase.Cumulative methane production,maximum methane production rate,pH value of digested feed solution and free ammonia(FA)concentration showed a trend of increasing first and then decreasing.The electrical conductivity,the concentrations of acetic acid,butyric acid,lactic acid and total ammonia nitrogen(TAN)showed a decreasing trend.Under the condition of58.1 g VS/L of chicken manure,the biochar dosage of 3.5%VS could effectively avoid the accumulation of organic acids and keep the concentrations of acetic acid,butyric acid and lactic acid at a low level,which decreased by 25%,50%and 45%,respectively,compared with the highest level.(2)Effects of biochar on the characteristics of continuous anaerobic digestion and microbial community structure.The results showed that biochar mediated the conversion of soluble organic carbon/acetic acid to methane,at the same time as the conversion of macromolecular insoluble substrates.The average daily methane production was increased by 12%with biochar supplement.Biochar alleviated propionic acid accumulation with its concentration decreased by 57%.Within the tolerance threshold range of AD system,the concentration of ammonium ion in the system was increased to neutralize the accumulated organic acids and strengthen the buffer capacity.Under a high concentration of propionic acid,biochar enriched important hydrolytic and acid-producing bacteria,as well as Methanosarcina that can metabolize various nutrients through direct interspecific electron transport pathway.(3)Regulation effect of biochar coupled with nano-iron on anaerobic digestion under acid inhibition.The results showed that addition of 2 g/L glycerin trioleate(GTO)significantly inhibited the cumulative methane production(CMP)from AD of chicken manure,showing 26%decrease.Biochar coupled with 6 g/L nano-iron supplement exhibited the highest CMP(315 mL/g VS),increased by 51%in comparison with the control group under acid-inhibited.The stability of total organic carbon concentration,pH value and total ammonia nitrogen concentration in biochar coupled with nano-iron mediated AD system was weaker than that in biochar mediated AD system,but stronger than that in nano-iron mediated AD system.With the excellent carrier characteristics of biochar,the enhancement effect of nano-iron on anaerobic digestion exerted stably.Mono-AD of chicken manure without GTO exhibited the lowest values of TAN concentration and electrical conductivity among all treatments,indicating that more NH₄⁺was needed to enhance buffering effect under fatty acid accumulation.(4)Regulation effect of biochar coupled with nano-iron on anaerobic digestion under synergistic inhibition of ammonia and acid.The addition of 5 g/L NH₄Cl and 2 g/L GTO significantly inhibited AD of chicken manure,with CMP decreased by 29%.Biochar coupled with 6 g/L nano-iron achieved CMP of 292 mL/g VS,increased by 45%compared with the control.The co-inhibition of ammonia and acid hindered the methanation process of organic matter,led to the accumulation of total organic carbon in AD system.Biochar coupled with nano-iron exhibited better regulation effect on this phenomenon,and promoted the conversion of organic matter to methane.Under co-inhibition,biochar supplement alleviated the drastic fluctuation of total organic carbon concentration.Adding1 g/L nano-iron in AD system promoted hydrolytic acidification,but the enhancement effect on methane production was not obvious.The addition of 6 g/L nano-iron was helpful for the synergism of acid production and methane production,whose equilibrium state can be reached quickly.(5)Microbiological mechanism of biochar coupled with nano-iron mediated anaerobic digestion under ammonia/acid inhibition.The results showed that under the condition of acid inhibition,the diversity of anaerobic digestion microorganisms increased significantly,while the relative abundance of Euryarchaeota decreased.Under the condition of co-inhibition,the relative abundance of nitrogen-metabolizing bacteria and Euryarchaeota increased.Under acid inhibition,biochar enriched Alkaliphilus that can reduce nitrate,thus promoting the nitrogen removal rate from AD system.Biochar coupled with 6 g/L nano-iron supplement resulted in uniform distribution of Clostridium,Caldicoprobacter,Alkaliphilus and Corynebacterium under acid inhibition.Bifidobacteria and Corinthinia were enriched under the condition of co-inhibition,and the abundance of Euryarchaeota was increased as well.Enrichment of important acid-producing bacteria and metabolic function genes is the potential mechanism for enhancing the degradation of organic matter and methane production during AD.Biochar and nano-iron can directly replace a large number of redundant electron transfer processes between microorganisms,shorten the lag period of methane production,promptly consume ammonia nitrogen and organic acids,thus alleviating their accumulation.