Environ Sci Pollut Res DOI 10.1007/s11356-015-4423-9
RESEARCH ARTICLE
Combination of ozonation, activated carbon, and biological aerated filter for advanced treatment of dyeing wastewater for reuse Xiao-ling Zou 1
Received: 13 December 2014 / Accepted: 20 March 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Laboratorial scale experiments were performed to investigate and evaluate the performance and removal characteristics of organics, color, and genotoxicity by an integrated process including ozonation, activated carbon (AC), and biological aerated filter (BAF) for recycling biotreated dyeing wastewater (BTDW) collected from a cotton textile factory. Influent chemical oxygen demand (COD) in the range of 156 −252 mg/L, 5-day biochemical oxygen demand (BOD5) of 13.5−21.7 mg/L, and color of 58−76° were observed during the 20-day continuous operation. Outflows with average COD of 43 mg/L, BOD5 of 6.6 mg/L, and color of 5.6° were obtained after being decontaminated by the hybrid system with ozone dosage of 0.25 mg O3applied/mg COD0, 40 min ozonation contact time, 30 min hydraulic retention time (HRT) for AC treatment, and 2.5 h HRT for BAF treatment. More than 82 % of the genotoxicity of BTDW was eliminated in the ozonation unit. The genotoxicity of the BAF effluent was less than 1.33 μg 4-nitroquinoline-N-oxide/L. Ozonation could change the organics molecular structures, destroy chromophores, increase the biodegradability, and obviously reduce the genotoxicity of BTDW. Results showed that the combined process could guarantee water reuse with high quality.
Keywords Ozonation . Decolorization . COD . Mineralization . Genotoxicity
Responsible editor: Gerald Thouand * Xiao-ling Zou [email protected] 1
School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang 330013, Jiangxi, China
Introduction Dyeing wastewater refers usually to the wastewater which is produced during sizing of fibers, scouring, desizing, bleaching, washing, mercerization, dyeing, and finishing from textile and dyestuff industries (Ahmad et al. 2012). This wastewater consists of various pollutants including a high content of organic matter, surfactants, additives, and dyes. Most of dyes are toxic and potentially carcinogenic in nature and their exclusion from wastewater is a major environmental concern (Carneiro et al. 2010). Decontamination of dye effluents has been a major concern to the scientific community for a long time. The classical technique can be classified into four categories: physical methods, chemical/electrochemical oxidation, biodegradation, and combined. Among the physical processes commonly applied to the discoloration of dyeing wastewater, adsorption onto activated carbon (AC) is worth mentioning and has been widely used (Ahmad et al. 2012, 2013; Wang 2012; Ranjithkumar et al. 2014). However, the application of AC adsorbent is hampered by the high cost of its preparation and regeneration. Thereupon, much attention has recently been paid to the preparation of AC from low-cost wastes for the removal of dyes from wastewater (Gupta and Suhas 2009; Wang 2012). Nevertheless, the adsorption treatment can be summarized as the simple transfer of adsorbate from aqueous phase to adsorbent, without the occurrence of any decomposition. Biodegradation is a process of removing pollutants from the environment by using metabolic potential of microorganisms to decompose a wide variety of organic compounds. Many previous studies have reported the results of biological treatment of dye-containing wastewater due to its low treatment cost, simple operation, and maintenance (Silveira et al. 2011; Malachova et al. 2013; Hu et al. 2014; Prasad and Aikat
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2014; Saroj et al. 2014). However, biological treatment alone is generally not sufficient to remove color, surfactants, and recalcitrant organic fractions from dyeing wastewater to levels for direct discharge. Thereupon, additional specific steps are needed to achieve this objective. Ozone (O3) is a strong oxidant which reacts rapidly either through a direct reaction of molecular ozone or by a radical type reaction in water. More importantly, it does not produce any solid residue. With the development of O3 generation technology, the cost declines gradually. Hence, ozonation is increasingly applied as a pretreatment or a final polishing step for advanced wastewater treatment (Somensi et al. 2010; Lotito et al. 2012; He et al. 2013; Wu et al. 2015; Zhang et al. 2014). O3-biological treatment has evolved as one of the most promising processes among advanced wastewater treatment processes. However, the residue of O3 has a large negative impact on the microbial community of the subsequent treatment unit. It is reported that O3 decomposition could be enhanced by AC (Gül et al. 2007). Actually, the AC-O3 process is drawing increasing research attention among scholars because it can enhance both the removal of organic compounds and O3 decomposition (Li et al. 2009; de Oliveira et al. 2011, 2014). Thus, the introduction of AC in O3-biological treatment systems may lessen the negative effect of O3 on microorganisms and improve treatment performance of the system. This work investigated the feasibility of O3-AC-biological aerated filter (BAF) process for the advanced treatment and water reclamation of biologically treated dyeing wastewater (BTDW). The performance, characteristics, and genotoxicity changes of O3-AC-BAF were investigated.
Materials and methods Wastewater samples The BTDW used in this study was obtained from an activated sludge system based on anaerobic–anoxic–oxic (A2/O) process located in a cotton textile factory in Jiangxi Province, China. The effluent was taken from the final discharge point of the existing wastewater treatment plant. The effluent quality of the A2/O process is listed in Table 1. As shown, COD, BOD5, and color are still too high to reuse the effluent. Ozonation treatment Figure 1 shows a schematic representation of the ozonation setup. Ozone was generated from pure oxygen with an ozone generator (RQ-A-2 k, Shandong Ruiqing Ozone Equipment Company, China). The ozone yield capacity is 25 g/h with gas flow rate of 0–750 L/h. Ozonation was performed in a Plexiglass-made reactor (100 cm height and 7 L working
Table 1 Characteristics of the biologically treated dyeing wastewater effluent used in this study Parameter
Range
Average
Reuse standard
pH BOD5 (mg/L) COD (mg/L) BOD5/COD
7.2–7.5 13.3–24.6 145–273 0.06–0.1
7.4 17.4 186 0.09
6.5–8.5 ≤10 ≤60 –
NH3–N (mg/L) Color (deg)
0.48–3.10 55–82
1.73 66
≤10 ≤10
volume) with continuously bubbling O3 into the solution through a microporous diffuser fixed at the bottom of the reactor. The wastewater was introduced by a peristaltic pump in a lateral at the bottom of the column. The emission from the reactor flowed into two wash bottles, which were placed in series and contained 2000 and 1000 mL KI solution, respectively, to trap undecomposed O3 from the reactor. The inlet concentration of O3 was measured before each ozonation test. The outlet concentration was measured continuously during the experiment. AC treatment An AC reactor was installed close to the ozonation reactor. Both AC and ozonation reactors had the same size. The effluent of ozonation reactor was pumped into the AC reactor using a peristaltic pump (Fig. 1). The AC reactor was loaded with commercial granular activated carbon (GAC) as suspended media. The GAC sample had a total surface area of 930 m2/ g, an apparent density of 402 kg/m3, and an iodine number of 870. The lid was closed during operation to prevent O3 from escaping. The GAC was washed and dried overnight at 100 °C before each experiment to eliminate any residual acidity or basicity. The GAC was saturated beforehand with ozonation effluent in the AC reactor and then started experiments. BAF treatment The BAF setup mainly consisted of one air compressor and two parallel BAFs (No. 1 BAF and No. 2 BAF). No. 1 BAF was fed with AC effluent and No. 2 was fed with BTDW as control experiment (Fig. 1). The two BAFs were the same, made of plexiglass with an inner diameter of 15 cm and a height of 180 cm with microporous air diffusers fixed on the bottom of the BAF columns. Pebbles with diameter of 1– 3 mm were packed in the bottom of the BAF columns in order to enhance the proportion of air bubbles. Particle ceramsites with diameter of 4–6 mm and density of 2.20×103 kg/m3 filled the BAFs to a filling height of 100 cm. Each BAF had a working volume of 8.5 L. The overall operation temperature was 23–28 °C.
