| Coagulation is an important water treatment unit,which is widely used in water treatment,primary treatment and deep processing of municipal sewage.Magnetic coagulation is a way of strengthening coagulation,it has the advantages of small area,high removal efficiency,short residence time and low moisture content of wet sludge,leading to a lot of engineering applications in the primary treatment and deep processing sewage.But the research on magnetic coagulation is confined to the effect of practical process at present,rather than the mechanism and process optimization.The main research content of this paper is to explore the best process parameters of treating municipal sewage and the mechanism of the magnetic coagulation process,and optimize the design of the practical process.The experimental results show that the best order of dosing agents of magnetic coagulation is as follows:adding magnetic powder firstly,then adding PAC before mixing,finally adding PAM when flocculating;The optimal stirring parameters are recommended as below:fast mixing speed N1=600r/min,the mixing time T1=60s,slow mixing speed N2=250r/min,the mixing time T2=120s.The optimal dosage:PAC is 40mg/L,PAM is 1.0mg/L,the magnetic powder is 200 mg/L.The removal of COD by magnetic coagulation technology is mainly based on the removal of suspended solids,the removal rate of suspended COD is about 70%-90%,non suspended COD is about 0-35%.The removal rate of suspended and non suspended TP are about 50%-90%.Sludge recirculation will reduce the effect of magnetic coagulation;When the dosage is enough,the sand medium and magnetic powder medium both have obvious effect on water treatment,but the magnetic powder dosage is lower.The results of experimental study and characterization analysis showed that the magnetic coagulation mechanism is consistent with conventional chemical coagulation.The ability to strengthen the coagulation of magnetic particles is mainly due to the fact the magnetic flocs can improve the sedimentation rate and magnetic separation.The nucleation mechanism of magnetic powder is the nucleating agent will adsorb the stabilized particles because of its high surface energy,then gradually increasing its bulk.The control of the nucleation process includes the control of the metastable state of the particles and the parameter of the nucleating agent.For the combined municipal sewage,the potential of magnetic coagulation should be controlled at about-10mV,the magnetic powder dosage should be 200mg/L,and the magnetic particle size should be 30-75μm.When the municipal sewage was treated by magnetic coagulation process,the removal rate of COD,TP and SS respectively was 67.9%,73%and 79.1%;The amount of mud is related to the SS and the moisture content of the mud cake,and the recovery rate of magnetic powder is between 99.0%-99.4%.The optimization design of the magnetic coagulation process for municipal sewage mainly includes the following four aspects:（1）the removal efficiency of COD,TP and SS is related to water quality,when the influent COD is less than 350 mg/l,SS is less than 300mg/l,TP is less than 5 mg/l,the effluent design values of these indicators can reach the second level of urban sewage discharge standards,TP can reach the first level B standard;（2）When the volume of water is below 60 thousand m3/d,and the process is limited to an area of 250m2/（10000 m3/d）,it is recommended to high gradient magnetic separator to separate magnetic floc than the inclined tube sedimentation tank;（3）Optimized design on the size of the pool and the power of the motor can achieve better treatment effect,including that The G value of the mixing tank is increased to 541 s-1,the flocculation tank is reduced from 252 s-1;（4）The dosage of PAC,PAM and magnetic powder should be respectively controlled at 400kg/10000 m3,10kg/10000m3,20kg/10000 m3. |
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