Study On Fe₃O₄ Mediated Enhancement Of The Efficiency Of Directional Production Of Odd/even Carbon VFAs By Anaerobic Fermentation

The production of volatile fatty acids（VFAs）by anaerobic fermentation of organic wastes and directional regulation of their composition is an effective way of waste recycling.In the previous study,it has been made clear that swill waste oil and peanut meal is directed to produce odd carbon VFAs（OCFA）and even carbon VFAs（ECFA）,but there are still some problems such as insufficient hydrolysis of organic matter,low production efficiency in acidizing process and easy accumulation of ammonia nitrogen.Some studies have shown that the addition of Fe₃O₄ can effectively promote the process of hydrolysis and acidification,and has a certain effect on the degradation of ammonia nitrogen.In this paper,the directional acid production efficiency,the change law of various indexes,and microbial community under different dosages of Fe₃O₄ were analyzed,and the effect and mechanism of Fe₃O₄ on the directional acid production system of waste oil and raw meal were studied.The model tests of ASBR and CSTR were carried out at the dose of 5g/L to verify the feasibility and stability of the optimization effect of Fe₃O₄ in sustainable acid production.The results showed that the Fe₃O₄ of 5g/L improved the hydrolysis and acidification efficiency of organic matter in the two-directional acidogenic systems of waste oil and peanut meal,significantly increasing the concentration of target acid（sig.<0.05）,and maintained a high proportion.Under the 5g/L dose,the Yscod in the OCFA directional acid production system was between 89%and 97%,the overall level was higher than 69%of the blank group,and the peak concentration was 6246.39mgCOD/L,which was 25.39%higher than that of the blank group,accounting for 86.41%.Fe₃O₄ accelerated the transformation from SCOD to VFAs and OCFA in the hydrolysis of waste oil（correlation p<0.01）and showed an obvious numerical simulation relationship in the process of fermentation（R2>0.9）.The overall level of Yscod in the ECFA directional acid production system was 61.66%,which was higher than 58.98%in the blank group,and the peak concentration was 9072.69mgCOD/L,which was 13.10%higher than that in the blank group,accounting for 97.16%.The addition of Fe₃O₄ enhanced the utilization of organic matter in peanut meal,especially the hydrolysis and acidification of protein.The hydrolysis and acidification process conformed to the first-order reaction kinetic equation and Logistic model.The addition of Fe₃O₄ to the ECFA-oriented acid production system can also effectively reduce the concentration of ammonia nitrogen in the fermentation process.The results showed that the peak concentration of ammonia nitrogen in the reactor with a dosage of 5g/L and 10g/L was 127.67mg/L,162.27mg/L,and 173.16mg/L respectively,which decreased by 38.74%,22.13%,and 16.90%respectively compared with the blank group,and the overall ammonia nitrogen concentration level also decreased by 29.22%30.24%and 26.91%,respectively.Further analysis showed that there was a very significant correlation between Fe2+ concentration and ammonia nitrogen concentration in the system.Fe₃O₄ induced active electron transfer and promoted the degradation of ammonia nitrogen through the joint action of Fe2+ and Fe3+.Microbial analysis showed that Fe₃O₄ made the formation of microbial community structure develop in the direction of target acid production,and increased the abundance of key microorganisms.The dominant microorganisms related to OCFA production are Firmicutes,Veillonellaceae,and Lactobacillaceae,as well as Anaerovibrio and Megasphaera,while Firmicutes,Proteobacteria,and Actinobacteria are dominant microorganisms related to ECFA production.In addition,Fe₃O₄ also decreased the concentration of ammonia nitrogen in the fermentation system by mediating the enrichment of microorganisms related to ammonia nitrogen degradation,such as Actinobacteria,Pseudoclavibacter,and Acinetobacter.At the dose of 5g/L,the running efficiency of the model test is good and relatively stable.In ASBR system,the peak values of OCFA concentration in each cycle were 6268.95mgCOD/L,5792.73mgCOD/L,5069.43mgCOD/L,6701.84mgCOD/L and 8240.97mgCOD/L,respectively.During the operation,the overall OCFA concentration will increase with the cycle,and the Yscod will reach more than 70%,accounting for between 87.03%and 90.73%.ORP is about 100mV.The pH is between 4.7 and 5.0,and the ecological environment is relatively stable.In the CSTR system,the peak concentration of ECFA in the start-up phase（A）is 9634.72mgCOD/L,accounting for 98.17%of the total.The peak concentration of ECFA in the continuous flow stage（B）is 11251.36mgCOD/L,accounting for 94.67%of the total.During the operation,the concentration of ammonia nitrogen was maintained below 350mg/L,which was friendly to acidogenic microorganisms and was much lower than the inhibition concentration threshold of ammonia nitrogen.The metabolism of ammonia nitrogen in the reactor was good.