Efficient Treatment Of High Turbidity Wastewater In Water Supply Plant By Magnetic Flocculation

In water supply plants,a large amount of sludge water and backwash water would be produced,collectively referred to as high turbidity wastewater,which is enriched with many microorganisms,organic matters,and inorganic impurities,direct discharge will not only waste a lot of water resources but also seriously pollute rivers and soil.At present,conventional treatment technologies,such as coagulation-precipitation technology,micro-sand assisted precipitation technology,particle bed filtration technology,air flotation and membrane separation technology,all have problems such as large process areas,high energy consumption,complicated operation,unstable treatment effect,and low safety.As a new water treatment technology,magnetic flocculation technology has been widely used in the water treatment industry,it has the advantages of strengthening the flocculation effect,improving the flocs structure,speeding up the solid-liquid separation rate,simple operation and so on.In practical engineering applications,it can effectively reduce the construction area and has broad application prospects.In this study,two kinds of high turbidity wastewater with different water quality conditions were selected,and turbidity,chemical oxygen demand（COD）,and UV₂₅₄ were taken as the main inspection indexes,the small-scale and expanding experiment of magnetic flocculation were carried out to study the performance of high turbidity wastewater treatment by magnetic flocculation technology,and the mechanism of magnetic seed to enhance flocculation effect was discussed.Firstly,the high turbidity wastewater of 130～150 NTU was selected for the small-scale experiment.The effects of the dosage of polyaluminium chloride（PAC）,the dosage of magnetic seed（Fe₃O₄）,initial p H and sedimentation time on the removal of turbidity,chemical oxygen demand,and UV₂₅₄ of the wastewater were systematically investigated according to a single factor and orthogonal experiment,and the recycling performance of magnetic seed was explored.Results showed that under the optimal condition with 5 mg·L^-1 of PAC,20 mg·L^-1of Fe₃O₄,5 min of sedimentation time,and at initial p H 7～9,the turbidity decreased from140.8 NTU to 5 NTU,COD was reduced from 66.7 mg·L^-1 to 14.2 mg·L^-1,UV₂₅₄ decreased from 0.0430 cm^-1 to 0.0311 cm^-1,with the removal rates of 96.4%,78.7%,and 27.7%,respectively.Compared with conventional flocculation,the residual turbidity of the wastewater in the magnetic flocculation process decreased faster.The results of orthogonal experiment indicated that the primary and secondary order of parameters that affected the turbidity removal rates were PAC dosage,p H,Fe₃O₄ dosage,and sedimentation time.The results of the magnetic seed cycle experiment showed that the magnetic seed still had a good enhanced flocculation effect after being recycled three times,and the recovery rate can reach77.3%.The characterization results of the magnetic seed showed that the magnetic seed had no obvious change before and after use,and could be reused many times.Secondly,under the above optimum conditions,the morphological structure,phase composition,and magnetic properties of flocs were characterized,and the potential changes during the magnetic flocculation process were analyzed,in order to preliminarily reveal the mechanism of magnetic flocculation.Results indicated that magnetic seeds in the flocculation process could promote the destabilization and flocculation of colloidal particles.Magnetic seeds could form composite magnetic flocs with colloidal particles in wastewater by the chemical bond and intermolecular force.Owing to the magnetics of the composite flocs,magnetic flocculation could shorten the sedimentation time by the external magnetic field.Finally,based on the above experiments,the high turbidity wastewater of 270～350 NTU was selected for expanding experiment.The dosage of PAC,PAM,and magnetic seed were further optimized,and the morphological structure,fractal dimension,sedimentation performance,and particle size of flocs during the flocculation process were analyzed to investigate the growth and change of flocs,and further verify the feasibility of magnetic flocculation technology to treat high turbidity wastewater.Results showed that the optimal parameters were 5 mg·L^-1 of PAC,0.2 mg·L^-1 of PAM,and 30 mg·L^-1 of Fe₃O₄.Compared with the small-scale experiment,the dosage of PAC didn’t increase,PAM was introduced to strengthen the binding effect between magnetic seeds and flocs,and the dosage of magnetic seeds increased.Under the optimal dosing condition,the turbidity decreased from 310 NTU to 6 NTU,COD decreased from 177.8 mg·L^-1 to 11.6 mg·L^-1,UV₂₅₄ decreased from 0.0509cm^-1 to 0.0377 cm^-1,with the removal rates of 97.9%,93.5%and 26.0%,respectively.Compared with conventional flocculation,the trend of increasing turbidity removal rate and decreasing residual turbidity in wastewater during magnetic flocculation were more significant.The addition of magnetic seeds was beneficial to strengthen the flocculation process,and the growth and change of magnetic flocs were better,with compact structure,large volume,high fractal dimension,fast settling and compaction rate.