Đang tải... (xem toàn văn)
This study was conducted to evaluate the ozone and UV combination (ozone/UV) process for the reuse of sewage treatment plant effluent. The ozone/UV process was compared to the ozone alone and the UV alone processes by measuring a variety of parameters, such as UV absorbance at 410 nm (A410), COD, BOD, BOD/COD, TOC, specific UV absorbance (SUVA), and aldehydes. The organics present in sewage effluent were more effectively removed by hydroxyl radical (OH°) than by molecular ozone. The ozone/UV process was highly effective for the color removal, disinfection, mineralization of organics, and minimization of ozone by-product. It was also found that ozone/UV process equipped with high-intensity UV lamp was more efficient and economical than that with low-intensity UV lamp
Journal of Water and Environment Technology, Vol.1, No.2, 2003 Application of Ozone/UV Process for the Reclamation of Sewage Treatment Plant Effluent Byung Soo Oh, Sei Jun Park, Heung Gu Lee1, Kyoung Suk Kim, and Kyung Hyuk Lee2, Joon-Wun Kang* Yonsei University at Wonju campus, Korea E-mail: jwkang@dragon.yonsei.ac.kr, Fax: +82-33-763-5224 Research and Development Center, Kumho Tire Co Inc Research and Development Center, Samsung Engineering (* Author to whom correspondence should be addressed.) Abstract This study was conducted to evaluate the ozone and UV combination (ozone/UV) process for the reuse of sewage treatment plant effluent The ozone/UV process was compared to the ozone alone and the UV alone processes by measuring a variety of parameters, such as UV absorbance at 410 nm (A410), COD, BOD, BOD/COD, TOC, specific UV absorbance (SUVA), and aldehydes The organics present in sewage effluent were more effectively removed by hydroxyl radical (OH°) than by molecular ozone The ozone/UV process was highly effective for the color removal, disinfection, mineralization of organics, and minimization of ozone by-product It was also found that ozone/UV process equipped with high-intensity UV lamp was more efficient and economical than that with low-intensity UV lamp Key words Ozone/UV Process; Reuse; Hydroxyl Radical (OH°); Mineralization; Introduction Ozone is a strong oxidant and disinfectant that is commercially used for water treatment In actual practice, however, the direct reaction of ozone is quite selective in organic oxidation This is because ozone has a very low reactivity toward aromatics substituted with electron-withdrawing groups (-COOH, -NO2) and compounds with single bond (1) Therefore, the advanced oxidation processes - 141 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 (AOPs) involving the generation of hydroxyl radicals (OH°) have become the subject of numerous studies to overcome the limitation of ozone oxidation (2) The ozone/UV process is an effective technique to enhance OH° generation in ozone oxidation This process was developed in the early 1970s for the treatment of cyanide-containing wastes Some studies have shown that the process is more effective for the destruction of some organic compounds than ozonation alone and that it has significant potential as a water treatment (3, 4) Many authors proposed that some free radicals such as OH° might play an important role to destroy compounds that are refractory even to ozonation (5) In this study, the ozone and UV combination (ozone/UV) process is applied for the reuse of sewage effluent Therefore, the aim of this study is to evaluate the effectiveness of ozone/UV process for the treatment of sewage effluent compared to the other processes such as ozone alone and UV alone Materials and Methods Raw water characteristics Sewage effluent water from a sewage treatment plant in W city was used as sample The typical water quality characteristics (CODCr, BOD5, TOC, A254 (UV absorbance at 254nm), pH and alkalinity) of the sample are shown in Table Table Typical water quality characteristics of sewage effluent Parameters Values CODCr (mg/L) 15–21 BOD5 (mg/L) 2–6 TOC (mg/L) 5–15 A254 (cm-1) 0.12–0.16 PH 7.5 Alkalinity (mg/L) as CaCO3 153–165 Experimental procedure The experiments were carried out in a semi-batch reactor (0.5 L and 30 L, with low- and high-intensity UV