Reuse of Semiconductor Wastewater Using Reverse Osmosis and

Article pubs.acs.org/IECR

Reuse of Semiconductor Wastewater Using Reverse Osmosis and Metal-Immobilized Catalyst-Based Advanced Oxidation Process Jeongyun Choi and Jinwook Chung* R&D Center, Samsung Engineering Co. Ltd., Woncheon-Dong, Youngtong-Gu, Suwon, Gyeonggi-Do 443-823, Korea ABSTRACT: This report describes an efficient hybrid treatment process that recycles semiconductor wastewater by reverse osmosis (RO) and advanced oxidation process (AOP) with metal-immobilized catalysts based on activated carbon. High-loading organic wastewater was pretreated biologically using a membrane bioreactor, and the remaining organic compounds were filtered by RO and oxidized chemically by AOP. The recycling of the final effluent to ultrapure water (UPW) was evaluated using a pilotscale UPW production system. AOP with a metal catalyst improved the oxidation of low-molecular-weight organic compounds, such as acetonitrile and acetaldehyde, in electronics wastewater. The pilot-scale RO and AOP with a metal catalyst were performed under optimal conditions and satisfied the water quality standards for UPW. We conclude that the proposed hybrid process is a powerful tool that can be used to recycle electronics wastewater and satisfy the water quality standards for UPW.

1. INTRODUCTION The electronics industry was one of the major water consumption businesses to produce semiconductors or thinfilm transistor liquid crystal displays. Most water in this business was initially treated and purified to produce the purest grade water called ultrapure water (UPW) which was supplied to the manufacturing process of electronics products as a prime cleansing agent.1−3 Recently, this industry has been rapidly growing in Asia; consequently, the industry encountered environmental issues such as the limited water supply and the limitation of pollutant discharge into bodies of water.4,5 One of the effective methods for resolving water shortage issues is the reuse of wastewater that is planned to be discharged from the plant after proper treatment. Wastewater reuse is a useful way to prevent pollution of bodies of water by chemicals which do not exist naturally and to resolve water supply limitations at the same time. The properly treated wastewater should be supplied to the UPW production process to increase the quantity of water reuse because most water in the electronics industry is used to make UPW. Therefore, recycled water should have high quality so that it can be supplied to UPW production processes as raw water.6 Wastewater from the electronics industry is classified as inorganic and organic wastewater. Inorganic wastewater generated from chemical mechanical planarization and lithography processes is treated by chemical processes such as coagulation and precipitation. Because inorganic wastewater contains many kinds of volatile low-molecular-weight compounds, which are difficult to remove in UPW production process, its reuse is limited. Organic wastewater generated from etching, stripping, and cleaning processes is treated by biological processes after neutralization. Although it contains many organic substances, most of them are removed in the biological processes, and the remaining substances are removed easily in secondary processes, such as reverse osmosis (RO). However, water that is recycled from organic wastewater also contains a small quantity of volatile low-molecular-weight compounds, which deteriorates the quality of UPW. Main © 2014 American Chemical Society

components in low-strength wastewater are acetone, isopropyl alcohol, acetaldehyde, and methanol. Among these compounds, isopropyl alcohol, which is utilized in etching and washing panel surfaces in the display manufacturing process, is toxic to humans and relatively resistant to biodegradation.7,8 Advanced oxidation processes (AOPs) have been proposed as a method of removing volatile low-molecular-weight compounds. In this procedure, hydroxyl radicals are formed in the water to remove organic substances. Thus, AOPs can be used as a standalone treatment or a post- or pretreatment step in conventional processes to enhance the removal of recalcitrant compounds in wastewater, such as landfill leachate and livestock and industrial wastewater. However, general and representative AOPs, such as the UV/H2O2 and O3/H2O2 methods, require high costs to remove volatile low-molecular-weight organic substances at low concentrations.9,10 The Fenton reaction can be one alternative to treat recalcitrant materials. However, the continuous supply of the Fenton agent and production of a large volume of sludge were considered to be obstacles of field application. Recently, a heterogeneous Fenton reaction has been seen as an effective method of overcoming the sludge problem. Several studies synthesized the Fenton catalyst in which catalytic metal was attached on various carriers, such as brick grain,11 SiO2,12 zeolite,13,14 alumina,14 zerovalent iron,15 Fe3O4/CeO2 composite,16 Mn3O4-reduced grapheme oxide,17 sewage sludge,18 and copper ferrite.19 On the basis of the homogeneous Fenton reaction, we selected a backbone carrier, developed a solidphase AOP using metal-immobilized catalysts, and determined the optimal combination and conditions.20 Previous study conducted at the authors’ laboratory revealed that a heterogeneous Fenton system was effective in removing the low-molecular-weight compounds compared to conventional AOP system using UV. Therefore, in this study, a pilot Received: Revised: Accepted: Published: 11167