Environ Sci Pollut Res Fig. 1 Schematic diagram of the O3-AC-BAF system used in this study. 1, O2 tank; 2, O3 generator; 3, influent tank; 4, peristaltic pump; 5, ozonation reaction tank; 6, first gas washing trap (KI, 2 %); (7) second gas washing trap (KI, 20 %); 8, AC tank; 9, No. 1 BAF column; 10, No. 2 BAF column; and 11, air compressor
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The two BAFs were inoculated with activated sludge from the cotton textile factory, and then filled with the BTDW and supplemented with sugar (5 g/L). The air diffuser was turned on and DO was controlled to be 2–4 mg/L. After 2 days, each reactor was connected to a circulation pump and was recirculated at a rate of 1.5 L/h. Every 2 days, sugar (5 g/L) and nutrients (CaCl2, MgSO4, FeCl3, each 0.1 g/L) were added into the BAFs to sustain the growth of microorganisms. During the acclimation stage, excess sludge was removed daily. Every day, the reactors were aerated for 22 h, followed by 2 h of quiescent condition. After 8 days, the BTDW was continuously fed into the reactors. The parameters of the two BAFs during the incubation time were gas/water ratio of 4:1 and hydraulic retention time (HRT) of 2.5 h. The sludge retention time in the reactors was maintained at 8 days by sludge withdrawal from the reactors once every day.
Analytical methods Water quality parameters were measured according to standard methods (China EPA 2002). Briefly, COD was determined with K2Cr2O7 and H2SO4 in a 1:1 ratio by the open reflux method with AgSO4 as a catalyst and HgSO4 to remove Cl− interference. Five-day biochemical oxygen demand (BOD5) was determined through the oxygen consumption of bacteria breaking down organic matter in the sample over a 5day period under standardized conditions. NH3–N was determined by Nessler’s reagent colorimetry. pH values were measured using a PHS-3C precision pH meter (Shanghai Precision Scientific Instrument Co., Ltd., China). After saturation with ozonation effluent, the amount of pollutants adsorbed onto GAC was calculated. Ozone concentration was determined using potassium iodide and Na2S2O3 titrometric method. Color was measured by dilution method, i.e., the colored sample (V1) was diluted by distilled water (V2) to the extent similar to achromic distilled water. Then the color value of original
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sample was (V1 +V2)/V1 (times), and results were expressed as x°, where 1° indicated the sample was as clear as pure water. Temperature and dissolved oxygen (DO) were routinely monitored during the entire experimental period. Additionally, a ultraviolet–visible (UV-vis) spectrophotometer (UNICO2802; United International Ocean Shipping Agency Ltd., Shanghai, China) was used to perform UV-vis scanning of the wastewater. Genotoxicity measurement of wastewater samples were conducted using β-galactosidase following the method of Wu et al. (2010). The dimethylsulfoxide solution of 4nitroquinoline-N-oxide (4-NQO) was used as positive control. The genotoxicity of the samples was standardized as the equivalent 4-NQO concentration. Measurements of the above parameters were performed by at least three measurements for each sample and the results presented here represented the average values± standard deviations.
Results and discussion BAFs start-up After about 2 weeks of operation, light reddish brown biofilm was observed on the surface of ceramsites at the reactor bottom near the inlet end, and some ceramsites were covered with small amounts of white floc. With the extension of operating time, biofilm gradually stretched over the BAFs from the bottom to the top. After continuous BTDW feeding for about 25 days, removal efficiencies of COD and color were maintained at around 30 and 40 %, respectively (Fig. 2). Additionally, microscopic images showed a dense population of eukaryotic organisms such as rotifers, nematodes, and paramecium and a small number of filamentous bacteria present in the
Environ Sci Pollut Res Influent
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Fig. 2 Time course of COD concentration and color and their removal during the start-up stage
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biofilm (figure not shown). These results demonstrated the completion of start-up stage and successful biofilm formation. Effect of ozone dosage on ozonation Ozone dosage and contact time are the two key parameters influencing ozonation performance. Thus, these two parameters were optimized in a batch mode using the ozonation reaction tank without connection to the AC tank and BAF column. The ozonation experiments with the BTDW were conducted for O3applied/COD0 ranging from 0.05 to 0.40 on the conditions of the 40-min and natural pH (7.3). As shown in Fig. 3a, COD removal increased with the increment in O3 dosage. The COD removal extent can be divided into two stages by the dosage of the O3applied/COD0 ratio. The first stage from 0 to 0.25 ratio corresponded to the partial oxidation of organic compounds with a lower COD removal degree than that of the second stage from 0.25 to 0.40, the latter indicating the predominance of organic substance mineralization under higher O3 dosage conditions. This finding was also verified by the variation of BOD5/COD with O3 dosage (Fig. 3a). At the O3 dosage corresponding to the 0.25 O3applied/COD0, the
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Fig. 3 a Variations of BOD5/COD and the removal of COD and color with different O3 dosages. b Variations of BOD5/COD, COD removal, and O3 utilization efficiency at different contact times. The initial values of COD and color were 175 mg/L and 68 times, respectively
BOD5/COD ratio was maximum, close to 0.42. However, the BOD5/COD ratio decreased when the O3 dosage was higher than 0.25 O3applied/COD0. This is because the excess amount of O3 can mineralize the biodegradable organics formed during the ozonation. Thus, this O3 dosage can be deemed optimum as it chemically yielded excellent BOD5/ COD enhancement while limiting COD mineralization. On the other hand, the color removal efficiency had reached 86.5 % at 0.25 O3applied/COD0, whereas further increasing O3 dosage only resulted in slow increase of color removal. Thereupon, the optimum O3 dosage considering running cost was 0.25 O3applied/COD0. This value was lower than that reported by Zhang et al. (2014), who obtained an optimum O3 dosage of 0.3 O3applied/COD0 for ozonation pretreatment of biotreated coking wastewater. This difference was ascribed to the discrepancy between the wastewater characteristics.
