Study On Buoyancy Enrichment Of Cyanobacteria-Ultrasonic Pretreatment Process Conditions

Mechanized salvage is an emergency measure to deal with the problem of large scale outbreak of cyanobacteria bloom.How to realize the separation of algae from water on a large scale is a key factor that restricts the engineering application of the mechanized removal technology for cyanobacteria bloom.The buoyancy enrichment of cyanobacteria——ultrasonic pretreatment process was designed in view of the characteristics of cyanobacteria,such as gas vesicles and colloidal coating,which can hardly be removed through coagulation and precipitation.In this process,cyanobacteria were enriched by their floating characteristic,which could reduce the pressure of the subsequent processing unit,and realize the large scale algal water treatment.In addition,the low intensity ultrasonic wave was used to pretreat cyanobacteria,which could destroy the gas vesicles,improve the sedimentation efficiency and reduce the application of chemical agents,finally achieving the goal of large-scale and pollution-free algal water separation.In this study,cyanobacteria of Dianchi Lake was used as the treatment object,whereas the cyanobacteria enrichment coefficient,algal cell sedimentation rate and water quality changes were used as the measurement indexes to determine the optimal parameters of the process,with an aim to provide data basis for the engineering design.Our research results showed that:(1)The buoyancy enrichment technology of cyanobacteria was employed to enrich cyanobacteria in Dianchi Lake.The single factor and BBD response surface experiment had determined the order of various influence factors of the cyanobacteria buoyancy enrichment process,and the optimal process parameters were obtained.The results suggested that,the influence of various factors on the enrichment effect of cyanobacteria followed the order of the retention time>flow rate>algae density,and there was no interaction between different factors.The optimal process parameters for the buoyancy enrichment of cyanobacteria were as follows:the flow rate of 0.91 cm/s,the retention time of 38 min,and the initial algae density of 32×10~8 cells/L.At this time,the cyanobacteria enrichment coefficient was 3.40 times.(2)The distribution of cavitation intensity in the ultrasonic pretreatment reactor was measured by the staining method,which indicated that,when 10 uniformly arranged transducers radiated simultaneously,the overall ultrasonic cavitation distribution in the reactor was relatively uniform due to the superposition of sound waves,as well as the vibration and reflection of the reactor side wall.When the effective water depth was 19.5cm(integer multiple of the half wavelength)and the original algae density was less than2.4×10~9 cells/L,the sound intensity attenuation was slow in the ultrasonic pretreatment reactor,which could achieve a good application effect for cyanobacteria pretreatment.(3)The destruction effect of ultrasound on gas vesicles conformed to the pseudo-first order kinetics model;the first order reaction rate constant k was increased with the increase in the ultrasonic power density,and gradually reached saturation.In addition,low power ultrasonic treatment for a short time(5.0~16.7 W/L,5 s)could break the gas vesicles,which could then improve the settling performance of cyanobacteria colonies,and reduce the water pH,with low influence on the DTN and DTP concentrations(<5%).Furthermore,under the premise of guaranteeing safe effluent,the optimized treatment conditions of Dianchi Lake cyanobacteria were determined to be the ultrasonic power density of 16.7 W/L and the irradiation time of 5 s,after taking various factors(such as the treatment efficiency and economic cost)into comprehensive comprehension.Under such conditions,the gas vesicles destruction rate and sedimentation rate of cyanobacteria were 84%and 80%,respectively.(4)Pretreatment with low power ultrasound for a short time(5 s,16.7 W/L)could reduce the coagulant dosage by 40.6%,when the coagulant dosage was 79 ng Al/(104cells),the turbidity of effluent,SS and Chla removal rates were 5 NTU,8 mg/L and 94%,respectively.