February 25, 2014 June 15, 2014 June 19, 2014 June 19, 2014 dx.doi.org/10.1021/ie500805x | Ind. Eng. Chem. Res. 2014, 53, 11167−11175

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematics of RO, solid-phase AOP, and UPW production system for the reuse of semiconductor wastewater.

approximately 55% of the initial metal content, i.e., approximately 45% of the active metal adhered solidly to the activated carbon. The details of the preparation are described in the Choi et al.20 2.2. Determination of Operational Factors in SolidPhase AOP. 2.2.1. Effect of H2O2 Concentration. The TOC removal from actual wastewater with 0.2 mg L−1 TOC (ROtreated water) was measured at various H2O2 concentrations (170, 250, 500, 700, and 900 μg L−1). A continuous test was performed using a water-jacketed glass column reactor with an internal diameter of 4 cm and a height of 10 cm that was packed with 50 g (22.5 mL) of a metal-immobilized catalyst. The pH was adjusted to 3 using nitric acid, and the space velocity was fixed at 6 h−1. 2.2.2. Effect of Space Velocity. The space velocity refers to the quotient of the entering volumetric flow rate of the reactants divided by the catalyst bed volume, which indicates how many reactor volumes of feed can be treated in a unit of time. First, to determine the effect of space velocity, the same glass column reactor as in the previous experiment was used. The initial H2O2 concentration and pH were 400 μg L−1 and 3, respectively. The space velocity condition was controlled at 5, 6, 7, and 9 h−1. The space velocities of 3, 6, 7, and 9 h−1 were converted into contact times of 20, 10, 8.6, and 6.7 min, respectively. 2.2.3. Effect of Linear Velocity. To determine the effect of linear velocity on the AOP reaction, glass column reactors with diameter/height ratios of 1/7, 1/1.5, and 1/0.2 were used to examine TOC removal by linear velocity (8.7, 24.2, 90.7 cm h−1). The volume of all reactors was fixed at 200 mL, into which 50 g (22.5 mL) of metal-immobilized catalyst was packed. The contact time in the column was 10 min (i.e., space velocity was 6 h−1). The initial H2O2 concentration and pH were 400 μg L−1 and 3, respectively. 2.2.4. Effect of Reactor Configuration. To assess the performance of the metal-immobilized catalyst by reactor configuration, a 1- or 2-pass lab-scale column was configured. Specifically, the removal of TOC and low-molecular-weight organic compounds (acetone and acetonitrile) was monitored.

test was conducted to verify the capability of solid-phase AOP to remove volatile low-molecular-weight organic substances in the actual wastewater discharged from semiconductor manufacturing industry. Specifically, operational factors, such as hydrogen peroxide concentration, space velocity, linear velocity, and reactor configuration, which are the major operational factors of solid-phase AOP, were determined. The produced reused water produced from the pilot-scale plant was also supplied as raw water for the UPW production process to examine if the reused water is of satisfactory UPW quality.