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Effect of contact time on ozonation Contact time is an important parameter for ozonation because it can determine the size of the reactor in the practical applications. Thus, batch experiments of BTDW ozonation were performed corresponding to seven different contact times from 10 to 70 min. The optimum O3 dosage (0.25 O3applied/ COD0) was used with natural pH (7.3) in all cases. The feed flow rate of O3 was regulated to adapt the duration of the contact time during each trial with the optimum O3 dosage. Figure 3b shows variations of BOD5/COD, COD removal, and O3 utilization efficiency at different contact times. As shown, the COD removal efficiency increased in a greater extent during the first 40 min. For longer durations, the slow increment in removal percentage was observed. For instance, the COD removal efficiency increased from 7.3 % for 10 min to 33.6 % for 40 min. Although the COD removal efficiency reached about 38.4 % within 70 min, the option was not optimum and not economic for ozonation pretreatment. This is because the primary objective of ozonation was to improve BTDW biodegradability instead of mineralization of organic substances. In addition, as the ozonation time increased from 30 to 50 min, the rise of BOD5/COD was slow, even presenting a slight decrease during the ozonation time from 50 to 70 min (Fig. 3b). On the other hand, the ozone utilization efficiency increased with the increase of contact time. However, the ozone utilization efficiency increased slowly when the contact time was longer than 40 min. Thereupon, 40 min contact time was reasonable in the present study. Optimization of AC load for O3 residue decomposition To exclude possible inhibitory effect of O3 residue in the ozonation effluent on the microbial community of the subsequent BAF unit, a series of experiments were performed to examine influence of HRT on decomposition efficiency of O3 residue. The ozonation effluent under the optimum conditions (40 min and 0.25 O3applied/COD0) was continuously fed into the AC reactor using a peristaltic pump at various HRTs. The AC reactor was saturated with the ozonation effluent in advance. As listed in Table 2, most of O3 residue (96.4 %) could be destroyed at 30 min HRT, and further extending HRT did not significantly enhance O3 decomposition. Thus, a HRT of 30 min was selected as a suitable parameter for the AC treatment. Performance and characteristics of the O3-AC-BAF process To investigate the performance of the integrated ozonation, AC and BAF processes in treating the BTDW, a 20-day experiment was performed using the hybrid system (Fig. 1). The
Table 2 Variations of removal efficiency of O3 residue in the ozonation effluent at different HRTs
HRT (min)
O3 removal (%)
10 20 30 40 50
42.6 74.8 96.4 98.2 99.3
60
99.8
ozonation was operated in a batch mode, while the AC and BAF processes in continuous mode. Namely, the BTDW was introduced into the ozonation tank in a batch mode and was treated under the optimum conditions (40 min and 0.25 O3applied/COD0). Afterwards, the effluent was continuously fed into the AC and BAF reactors. The HRT of AC and BAF was controlled at 30 min and 2.5 h, respectively, by adjusting the pumping rate and filling ratio of AC reactor. Meanwhile, the performance of the BAF alone was also conducted as control under the same operational conditions. Figure 4 shows changes of physicochemical characteristics COD, BOD5, and color of influent and effluent from the O3AC-BAF. As shown, the water qualities of ozonation effluent were relatively stable despite of their fluctuations in the BTDW. This is attributed to the feature of adjustable O3 dosage on the basis of BTDW COD. Figure 4a shows the COD removal characteristics of the experiment. The COD of BTDW was in the range of 156− 252 mg/L, which reduced to average 115, 94, and 43 mg/L in the effluents of ozonation, AC, and No. 1 BAF, respectively, corresponding to a total removal of 72.4−82.9 %. When the BTDW was treated directly by No. 2 BAF, the effluent COD was in the range of 113−195 mg/L, and the COD removal efficiency was 22.6−27.6 %, indicating the low biodegradability of the BTDW used in this study. The COD reduction of ozonation effluent after passing through the AC column was ascribed to the oxygen reactions resulting from the ACcatalyzed decomposition of O3 residue (Li et al. 2009; de Oliveira et al. 2011, 2014). Figure 4b shows the BOD5 removal characteristics of the experiment. The BOD5 of BTDW was in the range of 13.5− 21.7 mg/L, which was changed to average 56, 53, and 6.6 mg/ L in the effluents of ozonation, AC and No. 1 BAF, respectively, corresponding to a total removal of 51.1−69.6 %. When the BTDW was treated directly by No. 2 BAF, the effluent BOD5 was in the range of 5.3−7.5 mg/L. Figure 4c shows the color removal characteristics of the experiment. The color of BTDW was in the range of 58− 76°, which reduced to an average of 8.8°, 8.4°, and 5.6° in the effluents of ozonation, AC and No. 1 BAF, respectively, corresponding to a total removal of 90.3−92.6 %. When the BTDW was treated directly by No. 2 BAF, the effluent color was in the range of 36−45°.
Environ Sci Pollut Res
(A)
Fig. 4
Changes of physicochemical characteristics of a COD; b BOD5; and c color from the O 3 -AC-BAF and BAF alone processes. Experimental conditions: ozonation contact time = 40 min; ozone dosage=0.25 O3applied/COD0; AC HRT=30 min; BAF HRT=2.5 h
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Obviously, the hybrid process obtained higher treatment efficiency than the BAF alone. This is mainly due to that ozonation could increase the biodegradability of BTDW, improving the efficiency of the following biological treatment, despite of the incomplete mineralization of organics by ozonation. Ozone is efficient in decolorizing textile effluents. Ozone is frequently used for decolorizing dyeing wastewater because it attacks conjugated double bonds associated with color (Zhang et al. 2007; Turhan et al. 2012; Zhou et al. 2013; Khuntia et al. 2015). In addition, the use of O3 for the enhancement of biodegradability of biorefractory organic wastewater contaminants have been demonstrated in the past for various industry effluents (Somensi et al. 2010; Lotito et al. 2012; Wu et al. 2015; Zhang et al. 2014).
