The Efficient Removal Of Cyanobacterial Cells By Polyaluminum Ferric Chloride Coagulation With Low Cells Damage

The natural eutrophication is becoming more and more serious because of human activities. The outbreaks of cyanobacteria algae bloom are more and more frequent and expand. The source of drinking water is under the threat of cyanobacteria blooms. The safety of drinking water attracts more and more attention. Large amount of algae cells and toxic metabolites release led to the severe deterioration of water quality, which threatened the life health of human beings. Microcystis aeruginosa is the common and important dominant strains in the water body occurred cyanobacterial blooms. In addition, Microcystis aeruginosa is the main algae strains producing toxin which is the most toxic microcystins. Most MCs contained in cyanobacterial cells, thus to eliminate the toxin release, it is important to remove intact M. aeruginosa cells. When the high algae-laden water is treated in drinking water plant, the integrity of cyanobacteria cell and the release of algal toxins should be concerned.Conventional coagulation treatment, the key unit in removing cyanobacterial cells in drinking water treat plants (DWTPs), is efficient in removing cells. In this paper, polyaluminum ferric chloride (PAFC) was used to coagulate the high algae raw water, and the optimal coagulation conditions were obtained. Under the best operating conditions, the integrity of the Microcystis aeruginosa cells was investigated during coagulation and flocs storage period. The cells viability, extracellular microcystins, scanning electron microscopy, and three-dimensional fluorescence matrix spectroscopy analysis were applied to evaluate the effects of coagulant dose and floc storage time on the integrity of Microcystis aeruginosa. Cyanobaceteria could grow normally for a long time in the flocs, so the anaerobic treatment method was used to remove the microcystins contained in the flocs. The main results indicated that:(1) When the algal cell density is about 106 cells/mL, the optimal coagulation condition results are:PAFC dosage is 15 mg/L, rapid mix intensity is 250 rpm, rapid mix time is 1 min, slow mix intensity is 40 rpm and slow mix time is 20 min; settling time is 20 min, and the algal removal rate is 99%.(2) PAFC dosage, rapid mix intensity, slow mix intensity, and slow mix time have a great influence on algae removal efficiency, while rapid mix time in this range has low influence.(3) After coagulation settlement, cyanobacteria cells were completely removed from simulated water and transferred into coagulation flocs. Mechanical agitation conditions and the optimal PAFC dose do not cause obvious damage of cyanobacteria cells.(4) When the storage time of coagulation floc was not exceeding 12 d, the concentration of extracellular MCs is low. Meanwhile, SEM photos showed cyanobacteria cells were intact. However, their normal growth was inhibited and the release of intracellular organic matter was prevented.(5) Under anaerobic storage conditions (initial pH=8, Temperature=35℃), the degradation rate of total MCs in floes reached to 99% in 6 d and the concentration of MC-LR reduced to 1.16μg/L.