2. MATERIALS AND METHODS 2.1. Preparation of Catalyst. The metal catalyst was prepared by incipient wetness impregnation of activated carbon (SLS-100, 8−30 mesh, Samchully Activated Carbon Co., Korea) with an aqueous solution of metal nitrate, such as Fe(NO3)3·9H2O (SHOWA) and Al(NO3)3·9H2O (JUNSEI). The catalyst was prepared as follows. (i) Nitrate salts of metals were added to 200 mL of distilled water. (ii) Activated carbon (50 grams) was added to the transition-metal solution and mixed for 20 min using an ultrasonic device (DH.WUC.D22H, Daehan Science, Korea), during which the transition metals adhered to the pores of the activated carbon. (iii) The supernatant was removed from the mixed solution of metal and activated carbon, and the mixture of residual activated carbon and transition metals was rinsed 5 times with distilled water to remove any remaining transition-metal solution that did not adhere to the activated carbon. (iv) The moisture in the mixture of activated carbons and transition metals was removed in an oven at 60 °C for 12 h (WOF-155, Daehan Science, Korea). (v) The mixture was calcinated at 200 °C for 3 h and 1 °C min−1 using a sintering furnace (YSE-3000, Youl San, Korea) to remove nitrates. After the catalysts were synthesized, the residual metal that adsorbed onto the activated carbon or did not firmly immobilize to the surface of the activated carbon was washed from the catalysts using a pH 3 solution. The washes were repeated until the Fe and Al concentrations were below 0.05 mg L −1 . The amount of extracted active metals was 11168

dx.doi.org/10.1021/ie500805x | Ind. Eng. Chem. Res. 2014, 53, 11167−11175


Article

Table 1. Characteristics of Semiconductor Wastewater after MBR item (unit)

values

item (unit)

values

pH conductivity (μS cm−1) turbidity (NTU) alkalinity (mg L−1 as CaCO3) hardness (mg L−1 as CaCO3) TOC (mg L−1) NH4+−N (mg L−1) NO3−−N (mg L−1) T−N (mg L−1) Cl− (mg L−1)

6.52 ± 0.2 2 352 ± 1 000 0.51 ± 0.4 146.2 ± 9 35.5 ± 10 1.67 ± 0.4 0.16 ± 0.08 1.79 ± 1 6.45 ± 5 14.6 ± 2

SO42− (mg L−1) Ca2+ (mg L−1) Mg2+ (mg L−1) Na+ (mg L−1) K+ (mg L−1) Ba2+ (mg L−1) Sr2+ (mg L−1) Al3+ (mg L−1) Fe2+ (mg L−1) Mn2+ (mg L−1)

732 ± 200 211 ± 50 1.42 ± 0.2 415.2 ± 200 1.44 ± 0.6 0.07 ± 0.02 0.03 ± 0.01 0.08 ± 0.06 0.05 ± 0.04 3.37 ± 1

determine the optimal operating conditions. H2O2 was injected into the inlet pipe of both reactors. 2.3.3. UPW Production System. A pilot-scale UPW plant was generated to determine whether the water that was produced from the combination process (RO + solid-phase AOP) could be used as raw water for UPW production. The UPW plant had a 0.8 m3 day−1 capacity and consisted of a pretreatment block, a makeup block, and a polishing block. The pretreatment process entailed activated carbon, microfiltration, and membrane degasification steps; the makeup process comprised microfiltration, RO, UV oxidation, and ion exchange; and the polishing process consisted of UV oxidation, anion polishing, and mixed-bed polishing. Because the pH in the effluent from the solid-phase AOP was 3, NaOH was injected ahead of the RO inlet to adjust the pH to neutral. A 4 in. membrane (RE8040-UL, Woongjin Chemical Co., Korea) was used for the RO in the UPW plant and installed in a 2-pass configuration. 2.4. Sampling and Analysis. Samples were taken from all reactors at appropriate intervals and passed immediately through a glass fiber filter (Whatman, GF/C). The TOC concentration was measured with an online TOC meter (Anatel A-1000, HACH Co., Korea). The turbidity, conductivity, alkalinity, hardness, and total nitrogen were measured using standard water and wastewater methods.21 Isopropyl alcohol, acetone, acetonitrile, acetaldehyde, ethanol, methanol, methyl ethylketone, tetrahydrofuran, and chloroform concentrations were measured on a purge and trap gas chromatograph (7890N, Agilent Technology, U.S.) that was equipped with DB624 (0.32 mm i.d., 1.8 μm file thickness, and 30 m length). The pressure was 120 kPa helium, and the temperature program of the flame ionization detection (FID) system was as follows: 45 °C (9 min), ramp 10 °C min−1 up to 90 °C and 20 °C min−1 up to 260 °C (10 min). Chloride, sulfate, fluoride, ammonium, and nitrate concentrations were measured by ion chromatography (IC) (Dionex 3000, Camberley, U.K.). The amounts of metals (Fe, Al, Ba, Mg, K, Ca, and Na) were measured by inductively coupled plasmaoptical emission spectrometry (Ultima ICP) (Horiba Jobin-Yon Inc., U.S.).