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Figure 5a shows the genotoxicity changes in influent and effluents of ozonation, AC, No. 1 BAF, and No. 2 BAF. The genotoxicity of BTDW was approximately 38.40 μg 4-NQO/ L. It was reported that the genotoxicity of dyeing wastewater originated primarily from tetrahydropyran derivatives, benzoic acid derivatives, surfactants, etc. (Giorgetti et al. 2011). In this study, ozonation could greatly reduce the genotoxicity of BTDW, which decreased to 6.83 μg 4-NQO/ L after ozonation, probably due to the partial elimination of the toxic organic fractions during this treatment process. More than 82 % of the genotoxicity of BTDW was eliminated in the ozonation unit. This result was in agreement with the report of Somensi et al. (2010) who demonstrated a significant reduction in the toxicity of textile wastewater samples after ozonation. In this study, the genotoxicity of wastewater was further reduced to 5.26 and 1.33 μg 4-NQO/L when subsequently treated by AC and No. 1 BAF, respectively (Fig. 5a). The total removal of genotoxicity was 96.5 % by the hybrid system. The remaining genotoxicity could be ascribed to the recalcitrant compounds, and also to the inorganic fractions such as metal ions. However, the genotoxicity reduced slightly to 32.58 μg 4-NQO/L when directly treating the BTDW by No. 2 BAF, and only 15.2 % of genotoxicity can be removed during this process. UV-vis spectra
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Figure 5b shows the efficiency of O3-AC-BAF treatment as determined by periodic UV-vis absorbance spectra (from 200 to 700 nm) collections. The UV spectrum of BTDW shows the characteristic bands of benzene rings (around 210 nm) and
Environ Sci Pollut Res
naphthalene rings (around 310 nm). The color of the dyestuff arises mainly from the aromatic π-system (Turhan et al. 2012). Figure 5b shows that the intensity of bands at 210 and 310 nm was greatly reduced by ozonation, indicating the efficient ring opening of dye molecules. In the visible light region, the absorbance also remarkably declined after ozonation, and the absorbance spectrum became smoother. This suggested that the aromatic π-system of organics in the wastewater was efficiently oxidized by ozonation. However, the UV-vis absorbance reduced slightly when directly treating the BTDW by No. 2 BAF. These results were in accordance with the comparison of color removal by the hybrid system and BAF alone (Fig. 4c). In the present study, ozonation treatment showed limited mineralization of the organic substances concerning the COD
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removal but high color removal efficiency for the dyeing wastewater. This illustrated that only chromophores were destructed and reduced instead of completely oxidizing the organic molecules to CO2 and H2O.
Conclusions This study was performed to investigate the effect of ozonation as well as the O3-AC-BAF process on COD, BOD5, color, genotoxicity, and UV-vis absorbance of BTDW. Results demonstrated that the operational parameters including the ozone dosage of 0.25 mg O3applied/mg COD0, 40 min ozonation contact time and 30 min HRT for AC were recommended for the hybrid O3-AC-BAF process. This was highly effective in removing the COD, BOD5, color, and genotoxicity of BTDW simultaneously; the removal efficiencies of which reached about 72.4 − 82.9, 51.1 − 69.6, 90.3 − 92.6, and 96.5 %, respectively, during the 20-day treatment period. The water quality of the final effluent could satisfy given reuse requirements of textile industry. Therefore, the O3-AC-BAF process is a promising solution for enhancing the overall treatment efficiency of BTDW. Acknowledgments This study has been funded jointly by the Jiangxi Provincial Natural Science Foundation of China (grant No. 20132BAB213026), the Jiangxi Provincial Science and Technology Support Project of China (grant No. 20122BBG70081-1), and the Science Youth Science and Technology Foundation of Education Department of Jiangxi Province (GJJ13309).
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Fig. 5 Changes of a genotoxicity and b UV-vis spectra in the BTDW, ozonation effluent, No. 1 BAF effluent and No. 2 BAF effluent from the O3-AC-BAF and BAF alone processes. The samples were collected at day 16
References Ahmad AA, Idris A, Hameed BH (2012) Color and COD reduction from cotton textile processing wastewater by activated carbon derived from solid waste in column mode. Desalin Water Treat 41:224–231 Ahmad AA, Idris A, Hameed BH (2013) Organic dye adsorption on activated carbon derived from solid waste. Desalin Water Treat 51: 2554–2563 Carneiro PA, Umbuzeiro GA, Oliveira DP, Zanoni MVB (2010) Assessment of water contamination caused by a mutagenic textile effluent/dyehouse effluent bearing disperse dyes. J Hazard Mater 174:694–699 China EPA (2002) Analysis methods for the examination of water and wastewater, 4th edn. Chinese Environmental Science Press, Beijing (in Chinese) de Oliveira TF, Chedeville O, Cagnon B, Fauduet H (2011) Degradation kinetics of DEP in water by ozone/activated carbon process: Influence of pH. Desalination 269:271–275 de Oliveira TF, Cagnon B, Chedeville O, Fauduet H (2014) Removal of a mix of endocrine disrupters from different natural matrices by ozone/activated carbon coupling process. Desalin Water Treat 52: 4395–4403 Giorgetti L, Talouizte H, Merzouki M, Caltavuturo L, Geri C, Frassinetti S (2011) Genotoxicity evaluation of effluents from textile industries of the region Fez-Boulmane, Morocco: a case study. Ecotoxic Environ Safety 74:2275–2283
Environ Sci Pollut Res Gül Ş, Özcan Ö, Erbatur O (2007) Ozonation of CI Reactive Red 194 and CI Reactive Yellow 145 in aqueous solution in the presence of granular activated carbon. Dyes Pigments 75:426–431 Gupta VK, Suhas (2009) Application of low-cost adsorbents for dye removal–A review. J Environ Manage 90:2313–2342 He Y, Wang X, Xu J, Yan J, Ge Q, Gu X, Jian L (2013) Application of integrated ozone biological aerated filters and membrane filtration in water reuse of textile effluents. Bioresour Technol 133:150–157 Hu DX, Cui MH, Chen ZB, Tian Y, Cui YB, Ren NQ, Ran CQ, Sun HJ (2014) Performance of a novel HABR–CFASR system for the biological treatment of mixed printing and dyeing wastewater (MPDW). Desalin Water Treat 52:5553–5562 Khuntia S, Majumder SK, Ghosh P (2015) A pilot plant study of the degradation of Brilliant Green dye using ozone microbubbles: mechanism and kinetics of reaction. Environ Technol 36:336–347 Li X, Zhang Q, Tang L, Lu P, Sun F, Li L (2009) Catalytic ozonation of pchlorobenzoic acid by activated carbon and nickel supported activated carbon prepared from petroleum coke. J Hazard Mater 163: 115–120 Lotito AM, Fratino U, Bergna G, Di Iaconi C (2012) Integrated biological and ozone treatment of printing textile wastewater. Chem Eng J 195: 261–269 Malachova K, Rybkova Z, Sezimova H, Cerven J, Novotny C (2013) Biodegradation and detoxification potential of rotating biological contactor (RBC) with Irpex lacteus for remediation of dyecontaining wastewater. Water Res 47:7143–7148 Prasad SS, Aikat K (2014) Study of bio-degradation and biodecolourization of azo dye by Enterobacter sp. SXCR. Environ Technol 35:956–965 Ranjithkumar V, Sangeetha S, Vairam S (2014) Synthesis of magnetic activated carbon/α-Fe2O3 nanocomposite and its application in the removal of acid yellow 17 dye from water. J Hazard Mater 273:127– 135 Saroj S, Kumar K, Pareek N, Prasad R, Singh RP (2014) Biodegradation of azo dyes Acid Red 183, Direct Blue 15 and Direct Red 75 by the isolate Penicillium oxalicum SAR-3. Chemosphere 107:240–248
