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Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater a
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Azahara Espejo , Almudena Aguinaco , J. F. García-Araya & Fernando J. Beltrán
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Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, Badajoz, Spain Published online: 05 May 2014.
To cite this article: Azahara Espejo, Almudena Aguinaco, J. F. García-Araya & Fernando J. Beltrán (2014) Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:9, 1015-1022, DOI: 10.1080/10934529.2014.894845 To link to this article: http://dx.doi.org/10.1080/10934529.2014.894845
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Journal of Environmental Science and Health, Part A (2014) 49, 1015–1022 Copyright Ó Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.894845
Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater AZAHARA ESPEJO, ALMUDENA AGUINACO, J.F. GARCIA-ARAYA and FERNANDO J. BELTRAN
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Departamento de Ingenierıa Quımica y Quımica Fısica, Universidad de Extremadura, Badajoz, Spain
Sequential treatments consisting in a chemical process followed by a conventional biological treatment, have been applied to remove mixtures of nine contaminants of pharmaceutical type spiked in a primary sedimentation effluent of a municipal wastewater. Combinations of ozone, UVA black light (BL) and Fe(III) or Fe3O4 catalysts constituted the chemical systems. Regardless of the Advanced Oxidation Process (AOP), the removal of pharmaceutical compounds was achieved in 1 h of reaction, while total organic carbon (TOC) only diminished between 3.4 and 6%. Among selected ozonation systems to be implemented before the biological treatment, the application of ozone alone in the pre-treatment stage is recommended due to the increase of the biodegradability observed. The application of ozone followed by the conventional biological treatment leads high TOC and COD removal rates, 60 and 61%, respectively, and allows the subsequent biological treatment works with shorter hydraulic residence time (HRT). Moreover, the influence of the application of AOPs before and after a conventional biological process was compared, concluding that the decision to take depends on the characterization of the initial wastewater with pharmaceutical compounds. Keywords: Aerobic processes, biodegradation, emerging contaminants, environmental preservation, ozone advanced oxidation processes, wastewater treatment.
Introduction In a previous article [1] the sequential treatments of conventional aerobic biological oxidation and advanced oxidation of a wastewater effluent from the primary sedimentation unit of the wastewater treatment plant of Badajoz (Spain) was studied to check the importance of this sequential treatments for the removal of nine pharmaceutical compounds previously doped into the wastewater. The compounds were acetaminophen (AAF), antipyrine (ANT), caffeine (CAF), carbamazepine (CRB), diclofenac (DCF), hydrochlorothiazide (HCT), ketorolac (KET), metoprolol (MET) and sulfamethoxazole (SMX). These compounds are used both for medical or social purposes and are commonly identified in urban wastewaters.[2-4] The removal of these contaminants, also called emergent contaminants (ECs), is necessary due to their potential
Address correspondence to Almudena Aguinaco, Departamento de Ingenierıa Quımica y Quımica Fısica, Universidad de Extremadura, Avenida de Elvas S/N, 06071 Badajoz, Spain; E-mail: [email protected] Received November 25, 2013.
hazardous effect some them have shown to disrupt endocrine systems of living beings.[5,6] Best technologies to remove ECs are membrane, activated carbon adsorption and advanced oxidation processes, that is, tertiary treatments methods. However, advanced oxidation processes (AOP) are the only technologies able to completely remove these contaminants from water. In the previous article,[1] after the aerobic biological oxidation, the action of some AOPs: combinations of ozone with UVA light and iron catalysts (Fe(III) and Fe3O4) was studied. However, in order to complete the study, the sequential combination of these two oxidation systems first applying the AOP and then the biological step was thought to be necessary. This sequential order of combination could be a good option in some cases. For example, the use of AOPs may be recommended in a pretreatment stage if they convert the initially persistent organic compounds into more biodegradable intermediates, which would then be treated in a biological oxidation process with a better efficiency and lower cost.[7,8] Thus, in this work, an urban wastewater doped with the nine ECs mentioned above, has been chemically and biologically treated in two sequential steps with different AOPs and activated sludge, respectively. Ozone alone (O3) and combined with UVA radiation (Black light: BL) and
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two iron catalysts (O3/BL/Fe(III), O3/BL/Fe3O4, photocatalytic ozonation) have been the AOPs applied. These two catalysts were selected knowing the effectiveness of iron oxides as catalysts for this type of AOPs. Fe(III) combined with radiation and ozone could be an alternative to photo-Fenton reaction and Fe3O4 is a mixed oxide of Fe (II) and Fe(III) with magnetic properties that has the appeal of being a solid easy to remove from the medium. Additionally, the ecotoxicity of the chemical and biologically treated samples and some kinetic aspects are also addressed.
