| The ultrafiltration technology is promising in water treatment industry, but the existence of membrane fouling will affect the economy and safety of its use. So to explore the formation mechanism of membrane fouling and seek effective way to control membrane fouling has become a hotspot in this field. This study identifies the typical Fluorescent natural organic matter fractions for the PVDF membrane in raw water, and two pretreatment methods are selected: coagulation pretreatment and powdered activated carbon(PAC), to investigate the membrane fouling control strategies.This research collected ten kinds of raw water samples from different seasons and water sources. Raw water were directly filtrate through PVDF membrane and trans-membrane pressure(TMP) were recorded to calculate the Rirr and Rrev. It is shown that the removal efficiency of DOC and UV254 has strong Positive correlation with their initial concentration, which means that soluble organic matter is the main reason for the formation of membrane fouling. Fluorescence EEM spectroscopy matrix coupled with parallel factor analysis were used to identify the main fluorescent components in raw water. The results show that there are four fluorescent components:component one(Em:390~430 nm,Ex:220~240(300~330) nm))、component two(Em:330~350 nm,Ex:220~235 nm(270~290 nm))、component three(Em:450~500 nm,Ex:240~280(370) nm)、component four(Em:290~330 nm,Ex:220~240nm(275 nm)). Components one and four are microbially and terrestrially derived humic-like matter respectively. Components two and four are Tryptophan and Tyrosine protein-like substance. Among which the Component two have strong Positive correlation with irreversible membrane fouling. Multiple linear regression were used to assess the combined contribution of two components represent of protein-like substances and humic-like substances respectively. The results improve the existence of combined contribution of component two and three. Chemical cleaning by Na Cl O and Na OH can prove the conclusion above.Three kinds of coagulants such as polyaluminium chloride and ferric chloride and ferrous sulfate were compared by the removal rate of the organic matter in raw water. The polyaluminium chloride were identified as the best one. For the fluorescent component 1 & 3 representing of humic-like substances, fluorescent component 2 representing of protein-like substances, polyaluminium chloride can remove respectively 75.6% and 263.8%, 15.3% higher than that removed by ferric chloride. By adjusting the p H value with Na OH & HCl, we found at the low p H condition the higher removal rate of UV254 and DOC and low Rirr were achieved than the condition at high p H value, proved that the Zeta potential value had a great influence on the coagulation progress. Compare conventional coagulation process with micro-flocculation with UF in different coagulant dosage, we found that when the polyaluminium chloride dosage is less than 2 mg/L, the flocs in membrane pool will alleviate the membrane fouling, when dose more than 3 mg/L, the membrane fouling is aggravate by the flocs. The SEM images show that the flocs coated in the surface of membrane will maitain algae and other impurities.Two Powdered activated carbon adsorption model fitting were carried out, among which the Freundlich model is proved better than the Langmuir model. The humic-like substance adsorbed fast than the protein-like substance. In the study of adsorption kinetics, the pseudo-second-order kinetic model was justified better than the pseudo-first-order kinetic model. When the dosage of Powdered activated carbon was 10 mg/L, the adsorption rate is the fastest. When the dosage of Powdered activated carbon was less than 30 mg/L, the PAC in the membrane pool will alleviate the membrane fouling, but when the dosage was over 40mg/L, the PAC will aggregate the fouling badly. The SEM images show that the powder gathered as the carbon layer on the membrane surface will intercept the impurities such as algae in the water followed by aggravation of membrane fouling. |
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