Research On The Operational Efficiency And Mechanism Of Integrated Photocatalytic/Coagulation System For The Removal Of Natural Organic Matter (NOM)

Natural organic matter（NOM）mainly refers to a category of macromolecular organic matter produced by the biochemical reactions of plant and animal residues during natural cycles.The major component of organic matter in natural waters is humus.Currently,a variety of disinfection byproducts（DBPs）associated with humic acid（HA）were found in drinking water worldwide.In addition,NOM can promote the regrowth of bacteria in water distribution pipes,which seriously threatens the safety of drinking water.As a pivotal part of the water treatment process,coagulation can effectively remove turbidity,suspended organic matter and hydrophobic components in NOM.As an advanced oxidation technology,photocatalysis has significant effect on the removal of dissolved NOM from water,but the second pollution can be caused by the presence of the photocatalyst.Based on this,combined with the characteristics of coagulation and photocatalytic process,the visible light responsive photocatalyst Bi Fe O₃was taken as an example to construct a photocatalytic/coagulation integrated system,and the operation efficiency and mechanism of NOM removal were studied.The following conclusions were obtained:（1）The integrated photocatalytic/coagulation system was constructed.At the optimum dosage of polymeric aluminium chloride（PAC）,the photocatalyst dosage had little effect on the effluent,with UV₂₅₄ removal rates of 85%or more being achieved.When the dosing interval of photocatalyst and coagulant was 3 min,the effluent quality can be significantly reduced.The UV₂₅₄ removal rate was 89.48%and the effluent UV₂₅₄ was 0.020.Photocatalytic/coagulation integrated system can avoid the mutual containment between coagulation and photocatalysis,effectively remove humic substances in source water,and reduce the generation of DBPs at the concentration level ofμg·L^-1.（2）For the flocculation process of photocatalytic/coagulation integrated system,the increase of hydraulic stirring speed lead to the increase of collision efficiency between particles,which can improve the hydraulic conditions of floc growth and change the morphological characteristics of floc complex such as average particle size and fractal dimension.However,too high speed will produce large shear force in the area around the blade,which affected the further growth of flocs and even deteriorates the coagulation effect.The flow field optimization results of different coagulation speeds showed that the optimal hydraulic stirring speed in the flocculation stage was 60 rpm.The average particle size analysis results of floc recognition are consistent with the flow field simulation results.（3）In the process of photocatalytic/coagulation integration,the average particle size of floc complex increased and the two-dimensional fractal dimension decreased compared with that in the single coagulation process.Among them,the average particle size of floc compound was stable at about 0.61 mm,and the two-dimensional fractal dimension was maintained at about 1.38.Through the characterization analysis of the floc complex,it is found that with the increase of the growth time of the floc complex,the floc particles gradually occupy the pores and surface of the interlayer structure of the photocatalyst,and form the floc complex with petal-like Bi Fe O₃ as the core to realize the rapid settlement of the floc.In addition,the floc complex has good stability after washing and excellent reusability.The aim of this research was to construct an integrated photocatalytic/coagulation system and to investigate its operational efficiency and mechanism.Firstly,an integrated photocatalytic/coagulation system was constructed to investigate its efficacy in treating NOM in raw water.Secondly,combined with beaker test and computational fluid mechanics,the flocculant recognition module based on machine vision detection system was improved,and hydraulics optimization and morphology change characteristics of the photocatalytic/coagulation integrated system were studied.Finally,the structural change characteristics of flocs complex in the photocatalytic/coagulation integrated system were revealed from the microscopic point of view,the mechanism of action on the treatment effect of source water was clarified,and optimized the methods and modes for recycling photocatalyst,providing a theoretical basis for the realization of low consumption and efficient improvement of source water quality.