Controlling disinfection by-products and organic fouling by integrated ferrihydrite–microfiltration process for surface water treatment
Graphical abstract
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.
Section snippets
Raw waters
Four source waters were assessed in this study: namely CH1 and CH2, collected from two reservoirs in Chichijima Island (27°N); HA, collected from one reservoir in Hahajima Island (26°N); and AR, collected from the downstream of Ara River (35°N), Saitama prefecture, to represent a mainland surface water in a temperate region of Japan. All samples were pre-filtered through a 0.45 μm PTFE filter (JHWP09025, Merck Millipore Corp., Darmstadt, Germany), hereafter referred to as raw waters (Tables 1
Raw water characteristics and DOM properties
DOM in water sources of the sub-tropical islands presented in higher concentrations of 5.48–6.60 mg-C/L than that in temperate climate (1.02 mg-C/L) (Table 1). CH1, CH2 and HA contained mainly aromatic DOM with high SUVA254 of 3.63–4.14 L/mg-m and were rich in bromide, whereas AR, representing a mainland surface water, had moderate SUVA254 of 2.55 L/mg-m and low bromide concentration.
The origins of DOM were examined by the FI and β/α index. The FI value indicates whether DOM is of more microbial
Conclusions
The following conclusions can be drawn from this study:
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FH exhibits better performance in mitigating membrane fouling than PAC. Both FH and PAC decreased the rate of initial organic fouling by removing polysaccharide-like compounds.
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FH has lower specific cake resistances than PAC.
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DBP formation potential in FH-treated waters is mostly lower than that in PAC-treated waters and, thus, the lifetime cancer risk from THMs through the ingestion route is lower with FH compared to PAC.
Acknowledgements
The authors would like to thank the water treatment plant managers of Ogasawara Village for water sampling. This work was supported by JSPS Grant-in-aid for Scientific Research Nos. 26820221 and 26303013. The FE-SEM analysis was supported by “Nanotechnology Platform” (Project No. 12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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2022, Science of the Total EnvironmentCitation Excerpt :Also, microfiltration membranes have a larger post size than that of the natural organic matter molecules so they can be considered to be ineffective for the removal of natural organic matter. Along with that natural organic matter has a tendency to be bonded with each other in the pores and it will deposit on the surface of the membrane which can cause pore blockage (Chellam and Sari, 2016; Osawa et al., 2017). This pore blockage for the problem can be completed by pre-treatment using coagulation or flocculation.