A pilot-scale hybrid municipal wastewater reclamation system using combined coagulation and disk filtration, ultrafiltration, and reverse osmosis: Removal of nutrients and micropollutants, and characterization of membrane foulants

Bioresource Technology 141 (2013) 109–116

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A pilot-scale hybrid municipal wastewater reclamation system using combined coagulation and disk filtration, ultrafiltration, and reverse osmosis: Removal of nutrients and micropollutants, and characterization of membrane foulants Kangmin Chon a,b,⇑, Jaeweon Cho a,c, Ho Kyong Shon d,⇑ a

School of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland c School of Civil and Environmental Engineering, College of Engineering, Yonsei University, Yonsei-ro 50, Seoul 120-749, Republic of Korea d School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney (UTS), Broadway, P.O. Box 123, Sydney, Australia b

h i g h l i g h t s  Particulate materials are effectively removed by CC–DF process.  Removal of micropollutants by RO is governed by the MW, Log D, and charge characteristics.  EfOM with small MW contributes to fouling formation of the UF and RO membranes.  Hydrophilic fractions are responsible for fouling formation of UF and RO membranes.  Residual coagulants after CC–DF contribute to fouling formation of UF membrane.

a r t i c l e

i n f o

Article history: Available online 6 April 2013 Keywords: Combined coagulation–disk filtration process Ultrafiltration Reverse osmosis membrane Micropollutants Membrane foulants

a b s t r a c t A pilot-scale municipal wastewater reclamation system using combined coagulation and disk filtration (CC–DF), ultrafiltration (UF), and reverse osmosis (RO) membrane has been built to investigate removal of water contaminants and fouling mitigation. The reclaimed water using the pilot system could meet draft regulations on wastewater reuse of the California Department of Public Health (DOC: 0.5 mgC/L; TN: 5 mgN/L). The removal of micropolluants by the CC–DF process and UF could not be evaluated by their MW, Log D, and charge characteristics. However, they were identified as governing factors affecting the removal of micropollutants by the RO. The CC–DF process might effectively remove particulate materials capable of contributing to cake layer formation on the UF membrane surfaces but the residual coagulants provided a strong effect on fouling formation of the UF membrane. Thus, hydrophobic fractions of the desorbed UF membrane foulants were higher than those of the desorbed RO membrane foulants. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Municipal wastewater reclamation (MWR) has received increasing attention as a sustainable water resource for its potential to meet the continuously growing demands for goodquality water (Asano et al., 2007). However, the use of reclaimed ⇑ Corresponding authors. Addresses: Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, 8600 Dübendorf, Switzerland. Tel.: +41-58-765-5083; fax: +41-58-765-5210 (K. Chon), School of Civil and Environmental Engineering, Faculty of Engineering and Information Technology, University of Technology, Sydney (UTS), Broadway, PO Box 123, Sydney, Australia. Tel.: +61-29514-2629; fax: +61-2-9514-2633 (H.K. Shon). E-mail addresses: [email protected] (K. Chon), [email protected] (H.K. Shon). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.03.198

water from treated municipal wastewater can pose potential health effects associated with microbial pathogens, heavy metals, pharmaceuticals, personal care products (PPCPs), endocrine disrupting chemicals (EDCs), and other persistent organic compounds, as secondary effluents are considered a major source of these contaminants (Chon et al., 2012c; Alturki et al., 2012). Recent studies on environmental toxicology and pharmacology reported that chronic exposure to micropollutants (i.e., PPCPs, EDCs, pesticides, and disinfection byproducts) may lead to long-term health risks (Tanaka et al., 2001; Kolpin et al., 2002). Nevertheless, there are still no regulations pertaining to the concentrations of micropollutants in recycled water (Snyder et al., 2003). Bellona and Drewes (2007) reported that reverse osmosis (RO) membrane processes are required in order to remove micropollutants from