lamp, respectively) equipped with a low-pressure Hg arc lamp (WEDECO, Germany) Ozone was obtained from oxygen through the ozone generator (OZONIA, USA) and was continuously diffused at a flow rate of 0.5 L/min into the photo reactor In the 0.5 L reactor, ozone was directly injected into the UV reactor and was mixed up by the magnetic stirrer On the other hand, - 142 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 the 20 L reactor was composed of the ozone reactor (22 L) and UV reactor (8 L) The sample was circulated at a flow rate of 30 L/min through the centrifuge pump These reactors were both able to operate the ozone alone and UV alone processes Figure shows the schematic diagram of the experimental system O3 decomposer and Vent O3 monitor O3 generator UV lamp (High intensity) 30L reactor O2 UV lamp (Low intensity) Cooling 500mL reactor Ozone/UV reactor UV reactor Ozone reactor stirrer pump Figure Schematic diagram of the experimental system Results and Discussion Evaluation of several processes: Ozone alone, UV alone, and Ozone/UV Color removal: UV absorbance at 410 nm (A410) was used as a parameter for color measurement (6) Results show that both ozone alone and ozone/UV processes were significantly effective, showing 80% reduction of A410 after 10 of run time On the other hand, the UV alone process did not show much color change Therefore, for color reduction, the ozone alone process was considered as the best process among the three However, it should be noted that other parameters should also be considered in the treatment of sewage effluent water A relatively high water quality must be achieved with the end goal of reusing the sewage effluent water for general household purposes The color removal efficiency of each process (ozone alone, UV alone, and ozone/UV) in the sewage effluent water is compared in Figure - 143 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 1.2 Abs(UV410 C/Co) 0.8 Ozone alone Ozone/UV UV alone 0.6 0.4 0.2 0 10 12 Time(min) Figure Degradation of color (A410) by ozone, UV, and ozone/UV processes (ozone dose rate = 1.6mg/L-min; UV dose = 0.4W/L) Oxidation of organics in sewage effluent: In Figures (a) and (b), the removal efficiency of COD and the variation of specific UV absorbance (SUVA), which can be calculated by dividing A254 with DOC, are compared for the ozone alone, UV alone, and ozone/UV combination processes The ozone alone process was performed at the condition of aqueous pH and to investigate the effect of different pH in direct ozone and OH° reactions with organic matters In Figure (a), up to 90% COD was removed by the ozone/UV process and up to 60% and 40% by ozone alone process at pH and 4, respectively It was also found that the effect of UV irradiation could be negligible for the COD removal According to Hoigné et al., the mechanism of the reaction of ozone with substances in the water can be divided into two distinct pathways (7) The first route is the direct attack of molecular ozone The second route is the indirect reaction of OH° formed by ozone decomposition In sewage effluent, the following reaction pathway of sewage effluent organic matters (EfOMs) of ozone and OH° are to be considered EfOMs + O3 → products [1] EfOMs + OH° → products [2] Ozone oxidation in pH could reflect the direct reaction between the NOMs and the molecular ozone At pH 7, NOMs would not only be decomposed by molecular ozone but also by OH°, which can be formed by the ozone decay - 144 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 Therefore, the difference in the removal efficiency between pH and could be explained as the effect of OH° Results show that COD removal in ozone/pH and ozone/UV processes was enhanced up to 20% and 50%, respectively, as compared to ozone/pH This implies that the AOPs involving OH° production could be a promising technique for treatment of sewage effluent and the ozone/UV process could enhance organic removal through more OH° production Figure (b) shows the change of SUVA during the treatment of sewage effluent with each process This value could provide insights into the characteristics of natural water such as aromatic contents per unit concentration of organic carbon, hydrophobicity, and