Reverse Osmosis Membrane Fouling Caused By Biopolymers: Mechanisms And Mitigation By Ozone Oxidation-coagulation Pre-treatment

Reverse osmosis(RO)technology is widely investigated in secondary effluent wastewater reclamation.However,membrane fouling is the main drawback to restrict the popularization and application of this technology.Plenty of studies concluded that biopolymers in secondary effluent were one of the major foulants causing RO membrane fouling and pre-treatment of wastewater was the main method to alleviate membrane fouling.Recently,ozone oxidation and ozone oxidation-coagulation integrated technology achieved widely application in pre-treatment of low-pressure membrane filtration due to the excellent degradation or removal efficiency of biopolymers.However,it was rare and controversial for the mitigation of RO membrane fouling by above pre-treatments.In this thesis,bovine serum albumin(BSA)and sodium alginate(SA)were used to represent protein and polysaccharides in biopolymers,respectively.The effect of ozone oxidation,coagulation and their integrated technology on RO membrane fouling was studied to elucidate the mitigation mechanism of membrane fouling.Meanwhile,the mitigation characteristics of RO membrane fouling for filtration of real textile dyeing secondary effluent was investigated by above pre-treatments.The main content and conclusion were as follows:(1)Effect of ozone oxidation on the mitigation of RO membrane fouling(including membrane flux and fouling reversibility)caused by BSA,SA and BSA+SA(mass ratio=1:1)was investigated.The adsorption behavior of biopolymers on membrane surface was measured by in-situ membrane streaming potential measurement in order to clarify the mitigation mechanism of RO membrane fouling.The results exhibited that:(1)ozone oxidation can alleviate RO membrane fouling by BSA,SA and BSA+SA,and markedly increased the reversible ratio of BSA and BSA+SA fouling by as highest as 66.4%and 29.2%,respectively,while enhanced that of SA fouling by only 1.1%-8.0%.The increasing ozone dose was beneficial to mitigate membrane fouling,however,membrane fouling did not alleviate further when ozone dose was higher than 0.15 mg O₃/mg DOC;(2)ozone oxidation decreased zeta potential and hydrophobicity of biopolymers.The stronger electrostatic repulsion force and weaker hydrophobic interaction force between membrane and foulants are the main fouling mitigation mechanism.Membrane streaming current and membrane surface fouling layer analysis displayed that the reduced deposition of foulants on membrane surface after pre-ozonation was the direct reason for membrane fouling alleviation.(2)Effect of calcium or magnesium ion on ozone oxidation for mitigation of RO membrane fouling caused by BSA,SA and BSA+SA(mass ratio=1:1)was studied.The results showed that:(1)when calcium and magnesium ion concentration was 0.5 m M,membrane fouling caused by BSA,SA and BSA+SA reduced with the increasing ozone dose.However,excessive ozone dose was detrimental to fouling mitigation;(2)when calcium or magnesium ion concentration increased from 0.5 m M to 2.0 m M,pre-ozonation markedly reduced fouling by BSA+Ca²⁺/Mg²⁺,SA+Mg²⁺and BSA+SA+Mg²⁺,but negligibly mitigated fouling by BSA+SA+Ca²⁺and aggravated irreversible fouling by SA+Ca²⁺;(3)ozone oxidation reduced zeta potential and hydrophobicity of biopolymers,thus leading to enhanced electrostatic repulsion and weakened hydrophobic interaction,which was beneficial to membrane fouling mitigation;On the other hand,ozone oxidation increased the bridging action of calcium and magnesium ion between biopolymers and membrane,which was detrimental to membrane fouling alleviation.Since the interaction force between calcium ion and SA was strong,effect of bridging action was stronger than electrostatic interaction and hydrophobic interaction,thus causing more severe membrane fouling;however,the interaction forces of calcium ion-BSA,magnesium ion-BSA and magnesium ion-SA were relatively low,thus effect of bridging action was weaker than electrostatic interaction and hydrophobic interaction,which led to fouling reduction in different degrees.(3)The characteristic of coagulation and ozone oxidation-coagulation integrated pre-treatment for the mitigation of RO fouling caused by BSA,SA and BSA+SA(mass ratio=1:1)was investigated.Meanwhile,the effect of calcium ion was explored.The results exhibited that:(1)when calcium ion concentration was 0.5 m M,the sequence of mitigation degree of RO membrane fouling caused by biopolymers after coagulation was as follows:BSA+SA>SA>BSA;(2)when calcium ion concentration is 0.5 m M,ozone oxidation can improve the mitigation degree of BSA fouling by coagulation,but reduce that of SA and BSA+SA fouling by coagulation.Moreover,this phenomenon became more remarkable with the increasing ozone dose;(3)when calcium ion concentration increased from 0.5 m M to 2.0 m M,coagulation enhanced the mitigation degree of BSA and BSA+SA fouling,but reduced that of SA fouling;After ozone oxidation-coagulation integrated pre-treatment,mitigation degree of BSA,SA and BSA+SA fouling all increased with the increasing calcium ion concentrations.However,when calcium ion concentration increased to 4.0 m M,BSA fouling aggravated after coagulation and ozone oxidation-coagualtion treatments.(4)Effect of ozone oxidation,coagulation and ozone oxidation-coagulation pre-treatments on mitigation of RO membrane fouling caused by textile dyeing secondary effluent was investigated,meanwhile,the pre-treatment efficiency of ozone oxidation was also studied after removing calcium and magnesium ion by softening method,the results showed that:(1)the removal efficiency of DOC was low,while that of fluorescent organic matter and UV₂₅₄ was remarkable after ozone oxidation.The removal efficiencies of DOC,fluorescent organic matter and UV₂₅₄ in Ef OM were all relatively low after coagulation.Compared to ozone oxidation,removal efficiency of organics increased negligibly after ozone oxidation-coagulation integrated treatment;(2)the concentration of calcium ion in secondary effluent was approximately 1.12 m M,the bridging action enhanced after ozone oxidation,thus membrane fouling aggravated with the increasing ozone doses.After softening-ozone oxidation,the bridging action of calcium ion disappeared and ozone oxidation weakened the hydrophobic interaction,leading to remarkable reduction of membrane fouling with the increasing ozone doses.The quality of secondary effluent is complex,and secondary effluent organic matter(Ef OM)has relatively low molecular weight and relatively high hydrophilicity,thus they were difficult to be removed by coagulation,and RO membrane fouling caused by real wastewater cannot be mitigated effectively by coagulation and ozone oxidation-coagualtion treatment.