Optimization Of The Removal Technology For Microcystin Pollution In Drinking Water Treatment Processes

The increasing frequency and intensity of cyanobacterial blooms is a growing environmental and human health concern. The most common toxins produced by cyanobacteria are microcystins (MCs), a class of hepatotoxic monocyclic heptapeptides. MCs have been the cause of several poisonings of livestock and wildlife around the world, and they also posed a health hazard for humans through the use of drinking water. When orally ingested MCs, they are actively absorbed to hepatic cells, irreversibly inhibit protein phosphatase1, subsequently leading to disruption of cell structures, intrahepatic hemorrhage and death. Since MCs are potent hepatotoxins, controlling of their levels in drinking water became a great important issue. Previous studies have suggested various techniques for the control of MCs in drinking water, such as coagulation, flocculation, filtration, activated carbon adsorption and disinfection. These treatment processes were efficient in removing cyanobacteria cells and internal MCs, but were unreliable for dissolved MCs due to the fragility of cyanobacteria cells (inappropriate operations probabily lead to secondary pollution). Consequently, it is still necessary for new or modified water treatment methods to eliminate these toxins. In this research, we focused on the "the optimization of the removal technology for MCs pollution in drinking water treatment processes", including the lysis of Microcystis aeruginosa in coagulation and sedimentation, the dewatering of algae containing sludge and MC-DBPs formation and toxicity in disinfection.This study has five chapters, just as follows:In chapter one, we described the physical and chemical characteristics, the generation mechanisms and the current pollution status of MCs; summarized the typical biotoxicities and mechanisms, the related water quality standards and detection methods of MCs; we also introduced the control strategies and technologies directed against nutrients, algae cells/microcystins in raw water, in conventional/deep treatments of drinking water. Based on literature review, we proposed the researching aims, meanings, methods and contents of our work.In chapter two, we simulated the coagulation-sedimentation process and assessed the effects of coagulant dose, shear, and floc storage time on the integrity of Microcystis aeruginosa FACHB-905, to effective control cyanobacteria lysis both in coagulation and floc storage processes. On this basis, we investigated the release of intracellular MCs, clarified the breakage mechanism of cyanobacteria cells, and optimized coagulant dosage, stirring parameters and floc deposition time. Under the optimum coagulation conditions for cyanobacterial cells removal (for AICl3, coagulant dose15mg/L, rapid mix250r/min for1min, slow mix20r/min for20min; for PACl, coagulant dose4mg/L, rapid mix150r/min for2min, slow mix40r/min for30min), all cells were removed intactly by the surface charge neutralization with AlCl3/PACl and there was no additional release of MCs into the treated water. The formation of AICl3flocs brought a protection for cyanobacterial cells and reduced their breakage in a certain degree, however, they should also be treated or disposed within6days to avoid the lysis of cells and additional release of MCs. While in the PACl coagulation-sedimentation process, PACl could destroy the protective effects of EPSs produced by M. aeruginosa cells, inducing obvious damage to the cells and leading to a large amount of MCs release above background concentration. This study is not only significant for the effective removal of cyanobacterial cells in natural blooms, but also instructive for the safe treatment of coagulation flocs (secondary pollution) in drinking water treatment plants.In chapter three, we further investigated the characteristics of algae lysis and microcystin release in the filtration of algae containing sludge formed in coagulation treatment. By evaluating filtration efficiency (time and average rate), turbidity and MCs concentration in different operating conditions, the influences of mechanical action and physical/chemical effects on algal cell integrity were explicated, and the operation conditions were also optimized for vacuum filtration. Experiments showed that sample loading volume had significant influence on sludge dewatering characteristics, sludge would gradually accumulated on the surface of filtration media, resulting in reduced filtration rate and algae lysis. In actual operation, the formation of sludge layer should be avoided. Hydrophilic filtration media with lower porosity should be choosen to enhance filtration efficiency and remove solid insoluble matter. Despite positive pressure filtration had a higher efficiency, it could induce the damage of the flocs and algae cells. In consideration of MCs release, vacuum filtration with low-destructibility should be choosen. With appropriate driving force (higher vacuum), algae containing sludge had higher filtration efficiency and stable MCs concentration and turbidity levels. Prolonging the storage time of sludge was conducive to improving the efficiency of sludge dewatering, but also enhanced MCs concentration and turbidity. For this reason, the storage time of algae containing sludge should be severely restricted in actual water supply factories (the storage times for PACl and AlCl3should not be more than2d and4d, respectively). This work gave deeper knowledge on the fate of intracellular MCs in the process of sludge dewatering, while concrete experimental results provided valuable references for algae containing sludge treatment and the re-utilization of sludge resource.The principal objective of chapter four was to provide an evaluation of the generative mechanism and biological toxicity of MC-DBPs involved in disinfection. The widespread and dangerous MCs, MCLR and MCRR, were selected as the target of disinfection treatment and its primary DBPs were identified by mass spectrometry, liquid chromatography/mass spectrometry and tandem mass spectrometry. In addition to the generative mechanism studies, the biological toxicity of MC-DBPs on protein phosphatasel (PP1) was evaluated by molecular toxicity experiments. Subject to disinfection, MCs could be reduced to the limit value (1.0μg/L) of WHO, but could also be oxidized to a variety of MC-DBPs. The types and distributions of DBPs were under the influence and restriction of MCs type, disinfectant dose and reaction time. With a comprehensive analysis of MC-DBP formation mechanism, it was not difficult to find MCs mainly subjected to the the addition reaction of Adda conjugated diene and dehydration reaction of some secondary products. Though most MC-DBPs had lower toxicity on protein phosphatase1than MCs, they still possessed certain biological toxicity. From the perspective of the drinking water safety, disinfection was valid regulate method for MCs, but the secondary pollution of MC-DBPs also deserved further attention. The evaluation technology on MC-DBPs established in this work was not limit to MCLR and MCRR, it also could be applied to other toxin types according to their distribution characteristics in raw water. This study offers valid technique support for MC-DBPs identifiation, contributes to a comprehensive cognition on their hazard, and thus has great significance to prevent and control the environmental risk induced by MCs.Finally, we summarized the research findings of above parts and discussed the future developments of the removal technologies on microcystin pollution in drinking water treatment processes. This study has enriched the research on the optimization for removal technology on MCs pollution in drinking water treatment processes, and provided some reference gists for the control of MCs and their potential biotoxicity.