molecular weight distribution of DOC (8) In this research, the water tested had a relatively low SUVA value (1.1–2.5 L/mg-m), indicating that the water contained hydrophilic and low-molecular-weight materials As shown in Figure 3, SUVA decreased up to 20, 60, 70, and 80% after UV alone, ozone/pH4, ozone/pH7, and ozone/UV combination processes, respectively This means that both ozone alone and ozone/UV processes could alter hydrophobic to hydrophilic and high-molecular-weight to low-molecular-weight organic matter (8) 1.4 Ozone alone Ozone/UV Ozone/pH4 UV alone COD(C/Co) 1.2 0.8 0.6 0.4 0.2 0 10 20 Time(min) (a) - 145 - 30 40 Journal of Water and Environment Technology, Vol.1, No.2, 2003 1.2 Ozone alone Ozone/UV Ozone /pH4 UV alone SUVA 0.8 0.6 0.4 0.2 0 10 20 30 40 50 Time(min) (b) Figure (a) Degradation of COD; (b) Variation of SUVA value by ozone, UV, and ozone/UV processes ([COD]o = 15~21 mg/L; [SUVA]o = 1.3~1.5 L/mg-m; ozone dose rate = 1.6mg/L-min; UV dose = 0.4W/L) Variation of biodegradability: To compare the biodegradability between ozone and ozone/UV processes, BOD and BOD/COD values were measured (Figure 4) In the ozone alone process, BOD increased from 2.2 to 2.5 mg/L at 15 mg/L ozone dose, after which its value gradually decreased with the increase of ozone dose In the ozone/UV process, BOD was rapidly reduced up to 80% with 50 mg/L of ozone dose The ratio of BOC/COD value was also compared as a parameter presenting biodegradability in the water (9) The ozone alone process showed that the BOD/COD value was approximately 4.5 times higher than its initial value at 15 after ozonation However, the ozone/UV process did not cause any significant change of BOD/COD value during the reaction time From this result, it was found that the ozone alone process was not effective in reducing BOD value However, it enhanced the biodegradability in the water, giving the possibility of pre-treatment process for biological treatment For the ozone/UV process, because both BOD and COD decreased to low level (0.3 and 0.8 mg/L) and no increase was observed, the water treated could be directly reused as relatively clean water - 146 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 2.5 Ozone alone BOD (C/Co) Ozone/UV 1.5 0.5 0 50 100 150 200 Time(min) (a) BOD/COD (C/Co) 4.5 Ozone alone Ozone/UV 3.5 2.5 1.5 0.5 0 15 Time(min) 30 60 (b) Figure (a) Variation of BOD; (b) BOD/COD values by ozone and ozone/UV processes ([BOD]o = 2~3 mg/L; [COD]o = 15-18 mg/L; ozone dose = 2mg/L-min; UV dose = W/L) Disinfection: The effect of disinfection by ozone, UV, and ozone/UV processes was investigated by detecting E coli in the sewage effluent The concentration of E coli was 980~1050 CFU/mL As shown in Figure 5, 3.6 mg/L of ozone dose and 0.83 W-min/L of UV dose were required for the 99% inactivation of E coli Results show that both the ozone and the UV were strong disinfectants, and the OH° formed by ozone/UV process could inactivate E coli present in sewage effluent This indicates that the ozone alone and the UV alone processes could be sufficiently used for disinfection of sewage effluent It was also found that the ozone/UV process showed similar effect for the E coli inactivation as compared to - 147 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 the other processes (ozone and UV alone), and enhancement through OH° production did not rise for the disinfection 1200 E.coli (CFU/mL) 1000 800 UV alone Ozone/UV Ozone alone 600 400 200 0 0.5 1.5 Time (min) 2.5 3.5 Figure Inactivation of E coli by ozone, UV, and ozone/UV processes (ozone dose = 3.6mg/L-min; UV dose = 0.4 W/L) Mineralization of organics by ozone/UV process TOC removal: TOC value was measured to compare the mineralization of organics present in sewage effluent (Figure 6) In the ozone alone process, TOC was degraded by 40% at 60 after ozonation, after which this value (2.4 mg/L) remained constantly until the run was finished In the ozone/UV process, TOC value was steadily decreased by 90% during the run time, showing prominent effectiveness for the mineralization of organics The oxidative effect of UV irradiation was insignificant, as shown by the 10% removal of TOC From this