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secondary effluents. In past research, it has been reported that the removal efficiencies of micropollutants by membrane processes are strongly influenced by characteristics of solutes, in terms of molecular size (length and width), acid disassociation constant (pKa), diffusion coefficient (Dp), hydrophobicity (Log Kow), and membrane properties, in terms of molecular weight cut-off, pore size, surface morphology (roughness), surface charge (zeta potential), and hydrophobicity (contact angle) (Kiso et al., 2000; Bellona et al., 2004). Yoon et al. (2007) found that the recovery rate of micropolluants is governed by the hydrophobic interaction between micropollutants and the membrane surfaces during filtration. In addition, some studies have shown that charged contaminants, including 9-anthrancene carboxylic acid, salicylic acid, dichloroacetic acid, trichloroacetic acid, and diclofenac were effectively removed by combining nanofiltration (NF) with an RO membrane (>90%) (Kimura et al., 2003). For municipal wastewater reclamation, among various wastewater treatment processes, the combination of microfiltration (MF) or ultrafiltration (UF) and RO is considered an attractive technology due to its high contaminant removal efficiency and cost effectiveness (Reith and Birkenhead, 1998; Chon et al., 2010). Although significant advances have been made in membrane technologies during the past decade, one of major issue in wastewater reclamation using membrane processes is membrane fouling, which is a primary source of flux decline (Chon et al., 2012a,b). Colloidal and suspended particulate materials rejected by membranes can form cake filtration layers on the membrane surfaces which can increase transmission pressure of the membranes and lead to permeate flux decline (Gabelich et al., 2002; Cho et al., 2005; Lee et al., 2009). The formation of fouling in membrane processes for MWR can be greatly attributed to residual organic materials in secondary effluents, known as effluent organic matter (EfOM), which is comprised of natural organic matter from drinking water, polysaccharides and proteins that are produced during biological wastewater treatments, such as when using activated sludge and/or membrane bioreactors (Shon et al., 2006a,b; Chon et al., 2011, 2013a). Even though many studies on wastewater reclamation using membrane processes have been performed in order to investigate the fouling mitigate and removal of water contaminants, with respect to dissolved organic carbon (DOC), total coliform level, potential for disinfection byproduct formation, turbidity, the presence of nitrogen species, metals, and metalloids (Yim et al., 2007), and the removal of micropolluants (Acero et al., 2012), the knowledge on a combined coagulation and disk filtration (CC–DF) process as a pretreatment of UF and RO membranes, in terms of removal on water contaminants and fouling mitigation, is still limited since most of previous studies have focused on the integrated membrane system comprising of MF/UF and RO membranes. Furthermore, an extensive investigation of the CC–DF process for MWR has yet to be conducted. Thus, this work has focused on the removal of micropollutants, the physicochemical characterization of EfOM in raw and treated water, and membrane foulants in the pilot-scale UF and RO membrane systems receiving the combined CC–DF process treated wastewater.

The main objectives of this study are to: (i) evaluate the viability of the CC–DF process for treating secondary effluents in terms of their potential for removing water contaminants, such as organic matter, metals, metalloids, nutrient (i.e., nitrogen and phosphorous), and micropollutants, and fouling mitigation in both low-pressure and high-pressure membranes (i.e., UF and RO membranes); (ii) to identify the removal behavior and mechanism of water contaminants in terms of organic, nitrogen compounds, metals, metalloids, and micropolluants through a pilot plant that uses membrane processes for municipal wastewater reclamation; and (iii) to investigate techniques to mitigate fouling according to the characteristics of EfOM and membrane foulants in pilotscale membrane processes for municipal wastewater reclamation. 2. Methods 2.1. Description of pilot plant for municipal wastewater reclamation A pilot plant for MWR was designed using the CC–DF process, comprised of a double layer of polyester open mesh fabrics with both 15 lm and 10 lm mesh sizes (PETEXÒ, Sefar, Heiden, Switzerland) (effective surface area of DF: 10.5 m2), and UF and RO membranes (Chon et al., 2012b). Secondary effluents (feed water) from the Ansan wastewater treatment plant (WWTP) (Ansan, Gyeonggido, Korea) including biological nitrification and denitrification processes were subsequently treated using coagulation with aluminum sulfate (final concentration: 3 mg/L) for 15 min and then directly filtered with the DF to remove aggregates and particles. The DF permeate was continuously treated using a hollow-fiber polyvinylidene fluoride (PVDF) UF membrane (CleanfilÒ-P75R, Kolon Membrane Corporation, Korea), which has a nominal pore size of 0.1 lm. Two UF modules (effective surface area of each UF module: 73 m2; permeate flux: 227 m3/day) were operated in dead-end mode, and backwashing was performed using 13.2 mg/ L of NaOCl with air stripping every 30 min for 30 s in order to minimize fouling. After backwashing, UF modules were drained for 20 s and then refilled with fresh water for 40 s. The UF permeate was further filtered using spiral-wound polyamide type thin-film composite (TFC) RO membranes (RE8040-FL, Woongjin Chemical Corporation, Korea) to remove micropollutants, heavy metals, and metalloids. Four serially connected RO modules (effective surface area of each RO module: 37.2 m2; permeate flux: 82 m3/day) were operated at a water recovery rate of 72.6%. Properties of the RO membrane are provided in Table 1 (Chon et al., 2012b). 2.2. Sample collection Four types of water samples were collected to assess the performance of the pilot-scale MWR system: feed water, DF, UF, and RO permeate. The collection of samples from the pilot plant for the analyses of bulk parameters (i.e., DOC, specific UV absorbance (SUVA), metals, and metalloids), nutrients (i.e., nitrogen and phosphorous), and EfOM components in the feed and treated wastewaters was conducted on a weekly basis. Two comprehensive

Table 1 Characteristics of the UF and RO membranes. Type

Pore size (lm)

Dimension (ID/OD) (mm)

Tensile strength (kg/fiber)

MWCO (Da)

Zeta potential at pH 7 (mV)

Contact angle (°)

Roughness (nm)

UF RO

0.1a N.A.

0.9/1.7a N.A.

>25a N.A.

N.A. 100–400b

N.A. 35.5b

N.A. 30.4b

N.A. 63.5b

N.A.: not available. a Kolon membrane. b Chon et al. (2012b).

