| Polyhydroxyalkanoate(PHA),a kind of biopolyester,can be obtained by biosynthesis by more than hundreds of microorganisms in nature,with high molecular as well as similar properties to traditional chemical plastics after extraction and recovery.Besides,PHA also has good biodegradability,which makes it is considered to be an advantageous substitute that is expected to replace the traditional chemical plastics derived from petroleum.Widely concern and studies have been carried out since it was founded,but its high production cost limits its large-scale application.The factors that determine the production cost of PHA are mainly fermentation production cost and extracted recovery cost.In the past three decades of the development of activated sludge synthetic PHA technology,large numbers of studies have solved lots of bottleneck problems limiting the output of activated sludge synthetic PHA,proving that the activate sludge PHA production mode was more effective to reduce the fermentation production cost than the pure culture mode.So,when the restricting problem,the output of PHA synthesis of activated sludge PHA synthesis had been developed and effectively solved,the extracted recovery cost of PHA will become a prominent factor restricting the large-scale development of activated sludge PHA synthesis.However,most of the previous research reports on the PHA extraction from activated sludge synthesis adopted the PHA extraction process in the pure culture mode,which was not suitable for the PHA extraction from activated sludge.Because the microbial composition structure of activated sludge is more complex than that of the pure culture system,it usually requires more energy consumption,reagent loss,and a longer extraction cycle.And it is hard to reach the ideal recovery effect like the pure culture mode at the same energy consumption level.Besides,halogenated solvents,the solvents could pollute the environment and be toxic and harmful to operators,are often used as PHA extractants in the pure culture PHA extraction process,which is inconsistent with the current social concept of green environmental protection and sustainable development.Therefore,it is necessary to develop a green and effective PHA extraction process for activated sludge and increase its related research efforts.The research purpose of this subject is to take the activated sludge of synthetic PHA as the research object,reduce the energy consumption of the PHA extraction process and shorten the PHA extraction cycle as the starting point,compare and optimize the process parameters as well as extraction performance of activated sludge PHA extraction process,to explore a simplified and feasible PHA extraction process that can be produced on a large scale.Two methods of extracting PHA from activated sludge were studied.One is the organic solvent extraction method,which uses a green organic solvent named dimethyl carbonate(DMC)with a low boiling point and less environmental pollution as the extraction agent of PHA to recover PHA from activated sludge.The second one is the chemical digestion method,that is,the chemical agent’s sodium hypochlorite and sodium dodecyl sulfate(SDS)is used for the compound digestion operation: sodium hypochlorite is used to decompose the cell wall and the membrane of activated sludge in the pretreatment stage,so that the content in the cell could be cracked and released,and the emulsifying performance of SDS is used to promote the dissolution of nonPHA components into water-soluble substances,then recover the PHA after the post purification.By detecting and calculating the recovery and purity of PHA,the experimental parameters are further optimized,and the optimized working conditions are amplified to verify the effectiveness of the two extraction methods in this experiment.The experimental results showed that the recovery and purity of PHA obtained by organic solvent extraction method using dimethyl carbonate(DMC)as extractant are high.The optimal working condition was: pretreatment with 25%(V / V)sodium hypochlorite for 30 min,adding activated sludge and DMC with the solid-liquid ratio of 0.025,reacting the mixture at 90 ℃for 60 min,post-treatment and purification with water or methanol for 5min,the recovery of PHA was 89.03%,and the purity of PHA product was 98.92%,The recovery performance was similar to that of chloroform solvent.The purity of PHA products obtained by the chemical digestion method using SDS as a digestion agent was about 70%.Through the experiment,the optimal working conditions are as follows: 10 m L concentration of 25%(v/v)sodiumhypochlorite for 30 min pretreatment,activated sludge dry weight: SDS dosage = 1:2 for 30 min chemical digestion,and the weight ratio of PHA extract and methanol dosage used in the posttreatment purification step was 2:1.The experiment showed that in the PHA extraction process of activated sludge,pretreatment operation and post-treatment purification operation were necessary.These two steps can greatly improve the extraction purity of PHA.The comparative analysis of the performance parameters of PHA extracted samples shows that the PHA samples obtained by organic solvent extraction with DMC were transparent membrane,had similar mechanical properties to polypropylene,good flexibility,and good thermoplastic properties.The PHA sample obtained by chemical digestion and extraction with sodium hypochlorite + SDS was in white powder form.Although it was not suitable for thermoplastic applications,it could be used as an intermediate platform for chemicals or be used in the production of other non-thermal polymers.Based on the above analysis,the two methods have their own advantages in cost and sample performance.In actual production,the corresponding extraction method can be selected in combination with the production cost,target product demand and production cost budget. |
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