result, it was confirmed that ozonation had a limitation of organic oxidation due to the selective reaction and the partial oxidation with organics by ozone (1) Therefore, ozone/UV should be considered for the treatment of sewage effluent water for reuse because it resulted in low level of organics - 148 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 1.2 TOC (C/Co) 0.8 0.6 0.4 Ozone alone 0.2 Ozone/UV UV alone 0 30 60 90 120 150 Time(min) Figure TOC removal by ozone, UV, and ozone/UV processes ([TOC]o = 5~8 mg/L; ozone dose = mg/L-min; UV dose = W/L) Formation and reduction of aldehydes: Aldehydes are known as major byproducts produced after ozonation Of the aldehydes, formaldehyde and acetaldehyde are mutagenic and carcinogenic for humans (10, 11) Therefore, the process using ozone is important to investigate the aldehyde trend The total concentration of four principal aldehydes—formaldehyde, acetaldehyde, glyoxal, and methylglyoxal—in sewage effluent was 51 µg/L Considering the results plotted in Figure 7, the ozone alone process showed an increase of aldehyde concentration up to 110 µg/L at 20mg/L ozone dose, after which the concentration did not go lower than 80 µg/L In the ozone/UV process, the aldehyde concentration increased by 90 µg/L at the initial run time and decreased by 28 µg/L at 15 The UV alone process did not have a significant in the aldehyde concentration These results indicate that because the ozone alone process has the side effect of by-products such as aldehydes, applying this process for the reuse of sewage effluent could be troublesome From a different standpoint, since aldehydes can be used as a surrogate of assimilable organic carbon (AOC) (12), the ozone alone process could be proposed as a pre-treatment for the biological treatment, like as the result of BOD increase (see figure 4) It should be noted that the ozone/UV process could be regarded as a safe technique for the reuse of sewage effluent and that it minimizes the side effect of ozonation - 149 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 120 Aldehydes (㎍/L) 100 80 Ozone alone Ozone/UV UV alone 60 40 20 0 10 20 30 40 50 Time(min) Figure Variation of aldehydes by ozone, UV, and ozone/UV processes ([Aldehydes]o = 51 µg/L; ozone dose = mg/L-min; UV dose = W/L) Effect of UV type in ozone/UV process The ozone/UV process employed in this study was equipped with two types of UV lamp, one for low pressure low intensity (0.4 W/L) and the other for low pressure high intensity (8 W/L) Figure shows the removal efficiency of COD and TOC by the two types of the ozone/UV process as a function of ozone dose Both COD and TOC values were rapidly eliminated with the increase of ozone dose when high-intensity UV lamp was used in the ozone/UV process To remove 90% of COD, the ozone/UV process with high-intensity UV lamp (ozone/UV-HI) required 100 mg/L ozone dose and 40 W-min/L UV dose On the other hand, for the lowintensity UV lamp (ozone/UV-LI), 780mg/L of ozone dose and 16 W-min/L were consumed The difference between the two types of UV lamps was clearly revealed from the result of TOC removal The ozone/UV-HI process showed 90% TOC removal at 300 mg/L ozone dose However, the reduction of TOC by the ozone/UV-LI process was merely 23% The high-intensity UV lamp showed somewhat higher consumption of electric power than the low-intensity UV lamp It could not only greatly lower the amount of ozone dosage, but also significantly enhance the efficiency of organic removal - 150 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 Low intensity (0.4 W/L) High intensity (8 W/L) CODCr (C/Co) 0.8 0.6 0.4 0.2 0 200 400 600 Ozone dose(mg/L) 800 1000 (a) TOC (C/Co) 0.8 0.6 0.4 Low intensity (0.4 W/L) High intensity (8W/L) 0.2 0 200 400 600 800 1000 Ozone dose(mg/L) (b) Figure Comparison of the removal efficiency of (a) COD and (b) TOC by lowand high-intensity lamps in ozone/UV process ([COD]o = 18~21; [TOC]o = 7~9 mg/L) Conclusions For the reuse of sewage effluent, the ozone/UV process showed excellent effectiveness for several aspects, such as color removal, disinfection, mineralization of organics, and minimization of ozone by-product (such as - 151 