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sampling campaigns were performed for the analysis of the target micropollutants. The samples were collected in duplicate and analyzed by liquid chromatography-tandem mass spectrometry. Furthermore, four membrane foulants were desorbed using 0.1 N NaOH and 0.1 N HCl from the UF and RO membranes: UF-base (extracted with 0.1 N NaOH), UF-acid (extracted with 0.1 N HCl), RObase (extracted with 0.1 N NaOH), and RO-acid (extracted with 0.1 N HCl). 2.3. Analytical methods The total suspended solids (TSS) were determined using standard methods, and the chemical oxygen demand (COD) was determined by the oxidation of organic contents using trivalent manganese (manganese III method). In addition, the DOC and total nitrogen (TN) were quantified using a catalytic combustion-based TOC analyzer equipped with a TN analyzer (TOC-VCPH with TNM1 unit, Shimadzu, Kyoto, Japan), aromatic chromophores of the EfOM and membrane foulants were evaluated using UV absorbance at 254 nm (UV-1601, Shimadzu, Kyoto, Japan), and SUVA was calculated from the ratio of UV absorbance at 254 nm to the DOC concentration. Ion chromatography (IC) (ICS-90, DX 120, Dionex, Sunnyvale, CA, USA) equipped with IonPac AS 14 and CS 12A columns (Dionex, Sunnyvale, CA, USA) was employed to measure the levels of anions (e.g., nitrite, nitrate and phosphorous) and cations (e.g., ammonium, magnesium, and calcium). The total phosphate (TP) was measured by using the molybdo vanadate method at 450 nm and the ascorbic acid methods at 880 nm. Inductively coupled plasma-mass spectrometry (ICP-MS) (7500ce, Agilent, Santa Clara, CA, USA) was used to measure the concentration of metals and metalloids. The removal of micropollutants by the pilot system was investigated using an automated solid phase extraction system (Caliper Corporation, Hopkington, MA, USA) with hydrophilic–lipophilic balance cartridges (30 mg packing, Oasis, Waters, Milford, MA, USA) and liquid chromatography (Alliance 2695, Waters, Milford, MA, USA) coupled with a Quattro micro triple-quadrupole mass spectrometer (Micromass, Waters,

Manchester, UK) (Chon et al., 2012c). The molecular weight (MW) distribution of aromatic and protein-like substances were ascertained using high-performance size-exclusion chromatography (HPSEC) with a Protein-Pak 125 separation column (Waters, Milford, MA, USA), UV detection (SPD-10AVP, Shimadzu, Kyoto, Japan), and fluorescence detection (RF-10AXL, Shimadzu, Kyoto, Japan) (Chon et al., 2011). In addition, major components of EfOM and membrane foulants were confirmed using a fluorescence spectrophotometer (F-2500, Hitachi, Tokyo, Japan), which provides a 3dimensional fluorescence excitation-emission matrix (3D FEEM). A freeze dryer (Ilshin BioBase, Gyeonggi-do, Korea) was used to convert liquid samples into powders, and the infrared (IR) spectra of the powdered samples were obtained using a Fourier transform infrared (FTIR) spectrometer (FT/IR-460 plus, Jasco, Tokyo, Japan) with a KBr pellet (Pike, Madison, WI, USA). Finally, fractionation was performed using Amberlite XAD-8/4 resins (Supelco, Bellefonte, PA, USA) to evaluate the relative hydrophobicity of membrane foulants from the UF and RO membranes of the pilot plant for MWR (Chon et al., 2013b). 3. Results and discussion 3.1. Pilot system performance 3.1.1. Organic and inorganic contaminants removal Variations in water characteristics, including pH, conductivity, TSS, DOC, UV absorbance, SUVA, nitrogen species, metals, and metalloids passing through the pilot system are summarized in Table 2. The conductivity, COD and DOC in the fee water were not effectively removed by the CC–DF process (removal of conductivity <1%; removal of COD <4%; removal of DOC <11%). However, it was effective removal of turbidity (>43%), implying that the CC– DF process could reduce cake layer formation on the UF membrane surfaces since the cake layer formation might be minimized by removing particulate materials in the feed water (Howe and Clark, 2002). The turbidity in the DF permeate was substantially reduced by the UF membrane (>96%), but less than 4% of DOC was removed

Table 2 Changes of water qualities through pilot-scale wastewater reclamation system (n = 3). Feed

DF permeate

UF permeate

RO permeate

Bulk parameters pH Conductivity (lS/cm) Turbidity (NTU) CODMn (mg/L) DOC (mgC/L) UV254 (cm) SUVA (L/mg m) Al (lg/L) As (lg/L) B (lg/L) Ca (mg/L) Cr (lg/L) Co (lg/L) Fe (lg/L) Mg (mg/L) Ni (lg/L) Si (mg/L)

7.1 (±0.1) 769.8 (±13.7) 1.43 (±0.09) 5.09 (±0.87) 6.58 (±0.10) 0.100 (±0.000) 1.51 (±0.02) 11.49 (±2.98) 1.61 (±0.10) 59.83 (±0.25)a 36.58 (±3.17)a 0.83 (±0.18) 2.23 (±0.44) 33.48 (±9.10) 13.70 (±0.82)a 7.48 (±1.81) 5.23 (±0.3)

7.3 (±0.1) 767.1 (±18.3) 0.83 (±0.01) 4.93 (±0.84) 5.86 (±0.12) 0.097 (±0.002) 1.66 (±0.06) 28.67 (±19.18) 1.57 (±0.11) 59.90 (±2.40)a 35.45 (±3.91)a 0.71 (±0.15) 2.28 (±0.55) 5.73 (±1.85) 13.23 (±1.31) 7.51 (±1.55) 4.39 (±0.20)

