Screening On Efficient Acetogenic Bacteria And Its Construction Of Composting Microbial System

Biogas, as a kind of clean and high heat value renewable biomass energy, has broad application prospects. The acetogensis is considered as the first rate-limited step in the process of anaerobic digestion. Some results on isolated acetogen bacteria were reported under pure culture conditions, however, they haven’t contributed too much to improve the performance of biogas process. So far, little study has been carried out to improve the efficiency of anaerobic digestion and speed up the methane production by establishing an atificial composite system. This is a very promosing rsearch direction.In order to overcome the difficulty in isolation of acetogen bacteria, the medium and the gas chromatography conditions were optimized and 34 pure cultures were obtained by preliminary screening. Furthermore, by rescreening 8 strains, consisting of Enterococci, Lactococcus lactis and Clostridium,whose acetic acid production can reach up to more than 5.0mmoL/L were separated. The strains ZQ-1, CD-2 and ZY-3 belonging to Clostridium glycolicum, Clostridium butyricum and Clostridium bifermentans, acetic acid production were 18.76.17.05 and 18.51mmoL/L.The influence of ecological factors on the growth and metabolism of the strains ZQ-1> CD-2、ZY-3 were investigated. With butyrate as carbon source and the yeast extract as nitrogen source, ZQ-1 has an optimal ability of growth and metabolism at 37℃with an initial pH 8.5. The acetic acid production is promoted in the condition of 0.2M Na+ and eligible concentration of Mn2+; With amylaceum as carbon source and the tryptone as nitrogen source, strain CD-2 could grow vigorously at 37℃with aninitial pH 8.0. At 37℃, acetogenic process is stimulate with addition of K+ in the medium; Strain ZY-3 shows the optimal ability at 30℃with an initial pH 7.5 when it with sucrose as carbon source. Acetogenic process was promoted at Na+concentration of 0.4M and eligible concentration of Fe2+.An artificial composite system Th3 was Constructed by three bacteria mixed culture which contained CD-2(Clostridium butyricum), ZY-3(Clostridium bifermentans), ZQ-1 (Clostridium glycolicum) of three. The optimum fermentation conditions for Th3 were as follows:initial pH was 7.5, sequence of inoculation was ZQ-1, ZY-3 and CD-2, inoculation ratio was 1:2:1 and inoculum was 6%.The highest yield of acetic acid can reach 10.91mmoL/ L·d when it was deep static cultured at 30℃. The metabolic process of atificial composite system Th3 is stable, and the productivity of acetic acid can reach 10.91mmoL/L·d. At 26℃, the total biogas production and productivity were improved significantly after the addition of Th3. The start-up phase was shortened in the range of 3-5 days.While the biogas process will not be affected if Th3 is added at the end of fermentation.The population structures of the biogas fermentation was observed by polymerase chain reaction-denaturing gradient gel electrophoreses (PCR-DGGE) analysis. In different periods, the predominate microorganisms, as well as the fermentation product and concentration, were different. It needs to be further studied on the species of microorganism.