Silveira E, Marques PP, Macedo AC, Mazzola PG, Porto ALF, Tambourgi EB (2011) Decolorization of industrial azo dye in an anoxic reactor by PUF immobilized Pseudomonas oleovorans. J Water Reuse Desalin 1:18–26 Somensi CA, Simionatto EL, Bertoli SL, Wisniewski A, Radetski CM (2010) Use of ozone in a pilot-scale plant for textile wastewater pretreatment: physicochemical efficiency, degradation by-products identification and environmental toxicity of treated wastewater. J Hazard Mater 175:235–240 Turhan K, Durukan I, Ozturkcan SA, Turgut Z (2012) Decolorization of textile basic dye in aqueous solution by ozone. Dyes Pigments 92: 897–901 Wang L (2012) Application of activated carbon derived from ‘waste’ bamboo culms for the adsorption of azo disperse dye: Kinetic, equilibrium and thermodynamic studies. J Environ Manage 102:79–87 Wu QY, Li Y, Hu HY, Sun YX, Zhao FY (2010) Reduced effect of bromide on the genotoxicity in secondary effluent of a municipal wastewater treatment plant during chlorination. Environ Sci Technol 44:4924–4929 Wu C, Gao Z, Zhou Y, Liu M, Song J, Yu Y (2015) Treatment of secondary effluent from a petrochemical wastewater treatment plant by ozonation-biological aerated filter. J Chem Technol Biotech 90:543– 549 Zhang F, Yediler A, Liang X (2007) Decomposition pathways and reaction intermediate formation of the purified, hydrolyzed azo reactive dye CI Reactive Red 120 during ozonation. Chemosphere 67:712– 717 Zhang S, Zheng J, Chen Z (2014) Combination of ozonation and biological aerated filter (BAF) for bio-treated coking wastewater. Separ Purif Technol 132:610–615 Zhou XJ, Guo WQ, Yang SS, Zheng HS, Ren NQ (2013) Ultrasonicassisted ozone oxidation process of triphenylmethane dye degradation: Evidence for the promotion effects of ultrasonic on malachite green decolorization and degradation mechanism. Bioresour Technol 128:827–830
RESEARCH ARTICLE
Combination of ozonation, activated carbon, and biological aerated filter for advanced treatment of dyeing wastewater for reuse Xiao-ling Zou 1
Received: 13 December 2014 / Accepted: 20 March 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Laboratorial scale experiments were performed to investigate and evaluate the performance and removal characteristics of organics, color, and genotoxicity by an integrated process including ozonation, activated carbon (AC), and biological aerated filter (BAF) for recycling biotreated dyeing wastewater (BTDW) collected from a cotton textile factory. Influent chemical oxygen demand (COD) in the range of 156 −252 mg/L, 5-day biochemical oxygen demand (BOD5) of 13.5−21.7 mg/L, and color of 58−76° were observed during the 20-day continuous operation. Outflows with average COD of 43 mg/L, BOD5 of 6.6 mg/L, and color of 5.6° were obtained after being decontaminated by the hybrid system with ozone dosage of 0.25 mg O3applied/mg COD0, 40 min ozonation contact time, 30 min hydraulic retention time (HRT) for AC treatment, and 2.5 h HRT for BAF treatment. More than 82 % of the genotoxicity of BTDW was eliminated in the ozonation unit. The genotoxicity of the BAF effluent was less than 1.33 μg 4-nitroquinoline-N-oxide/L. Ozonation could change the organics molecular structures, destroy chromophores, increase the biodegradability, and obviously reduce the genotoxicity of BTDW. Results showed that the combined process could guarantee water reuse with high quality.
Keywords Ozonation . Decolorization . COD . Mineralization . Genotoxicity
Responsible editor: Gerald Thouand * Xiao-ling Zou [email protected] 1
School of Civil Engineering and Architecture, East China Jiaotong University, Nanchang 330013, Jiangxi, China
Introduction Dyeing wastewater refers usually to the wastewater which is produced during sizing of fibers, scouring, desizing, bleaching, washing, mercerization, dyeing, and finishing from textile and dyestuff industries (Ahmad et al. 2012). This wastewater consists of various pollutants including a high content of organic matter, surfactants, additives, and dyes. Most of dyes are toxic and potentially carcinogenic in nature and their exclusion from wastewater is a major environmental concern (Carneiro et al. 2010). Decontamination of dye effluents has been a major concern to the scientific community for a long time. The classical technique can be classified into four categories: physical methods, chemical/electrochemical oxidation, biodegradation, and combined. Among the physical processes commonly applied to the discoloration of dyeing wastewater, adsorption onto activated carbon (AC) is worth mentioning and has been widely used (Ahmad et al. 2012, 2013; Wang 2012; Ranjithkumar et al. 2014). However, the application of AC adsorbent is hampered by the high cost of its preparation and regeneration. Thereupon, much attention has recently been paid to the preparation of AC from low-cost wastes for the removal of dyes from wastewater (Gupta and Suhas 2009; Wang 2012). Nevertheless, the adsorption treatment can be summarized as the simple transfer of adsorbate from aqueous phase to adsorbent, without the occurrence of any decomposition. Biodegradation is a process of removing pollutants from the environment by using metabolic potential of microorganisms to decompose a wide variety of organic compounds. Many previous studies have reported the results of biological treatment of dye-containing wastewater due to its low treatment cost, simple operation, and maintenance (Silveira et al. 2011; Malachova et al. 2013; Hu et al. 2014; Prasad and Aikat
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2014; Saroj et al. 2014). However, biological treatment alone is generally not sufficient to remove color, surfactants, and recalcitrant organic fractions from dyeing wastewater to levels for direct discharge. Thereupon, additional specific steps are needed to achieve this objective. Ozone (O3) is a strong oxidant which reacts rapidly either through a direct reaction of molecular ozone or by a radical type reaction in water. More importantly, it does not produce any solid residue. With the development of O3 generation technology, the cost declines gradually. Hence, ozonation is increasingly applied as a pretreatment or a final polishing step for advanced wastewater treatment (Somensi et al. 2010; Lotito et al. 2012; He et al. 2013; Wu et al. 2015; Zhang et al. 2014). O3-biological treatment has evolved as one of the most promising processes among advanced wastewater treatment processes. However, the residue of O3 has a large negative impact on the microbial community of the subsequent treatment unit. It is reported that O3 decomposition could be enhanced by AC (Gül et al. 2007). Actually, the AC-O3 process is drawing increasing research attention among scholars because it can enhance both the removal of organic compounds and O3 decomposition (Li et al. 2009; de Oliveira et al. 2011, 2014). Thus, the introduction of AC in O3-biological treatment systems may lessen the negative effect of O3 on microorganisms and improve treatment performance of the system. This work investigated the feasibility of O3-AC-biological aerated filter (BAF) process for the advanced treatment and water reclamation of biologically treated dyeing wastewater (BTDW). The performance, characteristics, and genotoxicity changes of O3-AC-BAF were investigated.
