Research And Application Of M-cresol Degradation In Wastewater Via Microalgae

M-cresol as a methylated derivative of phenol is a representative substance in phenolic compounds.And it is listed as one of the priority pollutants in China due to its high toxicity and corrosivity.M-cresol is more toxic than phenol and it can be harmful to humans and the environment at lower concentrations.Removal of phenolic compounds from wastewater by microalgae has the advantages of fast,stable,high efficiency and low energy consumption.It can convert phenolic compounds or other refractory organic compounds into non-toxic substances by means of biological enrichment and biodegradation,thus meeting the discharge requirements of chemical wastewater.In this study,the artificial resorcin simulated wastewater was taken as the research object,the effects of dominant algae species and initial algae density,initial medium pH value and initial m-cresol concentration on the biodegradation of m-cresol by microalgae was studied;and the changes of tolerance and removal efficiency of m-cresol by Chlorella pyrenoidosa before and after acclimation were compared;the metabolic pathway of m-cresol by Chlorella pyrenoidosa was explored and algae-membrane reactor was designed and built to investigate the effects of influent concentration of m-cresol and hydraulic residence time on the operation of the reactor.The conclusions are as follows:?1?Four microalgae with good degradation ability to phenolic substances were selected:Microcystis aeruginosa,Chlorella pyrenoidosa,Scenedesmus obliquus and Anabaena,which biodegraded m-cresol,respectively;the tolerance and degradation ability of four microalgae to m-cresol were observed and Chlorella pyrenoidosa had the best degradation efficiency of m-cresol.The growth of Chlorella pyrenoidosa and the removal efficiency of m-cresol under different initial algae densities(OD₆₀₀=0.2,0.4,0.6,0.8,1.0),different initial pH?pH=5.0,6.0,7.0,8.0,9.0?and different initial m-cresol concentrations?100 mg/L,200 mg/L,300mg/L and 400 mg/L?were studied.It was found that when the initial algae density OD₆₀₀=0.8,m-cresol could promote the growth of Chlorella pyrenoidosa,and 100 mg/L m-cresol could be completely degraded within 4 days.When the initial pH of the medium was 7.0,the density of Chlorella pyrenoidosa was 1.79 times of the initial algae density on the sixth day,and the removal rate of m-cresol was 100%,while the removal rate of m-cresol under acidic conditions was only 28%.The initial concentration of m-cresol has a great influence on the growth of Chlorella pyrenoidosa.The higher the initial concentration of m-cresol,the stronger the toxicity to Chlorella pyrenoidosa,and the longer the adaptation period of C.pyrenoidosa to the degradation of m-cresol.Through a series of experiments,it was finally determined that the optimum initial algal density of degrading m-cresol was OD₆₀₀=0.8,the optimum initial pH of the medium was 7.0,and the optimum initial m-cresol concentration was 100 mg/L.?2?The effects of algae domestication on the growth of Chlorella pyrenoidosa and the degradation of m-cresol were studied.The results showed that the adaptation period of the domesticated Chlorella pyrenoidosa was shortened,the density and chlorophyll a content of the algae were significantly increased;the chlorophyll a content of the algae under 300 mg/L and 400 mg/L m-cresol stress was 1.98 and 3.98 times higher than that of the non-domesticated algae,respectively.The tolerance of the domesticated algae to m-cresol increased,and the maximum lethal concentration of m-cresol increased from 500 mg/L to 600mg/L.Chlorella pyrenoidosa can completely degrade 400 mg/L m-cresol in 3-5 days,which indicates that domestication of algae species can effectively improve the degradation ability of microalgae.?3?The metabolic pathway of m-cresol in Chlorella pyrenoidosa was investigated.By using the qualitative analysis functions of FT-IR and GC-MS,the formation of intermediate products such as 3-hydroxybenzoic acid and gentian acid was detected during the metabolism of m-cresol.It was speculated that m-cresol might be degraded by gentian acid pathway.M-cresol first metabolized to 3-hydroxybenzoic acid under the action of phenol hydroxylase,and then it enters the gentian acid pathway for further cleavage,eventually forming pyruvate or fumaric acid to enter the glycolysis and tricarboxylic acid cycle?TCA?.?4?The algae-membrane reactor was designed and built,and the effect of m-cresol influent concentration and hydraulic retention time on the operation of the reactor was studied.The results showed that the removal efficiency of m-cresol and COD decreased with the increase of influent concentration of m-cresol,and increased with the extension of hydraulic retention time.When the influent concentration of m-cresol is less than 200 mg/L or the hydraulic retention time is greater than or equal to 3 days,the removal rate of m-cresol and COD can reach more than 90%.Its showed that the algae-membrane reactor was designed and built had a higher removal efficiency for m-cresol wastewater.After the reaction,the fouling status of membrane filaments was characterized,and it was found that the wastewater and microalgae particles in the reactor only slightly contaminated the surface of the membrane,and had little effect on the interior of the membrane pore.