Quantitative Study Of The Effect Of Magnetic Fields On Scale Deposition On Nanofiltration Membranes Via UTDR

The lack of water resource and the serious water pollution in the world are influencing the life and the development of society economic deeply. Nanofiltration and Reverse Osmosis is playing a more and more important role in resolving the lack of water resource and water purification. In the membrane processes, however, membrane fouling weakens the separate effect and limits the further extension of membrane separation technique. The study on the mechanism and the control of membrane fouling has been paid extensive attention to by boffins all over the world. This study introduces the magnetic water treatments into Nanofiltration process, and describes an existing ultrasonic technique for quantitative study of the effect of magnetic fields on the CaCO₃ scale deposition on the membrane surface during crossflow nanofiltration (NF).The effects of magnetic fields on some physico-chemistry properties of Ca(HCO₃)₂ solutions and the crystal morphology were investigated systematically. The permanent and electro magnetic fields used in the experiments are 0.4T and 0.02T, respectively. Results show that magnetic field enhances the conductivity and pH of the treated solution and its effect depends on the concentration of the solution. The influence of magnetic fields declines with the increase of solution concentration. The magnetic effect also affected by the flow rate and the pH of the solutions. In addition, the magnetic treatments promote the formation of aragonite and vaterite from calcite.The results obtained in the experiments of crossflow nanofiltration show a good relationship between the ultrasonic measurements and the development of CaCO₃ scale on the NF membrane surface. The amplitude of the ultrasonic signal increased with the deposition of the CaCO₃ scale on membrane. Furthermore, the ultrasonic technique is capable of measuring the rate of fouling layer formation under different treatment conditions, i.e. with non-magnetic field, permanent magnetic field and electromagnetic field. The permeate flux of NF membrane declined slower and the thickness of the scale layer obtained by ultrasonic and weight measurements was thinner in the experiments with PMF and EMF. The SEM and XRD analyses imply that the magnetic treatment suppresses the formation of calcite crystals and prefers to vaterite and aragonite. The CaCO₃ crystal deposited on membrane surface is lessregular and looser, resulting in the decrease in the thickness and density of fouling layer and the slower flux-decline of NF membrane. These correspond with the lower increase in the amplitude of the ultrasonic response signals, the effect of magnetic treatments is normally related to the the magnetohydrodynamic (MHD) mechanism.Overall, the independent measurements such as the flux-decline data, weight measurement and SEM analysis corroborate the ultrasonic measurements. The ultrasonic technique has provided an effective measure to evaluate the prevention and control of membrane fouling.