In the 1-pass configuration, a single reactor was operated with a 20 min contact time to examine TOC removal. In the 2-pass configuration, 2 reactors were connected in series to allow the effluent from the first-stage reactor to be injected into the second-stage reactor. The contact time of each reactor was set to 10 min. 2.3. Pilot-Scale Plant. As shown in Figure 1, the pilot-scale plant for the recycling of organic wastewater comprised RO and solid-phase AOP components; the RO process removes organic and electrically conductive compounds in MBR-treated water, and the solid-phase AOP can eliminate low-molecular-weight nonbiodegradable compounds from RO-treated water. In addition, a UPW production system was operated to determine if the recycled water from the combination RO and solid-phase AOP process could be used as raw water for UPW. 2.3.1. RO System. The pilot-scale RO system had a capacity of 36 m3 day−1 and used fouling resistance membranes (RE4040-FL, Woongjin Chemical Co., Korea). This system contained 30 membrane elements that were installed in 10 pressure vessels in series in a 4-stage (3−3−2−2) configuration. The RO elements were made of polyamide, each of which was 2.5 in. To control the pH of the RO system (pH 6.2) and prevent microbial growth, sulfuric acid, NaOCl (3 mg L−1), and biocide (3 mg L−1) were injected into the raw wastewater tank. Also, to prevent organic fouling and oxidation of the membrane by any unreacted and remaining H2O2, an antiscalant and sodium bisulfate (SBS, 10.5 mg L−1) were injected into the pipe ahead of the prefilter, respectively. The total recovery rate of the RO system was maintained at 70% without the recirculation of concentrates. The cleaning in place (CIP) of the RO membrane was performed when the differential pressure increased by 50% (2.5 bar) (organic and inorganic CIP). The organic CIP and inorganic CIP were performed by soaking 10% NaOH (pH 9.5−10) and 2.5% citric acid, respectively, in the RO module for 3 h. 2.3.2. Solid-Phase AOP System. A pilot-scale solid-phase AOP system with a 30 m3 day−1 treatment capacity was constructed and operated. The reactor was an upflow type to ensure uniform flow, comprising 2 reactors to allow effluent from the first-stage reactor to flow into the lower part of the second-stage reactor. Nitric acid was injected into the inlet pipe of the first-stage reactor to control and maintain the pH at 3 using an online pH meter (CRIUS, Process Instruments, U.K.). Because the pH of the effluent from the first reactor tank was the same as that of the influent, no additional nitric acid was injected into the second-stage reactor. On the basis of previous results, the removal of TOC and low-molecular-weight organic compounds was examined at various H2O2 concentrations to

3. RESULTS AND DISCUSSION 3.1. Characteristics of Semiconductor Wastewater. The organic wastewater was selected among the various types of semiconductor wastewater because the operating cost of its reuse is the lowest. A membrane bioreactor (MBR) was used to treat the wastewater, and the resulting average water quality was 1.67 mg L−1 of total organic carbon (TOC), 5.33 mg L−1 of chemical oxygen demand (COD), and 2352 μS cm−1 of 11169



