Effect of chlorine on adsorption ultrafiltration treatment for removing natural organic matter in drink2

Effect of chlorine on adsorption ultrafiltration treatment for removing natural organic matter in drink2

Journal of Colloid and Interface Science 274 (2004) 587–593 www.elsevier.com/locate/jcis Effect of chlorine on adsorption/ultrafiltration treatment for removing natural organic matter in drinking water Tae-Wook Ha, a Kwang-Ho Choo, b,∗ and Sang-June Choi b a Department of Environmental Engineering, Taegu Science College, Buk-Gu, Daegu 702-724, South Korea b Department of Environmental Engineering, Kyungpook National University, Buk-Gu, Daegu 702-701, South Korea Received 29 July 2003; accepted 5 March 2004 Available online 16 April 2004 Abstract In drinking water treatment, prechlorination is often applied in order to control microorganisms and taste-and-odor-causing materials, which may influence organics removal by adsorption and membrane filtration. Thus, the addition of chlorine into an advanced water treat- ment process using a hybrid of adsorption and ultrafiltration (UF) was investigated in terms of natural organic matter (NOM) removal and membrane permeability. A comparison between two adsorbents, iron oxide particles (IOP) and powdered activated carbon (PAC), was made to understand the sorption behavior for NOM with and without chlorination. Chlorine modified the properties of dissolved and colloidal NOM in raw water, which brought about lower TOC removal, during IOP/UF. The location of IOPs, whether they were in suspension or in a cake layer, affected NOM removal, depending on the presence of colloidal particles in feedwater. Chlorine also played a role in reducing the size of particulate matter in raw water, which could be in close association with a decline in permeate flux after chlorination.  2004 Elsevier Inc. All rights reserved. Keywords: Chlorination; Iron oxide; Adsorption; Ultrafiltration; Water treatment 1. Introduction Ultrafiltration (UF) processes are increasingly popular in drinking water treatment to produce better quality water and meet more stringent treatment regulations, particularly con- cerning removal of pathogens and turbidity [1–10]. How- ever, the widespread use of UF membranesin drinking water treatment is still limited due to two major drawbacks, mem- brane fouling and insufficient removal of disinfection by- products (DBPs) precursors [11–16]. Furthermore, several studies on the membrane treatment of surface, lake, and river waters have demonstrated that the DBPs precursors of nat- ural organic matter (NOM) were one of the major foulants of the membranes [17–22]. To overcome such problems caused by NOM in UF ap- plications, conjunctive use of adsorbents and membranes is thus becoming more attractive for water treatment because the adsorbents can capture and retain NOM before it reaches the membrane surface [13–15,23–28]. The combination of UF with powdered activated carbon (PAC) adsorption re- * Corresponding author. Fax: +82-53-950-6579. E-mail address: chookh@knu.ac.kr (K H. Choo). vealed that PAC could play a role in removing NOM from water but is not always helpful in reducing membrane foul- ing [14,29,30]. Additionally, a large amount of spent carbon sludge that should be disposed of was generated in the com- bined process. Thus, an alternative adsorbent that can be readily regenerated as such iron oxide particles (IOP) has re- cently been developed and applied to water treatment in con- junction with UF membranes [26–28]. The addition of IOP into UF systems contributed to both an increase in NOM re- moval efficiency and a decrease in membrane fouling. How- ever, the interaction of IOPs with NOM and membranes was not fully understood, nor was the effect of water chemistries on IOP adsorption well evaluated. Oxidation of raw water would have been done using chlo- rine or ozone to control metals and microbial growth in potable water treatment, which exerted an additional effect on the coagulation efficiency [31,32]. The oxidation step im- proved or deteriorated the coagulation process while chang- ing the nature of the water to be treated. No information is available, however, on whether preoxidation could affect the treatment efficiency by adsorption and membrane filtration for water treatment. 