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Article

Advanced Oxidation Processes and Nanofiltration to Reduce the Color and Chemical Oxygen Demand of Waste Soy Sauce

1
Department of Environmental and Chemical Engineering, Eco-Friendly Offshore Plant FEED Engineering Course, Changwon National University, 20 Changwondaehak-ro, Uichang-gu, Changwon-si, Gyeongsangnam-do 51140, Korea
2
School of Civil, Environmental and Chemical Engineering, Changwon National University, 20 Changwondaehak-ro, Uichang-gu, Changwon-si, Gyeongsangnam-do 51140, Korea
*
Authors to whom correspondence should be addressed.
Sustainability 2018, 10(8), 2929; https://0-doi-org.brum.beds.ac.uk/10.3390/su10082929
Submission received: 20 July 2018 / Revised: 3 August 2018 / Accepted: 15 August 2018 / Published: 17 August 2018
(This article belongs to the Special Issue Sustainable Wastewater Treatment Systems)

Abstract

:
Currently, the ozone (O3) oxidation efficiency in the treatment of waste soy sauce provides 34.2% color removal and a 27.4% reduction in its chemical oxygen demand (COD). To improve the O3 oxidation efficiency, hydrogen peroxide (H2O2) is used to cause a H2O2/O3 process. In H2O2/O3 process experiments, a previously optimized pH of 11 and applied O3 dose of 50 mg L−1 were used and the H2O2/O3 ratio was varied between 0.1 and 0.9 in intervals of 0.2. The results show that an H2O2/O3 ratio of 0.3 results in the highest efficiencies in terms of color removal (51.6%) and COD reduction (33.8%). Nanofiltration (NF) was used to pretreat the waste soy sauce to improve color removal and COD reduction. The results showed that NF with an NE-70 membrane results in 80.8% color removal and 79.6% COD reduction. Finally, the combination of NF and H2O2/O3 process resulted in the best treatment efficiency: 98.1% color removal and 98.2% COD reduction. Thus, NF & H2O2/O3 process can be considered as one of the best treatment methods for waste soy sauce, which requires high intrinsic color removal and COD reduction efficiencies.

Graphical Abstract

1. Introduction

Waste soy sauce has a high chemical oxygen demand (COD). It has an intense dark brown color due to the presence of caramel pigments and the occurrence of the Maillard reactions producing melanin or melanoidin [1,2]. Several processes are typically used to treat soy sauce wastewater including nitrification and denitrification, an activated sludge process, and biological treatments. However, it is difficult to control the stability of the soy sauce treatment processes because of annual fluctuations in the amount of waste soy sauce generated (47,914 tons were produced in 2016 compared to 37,232 tons in 2015 and 40,461 tons in 2014) [3]. Biological treatment removes only a small fraction of the organic matter that contributes to the color of the effluents [4,5,6]. Therefore, the wastewater discharged to the environment is colored, which is aesthetically displeasing; moreover, the wastewater could affect the ecosystem due to its non-biodegradability and recalcitrance [7]. Therefore, it is necessary to decolorize waste soy sauce before it is discharged.
In our earlier study, overcoming the limitations of ozone (O3) oxidation to reduce the color and COD of waste soy sauce was intensively investigated [8]. Up to 34.2% color removal and 27.4% COD reduction were achieved by O3 oxidation at a pH of 11 with an applied O3 dose of 50 mg L−1 [8]. However, the color removal and COD reduction were not thoroughly investigated as the salt water regenerated from the waste soy sauce was reused. Therefore, a combination of different techniques must be considered as an alternative treatment method for removing the color and reducing the COD of waste soy sauce as completely as possible. In addition, a high concentration of salt can be recovered from waste soy sauce through the use of advanced technologies.
Many researchers conduct O3 oxidation by adding hydrogen peroxide (H2O2) to boost oxidation to raise efficiency of the color removal and COD reduction in wastewater. Fahmi, M. R et al. reported that advanced oxidation processes (AOPs) better facilitate the removal of reactive red 120 than ozonation [9]. This indicates that H2O2 accelerates O3 activation by forming hydroxyl radicals, which quickly oxidize reactive red 120 [9]. Rollon, A. A. et al. reported that the AOPs combination of H2O2 and O3 are most effective, followed by UV/H2O2-treatment, UV/O3-treatment, O3-treatment, and UV-C treatment (which is the least effective) [10]. However, it is difficult to improve the efficiency of AOPs without creating oxidative conditions. This is because the O3 oxidation at the optimum applied O3 dose involves a large number of hydroxyl radicals reacting instantaneously with O3-demanding species (the target substances) [8]. In other words, there is a high instantaneous ozone demand (IOD) in high concentration COD products such as waste soy sauce. Therefore, a pretreatment is required to overcome this IOD that is generated during oxidation.
Other researchers have proposed the use of membrane-based filtration techniques to remove color and reduce the COD of high-organic-content wastewater. Zheng, Y. et al. reported that under optimum conditions (pressure of 0.8 bar and volume-concentrating factor of 4.0), a submerged nanofiltration (NF) system exhibited a steady permeate flux of 5.15 L m−2 h, a color reduction of 99.3%, and a COD reduction of 91.5% [11]. Abid, M. F. et al. (2012) achieved 93.77%, 95.67%, and 97% removal of red, black and blue dyes using a NF membrane under a pressure of 8 bar [12]. However, it is difficult to remove the color and organic matter completely using typical membrane processes other than reverse osmosis because chromophoric dissolved organic matter generally has various molecular weights [13,14,15]. Furthermore, despite research progress to date, membrane filtration has yet to be applied in removing the intense color to reduce the high COD of waste soy sauce. Membrane systems have been considered promising for the pretreatment of waste soy sauce before oxidation. However, there remain some technological challenges to be solved for this application, including developing a physicochemical technique of membrane filtration and an oxidation method that can overcome the limitations of biological treatments. In this study, we applied H2O2/O3 process to enhance color removal and COD reduction by oxidation and the H2O2/O3 ratio was optimized. In addition, NF was combined with H2O2/O3 process to maximize the color removal and COD reduction. Herein, we identify the best method for treating waste soy sauce from five different treatment methods that will guide future wastewater treatment strategies to allow the saline wastewater to be reused and mitigate the impact on the environment when it is discharged.