Materials and methods Wastewater samples The BTDW used in this study was obtained from an activated sludge system based on anaerobic–anoxic–oxic (A2/O) process located in a cotton textile factory in Jiangxi Province, China. The effluent was taken from the final discharge point of the existing wastewater treatment plant. The effluent quality of the A2/O process is listed in Table 1. As shown, COD, BOD5, and color are still too high to reuse the effluent. Ozonation treatment Figure 1 shows a schematic representation of the ozonation setup. Ozone was generated from pure oxygen with an ozone generator (RQ-A-2 k, Shandong Ruiqing Ozone Equipment Company, China). The ozone yield capacity is 25 g/h with gas flow rate of 0–750 L/h. Ozonation was performed in a Plexiglass-made reactor (100 cm height and 7 L working
Table 1 Characteristics of the biologically treated dyeing wastewater effluent used in this study Parameter
Range
Average
Reuse standard
pH BOD5 (mg/L) COD (mg/L) BOD5/COD
7.2–7.5 13.3–24.6 145–273 0.06–0.1
7.4 17.4 186 0.09
6.5–8.5 ≤10 ≤60 –
NH3–N (mg/L) Color (deg)
0.48–3.10 55–82
1.73 66
≤10 ≤10
volume) with continuously bubbling O3 into the solution through a microporous diffuser fixed at the bottom of the reactor. The wastewater was introduced by a peristaltic pump in a lateral at the bottom of the column. The emission from the reactor flowed into two wash bottles, which were placed in series and contained 2000 and 1000 mL KI solution, respectively, to trap undecomposed O3 from the reactor. The inlet concentration of O3 was measured before each ozonation test. The outlet concentration was measured continuously during the experiment. AC treatment An AC reactor was installed close to the ozonation reactor. Both AC and ozonation reactors had the same size. The effluent of ozonation reactor was pumped into the AC reactor using a peristaltic pump (Fig. 1). The AC reactor was loaded with commercial granular activated carbon (GAC) as suspended media. The GAC sample had a total surface area of 930 m2/ g, an apparent density of 402 kg/m3, and an iodine number of 870. The lid was closed during operation to prevent O3 from escaping. The GAC was washed and dried overnight at 100 °C before each experiment to eliminate any residual acidity or basicity. The GAC was saturated beforehand with ozonation effluent in the AC reactor and then started experiments. BAF treatment The BAF setup mainly consisted of one air compressor and two parallel BAFs (No. 1 BAF and No. 2 BAF). No. 1 BAF was fed with AC effluent and No. 2 was fed with BTDW as control experiment (Fig. 1). The two BAFs were the same, made of plexiglass with an inner diameter of 15 cm and a height of 180 cm with microporous air diffusers fixed on the bottom of the BAF columns. Pebbles with diameter of 1– 3 mm were packed in the bottom of the BAF columns in order to enhance the proportion of air bubbles. Particle ceramsites with diameter of 4–6 mm and density of 2.20×103 kg/m3 filled the BAFs to a filling height of 100 cm. Each BAF had a working volume of 8.5 L. The overall operation temperature was 23–28 °C.
Environ Sci Pollut Res Fig. 1 Schematic diagram of the O3-AC-BAF system used in this study. 1, O2 tank; 2, O3 generator; 3, influent tank; 4, peristaltic pump; 5, ozonation reaction tank; 6, first gas washing trap (KI, 2 %); (7) second gas washing trap (KI, 20 %); 8, AC tank; 9, No. 1 BAF column; 10, No. 2 BAF column; and 11, air compressor
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The two BAFs were inoculated with activated sludge from the cotton textile factory, and then filled with the BTDW and supplemented with sugar (5 g/L). The air diffuser was turned on and DO was controlled to be 2–4 mg/L. After 2 days, each reactor was connected to a circulation pump and was recirculated at a rate of 1.5 L/h. Every 2 days, sugar (5 g/L) and nutrients (CaCl2, MgSO4, FeCl3, each 0.1 g/L) were added into the BAFs to sustain the growth of microorganisms. During the acclimation stage, excess sludge was removed daily. Every day, the reactors were aerated for 22 h, followed by 2 h of quiescent condition. After 8 days, the BTDW was continuously fed into the reactors. The parameters of the two BAFs during the incubation time were gas/water ratio of 4:1 and hydraulic retention time (HRT) of 2.5 h. The sludge retention time in the reactors was maintained at 8 days by sludge withdrawal from the reactors once every day.
Analytical methods Water quality parameters were measured according to standard methods (China EPA 2002). Briefly, COD was determined with K2Cr2O7 and H2SO4 in a 1:1 ratio by the open reflux method with AgSO4 as a catalyst and HgSO4 to remove Cl− interference. Five-day biochemical oxygen demand (BOD5) was determined through the oxygen consumption of bacteria breaking down organic matter in the sample over a 5day period under standardized conditions. NH3–N was determined by Nessler’s reagent colorimetry. pH values were measured using a PHS-3C precision pH meter (Shanghai Precision Scientific Instrument Co., Ltd., China). After saturation with ozonation effluent, the amount of pollutants adsorbed onto GAC was calculated. Ozone concentration was determined using potassium iodide and Na2S2O3 titrometric method. Color was measured by dilution method, i.e., the colored sample (V1) was diluted by distilled water (V2) to the extent similar to achromic distilled water. Then the color value of original
11
4
4
sample was (V1 +V2)/V1 (times), and results were expressed as x°, where 1° indicated the sample was as clear as pure water. Temperature and dissolved oxygen (DO) were routinely monitored during the entire experimental period. Additionally, a ultraviolet–visible (UV-vis) spectrophotometer (UNICO2802; United International Ocean Shipping Agency Ltd., Shanghai, China) was used to perform UV-vis scanning of the wastewater. Genotoxicity measurement of wastewater samples were conducted using β-galactosidase following the method of Wu et al. (2010). The dimethylsulfoxide solution of 4nitroquinoline-N-oxide (4-NQO) was used as positive control. The genotoxicity of the samples was standardized as the equivalent 4-NQO concentration. Measurements of the above parameters were performed by at least three measurements for each sample and the results presented here represented the average values± standard deviations.