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.010 588 T W. Ha et al. / Journal of Colloid and Interface Science 274 (2004) 587–593 The comparison of sorption behavior between PAC and IOP would help understand the characteristics of adsorbents with different surface structures and functionalities. Analy- ses of the effects of chlorination of raw water during adsorp- tion and UF could also establish some correlation of NOM removal and membrane permeability with alteration of the physicochemical nature of NOM and colloids. It may thus provideinsight into the phenomenaoccurringat the interface among the membranes, the adsorbents, and the adsorbates present in intact and chlorinated waters. In this work, therefore, the characteristics of adsorptive NOM removal by IOPs were investigated in a combined IOP/UF system with and without chlorination. The adsorp- tion behavior of IOP was also compared with that of PAC at different adsorbent doses. In particular, the influence of prechlorination on IOP/UF was evaluated in terms of NOM removal and membrane permeability, since chlorine was of- ten dosed into raw water to control microbial growth and taste-and-odor-causing materials. Also, the effect of parti- cles in the system, such as IOPs and particulate colloids, on NOM removal efficiency was explored during UF. 2. Materials and methods 2.1. Raw water Water samples used for this study were obtained from the Maegok Water Treatment Plant, which receives raw water from the Nakdong River, the major water source of the city of Daegu, Korea. The water samples collected were moved to the laboratory and then stored at 4 ◦ C before use. The key water quality characteristics are given in Table 1. To remove colloidal particles from raw water, it was pretreated, using a 0.45-µm filter, and so the filtrate was defined as prefiltered water. 2.2. Adsorbents Two adsorbents, powdered activated carbons (PAC) and iron oxide particles (IOP), were employed in this work. A commercially available PAC adsorbent was purchased from Dae Jung Chemicals and Metals (Korea), while an amorphous IOP slurry (10 g/L as Fe) was prepared in the laboratory by neutralizing a ferric chloride solution using 5 N NaOH [33]. The PAC and IOP adsorbents used had av- erage surface areas of 1400 and 240 m 2 /g, respectively. Table 1 Quality of raw water from the Nakdong River Parameter pH Alkalinity (mg/L as CaCO 3 ) Hardness (mg/L as CaCO 3 ) Turbidity (NTU) DOC (mg/L) UV absorbance at 254 nm (cm −1 ) Value 7.4–7.8 59–71 95–97 31–39 2.4–2.9 0.071–0.079 2.3. Chlorination A sodium hypochlorite dosing solution with 1000 mg/L free chlorine was prepared from a 12% sodium hypochlo- rite solution. Chlorine dosages were changed over the range from 0 to 11mg/LasCl 2 at nativepH. Normally, a high con- centration of chlorine (11 mg/LasCl 2 ) was applied in order to expedite the chlorination reactions within a short period of time. The chlorination steps lasted 30 min and then resid- ual free chlorine was immediately quenched by the addition of sodium sulfite. 2.4. Adsorption tests To evaluate efficiencies of NOM removal by different ad- sorbents, such as IOP and PAC, adsorption isotherm tests were performed at various adsorbent doses. For IOP adsorp- tion tests, a certain amount of the stock IOP slurry (cor- responding to 0–100 mg/L as Fe) was placed into several 300-ml Erlenmeyer flasks and the solution was then mixed at 200 rpm and 20 ◦ C for 30 min. The solution was filtered through a 0.45-µm filter (Millipore, USA) and the filtrate samples collected were used for analyses. PAC adsorption tests with a PAC dosing range of 0–100 mg/L were per- formed in the same manner as described above. 