Control Of Irreversible Ultrafiltration Membrane Fouling By Using Mesoporous Adsorbent Resin

The combination of adsorption pretreatment with ultrafiltration is a promising water treatment technology with broad application prospect. However, adsorption pretreatment with frequently-used adsorbents, such as activated carbon, can not alleviate membrane fouling efficiently. Research and development of adsorbents that can effectively control hydraulicly irreversible UF membrane fouling is of great significance for the widespread application of adsorption-ultrafiltration process. This study employed a new adsorbent(mesoporous adsorbent resin, MAR) to mitigate irreversible membrane fouling, with powdered activated carbon(PAC) as reference. The influence of adsorbent characteristics, ionic conditions, natural organic matter(NOM) properties, membrane surface properties and process layout on the performance of adsorption pretreatment was systematically investigated. Meanwhile, the mechanisms of membrane control by adsorption pretreatment was discussed through the analysis of fouling resistance distribution, the migration of NOM, the observation of cake layer structure and the characterization of organic matter properties.The effects of ionic conditions on fouling control efficiency of MAR and PAC were evaluated using humic acid(HA) as the model foulant. The results showed that the interactions between organic molecules and the membrane and those between organic molecules increased with the rise of ionic strength and calcium concentration, which resulted in the decrease of fouling reversiblity. Meanwhile, the strengthen of the interactions between organic molecules and the adsorbents promoted the removal of organic matter and increased the fouling control efficiency of adsorption pretreatment. Adsorption pretreatment controls membrane fouling by reducing the mass of foulants deposited on/within the membrane. MAR could readily adsorb organic molecules with high molecular weight(MW) because of its mesoporous structure, whereas the microporous structure of PAC restricted the adsorption of organic molecules with high MW and it mainly removed low MW organic matter that could pass through the membrane. Therefore, as the increase of ionic strength and calcium concentration, fouling control efficiency of MAR pretreatment increased more rapidly and it was superior to PAC pretreatment in irreversible fouling control under most ionic environments examined in this study.Performance of adsorption pretreatment in mitigating membrane fouling caused by different NOM fractions was systematically investigated by using two membranes with different surface properties and HA, tannic acid(TA), bovine serum albumin(BSA) and sodium alginate(SA) as representatives of different NOM components. Meanwhile, the influence of whether adsorbent particels were removed before UF was also considered. The results indicated that although membrane fouling and its control by adsorption pretreatment varied significantly with NOM characteristics, fouling control efficiency of adsorption pretreatment was not influenced by membrane surface properties. No matter MAR particles were removed before UF or not, MAR adsorption exhibited remarkable performance in alleviating HA and BSA fouling, but was somewhat inefficiency in TA fouling control. In contrast, PAC pretreatment slightly ameliorated HA fouling when PAC particles were removed before UF, whereas HA fouling was exacerbated by PAC pretreatment with PAC particles present in UF feed water. Both TA and BSA fouling were moderately controlled by PAC adsorption, but the presence of PAC particles in UF feed water enhanced TA fouling whereas compromised BSA fouling control. Moreover, neither MAR nor PAC pretreatments visibly influenced SA fouling. The above analysis suggested that MAR pretreatment mainly mitigated membrane fouling caused by hydrophobic fractions with high MW, while PAC pretreatment alleviated membrane fouling caused by hydrophobic fractions with low MW better. However, neither of them could effectively control membrane fouling caused by hydrophilic fractions.Efficiency of adsorption pretreatment in alleviating membrane fouling during UF of natural waters was examined by using Songhua river water and algal organic matter(AOM) contaminated water as representatives of typical surface waters that are abundant in allochthonous and autochthonic organic matter respectively. Meanwhile, integrated adsorption-UF system imitating full-scale operation were used to carry out long-term filtration tests. The results showed that the two feed water differed significantly in NOM MW distribution, hydrophobic/hydrophilic distribution and fluorescence spectrum, but MAR adsorption outperformed PAC adsorption in membrane fouling control for both Songhua River water and AOM contaminated water. In the long-term UF experiments, membrane fouling in MAR adsorption-UF process was much lower than that in UF alone process for both feed water. In contrast, although membrane fouling in PAC adsorption-UF process was lower than that in UF alone process when treating AOM contaminated water, PAC particles and NOM molecules interacted with each other and accumulated on membrane surface significantly during UF of Songhua River water, resulting in much severer membrane fouling in PAC adsorption-UF process than in UF alone process.