2. Materials and Methods

2.1. Sampling Procedure

The waste soy sauce used in this study was provided by Monggo Foods, Inc. (Changwon-si, Gyeongsangnam-do, Korea).

2.2. Characterization

All samples were analyzed after removing the impurities with a 1.2 µm GF/C filter (Whatman). A seven compactTM pH/Ion S220 instrument (Mettler-Toledo GmbH, Greifensee, Switzerland) was used to measure the pH. The biochemical oxygen demand (BOD5), chemical oxygen demand (CODcr), total nitrogen (TN), and total phosphorus (TP) were measured using a US/DR3900 (320−1100 nm, Hach Company, Düsseldorf, Germany) kit assay according to the methods for testing water pollution set forth by the Ministry of Environment of Korea. The total organic carbon (TOC) was analyzed using a Shimadzu JP/TOC−5000A (Kyoto, Japan). The color was calculated as the transmittance of wavelength at 390, 400, 456 nm as measured using an Agilent Cary60 UV−VIS spectrophotometer (Santa Clara, CA, USA). The salinity was measured using an SB1500PRO instrument (0%−10%, HM Digital, Seoul, Korea) and a table salinometer.

2.3. Size Exclusion Chromatography (SEC)

SEC analysis was performed with high performance liquid chromatography (HPLC, LC600 Shimadzu) with UVA (SPD-6A Shimadzu) and DOC (Modified Sievers Turbo total organic carbon analyzer) detectors. The columns employed were a TSK-50S column, a Polyacrylamide Bio-Gel P-6 column, and a Waters Protein-Pak 125 silica column (Table 1) [16].
Molecules that were larger than the pore size of the packing material were excluded and eluted first, at the void volume. Smaller molecules were able to penetrate through the porous infrastructure and were attenuated, corresponding to a higher retention time [16].