Experimental
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Primary wastewater effluent and chemicals Chemicals, experimental conditions for AOPs, concentration of ECs spiked and the characteristics of the wastewater treated were those shown in the previous article.[1] The wastewater was collected from the primary treatment effluent of the Rinc on de Caya Wastewater Treatment Plant (WWTP) (Badajoz, Spain) that was spiked with nine selected ECs with an initial concentration of 200 mg L1 each one. Table 1 shows the main physicochemical properties of the wastewater used.
Experimental setup Experimental setup, procedures and analytical determinations for both AOPs and biological oxidation runs were as those described in the previous article [1] with the exception that chemical and biological reactor volume was 5 L. Ferrioxalate actinometry was used to determine the incident flux of radiation on the chemical reactor, that was found to be 2.36 0.05 ´ 105 Einstein min1. Additionally, in the present work, the generation of carboxylic acids and inorganic anions was monitored by ion chromatography using a Metrohm 881 Compact IC provided with a Metrosep A Supp 7 column (150 4.0 mm, particle size 5 mm) (Metrohm, Herisau, Switzerland). The injection volume was 20 mL. The program used for anionic analysis comprised a 15 min isocratic step with 0.6 mM Na2CO3, followed by a 9-min gradient program to reach a 90/10 (v/v) of a mixture of 0.6 mM Na2CO3 and 35.6 mM Na2CO3, respectively, and finally, another 26 min gradient program to end with a 60/40 (v/v) mixture of 0.6 mM Na2CO3 and 35.6 mM Na2CO3, respectively. The mobile phase flow rate was always kept at 0.7 mL min1.
Results and discussion Activated sludge
Advanced oxidation processes
Conventional activated sludge used as biomass in the aerobic biological treatment was also collected from the returning pipe of the activated sludge in Rinc on de Caya WWTP (Badajoz, Spain). Table 1. Wastewater characterization before pharmaceuticals spiking. Parameter pH Turbidity (NTU) Conductivity, mS cm1 COD, mg L1 BOD5, mgO2 L1 Biodegradability index (BOD5/COD) Alkalinity, mg CaCO3 L1 TOC, mg L1 Cl, mg L1 F, mg L1 N-NH4þ, mg L1 N-NO3, mg L1 NTOT, mg L1 PO43, mg L1 SO42, mg L1 Acetic acid, mg L1 Piruvic acid, mg L1 Propionic acid, mg L1
Value 7–8 72.8 828 228 90 0.28 275 56 67.06 1.452 2.53 12.32 64.80 5.56 39,26 38.94 1.984 1,192
Removal of pharmaceutical compounds from wastewater First, a series of UVA radiation-oxidation experiments of the pharmaceutical compounds mixture was carried out in a primary sedimentation effluent of a municipal wastewater. Regarding specific pharmaceutical compound removal, application of ozonation and photocatalytic ozonation processes, lead to concentrations below the detection limit of the HPLC method applied (2 mg L1) in 1 h of reaction. On the other hand, as expected, direct photolysis of any of the 9 compounds did not lead to positive results since these compounds do not absorb radiation in the wavelength emitted by the UVA lamp used in this work. Also, photocatalytic oxidation with the iron catalysts used did not either lead to any pharmaceutical degradation. Figures 1 and 2 show, as examples, the time concentrations profiles of AAF and HCT from experiments of ozonation and photocatalytic ozonation. The nine compounds studied behave in two different ways when treated with ozone processes. Thus, as observed from Figure 1 for AAF there are no significant differences in the removal rates with any ozone process applied (similar results were obtained in the case of ANT, CRB, DCF, KET and SMX). For ozonation systems the results, a priori, suggest that direct ozonation is the main mechanism of AAF, ANT, CRB, DCF, KET and SMX
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Table 2. Hatta values for the ozone-pharmaceutical direct reactions.a
CAAF/CAAF0
0.8
Pharmaceutical Compound
0.6
AAF ANT CAF
0.4
CRB DCF HCT
0.2 0.0 0
10
20
30
40
50
KET MET
60
Fig. 1. Time evolution of the remaining concentration of AAF during the ozonation processes applied to pharmaceutical compounds mineralization in wastewater. Conditions: temperature: 20 C, pH 3, (buffered systems), gas flow rate: 35 L h1, inlet ozone gas concentration: 13 mg L1, Fe(III) concentration: 2.8 mg L1, Fe3O4 concentration: 150 mg L1. Systems: ^ O3, ~ O3/BL/Fe3O4, & O3/BL/Fe(III).