Results and discussion BAFs start-up After about 2 weeks of operation, light reddish brown biofilm was observed on the surface of ceramsites at the reactor bottom near the inlet end, and some ceramsites were covered with small amounts of white floc. With the extension of operating time, biofilm gradually stretched over the BAFs from the bottom to the top. After continuous BTDW feeding for about 25 days, removal efficiencies of COD and color were maintained at around 30 and 40 %, respectively (Fig. 2). Additionally, microscopic images showed a dense population of eukaryotic organisms such as rotifers, nematodes, and paramecium and a small number of filamentous bacteria present in the
Environ Sci Pollut Res Influent
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Fig. 2 Time course of COD concentration and color and their removal during the start-up stage
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biofilm (figure not shown). These results demonstrated the completion of start-up stage and successful biofilm formation. Effect of ozone dosage on ozonation Ozone dosage and contact time are the two key parameters influencing ozonation performance. Thus, these two parameters were optimized in a batch mode using the ozonation reaction tank without connection to the AC tank and BAF column. The ozonation experiments with the BTDW were conducted for O3applied/COD0 ranging from 0.05 to 0.40 on the conditions of the 40-min and natural pH (7.3). As shown in Fig. 3a, COD removal increased with the increment in O3 dosage. The COD removal extent can be divided into two stages by the dosage of the O3applied/COD0 ratio. The first stage from 0 to 0.25 ratio corresponded to the partial oxidation of organic compounds with a lower COD removal degree than that of the second stage from 0.25 to 0.40, the latter indicating the predominance of organic substance mineralization under higher O3 dosage conditions. This finding was also verified by the variation of BOD5/COD with O3 dosage (Fig. 3a). At the O3 dosage corresponding to the 0.25 O3applied/COD0, the
BOD5/COD
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Fig. 3 a Variations of BOD5/COD and the removal of COD and color with different O3 dosages. b Variations of BOD5/COD, COD removal, and O3 utilization efficiency at different contact times. The initial values of COD and color were 175 mg/L and 68 times, respectively
BOD5/COD ratio was maximum, close to 0.42. However, the BOD5/COD ratio decreased when the O3 dosage was higher than 0.25 O3applied/COD0. This is because the excess amount of O3 can mineralize the biodegradable organics formed during the ozonation. Thus, this O3 dosage can be deemed optimum as it chemically yielded excellent BOD5/ COD enhancement while limiting COD mineralization. On the other hand, the color removal efficiency had reached 86.5 % at 0.25 O3applied/COD0, whereas further increasing O3 dosage only resulted in slow increase of color removal. Thereupon, the optimum O3 dosage considering running cost was 0.25 O3applied/COD0. This value was lower than that reported by Zhang et al. (2014), who obtained an optimum O3 dosage of 0.3 O3applied/COD0 for ozonation pretreatment of biotreated coking wastewater. This difference was ascribed to the discrepancy between the wastewater characteristics.
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Effect of contact time on ozonation Contact time is an important parameter for ozonation because it can determine the size of the reactor in the practical applications. Thus, batch experiments of BTDW ozonation were performed corresponding to seven different contact times from 10 to 70 min. The optimum O3 dosage (0.25 O3applied/ COD0) was used with natural pH (7.3) in all cases. The feed flow rate of O3 was regulated to adapt the duration of the contact time during each trial with the optimum O3 dosage. Figure 3b shows variations of BOD5/COD, COD removal, and O3 utilization efficiency at different contact times. As shown, the COD removal efficiency increased in a greater extent during the first 40 min. For longer durations, the slow increment in removal percentage was observed. For instance, the COD removal efficiency increased from 7.3 % for 10 min to 33.6 % for 40 min. Although the COD removal efficiency reached about 38.4 % within 70 min, the option was not optimum and not economic for ozonation pretreatment. This is because the primary objective of ozonation was to improve BTDW biodegradability instead of mineralization of organic substances. In addition, as the ozonation time increased from 30 to 50 min, the rise of BOD5/COD was slow, even presenting a slight decrease during the ozonation time from 50 to 70 min (Fig. 3b). On the other hand, the ozone utilization efficiency increased with the increase of contact time. However, the ozone utilization efficiency increased slowly when the contact time was longer than 40 min. Thereupon, 40 min contact time was reasonable in the present study. Optimization of AC load for O3 residue decomposition To exclude possible inhibitory effect of O3 residue in the ozonation effluent on the microbial community of the subsequent BAF unit, a series of experiments were performed to examine influence of HRT on decomposition efficiency of O3 residue. The ozonation effluent under the optimum conditions (40 min and 0.25 O3applied/COD0) was continuously fed into the AC reactor using a peristaltic pump at various HRTs. The AC reactor was saturated with the ozonation effluent in advance. As listed in Table 2, most of O3 residue (96.4 %) could be destroyed at 30 min HRT, and further extending HRT did not significantly enhance O3 decomposition. Thus, a HRT of 30 min was selected as a suitable parameter for the AC treatment. Performance and characteristics of the O3-AC-BAF process To investigate the performance of the integrated ozonation, AC and BAF processes in treating the BTDW, a 20-day experiment was performed using the hybrid system (Fig. 1). The
Table 2 Variations of removal efficiency of O3 residue in the ozonation effluent at different HRTs
HRT (min)
O3 removal (%)
10 20 30 40 50
42.6 74.8 96.4 98.2 99.3
60
99.8
ozonation was operated in a batch mode, while the AC and BAF processes in continuous mode. Namely, the BTDW was introduced into the ozonation tank in a batch mode and was treated under the optimum conditions (40 min and 0.25 O3applied/COD0). Afterwards, the effluent was continuously fed into the AC and BAF reactors. The HRT of AC and BAF was controlled at 30 min and 2.5 h, respectively, by adjusting the pumping rate and filling ratio of AC reactor. Meanwhile, the performance of the BAF alone was also conducted as control under the same operational conditions. Figure 4 shows changes of physicochemical characteristics COD, BOD5, and color of influent and effluent from the O3AC-BAF. As shown, the water qualities of ozonation effluent were relatively stable despite of their fluctuations in the BTDW. This is attributed to the feature of adjustable O3 dosage on the basis of BTDW COD. Figure 4a shows the COD removal characteristics of the experiment. The COD of BTDW was in the range of 156− 252 mg/L, which reduced to average 115, 94, and 43 mg/L in the effluents of ozonation, AC, and No. 1 BAF, respectively, corresponding to a total removal of 72.4−82.9 %. When the BTDW was treated directly by No. 2 BAF, the effluent COD was in the range of 113−195 mg/L, and the COD removal efficiency was 22.6−27.6 %, indicating the low biodegradability of the BTDW used in this study. The COD reduction of ozonation effluent after passing through the AC column was ascribed to the oxygen reactions resulting from the ACcatalyzed decomposition of O3 residue (Li et al. 2009; de Oliveira et al. 2011, 2014). Figure 4b shows the BOD5 removal characteristics of the experiment. The BOD5 of BTDW was in the range of 13.5− 21.7 mg/L, which was changed to average 56, 53, and 6.6 mg/ L in the effluents of ozonation, AC and No. 1 BAF, respectively, corresponding to a total removal of 51.1−69.6 %. When the BTDW was treated directly by No. 2 BAF, the effluent BOD5 was in the range of 5.3−7.5 mg/L. Figure 4c shows the color removal characteristics of the experiment. The color of BTDW was in the range of 58− 76°, which reduced to an average of 8.8°, 8.4°, and 5.6° in the effluents of ozonation, AC and No. 1 BAF, respectively, corresponding to a total removal of 90.3−92.6 %. When the BTDW was treated directly by No. 2 BAF, the effluent color was in the range of 36−45°.