7.4 (±0.1) 763.9 (±18.2) 0.03 (±0.00) 3.98 (±0.24) 5.66 (±0.12) 0.090 (±0.002) 1.59 (±0.03) 17.28 (±7.92) 1.58 (±0.09) 57.93 (±1.47) 35.70 (±1.42)a 0.75 (±0.18) 2.29 (±0.49) 5.70 (±0.20)a 12.81 (±0.82)a 6.62 (±0.89)a 4.38 (±1.68)a

6.2 (±0.2) 24.1 (±2.0) N.D. 0.11 (±0.10) 0.17 (±0.03) 0.002 (±0.000) 0.98 (±0.027) 2.10 (±0.71) 0.07 (±0.03) 48.23 (±5.70) 0.23 (±0.17) 0.18 (±0.03) 0.01 (±0.01) N.D. 0.05 (±0.01) 0.10 (±0.05) 0.17 (±0.08)

Nutrients TN (mgN/L) NO2 (mgN/L) NO3 (mgN/L) NH4+ (mgN/L) TP (mgP/L) PO43 (mgP/L)

7.37 N.D. 7.26 N.D. 2.02 1.15

7.27 N.D. 7.17 N.D. 1.83 0.84

7.07 N.D. 7.01 N.D. 1.27 0.80

0.73 N.D. 0.80 N.D. 0.09 0.02

N.D.: not detected; N.A.: not available. a n = 2.

(±0.07) (±0.11) (±0.23) (±0.19)

(±0.04) (±0.00) (±0.33) (±0.27)

(±0.12) (±0.01) (±0.72) (±0.32)

(±0.02) (±0.01) (±0.02) (±0.03)

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after application of the UF membrane. The RO membrane could remove approximately 97% of DOC, 96% of turbidity, and 61% of SUVA in the UF permeate. The removal of SUVA through the RO membrane might be attributed to the electrostatic repulsion between negatively charged humic substances and negatively charged surfaces of the RO membrane (Chon et al., 2012b; Her et al., 2007). No significant changes in metal and metalloid levels through the CC–DF process and the UF membrane were found. However, the aluminum concentration of the feed water (11.49 lg/L) increased by 28.67 lg/L after the CC–DF process, presumably due to residual aluminum sulfates from the coagulants. As expected, most metals and metalloids were effectively removed through the RO membrane, though the removal efficiency of boron was substantially lower (<17%) than the other metals and metalloids, as boron remains in the form of boric acid at neutral pH; therefore, increasing the pH to 10.5 is required in order to enhance the boron removal efficiency (Chon et al., 2012c). 3.1.2. Nutrients removal Changes in concentrations of nitrogen and phosphorous during the pilot system are provided in Table 2. The nitrate ion was the major nitrogen species in the feed water since nitrite and ammonium ions were oxidized into nitrate ion during biological nitrification and denitrification processes. The CC–DF process and the UF membrane were not effective for removal of nitrate ion (the CC– DF process <1%; the UF membrane <3%) but it was effectively removed by the RO membrane (>89%) due the steric exclusion of the RO membrane and electrostatic repulsion by the negatively charged RO membrane surfaces (Lee and Lueptow, 2001). Similar removal trends were found in the removal of phosphorous. The CC–DF process was ineffective for either removal of TP (<10%) or removal of phosphate ions (<30%) while the UF membrane showed a relatively high removal efficiency of TP (56%). In contrary to the CC–DF process and the UF membrane, both TP and phosphate ions in the UF permeate were completely removed through the RO membrane (removal of TP >89%; removal of phosphate ions >97%). Consequently, the reclaimed wastewater through the pilot plant could satisfy the draft regulations of California Department of Public Health (CDPH) on concentrations of TN (5 mgN/L) and DOC (0.5 mgC/L) for wastewater reuse (California Department of Public Health and Draft Regulations: ground water replenishment with recycled water, November 2011).