removal at these conditions. A contrary situation is noted in the case of HCT removal rates (Fig. 2) (similar results were obtained in the case of CAF and MET). Now, there are differences in the removal rates due to ozone processes. Then, for CAF, HCT and MET removal, indirect oxidation due to hydroxyl radicals seems to be, a priori, the main mechanism of oxidation which would support the fact that the photocatalytic ozonation system leads to the highest removal rate.
1.0
0.8 0.6
CHCT/CHCT0
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t, min
0.4 0.2 0.0 0
10
20
30
40
50
60
t, min
Fig. 2. Time evolution of the remaining concentration of HCT during the ozonation processes applied to pharmaceutical compounds mineralization in wastewater. Conditions and symbols as in Figure 1.
SMT
kO3, M1 s1
Ha
4.11106 1.92 6.15 105 (4.07 105)c 0.67 (0.54) 0.02 650d (0.25)e (4.1104) 0.42 3105 0.68 106 0.04 3.1103 (91.3)e (6.5103) 3.4 105 0.43 0.03 2 103 (330)e (0.01) 4.1 105 0.47 (3.17 105)e (0.41)
Reference [10] [11] [12] [13] [14] [14] [11] [11] [16] [17]
kL: 5 105 m s1, [9] DO3: 1.7 109 m2s1 [18] pH 7 unless indicated. Initial pharmaceutical concentration: 200 mg L1. c Rate constant at pH 2. d Rate constant at pH 8.1. e Rate constant at pH 3. (AAF), antipyrine (ANT), caffeine (CAF), carbamazepine (CRB), diclofenac (DCF), hydrochlorothiazide (HCT), ketorolac (KET), metoprolol (MET) and sulfamethoxazole (SMX). a
b
Kinetics aspects In this work, Hatta numbers of the reactions between ozone and the ECs were calculated. The Hatta number compares the relative importance of chemical reaction and mass transfer rates in a gas liquid reaction.[9] Thus, for an initial concentration of about 200 mg L1 the Hatta numbers of the ozone-pharmaceutical direct reactions were determined and results obtained are shown in Table 2. It should be noted that due to the lack of data found in literature on rate constants of direct ozone reactions at pH 3 most of Ha values have been determined with the rate constants at pH 7 and only a few of them at the pH of photocatalytic reactions, that is, at pH 3. As it can be seen from Table 2, Ha presents values higher than 0.3 except for the cases of CAF, HCT and MET ozone reactions. A value of Ha lower than 0.3 indicates low kinetic regime for the corresponding ozone direct reaction. Then, at these conditions, ozone could reach the bulk water or the catalyst surface. Thus, CAF, HCT and MET removal from water is due to two types of reactions: the ozone direct reactions and the hydroxyl radical oxidation. In the semibatch perfectly mixed reactor used in this work, the mass balance of these pharmaceuticals compounds can be expressed as follows:
dCM ¼ kD CO3 CM þ kHO COH CM dt
ð1Þ
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where CM is the concentration of pharmaceutical compound, CO3 is the dissolved ozone concentration in water, and kD and kHO are the rate constants of the reactions of ozone and hydroxyl radicals with the pharmaceutical compound, respectively. Accordingly, for these three pharmaceuticals, the percentage contribution of the direct ozone reaction (%DR) for their removal has been calculated using Eq. (2): kD CO3 CM %DR ¼ 100 dCM dt
ð2Þ
previous works.[19,20] pH¼3
hy
FeðIIIÞ þ H2 O ! FeðOHÞ2þ ! FeðIIÞ þ HO ð3Þ FeðIIÞ þ O2 !FeðIIIÞ þ O 2
ð4Þ
þ 2O 2 þ 2H !H2 O2 þ O2
ð5Þ
Ferrous ion formed trough reduction of ferric ion (see Eq. (3)) could react with ozone forming FeO2þ, which hydrolyzes generating more OH (Eqs. (6) and (7)).
