| Ultrafiltration(UF) is one of the ideal technologies to address water crisis and increasingly strict standards for drinking water quality, but the main operational problem encountered in membrane filtration systems is still membrane fouling. More attention on hydraulically irreversible fouling is paid to in full-scale drinking water treatment plants involving UF membrane as it concerns the stable operation of the plant. Hydraulic irreversible fouling can be controlled by effective hydraulic backwash, thus the usage of chemical cleaning could be reduced and the lifetime of UF membranes could be prolonged, promoting the application of UF membranes in water industry. Up to now, as a routine operation of UF membranes in drinking water treatment, researches about the effect of backwash on hydraulically irreversible fouling are quite rare.In this study, firstly, the UF permeate was used as backwash water to investigate the effects of foulant types, pretreatments and operating conditions on backwash performance of UF membranes. Results showed that backwashing with UF permeate could remove inorganic particles effectively with a backwash efficiency of above 90%, and it was also effective for yeast suspension and integral algae cells. A limited backwash efficiency was achieved for membranes fouled by organics, resulting in severe irreversible fouling, but the presence of particles improved the backwash efficiency to some extent. As for the role of pretreatments, the total fouling of UF membranes could be alleviated by improving water quality to some extent, and thus the hydraulic irreversible fouling was controlled. However, the backwash efficiency was greatly influenced by the types of pretreatments, and coagulation was the most effective method, resulting in the highest backwash efficiency and lowest irreversible fouling. Backwash performance could be improved to a certain extent by optimizing backwash operating condition(i.e., backwash duration, frequency and strength), but the backwash efficiency was still limited when UF permeate was used as backwash water, especially for membrane fouling caused by organics. This was verified by a large full-scale UF plant in practice.Secondly, aiming at organic foulants that could not be removed by conventional backwash, a comparative study on the roles of backwash compositions in membrane fouling alleviation was performed. Using humic acid(HA) and sodium alginate(SA) as feed water, various types of backwash water, including UF permeate, ultrapure water, Na Cl solution, Ca Cl2 solution and HA solution, were employed as the backwash water. The results indicated that the backwash water compositions had significantly affected the alleviation of membrane fouling by hydraulic cleaning. The backwash solutions containing divalent cations or UF permeate significantly decreased the backwash efficiency. Ultrapure water was superior to UF permeate in cleaning organic-fouled membranes, and the backwash water containing monovalent cation or organic exhibited a comparable(or even superior) backwash efficiency to ultrapure water backwash. Variations in total surface tension and Fourier transformed infrared(FTIR) spectra analysis confirmed that the(irreversible) fouling behavior of various compositions of backwash water. The hydraulic irreversibility of UF membrane fouling significantly varied with membrane properties(p ﹤ 0.05), with the irreversibility in the order of PVDF100 > PES100 > CA100> CA30 > CA10. Statistical analysis indicated that the composition of the backwash water had a greater effect than the membrane properties on the backwash efficiency. During the backwashing process, it was simultaneous for the release of organic foulants and Ca2+ from the fouling layer, and the hydraulic irreversible fouling was strongly correlated with the residual foulants amount which was closely related to the amount of residual Ca2+. Mechanism analysis suggested that the alleviation of membrane fouling by hydraulic cleaning was governed by various mechanisms. Ion exchange and release of electric double layer resulted in better cleaning performance when monovalent cation and ultrapure water were involved, respectively. In addition, the presence of competitive complexation involved in HA solution backwash made it more effective than ultrapure water backwash.Further, according to the results above, the role of salt solution backwash in alleviating membrane fouling caused by organics was investigated. The results suggested that the presence of multivalent(e.g., divalent or trivalent) cations in backwash water significantly deteriorated backwash behavior, resulting in a low backwash efficiency, but the adverse impact only occurred when the divalent cations reached a critical concentration. Monovalent salt backwashing proved to be an effective approach for membrane fouling alleviation, and the effectiveness was verified using different types of UF membranes, and by natural organic matter and effluent organic matter. The results about salt concentration suggested that the backwash efficiency was enhanced by increasing the concentrations of salt solutions, but there was a slight decline in backwash efficiency when the salt concentration exceeded 50 mmol/L, probably due to charge screening effect. However, even at the highest ionic strength(600 mmol/L), the backwash efficiency of salt solution was still higher than that of ultrapure water backwash. Membrane fouling was aggravated at lower temperature or higher permeate flux, but monovalent salt solution backwash was still effective for alleviating hydraulically irreversible fouling, with a high backwash efficiency. Meanwhile, the influences of temperature and permeate flux on the backwash efficiency of salt solution backwash were not significant(p > 0.05), but the influence of temperature on the backwash efficiency of UF permeate was relatively significant. As for the operating condition during backwash, the backwash efficiency of monovalent solutions could maintain a high level for the backwash parameters investigated, and the decrease in backwash efficiency only occurred when backwash duration decreased to a value. The results involving temperature, operating flux, and backwash parameters indicated the superior backwash performance of monovalent solutions than UF permeate.This thesis focused on the roles of backwash water quantity and quality in hydraulic irreversible fouling of UF membranes. Based on these results, the approach of backwashing with monovalent solutions for alleviating organic fouling was proposed. This thesis can provide supports for the control of hydraulic irreversible fouling by backwashing in drinking waterworks. |
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