2.5. UF membranes and stirred cell UF tests The UF membranes used in this study was made of poly- ethersulfone and had a molecular weight cutoff of 100,000 and an effectivesurface area of 28.7 cm 2 . All the membranes used were initially rinsed, using pure water as mentioned in the instructions provided by the manufacturer. As shown in Fig. 1, batch UF experimentswere performedusing a 180-ml stirred cell plus an 800-ml reservoir (Amicon 8200, USA). So the overall working volume of the batch unit was 980 ml. The applied pressure was kept at 0.49 bar using a nitrogen cylinder, while the stirring speed was adjusted to 160 rpm using a magnetic stirrer. During UF, the mass of permeate was measured using an electronic balance and recorded si- multaneously on an on-line personal computer to calculate permeation flux. Fig. 1. Schematic of a stirred-cell UF system unit with a reservoir. T W. Ha et al. / Journal of Colloid and Interface Science 274 (2004) 587–593 589 2.6. Analytical methods Feedwater and permeate samples were analyzed for UV absorbance at 254 nm and total organic carbon (TOC) con- centrations. Light absorbance at 254 nm, which is associated with the aromatic groups in NOM, is used as a surrogate parameter for monitoring the concentration of dissolved or- ganic matter in a fast and easy manner during water treat- ment [34]. The UV absorbance was determined using a UV/vis spectrophotometer (Hewlett Packard 8452A, USA) and the TOC concentration was measured using a TOC ana- lyzer (Sievers 8200, USA). In particular, the UV absorbance and the dissolved organic carbon (DOC) concentration of raw water were measured after removal of particulate matter using a 0.45-µm membrane (Millipore, USA). Free chlo- rine concentrations were determined by a DPD colorimetric method using a Hach spectrophotometer (DR2500, USA) and reagent pillows. When the prechlorination of raw water was applied in the experiments, the initial UV absorbance was defined as the UV absorbance of raw water after chlorination and used for the evaluation of UV absorbance removal efficiency. 3. Results and discussion 3.1. Comparison of sorption behavior of NOM onto IOP and PAC Fig. 2 shows UV removal efficiencies for treatment of prefiltered Nakdong River water at various dosages of IOP and PAC. The UV removal efficiency increased sharply, to approximately 40%, with increasing IOP dosages and nearly leveled off at such a small dosage as 10 mg Fe/L, whereas a gradual increase in UV removal was achieved with higher PAC dosages. The distinctively different trends in adsorp- tive NOM removal for IOP and PAC could be attributed primarily to the discrepancy in structures and sorption mech- Fig. 2. Comparison of UV removal efficiency between IOP and PAC adsor- bents: adsorption time, 30 min. anisms of the adsorbents. The majority of active sorption sites of nonporous IOP are located on the outer surface lay- ers, with an amorphous structure which is wide open to the bulk solution phase. So NOM removal by IOP sorption can occur rapidly through surface coordinative reactions with- out any limitation on mass transfer of NOM molecules. In case of PAC, however, most of the active surface sites are present inside the pores in which NOM sorption normally occurs through van der Waals attraction. The rate of sur- face diffusion of NOM molecules inside PAC pores should be very low because of high molecular weights, so the ad- sorption equilibrium would take more than a few days. In fact, the initial external mass transfer coefficient of IOP (1.17 × 10 −7 m/s), which was determined in a complete mixed reactor, was nearly one order of magnitude higher than that of PAC (1.26 × 10 −8 m/s). In this regard, IOP could be a better adsorbent for removing NOM within lim- ited hydraulic residence times. If the combination of adsorp- tion and membrane separation (e.g., UF or MF) is applied to drinking water treatment, residence times in the systems must be less than a few hours and thereby IOP would be more desirable due to faster adsorption. Fig. 3 compares the effect of prechlorination on UV re- moval efficiency by IOP and PAC during the treatment of prefiltered water. The UV removal efficiency for river wa- ter by IOP decreased with higher dosages of chlorine, while no reduction in UV removal efficiency occurred for PAC ad- sorption. A possible explanation of the difference between IOP and PAC adsorption abilities after chlorination is that chlorine reacts with NOM to form DBPs such as THMs, which are not adsorbable onto IOP but still adsorbable onto PAC. As a result, it was found that the prechlorination of raw water could have an effect on NOM removal efficiency by IOP. However, furtherstudies on the influence of chlorine during treatment of raw water by IOP adsorption and UF are needed (which will be discussed in the next sections). Fig. 3. UV removal efficiencies using IOP and PAC adsorbents at different