| The fresh water is vulnerable to harmful seasonal algal blooms,and the effect of algal blooms on drinking water safety has caused raising concerns in China.Traditional coagulation method for algal pollution removal has many disadvantages including the excessive residual aluminum,excessive chroma and the release of algal toxin.To solve this,CaCO3 crystallization method has been recently developed and widely adopted for algal pollution decontamination in the drinking water.In this study,CaCO3 crystallization method was introduced in Microcystis aeruginosa removal process.The removal performances under different experimental conditions have been investigated systematically,in addition,Microcystis aeruginosa removal mechanism was proposed.The research results will provide a new idea for efficient and green algae removal in drinking water treatment.The main conclusion of this research including:(1)The Microcystis aeruginosa removal rates could remain stable when using CaCO3 crystallization method.When the algal density was lower than 5×105 cell/mL,the dosage of Ca2+was 4 mM,the C/Ca ratio was 1.0,and the pH value was 9,the removal rate of algal cells was more than 98%.Even in the severe blooms(algal density was 2×106 cell/mL),the removal rate could also reach 84%.In contrast,adding Ca2+alone has little effect on algae removal.C/Ca ration and supersaturation index(SI)were demonstrated could greatly affect the removal performance.The increase of C/Ca ration and SI could result in the enhancement of algal removal.(2)The high-performance algal pollution removal in CaCO3 crystallization method could be attributed to the characteristics of the generated CaCO3 crystallization.The positively charged CaCO3 crystal particles have an electrically neutralizing effect on algal cells.The aggregation and sedimentation of CaCO3 crystal particles can trap and sweep the algal cells.When the SI is low,the removal of Microcystis aeruginosa by CaCO3 crystallization is mainly because of electric neutralization,when the SI is high,the removal is mainly dominated by net trapping and sweeping.In addition,CaCO3 crystal particles have high density,and can also be used as weighting agent to promote the sedimentation of algal flocs.(3)Compared with alumina salina flocs,the advantages of crystal algal flocs including compactness,low water content,excellent sedimentation performance and dehydration performance.Under the conditions of initial algal density of 2×106 cell/mL and algal removal rate of 80%,the flocs density,water content of 97.6%,two-dimensional fractal dimension of 1.6,settling rate of 0.39 cm/min and capillary water absorption time(CST)of 56 s were 1.10 g/mL,97.6%,1.6 g/mL,0.39 cm/min.The corresponding crystalline algal flocs were about 1.40 g/mL,71.4%,1.8,0.82 cm/min and 31 s,respectively.(4)Influencing factors such as C/Ca ratio,SI and initial algal density could greatly affect the characteristics of crystalline algal flocs.With the increase of C/Ca ratio and SI,the flocs of crystalline algae tend to be compacted,the particle sizes were increased,and the sedimentation and dehydration properties were gradually improved.The increase of cell density will lead to the increase of water content and the decrease of density of crystalline algal flocs,while the sedimentation and dehydration performance will gradually deteriorate.(5)The algal pollution removal mechanisms were related to the characteristics of crystalline algal flocs.Compared with electroneutralization,the crystalline algal flocs produced by net trapping and scanning have larger particle size,lower water content,better sedimentation and dehydration performance.At high SI(2.41),the main removal method was net trapping and sweep,and at low SI(2.06),the main removal method was electric neutralization.The particle size,water content and CST of crystallized algal flocs reached 240.5 μm,71.4%and 30 s for the former,and 152.0 μm,84.4%and 35 s for the latter,respectively.(6)The crystallization product was demonstrated was vaterite based on the XRD analysis results,and the grain diameters were in the range of 2-4 μm. |
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