Studies On Algal Removal Efficiency And Release And Migration Of Microcystins In Typical Algal Removal Process

At present,eutrophication and cyanobacteria bloom pollution are still one of the prominent water environment problems in China.When the cyanobacteria bloom in large areas,the timely removal of cyanobacteria in water by physical method,chemical method,biological method and other technologies is a necessary means to protect the water environment.At present,there are few reports about the systematic studies on the efficiency of various algal removal techniques.The problem of algal toxin dissolution which may exist in the process of algal removal has not been paid enough attention and solved effectively.In this paper,based on the experimental study of the efficiency of several typical algal removal processes(chemical removal of algal,ultrasonic removal of algal,biological removal of algal),the release of microcystins and its diffusion rule in water were emphatically analyzed,and a one-dimensional diffusion model of microcystins in water was established.The research results can provide important reference for the research on the method and parameter optimization of algal removal based on the control of algal toxin dissolution.Main research contents and conclusions are as follows:(1)5L of algal water was treated with ultrasonic waves of 28khz with sound intensity of 214.97 W/m2,863.12W/m2 and 1300.05W/m2,and the removal rate of algal cells under three sound intensities was 41.4%,56.7%and 78.7%,respectively,after 15min.The effect of algae removal increased with the increase of sound intensity.The reaction of ultrasonic wave to remove algae conformed to first-order kinetics.The reaction rates at each acoustic intensity were 0.033min-1,0.051 min-1 and 0.091 min-1 respectively.The improvement of ultrasonic intensity can effectively promote algal removal rates.(2)The algal removal performance of two chemical agents,copper sulfate and glutaraldehyde,and biological algal remover,was investigated by static test in a certain range(copper sulfate:0.5mg/L-8mg/L;Glutaraldehyde:20ug/1-80ug/L;Biological algal remover:0.5mg/L-4mg/L).The best concentration of copper sulfate was 4mg/L,and the removal efficiency of algal cells was 96%when the algal was treated at this concentration for 12d.The optimum concentration of glutaraldehyde was 80ug/L,and the optimum concentration of biological algal remover was 2mg/L.The removal effect on algal cells was 68.2%and 34.6%,respectively.(3)Under three acoustic intensities(214.97W/m2?863.12W/m2?1300.05W/m2),the release rule of microcystins in the process of ultrasonic algal removal was studied.The results showed that the microcystins release existed in all the algal removal processes under acoustic intensities.In 15 min,the process of toxin release was in good agreement with the linear equation.The higher the sound intensity,the more significant the microcystins released:when the initial algal cell concentrations were between 1.975 × 1 07cells/mL?2.105 × 107cells/mL,the release rates of microcystins were 0.267?g·L-1min-1?1.034?g·L-1lmin-1?2.263?g·L-1min-1,respectively.(4)The release of microcystins during the algae removal process by copper sulfate,glutaraldehyde and biological algicide was studied by the beaker test.The results showed that the concentration of extracellular microcystins in the water increased under the effect of the three agents,indicating that the release of microcystins was obvious in these processes.When the copper sulfate concentration was 4mg/L or 8mg/L,the concentration of microcystins increased faster than that under low copper sulfate concentrations.The initial concentration of microcystins in water was 24.93ug/L and 25.01ug/L,respectively.In the process of algal removal by glutaraldehyde,the release of extracellular microcytstins was obvious 4-6 days later,and the concentration of microcystins reached 110.41?194.55ug/L 8 days later.Under the condition of 4mg/L biological agent,the maximum concentration of extracellular microcystins was 87.01ug/L,higher than the concentrations under other dosages,and the amount of microcystins released to unit removed chlorophyll a was 55.52ug/mg.(5)Based on the algal removal efficiency,microcystins dissolution degree and removal rate,the efficiency of ultrasonic algal removal,copper sulfate algal removal,glutaraldehyde algal removal and biological algal removal were compared under the respective optimal conditions.The ultrasonic algal removal rate was 0.0913min-1,which was highest of these methods,and the efficiency was relatively higher as 78.76%.The amount of microcystins released by removing the unit algal cells was the lowest,only 1.78ug/mg.The algal removal efficiency of copper sulfate was the highest(96.07%),but the algal removal rate was relatively low(0.313d-1),and microcystins release was significant(77.53ug/mg)during the algal removal process.The rate of algal removal and the efficiency were the lowest,but the degree of microcystins release in the process of algal removal was relatively small(21.30ug/mg).Although the relatively ideal algal removal effect(68.13%)could be achieved by glutaraldehyde,the rate of algal removal is relatively low,and microcystins release was the severest in the process of algal removal(84.60ug/mg).(6)A simulated lake test model was used to investigate the diffusion law of microcystins at different radius,the results showed that from the point source,with the passage of time,the microcystins concentration at different positions first rose then gradually declined.Within 12 hours,the concentration of microcystins at different positions varied.According to the fitting results,at 0.45m,0.75m and 1m,the maximum values were reached at 14.2min,45.8min and 3.0h respectively,and the maximum values were 1.21ug/L,0.728ug/L and 0.567ug/L respectively.The diffusion of microcystins was related to the concentration gradient.The closer to the center,the larger the concentration gradient and diffusion flux,and the concentration of microcystins increased rapidly.As the distance increased,the concentration gradient became smaller and smaller.Based on Fick's law,a one-dimensional water quality model for microcystins diffusion was constructed,and the diffusion coefficient of microcystins was 0.379m2/h,and the relative error range between the model and measured value was between 3.7%and 19.9%.The variation trend of the two was consistent,and the model had a good prediction function.