Rasgos cognitivos relacionados con la lectura

The results reported here demonstrate a considerable effect of the feed water calcium concentration on membrane fouling. Fouling was most severe at 0.5 mM, whereas at no calcium or at a higher concentration (e.g. 4 mM) membrane fouling was considerably lower. Two major fouling mechanisms, pore blocking and cake layer formation, are as-

Membrane fouling in the nanofiltration of landfill leachate and its impact on trace contaminant removal

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sumed to take place. Both can be caused by solute-solute interactions (bridging and com- plexation) and solute-membrane interactions (bridging and charge neutralisation). The study also showed a significant influence of fouling on the rejection of BPA. Low rejection coincided with low fouling at 0 mM and 4 mM of calcium, respectively. Rejection increased commensurate with the extent of fouling. However, the highest rejection (at 1 mM of cal- cium) was not observed when fouling was most severe (at 0.5 mM of calcium). It was hy- pothesized that pore blocking, which induced an enhanced sieving effect, resulted in an increase in the rejection of BPA. On the other hand, the cake-enhanced concentration polarisation, which hindered back diffusion into the bulk solution, could eventually result in a lower BPA rejection.

Characterising humic acid fouling of nanofiltration membranes using bisphenol A as a molecular indicator

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3

Characterising humic acid fouling of nanofiltration

membranes using bisphenol A as a molecular indicator

3.1

Introduction

Municipal water recycling is an increasingly important strategy for the replacement or augmentation of potable water supplies in many parts of the world. The scope of water recycling schemes is broad, encompassing industrial and agricultural reuse applications, municipal reuse via dedicated recycled water distribution systems and even the direct supplementation of drinking water supplies. Furthermore, the role of membrane filtration in water recycling schemes is steadily growing due to the high water quality that can be achieved at moderate cost (Fane 2007, Verliefde et al. 2007a).

A major benefit of the deployment of membrane filtration technology in water recycling schemes is the removal of micropollutants which are ubiquitously detected in secondary treated effluent at trace concentrations. Despite the importance of membrane filtration processes in the water industry, current knowledge regarding the fundamental mecha- nisms that lead to the separation of trace organic contaminants remains limited. Recent review articles have highlighted the fact that these removal mechanisms are complex and can be governed by many factors including membrane characteristics, physico-chemical properties of the solutes, and solution chemistry (Bellona et al. 2004, Nghiem and Schäfer 2005). Moreover, understanding of the rejection behaviours of trace organic contami- nants under realistic conditions is still quite poor. In particular, very little is known about the effects of membrane fouling, which is inherent in full scale operation, on the rejection of trace organic contaminants (Bacchin et al. 2006, Fane 2007).

Fouling presents one of the most challenging issues to the design and management of membrane filtration systems. It has been defined as ‘the process resulting in loss of per- formance of a membrane due to the deposition of suspended or dissolved substances on its external surfaces, at its pore openings or within its pores (Koros et al. 1996). Although membrane fouling can be reduced through appropriate design, operation and cleaning, some degree of fouling is inevitable in full scale applications (Bacchin et al. 2006, Fane 2007). In municipal water recycling applications, fouling by associated organic matter is expected to be the most prevalent form of membrane fouling. Previous studies have demonstrated that organic fouling is most severe under conditions of low pH, high ionic strength, and particularly in the presence of calcium ions (Li and Elimelech 2004, Manttari

Characterising humic acid fouling of nanofiltration membranes using bisphenol A as a molecular indicator

51

et al. 2000, Schäfer et al. 1998, Yuan and Zydney 2000). Furthermore, fouling of NF mem- branes by organic matter is mainly attributed to pore blocking, which can be classified into three categories: complete pore blocking (blocking of a pore by solutes approximately the same size as the pore), incomplete or intermediate pore blocking, and standard pore blocking (gradual pore narrowing and constriction by adhesive solutes that are much smaller than the pore) (Al-Amoudi and Lovitt 2007). It is noteworthy that most existing studies have investigated organic and colloidal fouling with an emphasis on the impacts on permeate flux and inorganic salt rejection. The effects of membrane fouling on the rejection of trace organic chemicals have been only recently and briefly investigated. How- ever, it has been demonstrated that membrane fouling can significantly influence the sep- aration of trace organic contaminants particularly by NF membranes (Agenson and Urase 2007, Plakas et al. 2006). Xu et al. (2006) showed that membrane fouling caused by sec- ondary treated effluent significantly affected the rejection of trace organics by NF and ultra-low-pressure RO membranes, whereas it was less important for more conventional RO membranes. This observation is consistent with our previous investigation of the influence of organic fouling on the removal of hydrophilic pharmaceuticals by NF mem- branes (Nghiem and Hawkes 2007). In that study, we demonstrated that membrane pore size played an important role in governing the effects of fouling on the rejection of hydro- philic trace organics. Nonetheless, the impact of membrane fouling on other important rejection mechanisms, such as hydrophobic adsorption and electrostatic repulsion, re- mains poorly understood.

This study aimed to elucidate the effects of membrane fouling by close examination of the key mechanisms of rejection of bisphenol A by NF processes. BPA is a ubiquitous trace organic contaminant in secondary treated effluent, but was selected primarily because of its suitable physical dimensions and physicochemical properties to provide indicatively variable rejection behaviour under variable conditions. Three commercially available NF membranes with differ-rent average pore sizes were selected for this investigation. Mem- brane fouling was induced by a foulant mixture containing humic acid as a model organic foulant in a background electrolyte solution. The effects of membrane fouling on the re- jection of BPA were observed with respect to the membrane pore sizes and the fouling characteristics. Mechanisms possibly accountable for the effects of membrane fouling on BPA rejection were systematically investigated and proposed.

Characterising humic acid fouling of nanofiltration membranes using bisphenol A as a molecular indicator

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