| Proliferation of cyanobacteria is becoming a serious water quality problem in lakes and reservoirs worldwide.Many natural and artificial lakes have undergone eutrophication and suffered from blooms of harmful cyanobacteria.The presence of cyanobacteria and their metabolites in drinking water may degrade water quality and cause the production of unpleasant tastes and odours.More seriously,several cyanobacterial species are potent producers of a variety of toxins,poisonous to animals and humans.For toxic cyanobacteria removal,the coagulation-sedimentation process plays the most important role in drinking water plants.However,the coagulation and sedimentation of toxic cyanobacteria in conventional drinking water treatment is not satisfactory from other perspectives.The cyanobacteria and their metabolites can increase the electronegativity of the colloid surfaces,which increases the necessary dosage of cationic coagulant.On the other hand,the settling of cyanobacterial flocs in the sedimentation process needs a long time,due to the buoyancy of algal cells,which is adverse to efficient drinking water treatment.In addition,after coagulation cyanobacterial flocs settle in a sedimentation tank,forming cyanobacteria-containing sludge.By the influences of some physical,chemical and biological factors,the accumulated cyanobacterial cells may easily die and decompose during storage,releasing large amounts of toxins and other harmful substances into the sludge.It will cause the secondary pollution and affect the harmless disposal of cyanobacteria-containing sludge and the safe reuse of sludge water.In view of the poor sedimentation and low removal efficiency of cyanobacteria,quartz sand was used as ballasts to enhance the coagulation of harmful cyanobacteria by conventional inorganic coagulants(AICl3,FeCl3 and PAFC).The effects of quartz sand ballasted coagulation on the sedimentation of cyanobacterial flocs,the removal of cyanobacteria and their metabolites,and the behaviors of cyanobacterial cells during sludge storage were studied.Results showed that the quartz sand ballast can be incorporated into the cyanobacterial flocs during coagulation stage to increase the floc weight,effectively overcoming the problem of cyanobacterial cells'floatation,and significantly improving the sedimentation and removal of cyanobacteria flocs.Moreover,with the addition of quartz sand,the AlCl3,FeCl3 and PAFC doses could be reduced by 25%,10%and 40%,respectively.Compared with AICl3 and FeCl3,the combination of PAFC and quartz sand can obtain better cyanobacterial removal effect.At the optimal doses(1.5 mg/L PAFC and 50 mg/L quartz sand),all cyanobacterial cells can be coagulated and completely settled within 3 minutes.In the process of ballasted coagulation,quartz sand had different effects on the floc properties treated by different coagulants:quartz sand can reduce the particle size of flocs formed by AICl3 and FeCl3,but increase the size of PAFC flocs.Therefore,the simultaneous increase of the flocs density and flocs size was the reason why the combination of PAFC and quartz sand had better cyanobacteria removal efficiency.In addition,the quartz sand ballasted coagulation technology can capture more organic matter,including algal toxins,into the flocs during the coagulation stage,and improve thewater quality of the supernatant.At the same time,during sludge storage period,the flocs mixed with quartz sand has a stronger binding function for the organic matter,including algal toxins,in flocs,which can inhibit their release and guarantee the quality of the sludge water.However,excessive ballasted particles(200 mg/L quartzsand)will cause the premature rupture of cyanobacterial cells during sludge storage,which may be related to the increased collision frequency between excessive quartz sand and algae cells during the coagulation agitation phase(especially during the fast agitation phase).Due to the low level of Al,Fe concentration and stronger floc protection,the cyanobacterial cells in the flocs formed by the combination of PAFC and quartz sand can maintain integrity for a longer time.With the optimal doses(1.5 mg/L PAFC and 50 mg/L quartz sand)treated,cyanobacterial cells began to lyse after 10 days' storage,providing sufficient time for the safe disposal of algae-containing sludge.In view of the harm of cyanobacteria cells and their metabolites,this thesis explored the effects and mechanism of visible light catalyst N-TiO2 on the simultaneous degradation of cyanobacteria