Investigating The Fouling Behavior Of Ultrafiltration Membranes For Dissolved Organic Matters From Microforce Measurements And Mechanisms

Ultrafiltration(UF) technology has been widely used in the field of wastewater reuse and reclamation. However, the main restriction in the popularization and application of UF membranes is the phenomenon of membrane fouling. Therefore, there is an urgent need to clarify the fouling behavior.On the basis of theoretical and practical researches in membrane fouling at home and abroad, the fouling mechanisms of dissolved organic-matter foulants in UF membranes according to membrane–foulant and foulant–foulant microforces have investigated by our group. The main achievements are as follows.(1) The existing atomic force microscope(AFM) colloidal probe technology was improved. A probe made from polyvinylidene fluoride(PVDF) membrane material,model/actual organic-matter-coated colloidal probes, and a carboxyl colloidal probe were prepared with the aid of self-assembly devices. The PVDF colloidal probe and foulant-coated colloidal probe can be used to measure membrane–foulant and foulant–foulant interaction forces, thus establishing a solid foundation for clarification of the fouling behavior of organic foulants in UF membranes from microforces.(2) The PVDF colloidal probe and model organic-matter-coated probes were used to measure the interaction forces between PVDF and humic acid(HA)/sodium alginate(SA)/bovine serum albumin(BSA) and those between foulant and foulant. Results of AFM force measurements combined with the results of corresponding membrane fouling experiments revealed that the PVDF–HA interaction force was much weaker than the PVDF–SA or PVDF–BSA interaction force, and that the rate of decline in theflux of the HA-fouled membrane was a minimum in the initial filtration stage. This phenomenon suggests that membrane fouling is mainly controlled by the interaction between membrane and foulant during the initial filtration stage. In addition, the average BSA–BSA, SA–SA, and HA–HA adhesion forces were respectively 0.41, 0.14,and 0.10 mN/m. The BSA–BSA interaction force was much stronger than the SA–SA and HA–HA interaction forces, and the pseudostable fluxes of SA- and HA-fouled membranes were significantly greater than that of the BSA-fouled membrane. It was clear that the membrane fouling behavior was completely controlled by the foulant–foulant interaction force in the later filtration stage.(3) The cake layer structure and the flux recovery rate of SA-, HA-, and BSA-fouled membranes were investigated. The cake layer on a HA-fouled membrane surface was looser and more porous than that for other fouled membranes, while the flux recovery rate reached 100%. The cake layer on an SA membrane surface was relatively less porous, and the flux recovery rate of the SA-fouled membrane exceeded80%. For the BSA-fouled membrane, there were almost no pores on the membrane surface and the cake layer surface seemed to be much more compact, and the flux recovery rate was less than 40%. Considering these results in conjunction with the results of interaction forces, it seems that the higher the foulant–foulant adhesion force,the more compact the corresponding structure’s cake layer and the lower the flux recovery rate. In contrast, a lower foulant–foulant adhesion force will result in a looser cake layer and a higher flux recovery rate. These results imply that physically irreversible fouling is mainly dependent on foulant–foulant interactions.(4) The organic matters in secondary effluent(EfOM) were isolated into hydrophobic(HPO), hydrophilic(HPI), and transphilic(TPI) fractions. In conjunction with PVDF colloidal probes, an AFM was used to measure the interaction forces between PVDF and different EfOM fractions quantitatively. The HPI–HPI, HPO–HPO,and TPI–TPI interaction forces were determined using the AFM in conjunction with corresponding EfOM-fraction-coated colloidal probes. We combined this analysis with corresponding fouling experiments to identify the EfOM fractions responsible for PVDF UF membrane fouling. Results show that hydrophilic and hydrophobic fractions were the dominant fractions responsible for membrane fouling and flux decline in theinitial and later filtration stages, respectively, which was mainly attributed to the stronger PVDF hydrophilic fraction and intrahydrophobic fraction interaction forces.The flux decline rate and extent of the TPI fraction were lower over all filtration stages,while the PVDF–TPI adhesion force and the intra-TPI-fraction interaction were weaker than the forces of HPO and HPI fractions, respectively.(5) The HPO–HPI, TPI–HPO, and TPI–HPI interaction forces were determined using an AFM in conjunction with a corresponding EfOM-fraction-coated colloidal probe. Considering the above results in conjunction with the results of the interactions between the same EfOM fractions, it is interesting to note that the forces increase in the order of HPI–HPI < HPO–HPI < HPO–HPO, TPI–TPI < TPI–HPO < HPO–HPO, and TPI–TPI < TPI–HPI < HPI–HPI. These results imply that, for fractions A and B where the A–A interaction force is stronger than the B–B interaction force, the interaction forces increase in the order B–B < A–B < A–A. This phenomenon indicates that the interfoulant-fraction(A–B) adhesion force is distributed within the range of corresponding intrafoulant-fraction adhesion forces.(6) To determine further the fouling behaviors of organic matters at different salt concentrations, fouling experiments were carried out with PVDF UF membranes and BSA over a range of salt concentrations. The interaction forces, the adsorption behavior of BSA on the membrane surface, and the structure of the BSA-adsorbing layers at corresponding salt concentrations were investigated. Results indicate that, as the salt concentration increased from 0 to 1 mM, there was a decrease in the PVDF–BSA and BSA–BSA electrostatic repulsion forces, resulting in a higher deposition rate of BSA onto the membrane surface, and the formation of a denser BSA layer; consequently,membrane fouling was enhanced. However, at salt concentrations of 10 and 100 m M,membrane fouling and the BSA removal rate decreased significantly. This was mainly due to the greater hydration repulsion forces, which weakened the PVDF–BSA and BSA–BSA interaction forces and reduced the hydrodynamic radius and increased the diffusion coefficient of BSA. Consequently, BSA passed more easily through the membrane and into the permeate. There was less accumulation of BSA on the membrane surface. A less rigid and more open structure of the BSA layer formed on the membrane surface. In the field of membrane fouling, hydration repulsion wasintroduced for the first time.