| The large-scale application of ultrafiltration technology in water treatment has been obstructed by membrane fouling.Applying appropriate pretreatment technology can not only improve the quality of membrane effluent,but also reduce the formation of membrane fouling to save the maintenance cost.And because of the good suitability of powdered activated carbon adsorption and coagulation,the effluent of powdered activated carbon and coagulation combination could pass through ultrafiltration membrane unit without precipitation,which can effectively control the formation of membrane fouling while shortening the process flow and reducing the cost of infrastructure.On the basis of coagulation/powdered activated carbon combination,this paper has investigated the influence on the membrane fouling behavior and mechanism,and further highlights the advantages of the short process coagulation/powdered activated carbon combination in the field of pretreatment technology of ultrafiltration.By changing PAC and PACl dosage,pH value and inorganic substance concentration,the membrane fouling control ability of coagulation-adsorption(coagulation first and adsorption then,C-A)and adsorption-coagulation(absorption first and coagulation then,A-C)was studied.The water treatment characteristics which are closely related to the membrane fouling behavior have been discussed to revealed the reasons for the difference membrane fouling control ability of C-A and A-C under different operating conditions.The results showed that both C-A and A-C can effectively improve the quality of the membrane effluent,control the decrease of membrane flux in ultrafiltration and reduce the production of irreversible membrane fouling.When the dosage of PAC was 50mg/L and PACl was 12mg/L,the specific flux of the ultrafiltration membrane after five cycles pretreated by C-A was 0.769,and the irreversible fouling was 0.058;the specific flux of ultrafiltration membrane pretreated by A-C was 0.793,and the irreversible fouling was 0.036.At the same time,changing the dosage of PAC and PACl would not affect the superiority of A-C to the control of membrane fouling compared with C-A.Due to the PAC which adsorbed the organic matter first could take part into the formation of flocs as core,the flocs produced in A-C process would have larger particle size,larger strengthe,more irregular structure and better permeability compared with flocs which formed in CA process at the same dosage conditions.Meanwhile,the effluent of A-C process would have less small molecule and macromolecular organic matter and stronger hydrophilicity.Pretreating by A-C process,the structure of the cake layer on the membrane surface was more loose,the porosity was higher,the hydraulic compression ability was better,the probability of the direct contact between the organic matter and the membrane surface was less,the force was weaker,the recovery of the membrane surface and the membrane hole could be better guaranteed.Therefore,comparing with C-A,A-C process could better control the formation of membrane fouling at the same PAC and PACl dosage conditions.When varying the pH value,the changes in the concentration of H+ and OHin the water would change the participation degree and form of the PAC particles into the formation of the flocs,thus changing the membrane fouling control ability of C-A and AC.In addition,while kaolin clay existed in water,kaolin would compete with PAC particles to participate in the formation of flocs.Although the existence of kaolin would lead serious membrane flux decrease and irreversible membrane fouling increase,it would not affect the advantages of membrane fouling control ability of A-C.The experimental study showed when ultrafiltring the C-A or A-C effluent,the membrane fouling mechanism would be controlled by complete blocking,intermediate blocking,standard blocking and cake layer filtration model simulaneously.The operating conditions,such as PAC and PACl dosage,pH value and inorganic substance concentration could lead to the change of the characteristics of C-A and A-C,which resulted in the different superiority of the four mechanisms in the ultrafiltration behavior,finally causing the different degree of membrane fouling. |
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