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electrical conductivity. Also, the concentrations of calcium and sulfate ions were 211 and 732 mg L−1, respectively (see Table 1). The concentration of calcium ions was high because Ca(OH)2 was added in the pretreatment to reduce the fluoride ion concentration before biological treatment of the organic wastewater. The sulfate concentration was high because sulfuric acid was used to control the pH in the wastewater treatment plant. Because high concentrations of calcium and sulfate ions can affect the formation of potassium sulfate, an antiscalant must be injected to prevent inorganic fouling during RO. No other compounds, except for calcium and sulfate, interfere with RO. The quality of MBR-treated water met the standards for the industrial use of reused water (as industrial cooling water), but it was deemed to require additional unit processes to supply the reused water as raw water for UPW production. 3.2. Verification of UPW Production System. Before we determine the feasibility of using recycled water as raw water for UPW production, the surface water that was supplied to semiconductor manufacturing facilities was examined to assess whether the performance of the pilot-scale UPW system was comparable to an actual UPW production facility. The activated carbon filter and mixed bed polishing filter were replaced every 15 days and 5 days, respectively. The CIP of the RO membrane was performed when the differential pressure increased by 2.5 bar (3 months after start-up). The water quality of feedwater which was supplied from a reservoir in the middle-west area of Korea varied from 1.1 to 1.8 mg L−1 as TOC during the pilot test period. As shown in Figure 2a, the average TOC concentration in water that was produced through pilot-scale UPW system was 0.70 μg L−1, and the maximum TOC concentration was 0.98 μg L−1, lower than the UPW quality standard for semiconductor UPW production (1 μg L−1 TOC). This difference of TOC concentration was caused by the fluctuation of feedwater. In addition, the resistivity, which reflects the concentration of ions in water indirectly, remained stable at 18.3 MΩ cm. Thus, the pilot-scale UPW system is suitable for verifying the feasibility of using recycled water to produce UPW. 3.3. Evaluation of UPW Quality with the Permeate of RO. Figure 3 shows the results of the RO operation using MBR-treated water for 75 days before the CIP. The electrical conductivity in the influent (MBR-treated water) and the permeate of the RO was 4060−4880 and 123−191 μS cm−1, respectively; the average salt rejection rate was 96.4% (see Figure 3a). The MBR-treated water contained up to several hundreds of milligrams per liter of chloride, sulfate, calcium, and sodium, of which the RO membrane removed 95.7%, 98.7%, 99.0%, and 96.5%, respectively. Figure 3b shows the TOC removal of the RO pilot-scale plant. The TOC in the influent and the permeate of the RO was 800−1000 and 82− 220 μg L−1, respectively, for an average TOC removal efficiency of 88%. Figure 3c shows the change in operating pressure and the differential pressure during the operation. The mean pressure in the RO was 10.7 bar and did not change significantly during operation. The differential pressure in the RO process (ΔP) maintained at 1.7 bar initially and rose gradually to 2.5 bar during the 75 days of operation. Also, after the CIP, the difference in pressure was recovered immediately. The RO-permeated water injected to the pilot-scale UPW system tested whether it can be used as raw water for UPW production (see Figure 2b). The average TOC concentration

Figure 2. Water quality of UPW production using (a) surface water and (b) RO-treated water.

produced through pilot-scale UPW system was 1.43 μg L−1 and exceeded the UPW quality standard for semiconductor UPW production. This result means that significant amounts of lowmolecular-weight compounds in treated wastewater were not properly removed through the RO system. Therefore, the additional treatment process is required to satisfy the water quality of UPW because the RO-permeated water was unable to be used as raw water for UPW production. 3.4. Optimization of Solid-Phase AOP. 3.4.1. Effect of H2O2 Concentration. In a previous study, the optimal pH in the AOP using metal-immobilized catalysts was less than 3.20 The H2O2 concentration is an especially important factor in an AOP using metal-immobilized catalysts because the removal of organic compounds is linked to the quantity of hydroxyl radicals that are generated from the catalyst and H2O2. As the H2O2 concentration increased, the production of the hydroxyl radicals rose, accelerating the oxidation of organic compounds. However, at 700 μg L−1 H2O2 or greater, the efficiency of removal of organic compounds declined (see Figure 4a), likely because the hydroxyl radicals that were produced from H2O2 were consumed in reactions between themselves or with H2O2, rather than in the oxidation of organic compounds. This explanation implies that the excessive H2O2 resulted in a scavenger effect of the hydroxyl radicals and decreased the removal efficiency of organic compounds. The H2O2 concentration showing scavenger effect of OH• radical 11170



