| The traditional sewage treatment technology has complex process flow and high cost,and it is difficult to effectively utilize the nutrients in the sewage.However,microalgae are highly adaptable to the environment,easy to cultivate,have good tolerance to different concentrations of sewage,and can effectively absorb organic matter,nitrogen,phosphorus and other substances in the sewage during the growth process.Coupling sewage treatment with microalgae cultivation can effectively treat sewage and reduce the cost of sewage treatment.The harvested microalgae can be used as raw materials for the production of biodiesel,nutrients,medicines,cosmetics,etc.This technology has a broad development and application prospects.However,the harvesting of microalgae is a difficult problem that restricts the large-scale application of the coupling technology of microalgae cultivation and sewage treatment.Although there are many microalgae harvesting processes,most of them have problems such as high cost and low efficiency.In this study,we selected Chlorella pyrenoidosa,which is capable of both photosynthetic autotrophic growth and heterotrophic growth of organic matter in the environment,strong environmental adaptability and high oil content,and domesticated and cultivated it by using the sewage from the sewage treatment plant.The domesticated algae species were inoculated into three types of sewage: the effluent from the primary sedimentation tank,the effluent from the secondary sedimentation tank,and the sludge digestion liquid of the sewage treatment plant.The growth of Chlorella in three kinds of sewage and the treatment effect of organic matter,nitrogen and phosphorus in three kinds of sewage were systematically compared.After the completion of the experiment,the microfiltration membrane was used to harvest the chlorella in the three kinds of sewage,and the membrane pollution problem in the process of the microfiltration membrane to harvest the chlorella was studied.The main research content and research results are as follows:(1)The pollution tolerance of domesticated Chlorella was greatly improved.The domesticated algae species were inoculated into the effluent of the secondary sedimentation tank,the effluent of the primary sedimentation tank,and the sludge digestion solution according to the inoculation ratio of algae species: sewage = 1:9.Chlorella in the effluent of the secondary sedimentation tank and the effluent of the primary sedimentation tank could grow for about 4 days and 11 days respectively,while the chlorella in the sludge digestion fluid could still grow normally at the end of the experiment,the growth period was the longest,and the biomass obtained was the highest.The extracellular organic matter(EOM)secreted by microalgae during the growth process is the main substance that cause membrane fouling.The total EOM concentration when the dry weight of Chlorella cells in the effluent from the secondary sedimentation tank,the effluent from the primary sedimentation tank and the sludge digestion reached the maximum was 52.86 mg/L,107.68 mg/L,and 154.28mg/L,respectively.During the growth of Chlorella,algae cells can accumulate oil.At the initial stage of inoculation,the oil content of algae cells in the three types of sewage decreased,and at the 7th day of cultivation,the content of microalgae oil in the three types of sewage increased.At the end of the culture,the oil content of algae cells in the effluent from the secondary sedimentation tank,the effluent from the primary sedimentation tank,and the sludge digestion solution were 21.21%,20.72%,and 15.43%,respectively.Although the oil content of Chlorella in the sludge digestion solution was low,the total oil content of the harvest was higher due to the high concentration of Chlorella.(2)Chlorella has a good and stable removal effect on ammonia nitrogen,total nitrogen,phosphate and total phosphorus in sewage.The highest removal rates of ammonia nitrogen in the effluent of the primary sedimentation tank,the effluent of the secondary sedimentation tank,and the sludge digestion solution could reach 99.29%,94.58%,and 87%,respectively;the total nitrogen removal rate could reach 99.45%,80.14%,and 82.33%,respectively;phosphate removal rate could reach 98.29%,92%,91.73%,respectively;the total phosphorus removal rate could reach 97.35%,85.45%,86%,respectively.However,the removal effect of chlorella on nitrate nitrogen and COD in sewage was not ideal.The study found that the removal rates of ammonia nitrogen,total nitrogen,phosphate,and total phosphorus in the effluent from the secondary sedimentation tank reached 94.58%,80.14%,92%,and 85.45% within four days of inoculation.After four days,chlorella could not grow normally.Therefore,when chlorella is used for advanced treatment of the effluent from the secondary sedimentation tank,it is possible to consider shortening the chlorella culture period.(3)Using microfiltration membrane to recover the chlorella in the effluent of the primary sedimentation tank,the effluent of the secondary sedimentation tank,and the sludge digestion solution.Through the critical flux experiment,continuous filtration experiment,FTIR and SEM analysis,it was proved that EOM was an important substance that causes membrane fouling.The higher the EOM content,the more serious the membrane fouling.Protein and polysaccharide are the main substances of EOM.By measuring the reversible and irreversible EOM content on the membrane,it was found that compared with polysaccharide,protein was more likely to be adsorbed on the membrane and cause membrane fouling. |
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