chlorine dosages: chlorination time, 30 min; IOP dose, 100 mg/LasFe; PAC dose, 80 mg/L. 590 T W. Ha et al. / Journal of Colloid and Interface Science 274 (2004) 587–593 (a) (b) Fig. 4. Effect of prechlorination on UV and TOC removal efficiencies in IOP/UF and UF only, treating (a) prefiltered and (b) raw waters: IOP dose, 100 mg/L as Fe; chlorine dose, 11 mg/LasCl 2 ; chlorination time, 30 min. 3.2. Interaction of chlorine with dissolved and colloidal NOM in IOP/UF To examine the interaction of chlorine with dissolved NOM and colloidal particles during IOP/UF and UF alone, UV and TOC removal efficiencies using prefiltered and raw water were compared and are shown in Fig. 4. During IOP/UF treatment of prefiltered water samples, there was a negligible decrease in TOC removal efficiency with chlori- nation, though the UV removal efficiency decreased by ap- proximately 10%. For raw water, however, the TOC removal efficiency also decreased comparatively significantly with prechlorination in IOP/UF. On the other hand, in UF treat- ment alone, the UV removalefficiency declined slightly with prechlorination, but negligible TOC removal occurred for both waters. Thus, the lower UV and TOC removal efficien- cies with prechlorination in IOP/UF might be attributed to a decrease in the reactivity (adsorption ability) of NOM after chlorination. The specific UV absorbance (SUVA) related Fig. 5. Variation of UV of different water samples with time during chlo- rination: chlorine dose, 11 mg/LasCl 2 . Particles collected from raw water were obtained using a 0.45-µm filter and then resuspended in pure water. to the dissolved portion of organic matter in river water de- creased with higher chlorine doses (e.g., from 1.95 L/mg m with no chlorine addition to 1.57 L/mg m with a chlorine dose of 11 mg/LasCl 2 ), suggesting that the reactivity of TOC in river water was lowered by chlorination. However, it was not enough to explain why the TOC re- moval efficiency for raw water decreased more significantly than that for prefiltered water. Thus, the variation of UV ab- sorbance differentials (UV) for three feedwater samples of raw and prefiltered waters and a particle suspension was examined in the course of chlorination time (Fig. 5). The UV value for the prefiltered and raw water samples de- creased from 0 to −2.1m −1 and 0 to −1.2m −1 , whereas UV increased from 0 to 0.65 m −1 with the suspension containing 30 NTU of colloidal particles from river water. These results implied that during chlorination the structure of dissolved NOM was modified, leading to a decrease of UV absorbance, but some organic matter could be released from colloidal particles to supply more carbon into the bulk liquid phase. When a feedwater sample containing 100 NTU of particles was chlorinated, UV increased up to 1.0 m −1 within 30 min of reaction time (data not shown). Conse- quently, during prechlorination the breakage of aromatic moieties in NOM molecules occurred without mineraliza- tion of NOM, which reduced the sorption capacity of NOM. The conversion of particulate matter to dissolved matter by chlorination should contribute to an increase of carbon con- tent in the dissolved NOM pool, resulting in lower DOC removal during IOP/UF. 3.3. Effect of IOP deposition and colloidal particles on NOM rejection Since IOPs injected into the IOP/UF system exist either in the bulk liquid phase or at the membrane surface, the ef- fect of IOP locations on NOM removal during IOP/UF was examined. As shown in Fig. 6, the UV and DOC removal T W. Ha et al. / Journal of Colloid and Interface Science 274 (2004) 587–593 591 Fig. 6. UV and DOC removal efficiencies using raw water during UF with IOPs in suspension and in deposited layers: IOP dose, 100 mg/LasFe. efficiencies with deposited IOPs were always at higher lev- els than those with suspended IOPs. It could be hypothesized that the formationof IOP cake layers at the membrane would contribute to further NOM rejection due to a sieving effect during IOP/UF. However, it was not clear that IOPs in the deposited layer had such similar sorption capacity for NOM as those in suspension. Any changes of the sorption capacity of IOPs located at different places were examined for raw water and are compared in Fig. 7. Different trends in NOM removal us- ing IOP-deposited