Combination of coagulation and ion exchange for the reduction of UF fouling properties of a high DOC content surface water
Introduction
Natural organic matter (NOM) consists of a complex mixture of organic compounds that can be basically divided into two main categories of molecules: identified biopolymers and humic substances. On the one hand, identified biopolymers, such as polysaccharides, amino sugars and proteins are mainly derivated from microorganisms and from the degradation of animal and plant tissues. On the other hand, humic substances are known to be yellow to black high molecular weight (MW) molecules, whose nature has not been properly chemically defined yet and that originate from a complex process called humification. From a practical point of view, the term humic substances generally refers to the organic material that is extracted according to the protocols defined by Thurman and Malcolm (1981) or Leenheer (1981).
The presence of NOM in source water adversely affects drinking water treatments and water quality of produced water. NOM is thus known to increase disinfectant and coagulant demand, generates potentially harmful disinfection by-products, fouls membrane and favors biological regrowth in the distribution network.
Conventional water treatment, using coagulation/flocculation, is the most frequent process for drinking water treatment targeting NOM and turbidity removal. However, alum or iron coagulation only removes a portion of NOM (40–70% of total organic carbon (TOC) removal depending on water quality and water treatment conditions (Croué et al., 1993; Edzwald, 1993; Owen et al., 1993; Eikebrokk, 1999)), especially organic compounds of higher MW (Lefebvre and Legube, 1993; Krasner and Amy, 1995; Bolto et al., 2002). As a consequence, drinking water utilities tend to improve or develop new technologies for producing waters with a low dissolved organic carbon (DOC) content.
While ultrafiltration (UF) is considered as a very effective technology for removing turbidity and bacteria, NOM rejection is often limited to high MW and highly charged hydrophobic organic compounds such as humic substances, and severe NOM fouling is experienced (Cho et al., 2000a, Cho et al., 2000b, Cho et al., 2002 2002; Schäfer et al., 2000; Ribau Teixeira and Rosa, 2003). In the UF range (1–500 kDa), NOM rejection depends greatly on the membrane MW cut-off (MWCO), and the lower the MWCO the higher the rejection. Besides steric exclusion and in the case of tight UF membranes (i.e. MWCO <10 nm), other mechanisms like charge effects (i.e. electrostatic repulsion) can also play a significant role in NOM rejection depending on NOM and membrane characteristics (Cho et al., 2000a, Cho et al., 2000b, Cho et al., 2002; Ribau Teixeira and Rosa, 2003). For instance, it has been shown that the higher the (negative) charge density of the membrane and the aromatic character of NOM (in terms of humic content and/or the specific UV absorbance or SUVA), the higher the rejection rate (Cho et al., 2000a, Cho et al., 2000b, Cho et al., 2002). Although DOC rejection rates by tight UF membranes can reach up to 90% with synthetic waters prepared with humic acids or with natural waters rich in humic substances (humic content >90%) (Cho et al., 2000a, Cho et al., 2000b, Cho et al., 2002; Ribau Teixeira and Rosa, 2003), NOM removal rates by UF membranes only ranged from 10% to 50% in a general way (Cho et al., 2000a, Cho et al., 2000b, Cho et al., 2002; Schäfer et al., 2000; Ribau Teixeira and Rosa, 2003). Therefore, appropriate pre-treatment for raw water treatment is often needed.
In very recent years, ion exchange processes have received considerable attention with the use of a new magnetic ion exchange resin: the MIEX® resin (Singer and Bilyk, 2002; Drikas et al., 2003; Fearing et al., 2004; Allpike et al., 2005; Humbert et al., 2005). Two main features make this strong anion exchange resin (AER) different from traditional ones (http://www.miexresin.com). First, the MIEX® beads (around 150 μm) are 2–5 times smaller than conventional AERs, which provide a greater external surface area and thus allow rapid sorption kinetics. Second, the MIEX® backbone contains a high proportion of a magnetic iron oxide compound, making fine resin beads agglomerate into larger, fast-settling particles. The MIEX® process corresponds to an innovative water treatment process based on this strong AER that incorporates contactors, a settler (where the separation between the resin and the treated water is operated) and a continuous resin regeneration/recirculation side process.
The objective of this study was to determine the efficacy of ion exchange for removing dissolved organic matter (DOM) from a high DOC content surface water and reducing its low-pressure membrane fouling properties (ultrafiltration membrane). The performances of selected anion exchange resins (MIEX® from Orica and IRA958® from Rohm and Haas) were compared with conventional treatment (i.e. coagulation/flocculation). The combination of conventional treatment and AER treatment was also investigated. DOM removal was monitored using UV absorbance at 254 nm (UV254), dissolved organic carbon (DOC) and high-performance size exclusion chromatography with UV detection (HPSEC/UV), DOC detection (HPSEC/DOC) and specific fluorescence detection (HPSEC/FLUO).
Section snippets
Characteristics of the studied waters
Raw waters were collected at different periods of the years 2003 and 2004 from the Villejean/Rennes drinking water treatment plant (WTP) in Brittany (France). Clarified waters were either collected at the plant or prepared from jar test experiments. At the WTP, the coagulation/flocculation step was operated using a mixture of 12.5 mg Fe/L (FeCl3) and 11 mg Al/L (Al2(SO4)3) at pH 5.8 followed by sand filtration, which correspond to the optimal conditions for DOM removal (based on lab-scale tests).
NOM removal by ion exchange: batch kinetic tests
Figs. 1a and b plot the removal of (a) DOC and (b) UV254 as a function of contact time for the Villejean raw water treated with IRA958® or MIEX® resins. Similar plots were obtained with all raw water samples collected at different periods of the year (not shown).
Both parameters show a rapid decrease as a function of time. DOC content of raw water dropped from 6.2 to less than 2 mg/L after 45 min of contact time with the two resins. In French regulation, the value of 2 mg/L corresponds to a
Conclusions
Although AER treatment was shown to be very effective in reducing the high DOC content of the Villejean raw water (80% DOC removal), both MIEX® and IRA958® resins were found to have no impact on the reduction of UF membrane fouling (100 kDa). By comparison, clarification (i.e. coagulation/flocculation) produced a higher DOC content water (about 2.5 mg C/L) as compared with AER treatment (about 1.5 mg C/L), but led to a more significant reduction of the membrane fouling properties of the raw water.
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