Controlling disinfection by-products and organic fouling by integrated ferrihydrite–microfiltration process for surface water treatment

Highlights

• This study delineated the role of FH in the hybrid adsorption–MF process.

• DOM in source waters of sub-tropical islands is greater than temperate region.

• FH reduced initial fouling caused by DOM when organic fouling is the main mechanism.

• FH + MF is more effective for raw water with DOM > 3 kDa than PAC + MF to control DBPs.

• Formation potential of CHCl₃, C₂HCl₂N, C₃H₃Cl₃O was controlled efficiently by FH + MF.

Abstract

Controlling organic fouling and removing disinfection by-product (DBP) precursors remain challenges for the hybrid membrane process. Whether ferrihydrite (FH), an amorphous iron oxide, can achieve both targets when used as an adsorbent is unknown. Batch experiments of the integrated FH adsorption–microfiltration (MF) process were performed to study the effects of organic components on mitigating membrane fouling and reducing DBP formation, in comparison to powdered activated carbon (PAC). Both FH and PAC decreased the rate of initial organic fouling by removing DOM causing pore-fouling, which was probably polysaccharide-like compound. Afterwards, when cake fouling predominates, FH had smaller specific cake resistances than PAC. FH + MF was more effective for raw waters containing DOM with larger molecular sizes (>3 kDa) than PAC + MF. DBP formation potential in FH-treated waters is mostly lower than that in PAC-treated waters. This promotes a greater reduction of lifetime cancer risk from trihalomethanes (THMs) through the ingestion route when compared to MF and PAC + MF by lowering the formation potential of bromodichloromethane that produces a high unit risk among THM species. In general, FH exhibits better performance than PAC at small filtered volumes (<336 L/m²).

Introduction

Membrane fouling is always a main concern in low-pressure membrane filtration i.e. microfiltration (MF) and ultrafiltration (UF). This problem also happens when MF or UF is used in natural water treatment, among which the main reason causing membrane fouling is natural organic matter (NOM) [1], [2], [3]. NOM, which is considered as precursors of disinfection by-products (DBPs) [4], can be classified into two categories: the humic and non-humic fractions [5], [6]. The humic fraction that comprises humic and fulvic acids is more hydrophobic than the non-humic fraction (comprising mainly amino acids, proteins and polysaccharides). A fraction of NOM adsorbed within the pores or onto the surface of the MF and UF membranes would trigger organic fouling that causes the loss of membrane permeability [1], [7]. Several past studies identified that humic acid is responsible for organic fouling (e.g. [3], [8], [9]) while a few studies indicated that a small fraction of DOM, which is probably polysaccharide-like compound, contributes to the organic fouling [1], [2], [7].

Ferrihydrite (FH) is a poorly crystalline iron oxide that has high reactivity and large specific surface area. While certain types of iron oxides have been used to remove dissolved organic matter (DOM) from surface waters, their potential for controlling membrane fouling and DBP formation in drinking water is unclear. Most of the previous research on the integrated adsorption–low-pressure membrane process using iron oxides has focused on fouling of relatively hydrophobic membranes by DOM surrogates or naturally occurring DOM (e.g. [7], [10]). These researches employed iron oxides for pre-adsorption, where the adsorbent is mixed with water and subsequently removed before the treated water is fed to the membrane [11], though a few studies have employed a direct filtration, wherein the suspension of iron oxides in water is directly applied to the membrane [9], [12]. Among various organic foulants, hydrophobic organic foulants were reported to be easily removed by adsorbents, whereas hydrophilic foulants such as polysaccharide-like compounds were not removed by mesoporous resin [7]. Thus, this study investigated if mesoporous FH can control the organic fouling including polysaccharide-like foulants.

Several researchers have claimed different NOM fractions removable by iron oxides, ascribed to diversity of NOM characteristics in natural waters. It was found that iron-oxide coating of adsorbents removed only small amounts of fulvic acid [13] and humic acid [8]. In some studies, iron-oxide coating of particles were effective in removing humic fraction of NOM and controlling chlorinated DBP formation [14] while FH particles adsorbed large NOM molecules (>30 kDa) and reduced reversible fouling of the 100-kDa UF membrane [11]. FH impregnation of powdered activated carbon (PAC) enhanced adsorption of phenolic compound in the presence of NOM [15]. In these studies, surface complexation–ligand exchange interactions are considered to play a key role in the adsorption process. A NOM fraction non-adsorbable to FH, however, could potentially form DBPs after reacting with disinfectants. There is no study in the past evaluating the effects of organic components on controlling both DBP formation and organic fouling.

The focuses of this study are experimental analysis on changes in the DBP components and mitigation of membrane fouling in the hybrid–MF treatment using FH. Four natural waters, which were taken from different sources in temperate and sub-tropical regions of Japan, have been evaluated to determine the potential for formation and relative distribution of DBP species, particularly brominated DBPs that are more toxic than their chlorinated analogues. The DBPs studied include regulated (i.e. trihalomethanes, THMs) and non-regulated DBPs (i.e. haloacetonitriles, HANs; haloketones, HKs) that have adverse health effects to humans. This is the first study that has reported the formation potential of THMs, HANs and HKs, as well as the lifetime cancer risks associated with an oral intake of THMs in chlorinated waters after the hybrid-MF treatment. This study is also filling the gap between efficient removal of DBP precursors and controlling membrane fouling in the hybrid MF-treatment process using FH. Organic fouling of the hybrid MF system, using a polyvinylidene fluoride (PVDF) membrane, was evaluated in the direct filtration approach in terms of initial fouling rate and relationships between major foulants and performance of FH in comparison with PAC.