Research On The Environmental Safety Of Water Bloom Control By Flocculation

The frequency of cyanobacteria blooms has been increasing in recent years in China, which is attributed to the eutrophication of fresh water bodies. Cyanobacteria blooms pose a series of problems on the functions of water body, and even threaten human health. Numerous techniques have been studyed for the control of water bloom. Flocculation is an efficient and inexpensive technique and has been widely used in water treatment. Previous studies showed that it can be used to control water blooms, especially with the natural or moditied clays as flocculator. However, the algal cells are not removed completely from water bodies after flocculation, and they still accumulate in the sediment of water bodies. The decomposition of the algal cells in the sediment, or the release of algal toxin may cause new environmental problems. It is necessary to investigat the environmental safety of water bloom conrol by flocculation. In this thesis, we studied the distribution and decomposition of the Microcystis aeruginosa in the water-sediment phase, and the release and degradation of Microcystin (MC) in the sediment to evaluate the effectiveness and environmental safety of this process by simulating the process of using flocculation in emergent control of algal bloom. The main content and results were as follows:1. The optimization method for extracting MC in sedimentsThe extraction method of MC from sediment is a precondition for the research of MC. In order to extract MC from sediment effectively, it's necessary to optimize the extraction method. The concentration of methanol, volume of extraction reagent, extraction time, number of extractions was optimized in this study. The result showed that optimal extraction method was:75% of methanol,7 mL,20 min, repeat three times, and the extraction yield could over 95%.2. Research on the environmental safety of water bloom control by flocculation under aerobic conditionsThe distribution and decomposition of Microcystis aeruginosa cells, and the release and degradation of intracellular microcystin under aerobic conditions were investigated through simulation experiments to evaluate the efficiency and environmental safety of this treatment process. Results showed that cells were efficiently removed from water column with polymeric aluminum chloride (PAC), sepiolite or both of them, amd the removal rate were more than 99%. Furthermore, PAC together with sepiolite effectively prevented the recruitment of deposited cells. The cells in sediments decomposed gradually after 24-28 days. Flocculation process neither accelerated cells decomposition nor slowed cell death. MC was degraded to be below the detected limit within 20 to 28 days after nine days delay. Flocculation had no negative effect on the release and degradation of MC, nor the TOC and TN concentration in the water phase. But in the late stage of experiment (about 20 days), TN and TOC concentration increased significantly, it was need more attention.3. Research on the environmental safety of water bloom control by flocculation under anaerobic conditionsThe distribution and decomposition of the cells, and the release and degradation of intracellular microcystin under anaerobic conditions were investigated to evaluate the efficiency and environmental safety of this treatment process. Results showed that cells were degraded to be below the detected limit within 24 days after eight days delay under anaerobic conditions. Flocculation process had no significantly affected on the decomposition of cells. The rapid decomposition of cells favored the removal of harmful algae. Furthermore, PAC and PAC together with sepiolite effectively prevented the recruitment of deposited cells. MC was degraded to be below the detected limit within 24 days after eight days delay under anaerobic conditions. Ninety percent of MC was degraded in four days, demonstrating that the microorganisms can efficiently degrade MC under anaerobic conditions. TN and TOC concentrations in the water phase would increase significantly (after 20 days), it was need more attention.