Screening And Hydrogen Production Characteristics Of Dominant Bacteria Through Dark Fermentation Enhanced By Magnetic Field

Hydrogen,which is one of clean energy,has been regarded as the most potential substitute for fossil fuel.Seeking large-scale,efficient and low-cost technology of hydrogen production is a common concern of researchers all over the world.Biohydrogen production by dark fermentation has already become a hotspot for researchers all over the world because of its wide source of raw materials,simple process and low cost.At the same time,some studies have shown that a certain intensity of magnetic field can promote the activity of cell enzymes and the growth of microorganisms,and effectively improve the effect of biological treatment.Compared with other enhanced biohydrogen production technologies,such as heat treatment,acid-base treatment,etc,magnetic field has the characteristics of low energy consumption and pollution-free.Therefore,it is of great practical meaning and application value to study the effect and characteristics of magnetic field on biological hydrogen production by dark fermentation.In this paper,two dominant hydrogen-producing bacteria were enriched and screened from surplus activated sludge(SAS)under certain magnetic field strengthening conditions,and two other dominant hydrogen-producing bacteria were screened from purchased hydrogen-producing bacteria.Then,the hydrogen-producing characteristics of the four strains were studied through batch experiments,under magnetic field.On this basis,compered with non-magnetic field biohydrogen system,the enhancement effect of magnetic field on hydrogen production from dark fermentation of glucose with SAS was studied,using continuous operation reactor.Finally,the enhancement effect of magnetic field and/or dominant bacteria on hydrogen production from dark fermentated food waste were discussed.The following conclusions are drawn:(1)The magnetic field is benefit for selective enrichment of hydrogen-producing dominant bacteria.Seven hydrogen-producing strains were successfully screened out by plate marking and pure medium culture,from the self-made dark fermentation reactor,with glucose as substrate and SAS as seed sludge,after 50 days of cultivation and domestication at 37±2? under magnetic field.They were all identified as Bacillus sp.belongs to Firmicutes by means of PCR-DGGE and 16S rRNA sequencing analysis.Then,two dominant strains for hydrogen-production were screened out,according to the results of hydrogen-producing performance tests,named BHO1(No.BH01)and BH03(No.BH03).(2)The enhancement effect of magnetic field on hydrogen-producing dominant bacteria of different fermentation types is quite different.The increased hydrogen-producing rate of ethanol-type dominant bacteria is 3-5 times that of mixed acid-type dominant bacteria.Compared with those of non-magnetized systems,the hydrogen-producing rates of ethanol-producing bacteria,i.e.BH01 and BH03,were increased of 36%and 32%respectively,with glucose as substrate,at the optimum hydrogen-producing conditions under magnetic field(54 mT,the magnetic flux density in the center of reactor),while those of mixed-acid producing bacteria,i.e.E.cloaca.e and E.aerogenes,increased only of 8%and 11%respectively.The maximum hydrogen-producing rate of BH01,BH03,E.cloacae and E.aerogenes were 1.07,0.86,0.68 and 0.67 mol H2/mol glucose respectively.(3)The enhancement of hydrogen production of dominant bacteria by magnetic field is mainly achieved by enhancing the activity of dehydrogenase and electron transfer chain,which would not affect the growth curve and hydrogen-producing fermentation types of the bacteria.The dehydrogenase activity of BH01 and BH03 was increased by 41%and 61%respectively,and the electron transfer chain activity of them also increased by 31%and 23%respectively at the optimum hydrogen production conditions and under magnetic field(54mT).(4)The hydrogen production process of four dominant strains was well fitted by Gompertz formula.According to Gompertz parameters,the external magnetic field can increase the maximum hydrogen production and hydrogen-producing rate of ethanol-producing bacteria and shorten the delay period.Under magnetic field(54 mT),the maximum hydrogen production of BHO1 and BH03 was 38%and 39%higher than that of the unmagnetized system,and the delay period was shortened by 0.4 h and 1.2 h,respectively.(5)Two identical dark fermentation hydrogen-producing systems are continuously operated,one is magnetic dark fermentation reactor(MDFR),and the other is non-magnetic dark fermentation reactor(NDFR).When the influent organic load was 2.0 kg CLD/(m3.d),the hydrogen-producing rate and the average COD removal rate could reach 0.35 nol/mol glucose and 40%in the MDFR,which were 45%and 11%higher than those of the NDFR,respectively.Moreover,the magnetic field can cause the change of the population structure of mixed microorganisms.The magnetic field has obviously enriclunent effect on Firmicutes.There were 40%of Firmicutes and 30%of Bacteroidetes in the microbial community of the NDFR,while 65%of Firmicutes and 20%of Bacteroidetes were observed in the microbial community of MFDR respectively.(6)The magnetic field can obviously improve the hydrogen-producing capacity of dark fermentation system with collected food waste and SAS.For instance,Under the combined action of magnetic field(54mT)and BH01,the hydrogen-producing rate of dark fermentation system reached 93.6 mL/g VS,22%higher than that of NDFR with BH01,38%higher than that of MDFR without BH01,and 44%higher than that of NDFR without BH01,Adding dominant bacteria,in the process of hydrogen production from fermented of kitchen wastes and SAS,may change the original fermentation type and turn to the predominant bacteria's fermentation type.