lected as target micropolluants and their removal through the pilot-scale MWR system was investigated, as summarized in Table 3. The DIC in the feed water was slightly removed by the CC– DF process (removal of DIC >17%) but it was ineffective for removing most of the target micropollutants (i.e., ATE, CBZ, CAF, DIL and SMX). Although negative removals were observed for ATE and DIL during the CC–DF process, the values were not significant (ATE < 6%; DIL < 3%). These removal trends were consistent with the previous study conducted by Vieno et al. (2006). They reported that coagulation is effective for removal of DIC but not effective for removal of ibuprofen, bezafibrate, CBZ, and SMX. In this study, MW, Log D, charge at neutral pH of the micropollutants were considered as major parameters affecting their removal by the UF and RO membranes. Thus, removal rates of the target micropollutants are plotted as a function of MW, Log D, charge at neutral pH in order to investigate dominant removal mechanisms (Fig. 1). Most of the micropolluants were not effectively removed using the UF membrane (<17%), with the exception of DIC and SMX. Concentrations of DIC and SMX in the DF permeate were considerably decreased after the UF membrane (removal of DIC >33%; removal of SMX >28%). However, there was no distinct relationship between the removal of target micropollutants by the UF membrane and their MW, Log D, charge at neutral pH. As expected, the RO membrane might achieve high removal efficiencies of all the target micropolluants and their removal was strongly influenced by the MW, Log D, charge at neutral pH. In spite of the similar MW, the negatively charged micropolluants (i.e., DIC and SMX) were more effectively removed by the RO membrane compared to the noncharged micropollutatns (i.e., CBZ, CAF, DIL) and/or positively charged micropollutants (i.e., ATE) due to the negatively charged surfaces of the RO membrane. In spite of the neutral charge at pH 7, approximately 98% of CBZ and 91% of DIL could be removed through the RO membrane since they were more hydrophobic (Log D of CBZ: 2.77; Log D of DIL: 2.13) than ATE (Log D: 2.14) and CAF (Log D: 0.55). The relatively low removal efficiency of CAF by the RO membrane (65%) compared with ATE (87%) was probably due to its low MW (194.2 g/mol), low hydrophobicity (Log D: 0.55), and neutral charge at pH 7 (Snyder et al., 2003). Among the target micropollutants, no florfenicol was detected in the feed and treated waters, indicating that it is not currently used as an anti-bacterial agent in Korea. 3.2. EfOM removal

3.1.3. Micropollutants removal Seven different kinds of micropolluants, including atenolol (ATE), carbamazepine (CBZ), caffeine (CAF), diclofenac (DIC), dilatin (DIL), florfenicol (FLO), and sulfamethoxazole (SMX), were se-

Changes in MW distributions of the feed and treated waters through the pilot system were investigated by HPSEC. The aromatic substances of EfOM ranged from 370 to 980 Da (feed: 370 Da

Table 3 Removal of micropollutants through the pilot plant for MWR (n = 2). Compounds (classification)

MWa (g/mol)

Atenolol (beta-blocker) Carbamazepine (anti-seizure) Caffeine (stimulant) Diclofenac (anti-arthritic) Dilatin (anti-seizure) Florfenicol (anti-bacterial agent) Sulfamethoxazole (antibiotic)

266.3 236.3 194.2 296.2 252.3 358.2 253.3

Log Pa (Log Kow) 0.16 2.45 0.07 4.51 2.47 N.A. 0.89

Calculated Log Pb Log Db 0.43 2.77 0.55 4.26 2.15 0.67 0.79

2.14 2.77 0.55 0.96 2.13 0.67 0.14

pKaa

Charge at pH 7b

MDLc (ng/L)

Feed (ng/L)

DF permeate (ng/L)

UF permeate (ng/L)

RO permeate (ng/L)

N.A. N.A. 10.4 4.15 8.33 N.A. N.A.

+1 0 0 1 0 0 1

1.2 0.7 1.5 1.3 1.2 0.7 0.6

206.6 (±9.1) 105.5 (±43.1) 54.1 (±27.1) 126.5 (±47.3) 60.3 (±28.2) N.D. 155.0 (±52.0)

218.1 (±28.2) 100.6 (±7.0) 52.2d 104.1 (±41.4) 61.6 (±16.0) N.D. 151.7 (±1.2)

194.4 (±60.3) 97.0 (±0.2) 43.3d 69.7 (±31.4) 60.4 (±40.2) N.D. 109.0d

26.3d 1.6 (±0.2) 15.0 (±0.1) N.D. 5.5 (±1.5) N.D. N.D.

N.A.: not available; N.D.: not detected. a ChemDplus Advanced (website) was referred for MW and experimental data of pKa, and Log P. b Software Calculator Plugins was used to calculate Log P, Log D at pH 7. c Method detection limit (MDL) was determined using LC/MS–MS with 5 and 10 ng/L of unlabeled standard trough SPE concentration factor of 1000 (T value: 2.764 for n 1 = 10). d n = 1.

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UF

RO

MW

320

100

300

80

280

60

260

40

240

20

220

0

200

-20

180 ATE (+1)

Removal (%)

(b)

MW (g/mol)

DF

120

CBZ (0) DF

CAF (0) UF

DIC (-1)

DIL (0)

SMX (-1) Log D

RO

120

4

100

3

80

2

60

1

40

0

20

-1

0

-2

-20

Log D

Removal (%)

(a)

-3 ATE (+1)

CBZ (0)

CAF (0)

DIC (-1)

DIL (0)

SMX (-1)

Fig. 1. Removal of the target micropollutants through the pilot plant as a function of (a) MW and charge at neutral pH, and (b) Log D and charge at neutral pH.

