Enhanced Coagulation By Ultrasound In Microcystis Aeruginosa-Laden Water:Effects And Mechanisms

Algae blooms caused by eutrophication has attracted a lot of attention in recent years.Although various measures on pollutant sources can clearly reduce eutrophication,recurrent eutrophication still exists in China and even around the world.Algal blooms are able to affect water ecology and function.In particular,the occurrence of algae blooms in source water would increase the difficulty of water treatment and affect the stability of drinking water treatment plants,as well as threaten the safety of drinking water due to the release of toxins and odorant substances.Ultrasonic treatment has attracted much attention because of its physical and chemical effects.There are many studies on sonication in controlling algae blooms and enhanced coagulation,but there are still no recommended parameter combinations.Based on the above research,this paper uses the dominant species,Microcystis aeruginosa,as the research object and concentrate on the effect of ultrasound on the subsequent processes,especially the coagulation and sedimentation.The main aspects are as follows:?1?Three typical coagulants?aluminum salt?PAC,Al₂?SO₄?₃?and iron salt?FeCl₃??for different algae-laden water treatment were studied through the jar-test.Results showed that the coagulation effects of the three coagulants were:FeCl₃>PAC>Al₂?SO₄?₃,and dosages were 80mg/L,10mg/L and 60mg/L respectively.The coagulation effects at different growth stages were:exponential growth phase>/?stationary growth phase 2>stationary growth phase 1.The results of jar-test on the different initial cell concentration showed that there were little difference in the remaining turbidity,and the sample with higher initial concentration had higher removal rate in turbidity and algae cell.?2?PAC with the dosage of 10 mg/L was selected.The jar-test was carried after ultrasonic treatment with different parameters to analyze the selection of ultrasonic parameters from the perspective of coagulation.Results showed that frequency,power and duration were all able to effect on the coagulation process.Among them,the effect of power and time were more significant,and excessive power and duration may cause an adverse effect on coagulation.It was recommended to treat at a lower power,and with less than 5 min irradiation time.The recommended ultrasonic parameters under the test conditions were:740kHz,0.02W/mL.?3?Further experiments were carried out on the change of morphology and substances of algae cells after sonication.The morphological changes of algae cells were observed by scanning electron microscopy.The substances changes were analyzed by three-dimensional fluorescence,the detection of protein and polysaccharide content,the changes of SUVA and zeta potentials of algae liquid.Results showed that under the experimental conditions,sonication did not cause algae cell lysis,but it induced changes in morphology and collapse of intracellular gas vesicle.Meanwhile,sonication caused changes of the protein content in AOM,especially in EOM,which directly affects the coagulation process.Further detection of free radical indicated that the effect of ultrasonic pretreatment on coagulation is the result of a combination of chemical and mechanical effects.?4?Ultrasonic-coagulation combined tests and pilot test showed that sonication can significantly increase the treatment effect at low doses of coagulants.The main mechanism of coagulation were bridging,sweeping and certain electrical neutralization in the case that the pH of the test raw water was alkaline.The pilot test with continuous operation for 4 hours showed that sonication can effectively enhance the removal of algae cells in the sedimentation stage?the removal rate of turbidity increased by about30%?and reduce the pressure of subsequent filtration.The research analyzed the influence of different ultrasonic parameters?frequency,power and duration?setting conditions on algae-laden water from the perspective of coagulation,and obtained the setting strategies of ultrasound parameters.Finally,a preliminary pilot test was carried out.This paper further explored the problem of ultrasonic parameter setting strategies when applying in drinking water treatment,which has certain significance for the expansion application of ultrasound.