2.4. H2O2/O3 Process

A schematic of the benchtop-scale reactor system used herein is shown in Figure 1. Experimental runs were performed in a 1 L Pyrex reactor with 500 mL of sample. The reactor was filled with 500 mL of waste soy sauce and agitated with a magnetic stirrer at 150 rpm. The optimum conditions for color removal and COD reduction by O3-based oxidation were a pH of 11 and an applied O3 concentration of 50 mg L−1 [8]. The O3-containing gas was supplied continuously for 30 min through a gas diffuser at the bottom of the reactor. An O3 trap containing a 2% potassium iodide solution was connected in series with the reactor to verify the O3 gas concentration in the outlet gas stream. A 0.1 N Na2S2O3 (sodium thiosulfate) solution was used as the reducing agent for the reverse titration in the trap. Subsequently, 0.1 N H2SO4 (sulfuric acid) was used to facilitate the reaction of the O3 in the liquid phase with the I2 [8]. All experiments were performed in duplicate at room temperature under a fume hood (for safety due to the presence of O3 gas). The pH, applied O3 concentration, and reaction time were kept constant while the H2O2/O3 ratio was varied between 0.1 and 0.9 in intervals of 0.2. Table 2 summarizes the parameters for the H2O2/O3 process experiments. In addition, the calculated vent gas of ozone by Equation (1) is shown in Table 3. The resulting color change and COD reduction were observed in 50 mL samples collected from the supernatant.
[ a ] eqNa 2 S 2 O 3 L 1 eqO 3 2 eqNa 2 S 2 O 3 48 gO 3 1 eqO 3 [ b ] mL [ c ] min 1 Reactor   volume   ( L )
where [a]: Na2S2O3 concentration, [b]: Na2S2O3 consumption, [c]: Contact time.

2.5. Nanofiltration (NF) System

The NF membranes used in this study were thin-film composite NF membranes, NE-70 and NE-90 (Toray Chemical, Korea). These membranes have different top layers, zeta potentials, wettabilities, and roughnesses as detailed in Table 4. The characteristics of the NF membrane were investigated in previous study [17].
A laboratory-scale dead-end NF membrane was used in this experiment. The membrane surface area was 0.015 m2. Waste soy sauce was fed into the filtration module by a gear pump. The filtration experiments were performed with a commercial NF module. Figure 2 shows a schematic of the NF system. In all experiments, a pressure of 1.5 MPa (15 bar) was applied at room temperature. The evolution of flux and rejection progressed over a period of 6 hours. For analysis and comparison, the measurements were taken after 1 h of filtration.

3. Results and Discussion

3.1. Characteristics of Waste Soy Sauce

The chemical properties of the sample were evaluated according to the guidelines for testing water pollution developed by the Ministry of Environment of Korea [18]. Table 5 shows the characteristics of the waste soy sauce. The pH value of 4.6 indicates that the waste soy sauce was acidic. The color was found to be 3810 TCU and the COD was measured to be 231.5 g/L. These results are similar to those reported in a previous study: 4038 TCU color and 229.1 g/L COD [8]. The T-N and T-P concentrations were measured to be 10.4 and 2.8 g/L, respectively. The salinity, 16.4%, was much higher than that found in other types of organic wastewater. In general, the high-salinity wastewater does not exhibit high removal efficiencies with biological treatment systems. This is because the performances of biological treatment processes are adversely affected by the negative effects of salt on microbial flora [19]. Moreover, the TOC, which is used as an indicator of water pollution, was found to be 57.6 g/L.