(intensity: 23,479 mV), 440 Da (20,658 mV), 690 Da (15,971 mV), 980 Da (15,892 mV); DF permeate: 370 Da (22,926 mV), 440 Da (20,229 mV), 690 Da (15,541 mV), 980 Da (15,249 mV); UF permeate: 370 Da (22,793 mV), 440 Da (20,220 mV), 690 Da (15,657 mV), 980 Da (15,316 mV); RO permeate: 370 Da (2,197 mV)). The florescence response was mainly composed of both small MWs (feed: 100 Da (intensity: 9,079 mV), 240 Da (69,157 mV), 350 Da (25,066 mV), 700 Da (13,313 mV); DF permeate: 100 Da (9,051 mV), 240 Da (68,963 mV), 350 Da (24,818 mV), 700 Da (12,951 mV); UF permeate: 100 Da (9,051 mV), 240 Da (68,963 mV), 350 Da (24,707 mV), 700 Da (12,951 mV); RO permeate: 220 Da (10,409 mV)) and large MWs (feed: 59,350 Da (intensity: 6,672 mV), DF permeate: 59,350 Da (3,768 mV); UF permeate: 48,036 Da (2,136 mV)). The UV and fluorescence intensities of EfOM in the DF permeate and the UF permeate was slightly lower than that of the feed water which could be attributed to the DOC removal by the UF membrane. In addition, a slight shift in MW distribution to lower sizes might be closely related to membrane sieving effects enhanced by the fouling layer formation on the UF membrane surfaces (Zhao et al., 2010). The small MW fractions of EfOM (<5,000 Da) are indicative of humic substances, whereas large MW fractions of EfOM (>21,000 Da) are indicative of non-humic substances (Her et al., 2007). From the MW distribution of aromatic and protein-like substances, it was confirmed that both small

and large MWs of EfOM were effectively removed by the RO membrane. Variations in fluorescent chromophores of the feed and treated waters passing through the pilot system were identified using 3D FEEM. It has been known that the protein-like fluorescence associated with bovine serum albumin (BSA) can be found at excitation (Ex) = 280 nm and emission (Em) = 340 nm and fluorescence peaks at Ex = 270 nm/Em = 450 nm (Suwannee River humic acid (SRHA) I), Ex = 315 nm/Em = 440 nm (SRHA II), Ex = 260 nm/Em = 440 nm (Suwannee River fulvic acid (SRFA) I), and Ex = 320 nm/ Em = 440 nm (Suwannee River fulvic acid (SRFA) II) are associated with humic-like fluorescence (Chon et al., 2010). The strong humic-like fluorescence was commonly found for the feed water (maximum intensity at Ex = 320 nm and Em: 420 nm: 4,058 mV), the DF permeate (maximum intensity at Ex = 320 nm and Em: 420 nm: 4,422 mV), and the UF permeate (maximum intensity at Ex = 320 nm and Em: 420 nm: 3,963 mV). However, its fluorescence intensity was substantially decreased after the RO membrane treatment (maximum intensity of the RO permeate at Ex = 320 nm and Em: 420 nm: 36 mV). These removal trends of EfOM in the raw and treated water samples were consistent with the removal of DOC and SUVA using the tested pilot system. Although it is difficult to characterize polysaccharides using HPSEC and 3D FEEM, the secondary effluents including a large amount of

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Table 4 Foulants characteristics of UF and RO membranes (n = 3).

DOC (mgC/m2) UV254 (/cm) SUVA (L/mg m) TN (mgN/m2) NO2 (mgN/m2) NO3 (mgN/m2) NH4+ (mgN/m2) Al (mg/m2) As (mg/m2) B (mg/m2) Ca (mg/m2) Cr (mg/m2) Co (mg/m2) Fe (mg/m2) Mg (mg/m2) Ni (mg/m2) Si (mg/m2)

UF-base

UF-acid

RO-base

RO-acid

63.15 (±2.43) 0.411 (±0.002) 1.34 (±0.05) 21.94 (±0.59) 0.86 (±0.09) 13.75 (±0.44) N.D. 6.04 (±0.33) 0.01 (±0.00) 0.68 (±0.02) 5.43 (±0.10) 0.03 (±0.00) 0.01 (±0.00) 0.00 (±0.00) 0.58 (±0.01) 0.03 (±0.00) 8.08 (±0.25)

14.51 (±0.23) 0.0081 (±0.000) 1.14 (±0.02) 10.67 (±0.24) N.D. 8.83 (±0.16) N.D. 40.76 (±8.17) 0.00 (±.000) 0.11 (±0.00) 127.46 (±2.15) 2.43 (±0.55) 0.05 (±0.00) 68.74 (±1.00) 31.12 (±5.20) 0.98 (±0.18) 11.96 (±0.19)

423.57 (±1.91) 1.292 (±0.003) 0.51 (±0.00) 30.79 (±0.24) N.D. 3.77 (±0.09) N.D. 8.99 (±0.47) 0.03 (±0.00) 0.55 (±0.02) 2.95 (±0.20) 0.04 (±0.00) 0.01 (±0.00) 1.47 (±0.08) 0.85 (±0.01) 0.08 (±0.00) 7.24 (±0.14)

12.87 (±0.10) 0.117 (±0.001) 1.53 (±0.02) 4.95 (±0.06) N.D. 4.69 (±0.06) N.D. 0.19 (±0.01) 0.00 (±0.00) 0.06 (±0.01) 129.25 (±32.16) 0.02 (±0.00) 0.06 (±0.00) 0.03 (±0.01) 28.07 (±0.95) 0.17 (±0.01) 18.09 (±1.94)

N.D.: not detected.