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Figure 3. (a) Salt rejection, (b) TOC removal, and (c) differential pressure in RO. Figure 4. Effect of (a) H2O2 concentration, (b) space velocity, and (c) linear velocity on TOC removal.

was significantly different depending on initial concentration and degradability of contaminants.22−24 Also, when high concentrations of H2O2 are not consumed in the oxidation reaction of organic compounds and its residuals are discharged, the next step or the environment will be affected. Thus, the optimal quantity of H2O2 must be determined. On the basis of these results, TOC removal was highest when the H2O2 concentration was approximately 1.2−2.7 times higher than the TOC concentration in the influent. Because the cost of H2O2 accounts for the largest portion of operating cost in most AOPs, the smallest amount of H2O2 that ensures a

certain removal efficiency of TOC must be added. Therefore, it can be concluded that the optimal ratio of H2O2 concentration to influent TOC concentration was 1.5. 3.4.2. Effect of Space Velocity. To apply the catalyst in a pilot-scale plant, the effect of linear and space velocity and the reactor configuration of the AOP must be determined with regard to the design of a pilot-scale plant. In enhancing the removal of organic compounds, the reaction between the 11171



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Figure 5. Removal of TOC with a (a) 1-stage and (b) 2-stage reactor configuration.

Table 2. Removal at Various H2O2 Concentrations by Pilot-Scale Solid-Phase AOP TOC 1 step

effluent (μg L−1)

2 step

H2O2 (μg L−1)

retention time (min)

H2O2 (μg L−1)

retention time (min)

200 300 400 500

10 10 10 10

100 100 100 100

10 10 10 10

influent (μg L−1) 95 90 92 104

± ± ± ±

3 4 4 3

1 step 57 45 36 42

± ± ± ±

2 step

6 5 4 5

39 28 14 22

± ± ± ±

removal efficiency (%)

4 2 3 3

58.9 68.8 84.7 78.8

± ± ± ±

5 3 3 3

Table 3. Characteristics of Low-Molecular-Weight Organic Compounds at Various H2O2 Concentrations in Pilot-Scale SolidPhase AOP (Retention Time per Step Was 10 min)a concentrations (μg L−1 as TOC) H2O2 conc. (μg L−1) 1 step400 2 step100 1 step400 2 step200 1 step400 2 step400

influent effluent

influent effluent

influent effluent

TOC (μg L−1)

MeOH

ACT

ACN

ACHO

EtOH

IPA

MEK

THF

CF

total

1 step

98 36

0.54 0

0.68 0.74

0.41 0.39

0.44 0.43

0.43 0.32

0 0

0.27 0

0.25 0.17

0.53 0

3.55 2.05

2 step

14

0

0.33

0.22

0.42

0.17

0

0

0

0

1.14

1 step

102 40

0.73 0.11

0.82 0.77

0.54 0.42

0.48 0.39

0.59 0.19

0 0

0.44 0

0.38 0

0.28 0

4.26 1.88

2 step

18

0

0.69

0.15

0.29

0.03

0

0

0

0

1.16

1 step

103 41

1.02 0

0.75 0.54

0.45 0.27

0.51 0.28

0.22 0.15

0 0

0.16 0.08

0.21 0.05

0.39 0

3.71 1.47

2 step

16

0

0.35

0

0.13

0.11

0

0

0

0

0.59

a

Methanol (MeOH), acetone (ACT), acetonitrile (ACN), acetaldehyde (ACHO), ethanol (EtOH), isopropylalcohol (IPA), methylethylketone (MEK), tetrahydrofuran (THF), and chloroform (CF).

