The Effect And Mechanism Of The Characteristics Of Algae-derived Organic Matter On The Coagulation-ultrafiltration Process

In recent years,with the rapid development of industry,a large amount of nutrients such as nitrogen and phosphorus in wastewater have been discharged into surface water,making the problem of eutrophication in our country increasingly serious.Coagulation-ultrafiltration technology is widely used in the treatment of algae-containing water due to its unique advantages（excellent treatment effect and stable effluent quality）.Good coagulation efficiency can not only effectively remove algae,but also alleviate membrane fouling in the subsequent ultrafiltration process.In this research,the most common and harmful cyanobacteria in the water body were selected,and the effect of the organic matter produced by cyanobacteria on the coagulation-ultrafiltration process was studied.The properties of the organic matter,flocs and membrane fouling during the coagulation-ultrafiltration process were studied.Through investigation,the influence mechanism of organic matter on the coagulation-ultrafiltration process of different substances were obtained.In addition,this study also used two agents commonly used in water plants:sodium hypochlorite and activated carbon to explore the effect of sodium hypochlorite-activated carbon process on the coagulation-ultrafiltration process of algae and its organic matter.The main research contents and results are as follows:（1）The interaction between cyanobacteria extracellular organic matter（EOM）and humic acid（HA）in the coagulation-ultrafiltration process was studied.The results showed that the coexistence of EOM and HA can improve the removal efficiency of two kinds of organic matter by coagulation and can alleviate membrane fouling caused by EOM.Zeta potential and Fourier transform infrared spectroscopy analysis showed that the interactions of EOM and HA can strengthen charge neutralization and reduce CH₂ chemical bonds,which had a positive effect on the coagulation process.EOM-HA floc had a more open and looser structure than EOM floc,which was more favourable in the ultrafiltration process.In addition,the XDLVO theory indicated that the coexistence of HA and EOM mainly reduced the acid-base（AB）interaction energy between pollutants and ultrafiltration membranes,thereby effectively alleviating membrane fouling.The beneficial interaction between EOM and HA can help to fully understand the impact of natural organic matter on the water treatment process.（2）The effect of EOM produced by two typical cyanobacteria:M.aeruginosa and Pseudoanabaena sp.on its ultrafiltration process was studied.The results showed that the EOM of two cyanobacteria had a broad molecular weight（Mw）distribution and the irreversible membrane fouling was basically caused by EOM.Moreover,humic acid and microbial metabolites were major components of EOM of two typical cyanobacteria.Since EOM could fill the voids of cake layers formed by the algal cells,EOM and algal cells played a synergistic role in membrane fouling.Fourier transform infrared spectroscopy analysis indicated that the CH₂and CH₃ chemical bonds played an important role in membrane fouling caused by EOM.The cake layer formed by the algal cells also could trap the organic matter produced by algae and alleviate some irreversible membrane fouling.The results also showed that although the cake layer formed by the algal cells caused severe permeate flux decline,it could play a double interception role with UF membrane and increase organic matter removal efficiency.Therefore,when using UF to treat algae-laden water,the balance of membrane fouling and organic matter removal should be considered to meet the needs of practical applications.（3）This study also used two agents commonly used in water plants:sodium hypochlorite and activated carbon to explore the effect of sodium hypochlorite-activated carbon process on the coagulation-ultrafiltration process of algae and its organic matter.The results showed that the rupture degree of different algae was different under the same sodium hypochlorite dosage and the rupture of Pseudoanabaena sp.was more serious due to the different cell membrane and EOM properties of M.aeruginosa and Pseudoanabaena sp..For M.aeruginosa,all combinations of sodium hypochlorite and activated carbon could basically improve the removal efficiency of organic matter and reduce the turbidity of the coagulation effluent.For Pseudoanabaena sp.,when the dose of sodium hypochlorite was higher than 1.5 mg/L,the degree of algae rupture was more serious,the removal efficiency of organic matter could decrease and the turbidity of the coagulation effluent could also increase to varying degrees.Pretreatment with only the addition of activated carbon can alleviate the membrane fouling in the early stage of ultrafiltration.However,the addition of sodium hypochlorite accelerated the rate of permeate flux decline and aggravated the membrane fouling.