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k¼8:2105 M 1 s1
Table 3 shows the values of%DR corresponding to the removal of CAF, HCT and MET. As it can be seen, free radical reactions are also responsible of oxidation, so photocatalytic ozonation processes are recommended to eliminate CAF, HCT and MET. On the other hand, for both ozone and O3/BL/Fe3O4 processes, the increase of reaction time leads to an increase of direct ozone reaction contribution. However, a different trend is observed when O3/BL/Fe(III) is applied. In this latter case, initially, the importance of the direct reaction contribution increases, but after 30 min of reaction it decreases. As it is known, formation of hydroxyl radicals has different possible ways. Thus, photolysis of these iron catalysts leads to hydroxyl radicals as has been shown in
Table 3. Relative importance of the direct reaction, %. O3 Process CO3, M
CAF
HCT
MET
4 10 20 30 45 60
5 107 2 106 4 106 5.7 106 6.75 106 7 106
0.02 0.07 0.12 0.14 0.43 0.07
3.0 14.5 22.3 19.7 100 100
15.2 67.3 100 100 100 100
4 10 20 30 45 60
O3/BL/Fe3O4 Process 0.004 5 107 0.011 1.05 106 0.014 2 106 0.010 2.7 106 6.91 3 106 71.6 3.3 106
1.5 3.3 6.0 4.6 100 100
7.6 18.7 30.6 26.7 100 100
4 10 20 30 45 60
O3/BL/Fe(III) Process 0.007 5 107 0.02 1.5 106 0.016 3 106 0.007 4.05 106 0 4.5 106 0 4.8106
0.01 5.2 5.0 3.2 0.92 0.17
9.9 27.0 23.8 19.9 11.4 0.45
t. min
FeðIIÞ þ O3 ! FeO2þ þ O2
ð6Þ
k¼0:013s1
FeO2þ þ H2 O ! FeðIIIÞ þ HO þ HO 2 ð7Þ Another possible contribution to form hydroxyl radicals is the ozone-hydrogen peroxide (ionic form) reaction. In the aqueous ozonation of unsaturated organics such as ECs studied in this work, hydrogen peroxide is likely formed as a result of aromatic ring and/or double carbon bond breaking in the ozone direct reactions with ECs.[21] In fact, in this work, hydrogen peroxide was experimentally detected in all oxidising systems studied reaching concentrations up to 1.2 104 M (4.1 mgL1). An important and fundamental aspect of AOPs is the concentration of hydroxyl radicals. In this work, this concentration was determined by applying Eq. (1) and hydroxyl radical reaction rate constants for CAF, HCT and MET, already reported in other investigation works.[12,15,16] Table 4 shows some of the results obtained. As can be seen from Table 4, concentration of hydroxyl radicals increases when ozone is combined with black light and iron photocatalysts as it could be expected according to Eqs. (6) and (7). Furthermore, HO radical concentration increases with time when Fe(III) is used. However, when O3/BL/Fe3O4 process is applied, hydroxyl radical concentration decreases after 30 min of reaction. These results justify the evolution of the relative importance of the ozone direct reaction shown in Table 3. Table 4. Hydroxyl radical concentration (M). Oxidation process t, min 4 10 20 30 45 60
O3/BL/Fe3O4
O3 13
1.29 10 1.69 1013 1.95 1013 2,94 1013 6.61 1014 4.05 1013
kOH-CAF: 5.9 109 M1 s1 kOH -HCT: 5.07 109 M1 s1 kOH-MET: 7.3 109 M1 s1.
13
3.84 10 4.90 1013 5.89 1013 1.11 1012 1.71 1015 6.83 1017
O3/BL/Fe(III) 2.53 1013 3.46 1013 9.18 1013 2.26 1012 1.02 1011 5.94 1011
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Ozone advanced and biological oxidation for pharmaceutical removal Table 5. Carboxylic acid intermediates identified after the application of chemical treatments.a Component Acetic acid Formic acid Oxalic acid Piruvic acid Propionic acid Succinic acid
O3
O3/BL/Fe(III)
O3/BL/Fe3O4
19.23 5.14 0.64 1.01 0.00 0.77
16.59 5.31 0.61 0.00 0.74 0.00
3.56 4.18 0.89 1.41 0.59 0.71
Units are mg L1.
