| With the increasingly serious problem of global nutrition,the treatment of high algal raw water has become one of the key problems concerned by drinking water plants.These excess algal cells not only increase the difficulty of water treatment and increase the operation cost of enterprises,but also easily penetrate the filter and release metabolites such as odors and algal toxins into the water or react with chlorine to produce disinfection byproducts,which directly threaten the safety of water supply.In this study,Microcystis aeruginosa was taken as the representative to investigate the cell integrity changes and the law of release and degradation of intracellular organic matter during the oxidation process.On this basis,the preoxidation conditions were optimized to achieve high efficiency coagulation and reduce the damage degree of preoxidation to algal cells.It provides technical support for drinking water plant to solve the problem of high algal raw water treatment.Firstly,in order to ensure the accuracy and timeliness of detection results,the detection methods of cell integrity and typical algal olfactory substance β-cyclocitral were optimized.The integrity of algal cells was analyzed by flow cytometry,and the changes in cell activity were identified by SYTOX green staining and autofluorescence.The amount of staining was25 μM and the reaction time was 3 min.The voltage of FSC,SSC,BL1 and BL3 used by flow cytometry were 120,240,240 and 320,respectively.Solid phase microextraction method was used to determine the concentration of β-cyclocitral.After optimization,25 m L sample was selected,30% sodium chloride was added,dimethoxy-triisopropyl pyrazine was extracted at70 ℃ for 60 min,and the lower limit of detection was 8.0 ng/L by gas chromatogram-mass spectrometry.Using raw water from drinking water plant as background,the relative standard deviations of 20ng/L and 40ng/L samples were 5.11% and 4.01%,respectively,and the recoveries were between 92.5 and 108.0%,which could meet the needs of the follow-up study.On this basis,the cell rupture and typical intracellular organic matter dissolution in microcystis cupric chloroides were studied.It was found that there was a lag between dissolution of organic matter and cell rupture,and the lag time decreased with the increase of oxidizer concentration.Under the action of low dose oxidizer,although the algal cell rupture phenomenon still exists,the dissolution rate of intracellular organic matter can be controlled at a low level within a certain period of time,indicating that if the degree of oxidation can be accurately controlled,it is possible to achieve the goal of pre-oxidation to aid coagulation but reduce the release of intracellular matter to avoid the deterioration of water quality.The effects of initial oxidizer concentration and hydraulic gradient on the oxidation process were investigated,and it was found that the increase of hydraulic gradient could accelerate the rate of algal cell rupture.When the chlorine concentration was 1 mg/L,the reaction would not cause the dissolution of intracellular chlorophyll under static condition,but more than 40% of the chlorophyll was dissolved under stirring condition,indicating that the coagulation process might enhance the pre-oxidation effect.Increase the probability of organic matter release inside the cell.The kinetic processes of chlorine decay,cell rupture and extracellular matter dissolution during oxidation reaction were fitted with the pseudo-first-order kinetic model.Finally,on the basis of the above studies,the coagulation-precipitation procedures and control parameters were optimized.When the three-stage coagulation was used,the dosage of aluminum sulfate was 13 mg/L and the concentration of oxidant was 0.8 mg/L,the coagulation could be effectively assisted and the release ratio of intracellular organic matter could be reduced.The turbidity of the algal water after precipitation by the optimized method is only 0.79,which is better than that without preoxidation.The formation potential of trihalomethane and β-cyclocitraldehyde are 2.98 mg/L and 493 ng/L respectively,which are only 39.7% and 40.5% of that of the water after traditional preoxidation coagulation and precipitation treatment.This study not only provides a new idea for the safe treatment of high algal raw water in drinking water plants,but also saves the amount of chemicals used while ensuring the coagulation efficiency,and effectively reduces the release proportion of intracellular metabolites of algae,so as to improve the quality of water supply and ensure the safety of water supply. |