cells and their metabolites in water,and compared the behaviors of filamentous C.raciborskii and unicellular M.aeruginosa under the same photocatalytic degradation conditions.Results showed that upon adding 200 mg/L N-TiO2 to a 5×106 cells/mL suspension of C.raciborskii,the cells were completely destroyed within 20 hours under visible light.Meanwhile,94%of the CYN and 87%of the TOM in C.raciborskii suspensions were degraded during the photocatalytic process.After treatment the water quality was markedly improved.For the degradation of C.raciborskii cells,the ROS produced by N-TiO2 must attack the cell membranes to induce cell lysis,and it then oxidizes the released intracellular organic matter and CYN.During this process of treatment with N-TiO2,C.raciborskii always adheres to the surface of N-TiO2 particles,shortening the distance between reactive oxygen species and algae cells,thereby improving the visible light catalytic activity of N-TiO2.By the attack of ROS,C.raciborskii filaments were firstly degraded into short fragments,then the cell membranes became coarse and the cells'activity levels were significantly reduced.The filament lengths at hour 0 were evenly distributed from 0 to 300 ?m,but when the C.raciborskii cells had been treated for 4 hours over 80%of the filament lengths were shorter than 100 ?m.Although the treatment was relatively satisfactory,its degradation rate was remarkably lower than that of the M.aeruginosa,which had been widely studied.Under the same photocatalytic conditions,C.raciborskii cells completely lysed and released all intracellular cyanotoxins after 12 to 14 hours' treatment.For M.aeruginosa,this process only needed 8 to 10 hours,and the removal rate and degradation efficiency of MCs were also significantly higher than that of CYN.Therefore,compared with M.aeruginosa,the properties of C.raciborskii cells and their metabolites in the raw water were more stable,which may increase the difficulty of drinking water treatment.Based on the above findings,this thesis combines ballasted coagulation with visible-light catalytic oxidation through the replacement of traditional ballast by N-TiO2 particles to simultaneously solve the poor sedimentation and low removal efficiency of cyanobacteria in drinking water treatment and the secondary pollution of cyanobacteria and its metabolism in sludge.Results showed that Cvanobacterial cells(1 ×106 cells/mL)can be thoroughly removed by dosing 7.5 mg/L PAFC and 200 mg/L N-TiO2 powder.In this process,80%to 85%algal cells can be separated from supernatant just in the slow agitation phase of coagulation,all the algal cells were able to rapidly settle within only 10 minutes after coagulation and the removal efficiency was as high as 97%.Compared to conventional drinking water treatment,the required PAFC dose was reduced to about half and the appropriate dose of N-TiO2 will not cause the issue of N-TiO2 residual.After coagulation and sedimentation,the cyanobacteria-containing sludge that is formed can be 'self-purified' under visible light due to the photocatalyst powder incorporated into cyanobacterial flocs by ballasted coagulation.During this period,sludge in containers with large free-surface area and low height("L-beaker")can be most effectively purified through photocatalysis due to the high-efficiency utilization of optical energy.In the L-beaker,most cyanobacterial cells in sludge can be destroyed within 8 hours.99%of total MCs,84%of TCOD and 85%of MLVSS can be degraded within 32 hours and their final contents were reduced to safe levels,which permits direct reuse for drinking water production.When the purified sludge was directly added to raw cyanobacteria-contaminated water,nearly half of the dosed coagulant in the initial treatment was still active.On the other hand,the reused photocatalyst can still effectively purify the newly formed cyanobacteria-containing sludge to a safe condition and the photocatalytic efficiency was just slightly lower than the initial treatment.In this thesis,the removal and mechanism of harmful cyanobacteria treated by ballasted coagulation technology and visible-light catalytic oxidation technology were systematically studied,and the two technologies were successfully combined to simultaneously achieve the purification of algae-containing water and the degradation and reuse of algae-containing sludge,which provides a promising idea for the removal of cyanobacteria in drinking water. |
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