and suspended systems were observed when prechlorination and/or prefiltration runs were per- formed. The DOC removal efficiency for the IOP-deposited system declined substantially when the colloidal particles present in raw water were removed, whereas that for the IOP-suspended system was not so much dependent on the absence of colloidal particles. This indicated that a signifi- cant loss of adsorption sites took place when IOPs formed a cake layer at the membrane surface. Thus it supported the above reasoning that higher DOC removal with the IOP- deposited system was mainly caused by the formation of a denser cake layer associated with colloids in raw water. In addition, it could be thought that crossflow UF that can keep IOPs in suspension would be better in regard to NOM removal compared to dead-end membrane filtration if raw water had a very low turbidity. 3.4. Effect of chlorination on membrane permeability Relative flux decline profiles for different treatment sys- tems using raw and filtered waters are shown in Fig. 8. The IOP/UF system had a flux more than two times that of UF alone. It was thus clear that IOP addition helped to enhance the membrane flux substantially, since IOP removed NOM that can otherwise cause membrane fouling [26–28]. On the other hand, the fluxes with the prechlorination of raw water were always kept at a relatively low level in both IOP/UF and (a) (b) Fig. 7. Effect of chlorine and colloidal particles on DOC removal during IOP/UF when (a) IOPs are in deposited cake layers and (b) in suspension: chlorine dose, 11 mg/LasCl 2 ; chlorination time, 30 min. UF-only systems, whereas the permeate flux was indepen- dent of chlorination when colloidal particles were removed prior to UF. Although a change of the nature of dissolved organic matter was obvious during chlorination, it did not affect membrane flux so much (Fig. 8). Considering the lower DOC removal efficiency with prechlorination shown in Fig. 4, a possible explanation for the above result was that the interaction of chlorine with colloidal particles in raw wa- ter may bring about a change of their characteristics (e.g., particle size reduction) leading to flux decline. As shown in Fig. 9, the size distribution of particles in raw water was shifted to lower ranges with chlorination, which can make the cake layer of particles at the membrane surface denser. This supports the above explanation about flux decline with chlorination. Accordingly, it was found that membrane flux was not sensitive to the change of physicochemical proper- ties of the DOC by chlorination, but it was affected by that of particulate matter characteristics. 592 T W. Ha et al. / Journal of Colloid and Interface Science 274 (2004) 587–593 (a) (b) Fig. 8. Effect of prechlorination on flux in IOP/UF and UF alone treating (a) raw and (b) filtered waters: chlorine dose, 11 mg/LasCl 2 ; chlorination time, 30 min. Fig. 9. Particle size distributions of raw water before and after chlorination: chlorine dose, 11 mg/LasCl 2 ; chlorination time, 30 min. 4. Summary and conclusions Prechlorination of river water during UF in combination with adsorption was conducted and evaluated with respect to NOM removal and membrane permeability.IOP had a larger sorption capacity for NOM at lower dosages than PAC, though the sorption capacity of IOP declined slightly after chlorination of raw water. The interaction of chlorine with dissolved NOM in raw water caused a decrease in UV ab- sorbance, while that with particulate matter did an increase in it. During the chlorination reactions, part of the particu- late matter was converted to dissolved matter, leading to an increase of dissolved carboncontentin the NOM total.These results contributed to the lower TOC removal efficiency by IOP/UF after chlorination. When the IOPs injected into an UF system formed a cake layer at the membrane surface, further NOM removal was achieved due to a sieving effect by the physical barrier. As colloidal particles were removed from raw water, however, a significant decrease in NOM re- moval efficiency occurred in the IOP-deposited system. It was revealed that IOPs in the cake layer were not so effective for adsorption as those in suspension because of their ag- gregation. 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