Investigation On Coagulation Behavior Of Polymerized Zirconium For Chloride Treating High Algae Water

In recent years,water eutrophication occurs frequently,and the deterioration of water quality caused by algae pollution poses a threat to drinking water safety.Therefore,it is urgent to develop high-efficiency,low-cost and green algae removal technology.Coagulation technology,as the most common water purification technology in drinking water treatment,can effectively remove most suspended substances including algae and a small part of soluble substances in water.Based on the biological affinity,excellent coagulant performance and wide distribution of zirconium,a new inorganic coagulant,Polyzirconium Chloride（PZC）,was prepared in this study.The coagulant behavior and mechanism of PZC in treating high algal water were studied.In this study,Microcystis aeruginosa,a common freshwater algae bloom species,was used as the experimental template algae species to systematically explore the coagulation effect and behavior of PZC in the treatment of high algal water in different growth periods,and the coagulation mechanism of PZC in algae removal was analyzed according to Zeta potential,SEM and other means.The flocs’growth rate,particle size,strength factor and recovery factor were studied by on-line monitoring of the PZC coagulation process.Finally,the effect of PZC combined withultrafiltration on treating algal water in different growth periods was investigated.Specific research contents and conclusions are as follows:（1）The effects of the alkalinity of the coagulant,the dosage,the type of the coagulant and the initial p H value of the water sample on the coagulation behavior of Microcystis aeruginosa in different growth periods were studied.The results showed that PZC was a highly efficient coagulant,and had a very significant coagulation effect on the Microcystis aeruginosa in each growth period,especially the removal rate of UV₂₅₄,TOC and fluorescent organic matter was much higher than that of PACl.The optimal basicity of PZC in logarithmic phase,stable phase and decay phase is 2.0,2.0 and 1.0,respectively.Compared with the algal water in the logarithmic phase and the stable phase,the optimal dosage of PZC coagulant in the decay phase increased significantly,and the optimal dosage of PZC in the three phase were 90 mg/L,100 mg/L and 190 mg/L,respectively.The optimal initial p H value of the three growing phase changed from neutral（7.0）to slightly alkaline（8.5）and then to strongly alkaline（9.0）.The Zeta potential values of the effluent under the optimal coagulation conditions of the algal water in each growth phase were all negative,which indicated that the coagulation mechanism of the algal water treated by PZC in different growth phase was not only electric neutralization,but also other mechanisms,such as net trapping and sweeping,which also played a coagulation role.（2）The coagulation dynamic process of PZC at high,medium and low doses and PACl under optimal conditions were monitored online by Mastersizer 3000 laser particle size analyzer,and the integrity of algal cells was observed by fluorescence microscopy and scanning electron microscopy.The results showed that PZC could not damage the integrity of algal cells during each growth phase,and avoided the release of algal toxins and other toxic substances caused by cell rupture.Compared with the optimal dosage of PACl,the supernatant of PZC with the optimal dosage of PZC was almost completely clarified after 30min of coagulation precipitation,and no redundant algal cells were suspended in the water,which indicated that the flocs generated by PZC were large and dense,and easier to aggregate and precipitate.Compared with the logarithmic phase and the decay phase,the flocs size and growth rate were higher in the stable phase treated by PZC coagulation.The Strength factor and Recovery factor of flocs after PZC coagulation decrease with the increase of growing period.（3）PACl was used as the control to study the effects of different dosage of PZC in the treatment of high algae water at different growth phase in the coagulation-ultrafiltration process,and the membrane fouling situation before and after coagulation was compared from two aspects of membrane flux and membrane fouling resistance distribution.The results showed that compared with the raw solution which was not pretreated by coagulation,the membrane filtration performance was improved and membrane fouling was reduced due to the removal of a large amount of organic matter in the high algae water after the pretreatment with zirconium salt,and the alleviating effect of PZC on membrane fouling was significantly higher than that of PACl.The membrane fouling of algal water in the logarithmic phase and stable phase is mainly the formation of external resistance,the formation of surface filter cake layer is the main reason to inhibit water samples from passing through the ultrafiltration membrane,and the irreversible internal fouling is mainly formed for the algal water in the decay phase.