| Summary: | 碩士 === 國立成功大學 === 環境工程學系碩博士班 === 91 === Nnanofiltration membrane can remove greater part of natural organic matter (NOM), precursor of disinfection by-products, and multivalent ions. Its rejection of monovalent ions is much lower than that of multivalent ions, and therefore retains some of the mineral substances in the permeate. However, the major problem of applications is the fouling of membrane. In this research, Rapid Bench Scale Membrane Test (RBSMT) is used to study the factors affecting the fouling of the NF membrane. Those factors studied include membrane surface morphology, cross-flow velocity, and water quality of the feed water, such as pH, type and concentration of the solute and suspended solids.
The results show the rougher the membrane surface is, the easier the fouling in valley, and unfavorable in flux recovery rate by cleaning. Increasing cross-flow velocity could reduce concentration polarization and fouling rate, and increase rejection rate for calcium and conductivity. However, its affect on dissolved organic carbons (DOC) rejection was minor.
And impurities rejection increased with increasing pH. This probably is due to the higher electrostatic repulsive force between impurities and membrane surface, as both usually gain more negative charge at higher pH. For feed water consisted of pure water and background electrolyte (0.003 M NaClO4), the flux has highest value at isoelectric point of the membrane. For feed water contained calcium and organics, flux was mainly affected by complexed calcium.
For single species impatrity testing, comparing the same concentration of salicylic acid, alginic acid, humic acid, 0.1 μm SiO2, 0.01 μm SiO2, kaolin, bentonite and Ca2+, results show that NF membrane have the same flux decline under the same operation condition, regardless of the kinds of solutes, except for salicylic acid. The higher the pH value, the higher the flux decline rate. For feed water contained organics without calcium, flux was mainly affected by their solubility in water. And when impurity concentration was in certain range, after cake formation and initial flux decline, the flux value would remain at certain value for long time, even without cleaning.
For dual impurities testing, it is noticed that systems containing dissolved organic matter (DOM) and calcium have the highest fouling rate. This is probably due to the formation of tenacious gel layer, made from the adsorption of DOM-Ca complex membrane surface. Therefore, in the preatment of NF, the removal of either DOM or Ca is more important than removing particles.
When DOM was mixed with Ca, the fouling of alginic acid was more serious than humic acid, probably because the quantity of complexation formation of the former was larger than that of the latter. This also indicates that when the source water of a NF system is entrophic, the pretreatment to prevent excessive fouling is especially important. Comparing the fouling phenomena with feed water under various concentration ratio of Ca and DOM, we notice that the role palyed by DOM is more significant than that of Ca or particles.
Finally, comparing the correlation between surface deposition quantity and flux decline rate versus that between calcium complexation qantity and flux decline rate, it was found the correlation between the latter was stonger, no matter the DOM involved was humic acid or alginic acid. Therefore, calcium complexation quantity may be used as fouling index in the field.
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