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a
The increase of OH radical concentration when Fe(III) photocatalytic ozonation is applied could be due to the formation of ferricomplexes with carboxylic acids of low molecular weight such as oxalic acid. Photolysis of these ferricomplexes to yield OH radicals, especially the ferrioxalate complex, has quantum yields higher than that of Fe (III) photolysis,[22,23] so that, their presence improves the process rate. Ferricomplex formation due to the presence of different carboxylic acids has been observed in this work as shown in Table 5 and also explains the decrease observed on total iron concentration with time as presented in Figure 3. Thus, from Figure 3, two main observations can be made: 1. Fe(II) concentration is very low and stays practically constant with time so that most of total iron concentration is mainly due to Fe(III) and 2: The decrease of total iron concentration, that is, Fe(III) concentration with time, especially from 30 min reaction, confirms the formation of ferricomplexes with carboxylic acids, main end products of ozonation processes. TOC, COD and biodegradability O3, O3/BL/Fe3O4 and O3/BL/Fe(III) systems only lead to 3.4, 4.3 and 6% mineralization in 1 h, respectively.
Thus, depending on the ozone processes previously applied, the initial TOC fed to the biological treatment was 55, 54.5 or 53.6 mg L1. On the other hand, COD degradation percentages were higher. Specifically, about 53, 38 and 36% COD reductions were achieved when O3, O3/BL/Fe3O4 and O3/BL/Fe(III) were applied, respectively. On the other hand, in order to quantitatively establish the oxidation capacity of the systems studied the average oxidation state of carbon (AOSC) and the increment observed in AOSC during the oxidation process (D[AOSC]) were also calculated as in the previous article.[1] Sarria et al.[7] reported that an increment of AOSC to 1.5 is characteristic of highly oxidized aliphatic compounds and readily degradable biocompatible organic substances such oxalic acid that can be treated in a conventional biological treatment. For the photocatalytic ozonation processes, D[AOSC] was 1 or 1.3 when Fe3O4 or Fe(III) was used as catalyst, respectively. Hence, ozonation is the process that leads to the highest oxidation state, reaching a value of 3.1 after 1-h reaction. It should be remembered, however, that these parameters and also the partial oxidation yield have to be taken with caution, as their values may also be due to the presence of inorganic substances that contribute to increased COD. Therefore, the biodegradability of the water treated has also been determined as the BOD5/COD ratio, which gives an estimation of how easy or difficult a subsequent aerobic biological treatment would perform (see Table 6). As it can be seen from Table 6 when photocatalytic processes are applied a slight increase in the biodegradability of treated wastewater was observed, while a 115% increase of this parameter is reached after the ozonation process. The low efficiency of photocatalytic ozonation treatments was not previously expected because the high hydroxyl radical concentration generated in these AOPs (see Table 4). This low efficiency can be due to the presence of several chromophores species (humic acids)[24] and OH scavengers like carbonates, bicarbonates and nitrates.[25] Thus, a priori, the recommended treatment in a pretreatment AOP stage would be ozonation alone, since this
Table 6. BOD5, COD and biodegradability as BOD5/COD for the secondary wastewater used before and after the chemical experiments.a System
Fig. 3. Time evolution of the concentration of total iron (lines and empty symbols) and ferrous ion (filled symbols) during photocatalytic ozonations applied. Conditions and symbols as in Figure 1.
Raw secondary wastewater with pharmaceuticals O3b O3/BL/Fe3O4b O3/BL/Fe(III)b
BOD5
COD
BOD5/COD
90
228
0.40
92 77 70
107 180 170
0.86 0.43 0.41
Units in mgO2 L1 for BOD5 and COD. Conditions as in Figure 1 after 1 h treatment.
a
b
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chemical process is able to convert the initially persistent organic compounds into more biodegradable intermediates, which would then be more easily removed in a subsequent biological oxidation process.