3.2. Optimization of H2O2/O3 Ratio

Our previous study revealed that a pH of 11 and an applied O3 concentration of 50 mg L−1 are optimal for color removal and COD reduction; these conditions were used in this study. H2O2 was combined with O3 to accelerate the oxidation of the organic molecules present in the wastewater. The H2O2/O3 ratio was varied between 0.1 and 0.9 to optimize the color removal by H2O2/O3 process; the results of this experiment are shown in Figure 3. As the H2O2/O3 ratio increased from 0.1 to 0.3, the color removal increased gradually from 41.2 to 51.6%. The H2O2/O3 ratio of 0.3 resulted in the greatest color removal. The color removal decreased sharply from 51.6 to 17.1% as the H2O2/O3 ratio increased above 0.3.
The lower residual color after oxidation with H2O2/O3 as the H2O2/O3 ratio increased was attributed to the increased formation of OH radicals [20,21,22,23]. Other studies have reported increased H2O2/O3 ratios and biodegradability after wastewater treatment with O3 and H2O2 [24,25]. The decreased color removal with H2O2/O3 ratios above 0.3 was attributed to the strong scavenging effects of carbonate (CO32−) and bicarbonate (HCO3). The results showed that an H2O2/O3 ratio of 0.3 was optimal for color removal (Yielding 51.6% color reduction) because of the high oxygenation capacity resulting from the suitable amount of hydroxyl radicals. Thus, it was confirmed that the H2O2/O3 process are more efficient (51.6%) in removing color than in O3-based oxidation (34.2%) [8].
Figure 4 shows the influence of the H2O2/O3 ratio between 0.1 and 0.9 on the COD reduction due to the O3 treatment of waste soy sauce. The results show that the COD reduction as a function of the H2O2/O3 ratio followed a similar trend to that of the color removal shown in Figure 3. The COD reduction was optimal (a 73.6 g/L reduction from 217.73 g/L) at an H2O2/O3 ratio of 0.3. This result confirms that a sufficient amount of OH radicals were produced at an H2O2/O3 ratio of 0.3. The H2O2 addition facilitated the production of OH radicals, resulting in a synergistic effect between the applied O3 concentration and the COD reduction [26]. The COD reduction effect decreased distinctly from 33.8 to 16.6% as the H2O2/O3 ratio increased above 0.3 as observed with the color removal. This was likely due to the OH radicals being consumed by the excessive amounts of H2O2 [27,28,29].
The results shown in Figure 3 and Figure 4 confirm that the color removal and COD reduction strongly depend on the H2O2/O3 ratio. However, the H2O2/O3 ratio influences color removal more than COD reduction. Moreover, an H2O2/O3 ratio of 0.3 was the most effective in terms of both color removal and COD reduction.

3.3. Nanofiltration (NF) System

3.3.1. Size Exclusion Chromatography Analysis

The COD reduction was not significant despite the use of a H2O2/O3 process because the COD of waste soy sauce is considerably higher than that of other types of waste and wastewater. In addition, there is a high IOD due to the high COD of the waste soy sauce. Thus, the initial COD of waste soy sauce makes it unsuitable for the oxidation treatment [30]. In addition, it is known that H2O2 interferes with the COD reduction by this process by consuming oxidation agents including potassium dichromate [31]. Thus, the molecular weight distribution of the total organic carbon was measured to determine the best membrane-based filtration pretreatment in the interest of improving the COD reduction effect of the H2O2/O3 process.
Figure 5 shows the molecular weight distribution in the waste soy sauce as measured via size-exclusion chromatography (SEC) using a method for detecting organic matter wherein the adsorption of the sample was measured at an ultraviolet (254 nm) wavelength [32]. The main peak shows the molecular weight of the organic matter ranged 750–2200 dalton.
From this data, the appropriate molecular weight cutoff (MWCO) for the NF membranes (NE-70 and NE-90) was selected to approve the COD reduction effect by H2O2/O3 process. To facilitate stable treatment by oxidation, the COD reduction by the membrane pretreatment was calculated to be over 70%. Thus, experiments were conducted using two membranes, NE-70 (71.8% of the expected COD removal) and NE-90 (73.2% of expected COD removal), confirming the effectiveness of the pretreatment in terms of color removal and COD reduction.

3.3.2. Color Removal and COD Reduction by Nanofiltration (NF)

In our earlier study, O3-based oxidation exhibited low color removal (34.2%) and COD reduction (27.4%) efficiencies at pH 11 with an O3 injection dose of 50 mg L−1 [8]. In this study, we used H2O2/O3 process and found that the removal efficiency was higher than that with than O3-based oxidation under the same conditions. However, the color removal and COD reduction were not complete even with H2O2/O3 process. Thus, we applied NF as a pretreatment to enhance the color removal and COD reduction by oxidation method.
Table 6 shows that color removal and COD reduction by NF was similar with the NE-70 and NE-90 membranes. The results show that the color removal and COD reduction were similar with both membranes even though the MWCO of the NE-90 membrane (210 daltons) is lower than that of the NE-70 membrane (350 daltons). The NE-90 membrane, which yielded a color reduction of 81.3% and a COD reduction of 80.7%, was slightly more effective for removing color and reducing the COD than the NE-70 membrane, which yielded a color reduction of 80.8% and a COD of 79.6%. However, the NE-70 is of sustainability usable than NE-90 for experiment. It can further be concluded that the NE-70 membrane is suitable as a pretreatment for waste soy sauce treatment as it improves the color removal and COD reduction.

