The Mechanism Of Synthetic PHAs Was Studied By Strengthening Activated Sludge By Feeding/Magnetic Field

As a biodegradable plastic,Polyhydrocylkantes is a kind of natural polyester synthesized entirely in microorganisms.Its great biocompatibility and other characteristicsmake it a potential bioplastic for reducing the use of petroleum-based plastics.Under unbalanced nutrient conditions,PHAs is synthesized as an energy storage substance in microbial cells,and this environment provides conditions for PHAs accumulation by activated sludge.In order to increase the accumulation of PHAs,many different strengthening methods and conditions have been applied to the synthesis of PHAs.However,there is still a lack of microscopically clear enrichment mechanism of PHAs dominant bacteria.The association between PHAs synthesis ability and specific strains is not clear,which restricts the large-scale application of PHAs synthesis technology.To further clarify the influence of microbial community on PHAs synthesis,activated sludge from the AAO process stage of the sewage treatment plant was inoculated with the reactor.In the experiment,anaerobic-micro-oxygen were used as oxygen supply mode,and the activated sludge was cultured under the conditions of feeding in the micro-oxygen section(R2)and external magnetic field(magnetic field intensity about 20 m T,R3).With the progress of the experiment and the increase of carbon source substrate concentration(sodium acetate was the carbon source of artificial water distribution),PHAs dominant bacteria were enriched.The experiment was conducted under abnormal nutrient conditions with C/N of 150 to explore the maximum PHAs synthesis capacity of PHAs dominant bacteria after the enrichment stage.Finally,the utilization ability of PHAs dominant bacteria to the actual fermentation broth was explored to provide the microbial basis for using cheap carbon source substrate as raw material for PHAs synthesis.The results showed that the mixed bacteria under the two selected environments had great substrate degradation ability and PHAs synthesis ability.Under normal nutrient ratio,the maximum PHAs content in feeding in the micro-oxygen section and external magnetic field were 30.87%DCW and 29.84%DCW,respectively,higher than that in blank condition(22.13%DCW).The maximum content of PHAs reached 63.05%DCW and 57.38%DCW under the unbalanced nutrient condition with C/N ratio of 150,respectively,which was higher than 46.58%DCW under the blank condition.However,with the increase of carbon source load,the mixed bacteria in R2 showed sludge bulking and the sedimentation became worse——SVI reached more than 80.But it was more stable in R3——SVI remained at about 30.At the stage of high carbon source load,the phosphorus removal capacity of the two enrichment systems decreased,and the maximum phosphorus release was only 14.45mg/L in R2 and 10.99mg/L in R3,indicating that the phosphorus accumulating bacteria lost their competitive advantage after environmental screening.It was also found that the mixed bacteria in R2 showed a new phosphorus absorption phenomenon——the phosphorus was immediately absorbed after the maximum phosphorus releasein within anaerobic stage.Experiments showed that the accumulation of PHAs was limited by using fermentation solution as substrate.The contents of PHAs synthesized in even-numbered fermentation broth were 17.13%DCW and 14.12%DCW under the condition of feeding in the limited-oxygen section and external magnetic field,respectively.And the conversion rate of substrate to PHAs reached 0.44 mg COD/mg COD and 0.46 mg COD/mg COD,respectively.The synthesis effect of PHAs in fermentation broth with substrate concentration of 1000mg/L was similar to that of influent COD of 600mg/L sodium acetate.Compared with acetate as the only carbon substrate,the proportion of PHV was increased because polysaccharide uptake in fermentation broth.After the screening of intensive conditions,the diversity of the enriched mixed bacteria decreased,and the simplification of the community structure meant that the function of the dominant bacteria developed towards the Unitary development of PHAs synthesis.Thauera(30.22%,aerobic PHAs synthetic bacteria)and Thiothrix(21.30%,denitrifying bacteria with PHAs synthetic ability)were the dominant bacteria enriched under the condition of feeding in the micro-oxygen section.Candidatus＿Competibacter(29.19%,a kind of denitrification bacterium with PHAs synthesis ability)and Ignavibacterium(10.75%,a kind of denitrification bacterium)are dominant bacteria enriched under applied magnetic field.Based on the slow rise of dissolved oxygen in the micro-oxygen section,a large number of bacteria with denitrification function were enriched under high carbon source load,which transformed the two activated sludge systems from phosphorus removal system to denitrification system.In the competitive relationship,PAOs are dominant under medium and low carbon source load,but the dominant bacteria turn to aerobic bacteria under the condition of feeding in the micro-oxygen section and GAOs in the external magnetic field environment.