Table 5 Fluorescence characteristics of the desorbed UF and RO membrane foulants. DOC (mgC/m2) UF-base

63.15

UF-acid

14.51

RO-base

423.57

RO-acid

12.87

Peak

Ex (nm)

Em (nm)

Intensity

Type of compounds

I II I II I II I II

340 280 330 280 330 280 330 280

430 340 400 340 410 340 410 340

12,950 9890 1716 1416 17,940 15,540 1757 1190

Humic-like fluorescence Protein-like fluorescence Humic-like fluorescence Protein-like fluorescence Humic-like fluorescence Protein-like fluorescence Humic-like fluorescence Protein-like fluorescence

polysaccharides should be considered as major UF and RO membrane foulants (Jarusutthirak and Amy, 2006).

3.3. The UF and RO membrane foulant characteristics 3.3.1. Organic and inorganic composition of the UF and RO membrane foulants Characteristics of the membrane foulants desorbed from the UF and RO membranes of the tested pilot system, including DOC, UV absorbance, SUVA, inorganic/organic species, and inorganic materials, are summarized in Table 4. In terms of DOC, membrane foulants were more effectively desorbed by an alkaline solution (0.1 N NaOH) than an acidic solution (0.1 N HCl), suggesting that cleaning-in-place (CIP) processes using an alkaline cleaning solution could easily be used to recover the flux of a fouled membrane (Yamamura et al., 2007). No significant difference was observed in the inorganic compositions of the desorbed UF and RO membrane foulants. However, the mass ratio of dissolved aluminum to DOC (mg Al/mg DOC) for the desorbed UF membrane foulants (UF-base: 0.096 and UF-acid: 2.809) was significantly higher than those of the desorbed RO membrane foulants (RO-base: 0.021 and RO-acid: 0.014), which indicating that residual aluminum-humic substance complexes from the CC–DF process had a great influence on the fouling layer formation of the UF membrane. From the inorganic compositions of the UF and RO membrane foulants, it can thus be postulated that some metals and metalloids, such as calcium, iron, magnesium, and silicon, were more efficiently extracted by the acid solution than the alkaline solution, which indicates that the formation of scaling might be reduced by cleaning using an acidic solution.

3.3.2. Molecular weight and fluorescence characteristics of the UF and RO membrane foulants The MW distribution of aromatic and protein-like substances from the UF and RO membrane foulants was confirmed by HPSEC. In the case of aromatic substances, their MW ranged from 820 to 1270 Da (UF-base: 980 Da (intensity: 3288 mV); UF-acid: 820 Da (17,277 mV); RO-base: 890 Da (20,187 mV), 1270 Da (19,390 mV); RO-acid: 1270 Da (16,829 mV)). The MW distribution of protein-like substances in the desorbed UF and RO membrane foulants was comprised of both small MWs (UF-base: 420 Da (intensity: 4988 mV); UF-acid: 290 Da (7627 mV); RO-base: 840 Da (10,648 mV); RO-acid: 460 Da (8304 mV)) and large MWs (RO-base: 26,590 Da (intensity: 3945 mV)). Even though the DOC concentration of RO-base (423.57 mgC/m2) was substantially higher than the other desorbed membrane foulants (UF-base: 63.15 mgC/m2; UF-acid: 14.51 mgC/m2; RO-acid: 12.87 mgC/m2), no significant differences were observed in terms of UV and fluorescence detection, which implies that RO-base might include a large hydrophilic fraction (e.g., polysaccharides and proteins). Based on the MW distribution of aromatic and protein-like substances, it could be confirmed that small MW fractions of EfOM contributed more strongly to the formation of a fouling layer on the UF and RO membranes in the test system. The fluorescence characteristics of the four desorbed UF and RO membrane foulants are provide in Table 5. Two strong fluorescence intensities were found in UF-base at Ex = 280 nm, Em = 340 nm (maximum intensity: 9890 mV) and Ex = 340 nm, Em = 430 nm (maximum intensity: 12,950 mV), UF-acid also had two pairs of maximum peaks at Ex = 280 nm, Em = 340 nm (maximum intensity: 1416 mV) and Ex = 330 nm, Em = 400 nm (maximum intensity: 1716 mV), and two strong fluorescent chromophores were

K. Chon et al. / Bioresource Technology 141 (2013) 109–116

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Fig. 2. Fluorescence excitation-emission matrix (FEEM) of reference materials, UF and RO membrane foulants.

Fig. 3. Fractionations using Amberlite XAD-8/4 resins of the desorbed foulants from the UF and RO membranes of the pilot plant.

exhibited in RO-base at Ex = 280, Em = 340 nm (maximum intensity: 15,440 mV) and Ex = 330 nm, Em = 410 nm (maximum intensity: 17,490 mV). In addition, RO-acid provided two maximum peaks at Ex = 280 nm, Em = 340 nm (maximum intensity: 1190 mV) and Ex = 330 nm, Em = 410 nm (maximum intensity: 1757 mV). The fluorescence excitation-emission matrix (FEEM) of the reference materials and desorbed UF and RO membrane foulants are exhibited in Fig. 2. The peaks in Group A (e.g., UF-base II, UF-acid II, RO-base II, and RO-acid II) appeared near BSA (reference material of protein-like substances) whereas the location of peaks in Group B (e.g., UF-base I, UF-acid I, RO-base I, and RO-acid I) were considerably biased to humic-like fluorescence (e.g., SRFA I and SRHA I). From these FEEMs, it could be confirmed that both the hydrophobic and hydrophilic fractions of EfOM contributed to the formation of fouling on the UF and RO membranes in the pilot system. 3.3.3. Functional group compositions and relative hydrophobicity of the UF and RO membrane foulants The IR spectra of the desorbed UF and RO membrane foulants were investigated using FTIR analysis. Four types of desorbed membrane foulants exhibited quite similar IR peaks. The peak in the range of 3500–3300 cm 1 is indicative of the O–H stretching of amides, and the peak of carbonyl groups (C@ @O) from primary

