ABSTRACT
In the future, the ever-growing demand for drinking water will lead many cities to implement indirect water reuse programs, where wastewater effluent is used to augment their drinking water sources. Pollution of those sources with organic micropollutants such as endocrine disrupting compounds (EDCs) and pharmaceutically active compounds (PhACs) which have been detected in water supplies and wastewater effluents around the world pose negative health effects for consumers and the environment (Kolpin et al., 2002; Snyder et al., 2003; Ternes et al., 2004). Membrane filtration technology, particularly nanofiltration (NF) and reverse osmosis (RO), has demonstrated promising results with the rejection of PhACs and EDCs. In order to examine the ability of RO membranes to retain PhACs and EDCs, Kimura et al. (2004) showed that the polyamide membranes exhibited better rejection than cellulose acetate membranes. Results from other investigations showed that, due to electrostatic repulsion, the rejection of negatively charged compounds was effective and varied from 89% to over 95% by NF membranes and exceeded 95% by ULPRO and RO membranes (Kimura et al., 2003b; Xu et al., 2005; Nghiem et al., 2006). The results from a study on removal of hormones and pharmaceuticals from treated sewage indicated that ozonation, microfiltration and nanofiltration were partially effective whereas RO treatment was the most successful in the removal of target residuals (Khan et al., 2004). Ozaki and Li (2002) showed that the rejection of organic compounds by ultra-low pressure RO (ULPRO) increased linearly with the molecular weight and molecular width, while investigating the rejection of DBPs, EDCs and PhACs by polyamide NF/RO membranes. Fouling may alter membrane surface characteristics in terms of the contact angle, zeta potential, functionality and surface morphology, which potentially affect transport of contaminants compared to non-fouled membranes; for instance Ng and Elimelech (2004) observed a decline in the rejection of hormones by RO membranes after colloidal fouling. Furthermore, findings of another study indicated that membrane fouling significantly affected the rejection of organic micropollutants by cellulose acetate RO, NF and ULPRO membranes (Xu et al., 2006). After the organic fouling of membranes, Agenson and Urase (2007) observed a decrease in rejection of high molecular weight (MW) neutral organics by NF/RO membranes; however, rejection of low MW compounds was reported to have increased. In addition, Makdissy et al. (2007) observed lower rejection of EDCs and personal care products (PCPs) by NF membranes fouled by surface water than by clean membranes. As many of these studies illustrate, membrane fouling has the potential to affect rejection mechanisms of
organic solutes as a result of modified electrostatic, steric and hydrophobic/hydrophilic solute-membrane interactions. However, reported results are complicated to follow due to the variety of foulants and particular interactions with each membrane type and feed water composition that leads to a diversity of explanations for observed rejections. This investigation attempts to overcome that diversity using defined groups of organic compounds, well-characterised polyamide nanofiltration membranes and a surrogate foulant. An emphasis on the interaction between clean membranes and compounds is given in this chapter, considering rejections after steadystate saturation of the membrane and adsorption on the membranes. In this chapter, there is a description of the use of sodium alginate as the foulant, a hydrophilic (anionic) polysaccharide, which forms a uniform film on the membrane surface altering the electric and hydrophobic membrane characteristics. In the next chapter, there is an explanation of the differences between the rejection of clean and fouled NF membranes using an additional foulant surrogate (dextran) and natural organic matter (NOM) from surface water.