Environ Sci Pollut Res
(A)
Fig. 4
Changes of physicochemical characteristics of a COD; b BOD5; and c color from the O 3 -AC-BAF and BAF alone processes. Experimental conditions: ozonation contact time = 40 min; ozone dosage=0.25 O3applied/COD0; AC HRT=30 min; BAF HRT=2.5 h
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Obviously, the hybrid process obtained higher treatment efficiency than the BAF alone. This is mainly due to that ozonation could increase the biodegradability of BTDW, improving the efficiency of the following biological treatment, despite of the incomplete mineralization of organics by ozonation. Ozone is efficient in decolorizing textile effluents. Ozone is frequently used for decolorizing dyeing wastewater because it attacks conjugated double bonds associated with color (Zhang et al. 2007; Turhan et al. 2012; Zhou et al. 2013; Khuntia et al. 2015). In addition, the use of O3 for the enhancement of biodegradability of biorefractory organic wastewater contaminants have been demonstrated in the past for various industry effluents (Somensi et al. 2010; Lotito et al. 2012; Wu et al. 2015; Zhang et al. 2014).
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Figure 5a shows the genotoxicity changes in influent and effluents of ozonation, AC, No. 1 BAF, and No. 2 BAF. The genotoxicity of BTDW was approximately 38.40 μg 4-NQO/ L. It was reported that the genotoxicity of dyeing wastewater originated primarily from tetrahydropyran derivatives, benzoic acid derivatives, surfactants, etc. (Giorgetti et al. 2011). In this study, ozonation could greatly reduce the genotoxicity of BTDW, which decreased to 6.83 μg 4-NQO/ L after ozonation, probably due to the partial elimination of the toxic organic fractions during this treatment process. More than 82 % of the genotoxicity of BTDW was eliminated in the ozonation unit. This result was in agreement with the report of Somensi et al. (2010) who demonstrated a significant reduction in the toxicity of textile wastewater samples after ozonation. In this study, the genotoxicity of wastewater was further reduced to 5.26 and 1.33 μg 4-NQO/L when subsequently treated by AC and No. 1 BAF, respectively (Fig. 5a). The total removal of genotoxicity was 96.5 % by the hybrid system. The remaining genotoxicity could be ascribed to the recalcitrant compounds, and also to the inorganic fractions such as metal ions. However, the genotoxicity reduced slightly to 32.58 μg 4-NQO/L when directly treating the BTDW by No. 2 BAF, and only 15.2 % of genotoxicity can be removed during this process. UV-vis spectra
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Figure 5b shows the efficiency of O3-AC-BAF treatment as determined by periodic UV-vis absorbance spectra (from 200 to 700 nm) collections. The UV spectrum of BTDW shows the characteristic bands of benzene rings (around 210 nm) and
Environ Sci Pollut Res
naphthalene rings (around 310 nm). The color of the dyestuff arises mainly from the aromatic π-system (Turhan et al. 2012). Figure 5b shows that the intensity of bands at 210 and 310 nm was greatly reduced by ozonation, indicating the efficient ring opening of dye molecules. In the visible light region, the absorbance also remarkably declined after ozonation, and the absorbance spectrum became smoother. This suggested that the aromatic π-system of organics in the wastewater was efficiently oxidized by ozonation. However, the UV-vis absorbance reduced slightly when directly treating the BTDW by No. 2 BAF. These results were in accordance with the comparison of color removal by the hybrid system and BAF alone (Fig. 4c). In the present study, ozonation treatment showed limited mineralization of the organic substances concerning the COD
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removal but high color removal efficiency for the dyeing wastewater. This illustrated that only chromophores were destructed and reduced instead of completely oxidizing the organic molecules to CO2 and H2O.
Conclusions This study was performed to investigate the effect of ozonation as well as the O3-AC-BAF process on COD, BOD5, color, genotoxicity, and UV-vis absorbance of BTDW. Results demonstrated that the operational parameters including the ozone dosage of 0.25 mg O3applied/mg COD0, 40 min ozonation contact time and 30 min HRT for AC were recommended for the hybrid O3-AC-BAF process. This was highly effective in removing the COD, BOD5, color, and genotoxicity of BTDW simultaneously; the removal efficiencies of which reached about 72.4 − 82.9, 51.1 − 69.6, 90.3 − 92.6, and 96.5 %, respectively, during the 20-day treatment period. The water quality of the final effluent could satisfy given reuse requirements of textile industry. Therefore, the O3-AC-BAF process is a promising solution for enhancing the overall treatment efficiency of BTDW. Acknowledgments This study has been funded jointly by the Jiangxi Provincial Natural Science Foundation of China (grant No. 20132BAB213026), the Jiangxi Provincial Science and Technology Support Project of China (grant No. 20122BBG70081-1), and the Science Youth Science and Technology Foundation of Education Department of Jiangxi Province (GJJ13309).
20
10
0 1 BTDW
Ozonation 2 effluent
3 AC effluent
#14BAF effluent
#25BAF effluent
Sample
(B)
Fig. 5 Changes of a genotoxicity and b UV-vis spectra in the BTDW, ozonation effluent, No. 1 BAF effluent and No. 2 BAF effluent from the O3-AC-BAF and BAF alone processes. The samples were collected at day 16
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