amides was detected in the range of 1680–1630 cm 1. The amides from peptidoglycans can be introduced into aquatic systems through the cell lysis of microorganisms (Lee et al., 2006). The O–H stretching of carboxylic acids derived from hydrophobic and/or transphilic fractions of organic matters appeared in the range of 3100–2900 cm 1 and 1440–1395 cm 1; however, UF-base showed a relatively strong IR peak of carboxylic acids at 1386 cm 1 (absorbance: 0.204) compared to other desorbed foulants (absorbance of UF-acid: 0.049; absorbance of RO-base: 0.080; absorbance of RO-acid: 0.074). The peak in the region of 2900–2800 cm 1 was the CH band of aldehydes, and the C–O stretching from alcohols associated with polysaccharides was found in the range of 1210– 1100 and 1075–1000 cm 1 (Lee et al., 2006). In addition, a relatively strong CH band of aromatics was only observed for UF-base in the region of 860–780 cm 1. Among the various functional groups, polysaccharides, carboxylic acids, and amide groups were found to be the major functional groups of the desorbed UF and RO membrane foulants. These results indicated that hydrophilic functional groups (i.e., amides and alcohols) were responsible for the fouling formation of the RO membrane while hydrophobic functional groups (i.e., carboxylic acids and aromatics) could strongly contribute to the fouling layer formation on the UF membrane surfaces.

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Structural analysis results of the desorbed foulants from the UF and RO membranes using Amberlite XAD-8/4 resins are provided in Fig. 3. The desorbed UF and RO membrane foulants were mainly composed for hydrophilic (HPI) fractions (UF-base: 44%; UF-acid: 73%; RO-base: 87%; RO-acid: 57%). However, the hydrophobic (HPO) fractions of UF-base (35%) were considerably higher than those of the UF-acid (20%), RO-base (HPO: 10%), and RO-acid (26%), which could be attributed to the residual aluminum–humic substance complexes in the DF permeate. These observations were consistent with results from the ICP-MS, HPSEC, 3D FEEM, FTIR analyses of desorbed membrane foulants. Based on the structural analysis of the desorbed UF and RO membrane foulants, it can be asserted that the relative hydrophobicity of membrane foulants is highly dependent on their functional group composition. As such, the strong IR peaks of carboxylic acids and aromatics in the desorbed UF membrane foulants are seen to strongly support these observations. 4. Conclusions The pilot system was effective for removal of most contaminants. Therefore, the reclaimed wastewater might satisfy draft regulations of the CDPH for wastewater reuse (DOC: 0.5 mgC/L; TN: 5 mgN/L). The removal of micropollutants by the CC–DF and UF could not be predicted by their MW, Log D, and charge characteristics while they played critical roles in the removal of micropollutants by the RO. The desorbed UF and RO membrane foulants mainly consisted of hydrophilic factions. However, the desorbed UF membrane foulants included more hydrophobic fractions than the desorbed RO membrane foulants due to the residual coagulants after the CC–DF. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012047029, No. 2011-0030842), and Australian Research Council (ARC). The authors would like to express deep appreciation to Mr. Seung Joon Kim of Kolon Engineering and Constructions for his support and assistance in the operating system. References Acero, J.L., Benitez, F.J., Real, F.J., Teva, F., 2012. Coupling of adsorption, coagulation, and ultrafiltration processes for the removal of emerging contaminants in a secondary effluent. Chem. Eng. J. 210, 1–8. Alturki, A., McDonald, J., Khan, S.J., Hai, F.I., Price, W.E., Nghiem, L.D., 2012. Performance of a novel osmotic membrane bioreactor (OMBR) system: flux stability and removal of trace organics. Bioresour. Technol. 113, 201–206. Asano, T., Burton, F.L., Leverenz, R., Tchobanoglous, G., 2007. Water Reuse: Issues, Technologies, and Applications. McGraw-Hill, New York, USA. Bellona, C., Drewes, J.E., 2007. Viability of a low-pressure nanofilter in treating recycled water for water reuse application: a pilot-scale study. Water Res. 41, 3948–3958. Bellona, C., Drewes, J.E., Xu, P., Amy, G., 2004. Factors affecting the rejection of organic soultes during NF/RO treatment – a literature review. Water Res. 38, 2795–2809. California Department of Public Health, Draft Regulations: ground water replenishment with recycled water, November 2011, Sacramento, CA, USA. Cho, J., Song, K.G., Ahn, K.H., 2005. The activate sludge and microbial substances influences on membrane fouling in submerged membrane bioreactor: unstirred batch cell test. Desalination 183, 425–429. Chon, K., Lee, S., Chon, K., Hussain, A.A., Cho, J., 2010. Developing organic fouling indices of microfiltration and nanofiltration membranes for wastewater reclamation. Desalin. Water Treat. 18, 61–70.

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