3.3.3. Flux Variation

To predict the lifetime of the nanofiltration (NF) membranes for waste soy sauce treatment, flux experiments were conducted. Figure 6 shows the flux decline of the two NF membranes during 4 hours of operation. The flux decreased sharply initially then steadily decreased after 45 min of operation. The flux of the NE-70 membrane was lower than that of the NE-90 membrane. This was attributed to the properties of the membranes: NE-90 has a higher contact angle (41.5 ± 3.7°) than NE-70 (22.6 ± 1.9°), showing that its surface is more hydrophobic [17,33,34]; this would cause hydrophobic organic compounds to adsorb onto the surface [17]. In addition, NE-90 has a higher surface roughness than NE-70, which results in a greater repulsive force between the membrane surface and the solute and, thus, a lower solute permeability [17].
These results show that NE-70 is more suitable for this application as its flux decline is lesser during the waste soy sauce treatment than that of NE-90 although the color removal and COD reduction were similar for the two membranes.

3.3.4. Comparison of Treatment Methods

To enhance the effectiveness of the O3 oxidation, we aimed to increase the OH radical production by adding H2O2 and applying NF as a pretreatment to overcome the limitation (i.e., the high COD concentration) of the H2O2/O3 process. Figure 7 and Table 7 show comparisons of color removal and COD reduction obtained by various treatment methods for waste soy sauce treatment. The results show that the NF & H2O2/O3 process resulted in the greatest color removal and COD reduction (98.1% and 98.2%, respectively) of the five methods considered. However, the residual color was 74.4 TCU and the residual COD was 4.2 g/L, which are considerably higher than those in ordinary wastewater. Therefore, additional treatment methods should be investigated in order to further improve the process.

4. Conclusions

In this study, the H2O2/O3 process were optimized for removing the color and reducing the COD of waste soy sauce. The H2O2/O3 process were conducted under optimized conditions (H2O2/O3 ratio or 0.3, pH of 11.0, and applied O3 dose of 50 mg L−1), resulting in 51.6% color removal and 33.8% COD reduction. This was primarily due to the high oxidation capability of O3 in the presence of the hydroxyl radicals introduced by the addition of H2O2. Moreover, an appropriate membrane was selected for waste soy sauce pretreatment based on the molecular weight distribution of the wastewater. The addition of this pretreatment resulted in 98.1% color removal and 98.2% COD reduction. Comparing with alternative methods, NF & H2O2/O3 process can be considered one of the best treatment methods for waste soy sauce, which requires particularly high degrees of color removal and COD reduction. These results can ultimately guide future research into the best wastewater treatment techniques that would allow the wastewater to be reused and mitigate the environmental impacts when it is discharged.

Author Contributions

H.-H.J. conceived and designed the experiments, analyzed the data, participated literature review, and preparation of the manuscript. G.-T.S. provided methodology of the research, participated in analyzed the data, and preparation of the manuscript. D.-W.J. offered suggestions on the concept and preparation of the manuscript.

Funding

This research was funded by the Gyeongnam Green Environment Center (16-4-20-22-8).

