Magnetic Sedimentation Algal Water Separation Process And Kinetic Study

Algae are the primary producer in water bodies.However,the harmful algal bloom,especially the occurrence of cyanobacteria bloom,also damages the aquatic ecological environment of aquaculture water,and then affect the quality of aquatic economic animals,and ultimately threaten human health.Therefore,the emergency treatment of algae bloom is needed around the world.Coagulation sedimentation method is a simple and effective emergency cyanobacteria bloom treatment process,but the traditional coagulation sedimentation method requires a long precipitation time,which limits its large-scale application.Magnetic coagulation sedimentation process has the advantages of short settling time,small footprint and low operational cost,so it has a wide application potential in the emergency treatment of cyanobacteria bloom.In this study,the effects of different dosage and their combinations on the removal efficiency of Microcystis flos-aquae were investigated,and the synergistic effect of polyaluminum chloride(PAC)and cationic polyacrylamide(CPAM)on magnetic powder loading was found in the removal process of Microcystis flos-aquae by magnetic coagulation.A high-speed camera was used to measure the settling velocity of magnetic floc,and the influence of different dosage of magnetic powder on settling kinetics was investigated.In order to reduce the unit water treatment costs and purify the coagulation harvested biomass,the recovery of aluminum salt from Microcystis flos-aquae biomass by acid washing was further studied.The main results are as follows:(1)The synergistic effect of PAC and CPAM and the effect of agitation on the formation of flocs in the process of water separation of magnetic coagulating sedimentation algae were investigated.It was found that the mixed addition of PAC and CPAM had a higher removal efficiency of Microcystis flos-aquae under lower addition conditions.In the process of dosing magnetic particle,the magnetic particles can be uniformly dispersed when the G value generated by stirring is greater than 181 s^-1.In the flocculation process,when the G value generated by stirring is 21 s^-1,the flocs with higher fractal dimension can be obtained after 5 min of coagulation,and the flocs with higher fractal dimension is beneficial to reduce the sedimentation rate.(2)The kinetics of magnetic coagulation sedimentation was studied.Studies have found that floc properties varies under the different reagent ratios.Size and fractal dimension of flocs increased with the increase of the dosage of PAC and CPAM.When the PAC dosage was 5 mg/L and CPAM dosage was 0.4 mg/L,the fastest sedimentation velocity achieved.The average particle size and fractal dimension of flocs were 827.05?m and 1.83?m,respectively.There is an obvious positive correlation between the average particle size and fractal dimension of flocs and sedimentation velocity.It was found that the separation efficiency of magnetic coagulation sedimentation increased with the increase of magnetic powder dosage.The sedimentation velocity increased significantly with the increase of dosage of magnetic powder.When the magnetic powder dosage reached 0.5 g/L,the sedimentation velocity of flocs reached 1.72 mm/s.The comparison of magnetic and traditional coagulation sedimentation shows that magnetic coagulation sedimentation has higher treatment efficiency than traditional coagulation sedimentation under the condition of high flow rate.When the settling efficiency is 90%or above,the hydraulic load rate of magnetic coagulation sedimentation surface is 5 times of that of non-magnetic sedimentation surface,reaching 7.81 m³/(m²·h).(3)Magnetic particle recovery from Microcystis flos-aquae biomass coagulation was studied using high-speed shear treatment.When the speed was 2000 rpm and the shear time was 1 min,the recovery rate of magnetic particles was 98.05±0.91%,and the removal rate of algae cells was97.85±0.58%.The recovered magnetic particles could be recycled for many times.It was found that under the condition of aluminum salt recovery of 90%or above,three times of washing with0.01 mol/L HCl and two times with 25 m L/g with 0.1 mol/L HCl were required for washing with PAC from Microcystis flos-aquae biomass.It is found that the recycled aluminum salt can be reused for 5 times,and its removal rate is always higher than that of newly prepared PAC and newly prepared aluminum sulfate at the same concentration.Further analysis enhanced of the coagulation of washing aluminum salts showed that the elution of extracellular polymer(EPS)enhanced the coagulation effect.The technical and economic analysis showed that the acid washing process to recover aluminum salt from Microcystis flos-aquae biomass after coagulation could significantly reduce the cost of algae water treatment from 0.72 CNY/m³to 0.42 CNY/m³.