The Migration And Transformation Of Cylindrospermopsis Raciborskii And Its Toxins During PAFC Coagulation

With the aggravation of water eutrophication,cyanobacteria bloom frequently break out in freshwater sources such as lakes,reservoirs and rivers,seriously threatening the safety of drinking water.In recent years,C.raciborskii has spread from the tropics and subtropics to temperate zones owing to the global warming and its strong physiological adaptability.The frequent C.raciborskii blooms in freshwater has gradually received widespread attention.C.raciborskii can produce toxic substances,such as CYNs.CYNs as a kind of hepatotoxin can cause toxic damage to liver,kidneys and thymus.If the toxin entered into drinking water,it would undoubtedly pose a serious threat to human health.However,as one of toxic cyanobacteria in many drinking water sources,it has not been drawn proper consideration in drinking water treatment plants so far.Coagulation-sedimentation is usually the effective unit operation for removing cyanobacterial cells in typical drinking water treatment plants.At present,researches on the removal of cyanobacteria in the coagulation process are mainly focused on Microcystis aeruginosa.But C.raciborskii's behavior during this process has still been shrouded in mystery.Researches on the migration and transformation of C.raciborskii in drinking water coagulation process are very little.It is noteworthy that C.raciborskii has some big differences from M.aeruginosa,such as in their morphology,physiology and toxicity.Therefore,the migration and transformation characteristics of C.raciborskii in the coagulation process might different from that of M.aeruginosa.In this situation,the efficient elimination of intact C.raciborskii cells and illustration of the toxin release mechanism during coagulation process are of critical importance for water treatment plants to ensure the safety of drinking water.This study simulated the water of high concentrations of C.raciborskii in the period of blooming and the C.raciborskii-containing water was treated with the polyaluminum chloride(PAFC)coagulation process.The algae removal efficiency and the physiological changes of cells were investigated during the coagulation process.The toxin release rule and physiological response mechanism of C.raciborskii were classified in sludge storage process.What's more,this study revealed the differences of the migration and transformation characteristics in the coagulation process between C.raciborskii and M.aeruginosa.The main findings are as follows:(1)In this study,the optimum coagulation conditions for the effective removal of C.raciborskii cells(106 cells/mL)are PAFC dosage 10 mg/L,rapid mixing for 1.5 min at 200 r/min and slow mixing for 25 min at 20 r/min.Standing for 40 min after coagulation.(2)C.raciborskii cells were effectively removed intactly under optimum coagulation conditions.PAFC at higher dosages(>10 mg/L)triggered additional cylindrospermopsins release.The mechanical agitation applied in the coagulation process has little effect on the C.raciborskii cells.(3)After coagulation,a large number of Cylindrospora cells were transferred into the sludge.In sludge storage,the coagulated C.raciborskii cells suffered severe oxidative damage,leading to significant cylindrospermopsins release after 6 days.It suggests that the sludge should be properly disposed within 6 days in order to reduce the secondary pollution by metabolites such as toxins.(4)C.raciborskii manifested different behaviors from M.aeruginosa during coagulation process:excess PAFC dosages induced extra CYNs release from C.raciborskii while the release of MCs from M.aeruginosa was not effected by PAFC dosage in coagulation.In sludge storage period,coagulated C.raciborskii cells were subjected to cellular damage and thus releasing CYNs from sludges after 6 d storage.In contrast,M.aeruginosa cells with PAFC coagulation had no obvious MCs release during sludge storage period.The different physiological responses of the two species are mainly due to the physiological and morphological differences between C.raciborskii and M.aeruginosa.C.raciborskii had a higher risk of toxin release in water production process than M.aeruginosa,which should deserve more attention.