Study On Characterics Of Filtration And Membrane Fouling In Nanofiltration Process

With the growing shortage of water resources and the more and more stricter of water quality standards carried out, it becomes more urgent to make research on the advanced treatment technology of drinking water and wastewater reuse. Membrane technology with much merits such as low energy consumption, high quality effluent, etc, has been paid greater attention. But membrane fouling will cause the decline of flux and the increasing of difference between inner and outer membrane pressure, operation cost and water production cost are increased accordingly, finally it will lead to limit the promotion of membrane technology. Many studies indicate that characteristics of membrane, solute, physicochemical properties of solution, operating conditions and the interaction among membrane-solute-solvent will reduce membrane separation performance and cause membrane fouling. Among these characteristics, molecular weight distribution and Hydrophilic / Hydrophobic properties of organic compounds have greater impacts on membrane fouling. In Oder to save the operation cost, it is necessary to study the influence of molecular weight distribution and Hydrophilic / Hydrophobic properties in nanofiltration process.In this experiment, the water spiked with humic acid and the water submitted to secondary treatment were employed as raw water for conducting nanofiltration membrane filtration test, respectively. The effects of different organic molecular weight distribution and Hydrophilic / Hydrophobic properties of raw water on nanofiltration membrane permeable flux,interception performance and the change of membrane surface resistance were investigated. We observed and analyzed the characteristic of membrane fouling in the process of nanofiltration filtration for 32 h and 54h respectively. The main results were shown as following:1) In this experiment, XAD-8 resin and Ultra - filtration membranes with different molecular cut-off weight were used to determine the molecular weight distribution of soluble and hydrophilic/hydrophobic organic compounds which exist in secondary effluent. As the result, the proportion of macromolecule organic compounds whose molecular weights are more than tens of thousands was relative smaller, yet low molecular weight organic compounds whose molecular weights are less than 2 kDa had the larger proportion with about 45.6% of the total amount of organic compounds. The amount of hydrophobic organic compounds in secondary effluent was relative large, yet the amount of hydrophilic organic compounds was relative small. Most of the hydrophilic and hydrophobic organic compounds were small molecules.2) In this part, the raw water containing organic compounds with different molecular weight distribution was employed for conducting nanofiltration membrane filtration test and the results showed that different molecular weight distribution of organic compounds in raw water had impacts on the attenuation extent of NF permeable flux and retention rates of membrane. Organics with molecular weight less than 2kDa caused the largest NF membrane flux decline, organics with molecular weight less than 100kDa caused largest organic retention rate. It can be inferred the permeable and the interception capacity of nanofiltration membrane were not only affected by organic molecular size, but also affected by another more factors such as organic nature, properties of the membrane surface and so on.3) In this part, the raw water containing organic compounds with identical molecular weight but different Hydrophilic / Hydrophobic properties was used to carry out the nanofiltration membrane filtration test. By determining membrane surface resistance, it indicated that hydrophobic material was the main reason for causing membrane flux decline and membrane surface resistance whereas hydrophilic material has negligible effects. Compared with hydrophilic organic compounds, hydrophobic organic compounds were easier to deposit on the membrane surface to form absorption layers, then it had great impact on membrane flux and retention rate by forming gel layer and adsorption layer. Hydrophilic organics with molecular weight less than 2kDa and less than 50kDa caused larger organic retention rate than that Hydrophilic organics with molecular weight less than 100kDa caused. So it can be inferred that affecting organic retention rate and membrane fouling was not only related to molecular weight distributing, but also closely related to hydrophilic / hydrophobic properties of organics.4) Compared the changes in membrane flux,retention rate and the proportion of the surface resistance of two operations for 32h and 54h respectively, we discovered that at the initial stage of membrane filtration, soluble organic matter accumulated on membrane surface and formed the concentration polarization resistance which caused a significant drop of membrane permeable flux and great increase of membrane retention rates and membrane surface resistance. As the continuous growing accumulation of organic matter on the membrane surface, solute on the membrane surface diffused and transferred from surface to the inner main-body solution, membrane filtration fall into a short-term special stage. In this stage, there were slight raise of membrane permeable flux and a little decline in retention rate and membrane surface resistance. Then the membrane fouling fall into a stabilization stage. In this stage the filtration cake layer formed on the membrane surface, which made the membrane fouling resistance reach the largest point; NF membrane flux,membrane retention rate and membrane resistance fluctuated mildly. As operate operation time increased, adsorption and gel surface resistance were became more and more larger, yet other resistance (including internal plugging holes and concentration polarization) increased first initially and then gradually decreased.