| In recent years,the production of food waste in my country has been increasing.Driven by the demand for energy recovery,anaerobic digestion has become the mainstream technology for energy conversion of food waste.The traditional fully mixed digestion system is easy to cause the loss of functional microorganisms under the condition of low hydraulic retention time(HRT),which affects the biogas production.The anaerobic membrane bioreactor(AnMBR),which combines anaerobic digestion and membrane filtration,can achieve efficient retention of functional microorganisms through the effective decoupling of HRT and sludge retention time(SRT),thus providing the possibility for efficient energy recovery.However,for wastes with high solid content such as food waste,the current research mostly adopts the low-frequency feeding method once or several times a day,which can easily cause excessive feeding impact under high load conditions,aggravating the inconvenience of acid production and methane production.balance,resulting in a decrease in methane conversion efficiency,or even rancidity.In addition,the low frequency feeding mode may also affect the membrane fouling rate due to the large impact load.Therefore,this study proposes a technical idea based on highfrequency feeding to alleviate the load shock and improve the efficiency of AnMBR in treating food waste.The changes of AnMBR operating efficiency and stability under four feeding frequencies of 0.25 hours,0.5 hours,1 hour and 2 hours were studied,and the membrane fouling characteristics of AnMBR under high frequency feeding were analyzed.The biochemical kinetic characteristics and microbial evolution law of AnMBR system under high frequency feeding,and finally put forward the idea of constructing AnMBR system based on high frequency feeding.The main conclusions are as follows:(1)When AnMBR runs stably until the organic load(OLR)is 5.34 g COD/(L·d)(HRT15 days,SRT 37.5 days),the feeding frequency is gradually reduced from once every0.25 hours to once every 2 hours.The measurement results of various indicators showed that with the decrease of the feeding frequency,the methane digestion performance changed little,and only a slight inhibition occurred when the feeding frequency was once every 2 hours.However,the system stability and the removal capacity of macromolecular organics decreased significantly with the decrease of feeding frequency.The removal rate of protein substances in the reactor decreased gradually from 98.11% to 94.65%;the removal rate of COD decreased gradually from 97.30% to 90.37%.At the same time,the concentration of VFAs continued to increase,the p H gradually decreased from 7.70 to6.96,the total alkalinity and effluent ammonia nitrogen concentration in the system also continued to decrease,and the buffer capacity decreased.When the feeding frequency is once every 2 hours,the TVFA value reaches 1.67 g COD/L,the propionate/acetate ratio rises sharply to 1.17,the bicarbonate alkalinity/total alkalinity decreases to 0.59,and the VFA/alkalinity ratio increases to 0.42 At this time,the stability of the digestion system is poor,and the anti-acidification ability is low.(2)The continuous monitoring results of the AnMBR transmembrane pressure difference(TMP)showed that when the OLR was 5.34 g COD/(L·d)(HRT 15 days),the membrane fouling rate ratio increased significantly with the decrease of the feeding frequency.When the feeding frequency was once every 2 hours,the membrane fouling rate entered a stage of rapid growth,and the value of d TMP/dt reached 1.23 KPa/d.This may be because as the feeding frequency decreases,the instantaneous load increases,the concentration of organic matter in the mixed solution increases rapidly,and the retained macromolecular protein substances increase.In addition,through the analysis of the polluted layer on the membrane surface,it can be found that the polluted layer on the membrane surface of AnMBR is mainly composed of more protein,a small amount of humic acid and other organic acids,and there are large sludge particles on the polluted membrane surface,with a roughness of 93.4nm.(3)By calculating the four-stage conversion rate of AnMBR under different feeding frequencies(including hydrolysis rate,acidification rate,acetification rate,and methanation rate),it can be seen that when the OLR is 5.34 g COD/(L·d),the feed The decrease in frequency was significantly correlated with the difference between the acidification rate and the methanation rate.When the feeding frequency was reduced from0.25 hours to 1 hour,the rate difference between the two gradually increased from 0.046 to 0.128.As the feeding frequency was further reduced to 2 hours,the difference between acidification and methanation rates increased to 0.172,while the methanation rate rapidly decreased from 0.840 to 0.746.This indicates that the VFAs produced in the acidification process of the matrix could not be converted into methane in time,and the accumulation of VFAs appeared obviously,and the reaction in the methanation stage was obviously inhibited.(4)The VFAs methanation characteristic experiment and the specific methanogenic activity experiment of the sludge mixture in AnMBR under different working conditions found that the OLR was 5.34 g COD/(L·d),and the feeding frequency was once every0.25 hours.The specific methanogenesis activity of the reactor with acetate,propionate and butyrate as substrates all reached the maximum value at the concentration of8 g COD/L,and the values were 0.368,0.304,0.445 g COD/g VSS·d,respectively.The methanation rate Faster and with almost no lag period,the degradation ability of VFAs in the digestion system is strong at this time.After the feeding frequency was reduced to once every 2 hours,the specific methanogenesis activities of acetate,propionate,and butyrate as substrates all decreased,and the maximum specific methanogenesis activities decreased to 0.263,0.249,and 0.339 g COD/g VSS·d,respectively.At the same time,the rate of methanation decreased,and the lag period of methane production increased significantly.It shows that the methanation process of VFAs is inhibited at this time,and the VFAs in the system cannot be utilized in time.In addition,the hydrophilic activity results showed that the microbial activity of the hydrogenotrophic methanogenesis pathway was significantly enhanced with decreasing feed frequency.(5)Analysis of microbial community structure under different feeding frequencies showed that when the OLR was 5.34 g COD/(L·d)(HRT 15 days),as the feeding frequency decreased from 0.25 hours to 2 hours,the degradation of VFAs was promoted.The relative abundance of mutual oxidative bacteria Candidatus Cloacamonas increased from 21.13% to 32.7%,because the concentration of VFAs to be degraded in AnMBR increased at this time.The relative abundance of Defluviitoga,which can degrade a large number of monosaccharides and polysaccharides and simultaneously produce acetate,ethanol and propionate,increased from 0.47% to 32.04%,which may be one of the reasons for the increase in the concentrations of acetate and propionate in the system.At the same time,the abundance of the acetate-trophic archaea Methanothrix decreased significantly from 37.16% to 29.62%,and the abundance of the hydrogen-trophic bacteria Methanomassiliicoccus increased significantly from 16.81% to 43.90%,indicating that the acetate degradation capacity in the digestion system changed at this time.Weak,the hydrogenotrophic pathway accounts for an increased proportion of the methanogenesis pathway.The results of RDA analysis also showed that the change in the abundance of Methanobacterium and Methanothrix may be one of the main reasons for the reduction of the acetate and butyrate degradation ability of the digestion system after the feeding frequency was reduced. |
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