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 aldehydes) In addition, the ozone/UV process equipped with high-intensity UV lamp (8W/L) resulted in 88% reduction in the amount of ozone dosage and the 57% enhancement of organic removal compared to the ozone/UV process with low-intensity UV lamp (0.4W/L) Because the organics present in sewage effluent was highly destroyed by OH°, the ozone/UV process was favored to mineralize organics, showing 90% removal of TOC The ozone alone process was effective for color removal but showed a limitation, which is organic oxidation Considering the results of the BOD/COD and aldehydes, the ozone alone process could be proposed as a pre-treatment for the biological treatment because ozonation was able to enhance the biodegradability in the water For the inactivation of E coli present in sewage water, the ozone alone and the UV alone processes showed good efficiency with only 3.6 mg/L ozone dose and 0.83 W-min/L UV dose, respectively If the aim of the treatment were for disinfection only, either the ozone alone or the UV alone process would be enough Acknowledgement This research was supported by a grant ( 4-1-1 ) from Sustainable Water Resources Research Center (SWRRC) of 21st century frontier R&D program through the Water Reuse Technology Center (WRTC) at Kwangju Institute of Science & Technology (K-JIST) References Bruno, L., A.R David and R B Deborah, Ozone in Water Treatment (Colorado, The United States of America: Lewis Publishers, 1991), p.12 Glaze, W.H., J.W Kang and D.H Chapin, “The Chemistry of Water Treatment Processes Involving Ozone, Hydrogen peroxide, and Ultraviolet Radiation”, Ozone Sci Eng 9(4):335-352 (1987) Peyton, G.R., F.Y Huang, J.L Burleson and W.H Glaze, “Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation General Principles and Oxidation of Tetrachloroethylene”, Environ Sci Technol 16:448-453 (1982) Glaze, W.H., G.R Peyton, S Lin, R.Y Huang and J.L Burleson, “Destruction of Pollutants in Water with Ozone in Combination with Ultraviolet Radiation Natural Trihalomethane Precursors”, Environ Sci Technol 16:454-458 (1982) Prengle, H.W., Jr, “Experimental Rate Constants and Reactor Considerations for the Destruction of Micropollutants and Trihalomethane Precursors by Ozone with Ultraviolet Radiation”, Environ Sci Technol 17:743-747 (1983) - 152 - Journal of Water and Environment Technology, Vol.1, No.2, 2003 Beltran, F.J., J.F Garcia-Araya, J Frades, P Alvarez and O Fimeno, “Effects of Single and Combined Ozonation with Hydrogen Peroxide or UV Radiation on the Chemical Degradation and Biodegradability of Debittering Table Olive Industrial Wastewaters”, Water Res 33(3):723732 (1999) Staehelin J and J Hoigné, “Decomposition of Ozone in Water: Rate of Initiation by Hydroxide Ions and Hydrogen Peroxide”, Environ Sci Technol 16:676-681 (1982) Karanfil, T., M.A Schlautman, and I Erdogan, “Survey of DOC and UV measurement practices with implications for SUVA determination”, Jour AWWA 94(12):68-80 (2002) Alvares, A.B.C., C Diaper and S.A Parsons, “Partial oxidation by ozone to remove recalcitrance from wastewaters – a review”, Environ Sci Technol 22(4):409-427 (2001) 10 Glaze, W.H., and H S Weinberg, Identification and Occurrence of Ozonation By-products in Drinking Water (Denver, USA: AWWARF, 1993), p8 11 Schechter, D.S., and P.C Singer, “Formation of aldehyde during ozonation”, Ozone Sci & Eng 17:53-69 (1995) 12 Kang, J.W and J.B Kim, “The formation of assimilable organic carbon (AOC) in ozonated water and the determination of AOC with acinetobactor calcoaceticus”, J KSWQ, May, 69-75 (1995) - 153 - ... study, the ozone and UV combination (ozone/UV) process is applied for the reuse of sewage effluent Therefore, the aim of this study is to evaluate the effectiveness of ozone/UV process for the treatment. .. considered in the treatment of sewage effluent water A relatively high water quality must be achieved with the end goal of reusing the sewage effluent water for general household purposes The color... sewage effluent and the ozone/UV process could enhance organic removal through more OH° production Figure (b) shows the change of SUVA during the treatment of sewage effluent with each process