3.4.3. Effect of Linear Velocity. As shown in Figure 4c, H2O2 was removed completely from the reactors at various linear velocities. As the linear velocity increased from 8.7 to 90.7 cm h−1, there was little difference in TOC removal efficiency. This implies that TOC removal did not differ significantly by diameter/height ratio of the reactors. Ultimately, the pilot-scale reactor was designed using the standards of diameter/height ratio for general activated carbon-packed column: 1/1.5. 3.4.4. Effect of Reactor Configuration. The 1-stage reactor had removal efficiency values of 58% for TOCs, 5% for acetone, and 31% for acetonitrile (Figure 5). In the 2-stage reactor, the removal of TOCs, acetone, and acetonitrile increased to 71%, 42%, and 92%, respectively. More than 90% of nonbiodegradable acetonitrile was removed in the 2-stage reactor, indicating that easily degradable organic compounds are removed in the 1-

optimized hydroxyl radicals that were produced from H2O2 and organic compounds is significant. A long reaction time can reduce TOC concentrations, but the optimal space velocity must be determined under a high reaction rate because an increase in reaction time requires a larger tank volume and more chemicals. However, as shown in Figure 4b, the decrease in TOC removal was not significant as the space velocity increased. With the increase in space velocity, H 2 O 2 consumption (removal efficiency) decreased slightly, indicating that the conversion time for H2O2 to hydroxyl radicals is insufficient because of short hydraulic retention time. Because TOC removal was nearly constant, regardless of the space velocity, but H2O2 discharge differed slightly, we conclude the optimal space velocity is 6 h−1, i.e., a retention time of 10 min. 11172



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stage reactor and that organic compounds that are not oxidized easily are removed in the 2-stage reactor. The reason for this difference can be implied to be the competition of hydroxyl radical between easily- and slowly degradable compounds.25,26 Thus, the 2-stage reactor is beneficial for treating the recycling water that contains a large quantity of low-molecular-weight organic substances. 3.5. Evaluation of UPW Quality with the Effluent from RO and Solid-Phase AOP. 3.5.1. Solid-Phase AOP. On the basis of the results of the previous experiment, we established the optimal operating conditions for the pilot-scale solid-phase AOP, wherein the concentration of the H2O2 that was added to the first-stage reactor was 200−500 μg L−1 and fixed at 100 μg L−1 in the second-stage reactor. The TOC removal bottomed out at 59% when the H2O2 concentration was 200 μg L−1 (1.5 times higher than the TOC concentration in the influent) (see Table 2). This finding indicates that hydroxyl radical scavengers, such as nitrite and carbonate, have adverse effects on the decomposition of organic compounds during continuous operation.27,28 When the H2O2 concentration was increased to 300 μg L−1, the TOC removal increased by approximately 17%. TOC removal peaked when the H2O2 concentration was 400 μg L−1, and the removal efficiency started to decline when the concentration rose to 500 μg L−1, likely because the excessive H2O2 scavenged the hydroxyl radicals. Thus, the H2O2 concentration for the first-stage solid-phase AOP was set to 400 μg L−1. After the H2O2 concentration for the first-stage reactor was fixed to 400 μg L −1, we determined the optimal H 2O 2 concentration for the second-stage reactor (see Table 3). The TOC removal by each reactor was examined while the H2O2 concentration in the second-stage reactor increased to 100, 200, and 400 μg L−1, but no significant difference was observed between conditions, indicating that TOC removal by the second-stage reactor is similar, regardless of its H 2 O 2 concentration. This result demonstrates that most easily degradable organic compounds in RO-treated water are removed in the first-stage reactor of the solid-phase AOP. Table 3 shows the removal efficiencies of low-molecularweight organic compounds for the solid-phase AOP. The removal of low-molecular-weight organic compounds peaked when 400 μg L−1 H2O2 was added to each of the first- and second-stage reactors. Specifically, the removal efficiencies of acetonitrile and acetaldehyde increased by 53.7% and 65.6%, respectively. In addition, the removal of low-molecular-weight organic compounds in the second-stage reactor rose 7.2% to 23.8% when the H2O2 concentration increased to 100, 200, and 400 μg L−1. These findings indicate that the second-stage system improves the decomposition of low-molecular-weight organic compounds. The continuous operation was performed for 6 months to assess the long-term stability of solid-phase AOP under optimal conditions, based on the experiments above, and TOC removal and the extracted metals concentrations were recorded. As shown in Figure 6a, the TOC concentration in the influent (RO-treated water) varied between 70 and 211 μg L−1 during the operation, and the average TOC concentration was approximately 110 μg L−1. The average TOC concentration in effluent from the solid-phase AOP was 23 μg L−1. The mean TOC removal efficiency was 79%. Even though TOC concentration in the influent fluctuated, TOC concentration in the water that was treated by solid-phase AOP was quite