Acknowledgments

This work was supported by the research grant of the Gyeongnam Green Environment Center (16-4-20-22-8).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of the O3-oxidation experimental system.
Figure 1. Schematic of the O3-oxidation experimental system.
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Figure 2. Schematic of the NF experimental system.
Figure 2. Schematic of the NF experimental system.
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Figure 3. Removal efficiency of color with various H2O2/O3 ratios during H2O2/O3 process.
Figure 3. Removal efficiency of color with various H2O2/O3 ratios during H2O2/O3 process.
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Figure 4. Removal efficiency of COD with various H2O2/O3 ratios during H2O2/O3 process.
Figure 4. Removal efficiency of COD with various H2O2/O3 ratios during H2O2/O3 process.
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Figure 5. Molecular weight distribution of waste soy sauce.
Figure 5. Molecular weight distribution of waste soy sauce.
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Figure 6. Flux decline of NE-70 and NE-90 in filtration of waste soy sauce.
Figure 6. Flux decline of NE-70 and NE-90 in filtration of waste soy sauce.
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Figure 7. Comparison of removal efficiencies from five treatment methods.
Figure 7. Comparison of removal efficiencies from five treatment methods.
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Table 1. Characteristics of tested columns.
Table 1. Characteristics of tested columns.
TypeColumn PackingParticle Size (µm)Separation Range (Da)Column Size (cm)
Hydroxylated organicProtein PAK 125101000–30,0000.78 × 30
PolyacrylamideTSK-50S30<5 × 1062 × 25
SilicaBio-Gel P-690–1801000–60000.5 × 90
Table 2. The experimental conditions for the H2O2/O3 process.
Table 2. The experimental conditions for the H2O2/O3 process.
ParameterValue
pH11
Applied O3 conc. (mg L−1)50
H2O2/O3 ratio (wt/wt)0.1, 0.3, 0.5, 0.7, and 0.9
Reaction time (min)30
Mixing speed (rpm)150
Sample volume (mL)500
Table 3. Quantitative assessment of O3 consumption.
Table 3. Quantitative assessment of O3 consumption.
Parameter1st Experiment2nd Experiment
Na2S2O3 conc.0.1
Na2S2O3 consumption1.42.9
Vent O3 conc. (mg L−1 min−1)1.122.32
Table 4. The characteristics of the NF membrane.
Table 4. The characteristics of the NF membrane.
MembraneMaterialMWCO (Da)Zeta Potential at pH 7 (mV)Contact Angle (°)Roughness (nm)
NE-70Sulfonated polyethersulfone350–47.222.6 ± 1.98.69
NE-90Meta-phenylene diamine210–38.741.5 ± 3.748
Table 5. Chemical characteristics of waste soy sauce.
Table 5. Chemical characteristics of waste soy sauce.
ParameterValue
pH4.4 ± 0.2
1 CODcr (g/L)231.5 ± 0.9
2 BOD5 (g/L)129.4 ± 6.6
3 TN (g/L)10.4 ± 0.7
4 TP (g/L)2.8 ± 0.3
5 TOC (g/L)57.6 ± 2.7
Salinity (%)16.4 ± 0.2
Color (TCU)3810 ± 130
1 COD: Chemical oxygen demand; 2 BOD: Biochemical oxygen demand; 3 TN: Total nitrogen; 4 TP: Total phosphorus; 5 TOC: Total organic carbon.
Table 6. Color removal and COD reduction by nanofiltration (NF).
Table 6. Color removal and COD reduction by nanofiltration (NF).
ClassificationTypeAmount of Removal (Removal Efficiency)
Color (TCU)COD (g/L)
Treated waste soy sauce (Removal efficiency)NE-702908.8 (80.8%)173.3 (79.6%)
NE-902926.8 (81.3%)175.7 (80.7%)
Table 7. Color removal and COD reduction by various treatment methods.
Table 7. Color removal and COD reduction by various treatment methods.
ParameterAmount of Removal (Removal Efficiency)
Color (TCU)COD (g/L)
O3 [8]1333.8 (34.2%)63.3 (27.4%)
H2O2/O3 process1857.6 (51.6%)73.6 (33.8%)
NF (NE-70)2908.8 (80.8%)173.3 (79.6%)
NF & O33344.4 (92.9%)206.2 (94.7%)
NF & H2O2/O3 process3531.6 (98.1%)213.8 (98.2%)

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Jang, H.-H.; Seo, G.-T.; Jeong, D.-W. Advanced Oxidation Processes and Nanofiltration to Reduce the Color and Chemical Oxygen Demand of Waste Soy Sauce. Sustainability 2018, 10, 2929. https://0-doi-org.brum.beds.ac.uk/10.3390/su10082929

AMA Style

Jang H-H, Seo G-T, Jeong D-W. Advanced Oxidation Processes and Nanofiltration to Reduce the Color and Chemical Oxygen Demand of Waste Soy Sauce. Sustainability. 2018; 10(8):2929. https://0-doi-org.brum.beds.ac.uk/10.3390/su10082929

Chicago/Turabian Style

Jang, Hyun-Hee, Gyu-Tae Seo, and Dae-Woon Jeong. 2018. "Advanced Oxidation Processes and Nanofiltration to Reduce the Color and Chemical Oxygen Demand of Waste Soy Sauce" Sustainability 10, no. 8: 2929. https://0-doi-org.brum.beds.ac.uk/10.3390/su10082929

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