Figure 6. Long-term stability of solid-phase AOP. (a) TOC removal and (b) Fe and Al concentration in effluent.

stable, suggesting that solid-phase AOP contributes significantly to stable water quality. Because the catalyst that was used in the solid-phase AOP contained Fe and Al, supported by activated carbon, the degree to which activated metal was extracted must be examined. As shown in Figure 6b, the levels of Fe and Al in the water that was treated by solid-phase AOP were below 30 μg L−1, and their concentrations were similar to those in RO-treated water that was supplied by solid-phase AOP. This finding implies that the sudden decline in catalyst performance by the extraction of active metals from the catalyst does not occur. The water that was treated by solid-phase AOP was compared with surface water that was used as raw water for UPW production. As shown in Table 4, the ion concentrations in water that was treated by solid-phase AOP were lower than those in surface water. However, nitrate concentration in the solid-phase AOP treated water was relatively higher than that in the surface water because HNO3 was used to reduce the pH in the solid-phase AOP. More than 99% of nitrate ions can be removed in RO during UPW production and do not remain in the final UPW. In conclusion, water that has been treated by solid-phase AOP can be supplied as raw water for UPW production. 3.5.2. UPW Production. Because the pH in water that was treated by solid-phase AOP was 3, NaOH was injected to increase the pH to 7−8, which is suitable for achieving a stable 11173



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standards. Thus, our hybrid process can be used for practical wastewater treatment and reuse in electronic wastewater as well as semiconductor wastewater.

Table 4. Difference in Water Quality of Surface Water and Solid-Phase AOP-Treated Water item

unit

surface water

solid-phase AOP treated water

pH TOC NO3− F− Cl− SO42− Ca Mg Na K Ba Al Fe

− μg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1

7.7 ± 0.5 1 390 ± 20 6.84 ± 1 0.015 ± 0.004 14.9 ± 3 18.4 ± 4 23.6 ± 7 4.31 ± 1 8.99 ± 3 2.28 ± 1 0.079 ± 0.004 0.104 ± 0.05 0.005 ± 0.001

3.1 ± 0.1 22.1 ± 4 0.04 ± 0.01 0.045 ± 0.009 2.51 ± 0.5 5.46 ± 2 0.097 ± 0.01 0.012 ± 0.003 6.47 ± 1 0.139 ± 0.5 0.054 ± 0.02 0.054 ± 0.01 0.017 ± 0.009

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-31-260-6053. Fax: +82-31-260-3800. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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production efficiency of UPW. As shown in Figure 7, the average TOC concentration of UPW production water was

Figure 7. Water quality of UPW production using the RO+solid-phase AOP treated water.

0.76 μg L−1, which is slightly higher than that of surface water. The range in variation of TOCs was smaller in the reused water versus surface water because the concentration of organic compounds in surface water varied by season and weather, whereas those in the reused water did not change significantly because of its complete removal by solid-phase AOP. Thus, the combination of RO and solid-phase AOP that we have developed can supply raw water for UPW production.

4. CONCLUSION A hybrid RO and solid-phase AOP system was developed to recycle semiconductor wastewater. In the solid-phase AOP, the optimal H2O2 concentration was 400 μg L−1 and the reactor required a 2-stage system to remove low-molecular-weight organic compounds, such as acetonitrile and acetaldehyde. However, other operational factors, such as space and linear velocity, did not significantly affect the removal of organic compounds. Approximately 80% of the organic compounds that remained after RO filtration were oxidized or removed by solid-phase AOP. During the 6-month operation, removal rates and low metal concentrations were maintained. Also, the wastewater that was treated in the hybrid process was supplied to a UPW system that produced UPW for semiconductor production facilities and satisfied the requisite UPW quality 11174



Article

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Reuse of Semiconductor Wastewater Using Reverse Osmosis and

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