Table 7. Ecotoxicity results of Daphnia Magna immobilisation tests. Effect (%) Chemical pre-treatment
In a second series of experiments, a biological treatment was applied to the non-treated and chemically treated urban wastewater. In Figure 4, the changes observed on TOC with time corresponding to the biological process are presented. COD also showed similar results. As it can be seen, about 54% TOC reduction after 7-h treatment was reached when non-pretreated water was used, against about 58% TOC reduction achieved after 3 h biological treatment when the wastewater was pretreated with ozone. On the other hand, regarding photocatalytic ozonation processes, after 1 h of reaction time results are similar to those obtained for the biological treated raw wastewater spiked with pharmaceuticals. However, as can be inferred from Figure 4, when photocatalytic ozonation is applied in a previous stage to biological treatment, TOC removal rate is stopped after a 1-h reaction time in the activated sludge process. This may be due to the generation of toxic intermediates, as can be deduced from ecotoxicity data (see later). In fact, no swimming inhibition in Daphnia Magna was observed when raw wastewater with pharmaceuticals was exposed, while, as can be seen in Table 7, the application of the aforementioned oxidation processes leads to intermediates which show high toxicity. It should be noticed that concentrations of carboxylic acids detected in this work were below the EC50.[26–29] Thus, as it was previously stated,[1] toxicity to D. Magna 1.0 0.8 TOC/TOC0
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Biological treatment
0.6
0.4 0.2 0.0 0
60
120
180
240
300
360
420
t, min
Fig. 4. Changes of TOC with time during biological oxidation of non-treated and chemically treated urban wastewater. Conditions: temperature: 20 C, pH: 7.5, Dissolved oxygen concentraNontion: 3.0 mg L1, Initial MLVSS: 1.0 g L1. Symbols: treated wastewater. Pretreatments: ^ O3, ~ O3 /BL/Fe3O4, & O3/BL/Fe(III).
O3/Fe(III)/BLa O3/Fe3O4/BLa
Time of treatment (min)
24 h
48 h
60 60
76.5 95
94 100
a
Conditions as in Figure 1.
must be due to formation of ferrioxalate complex in Fe (III) photocatalytic ozonation runs. On the other hand, when Fe3O4 photocatalytic ozonation is applied, the percentage of D. Magna immobilization should likely be due to other unknown intermediates, because neither iron leaching nor decrease of total iron concentration was observed.
Comparison of combined methods To determine the most recommended combined method to treat a secondary wastewater spiked with the nine pharmaceuticals, the influence of the application of AOPs before and after a conventional biological process was compared. However, no differences are observed in the removal of the nine pharmaceutical compounds by the application of the sequence of biological plus chemical or chemical plus biological treatments. Nonetheless, TOC and COD removal rates and the ecotoxicity were different depending on the type of sequential treatment applied. First, from results reported in the previous work,[1] amongst chemical processes applied after the biological treatment, similar evolution of TOC and COD with time was observed for photocatalytic ozonation treatments. Specifically, these processes led to about 69 and 71% TOC reduction after 480 min of sequential treatment and about 86 and 87% COD reductions were achieved with the Fe3O4 and Fe(III) photocatalytic ozonation processes, respectively. However, if ecotoxicity at the end of the sequential treatment is compared, only the wastewater treated with Fe(III) photocatalytic ozonation resulted in a given degree of ecotoxicity. When the sequential treatment consists of a chemical process (O3, O3/BL/Fe(III) or O3/BL/Fe3O4) followed by a conventional biological treatment (this work), the application of ozone alone in the pre-treatment stage is recommended since ozonation alone leads high TOC and COD removal rates, 60 and 61%, respectively, and allows the subsequent biological treatment works with shorter HRT. Moreover, it should be highlighted that using the biological treatment as the final step, no swimming inhibition of D. Magna was observed, regardless of the previous AOP applied.
Ozone advanced and biological oxidation for pharmaceutical removal Therefore, the selected combined method could be the application of a biological treatment followed by a photocatalytic ozonation process using Fe3O4 as catalyst or a previous step using ozone alone and a subsequent conventional biological treatment. The decision depends on the characterization of the initial wastewater with pharmaceutical compounds according to Council Directive 91/271/ EEC.[30]
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Conclusions From this work, the following conclusions can be derived: Application of any ozone process removes pharmaceuticals in a 1-h reaction. CAF, HCT and MET removal from water is due to two types of reactions: the ozone direct reactions and the hydroxyl radical oxidation. Photocatalytic processes only lead to a slight increase in the biodegradability of treated wastewater, while a 115% increase of this parameter is reached after the ozonation process. Aerobic biological oxidation of the non-pretreated urban wastewater spiked with the nine pharmaceuticals studied lead about 54% TOC reduction, against about 58% TOC reduction achieved after 3 h biological treatment when the wastewater was pretreated with ozone. TOC removal rate is stopped after a 1-h reaction in the activated sludge process when photocatalytic ozonation processes were applied in a previous stage to biological treatment, due to the generation of toxic intermediates. After the comparison of the influence of the application of AOPs before and after a conventional biological process, it can be concluded that the selected combined method could be the application of a biological treatment followed by a photocatalytic ozonation process using Fe3O4 as catalyst or a previous step using ozone alone and a subsequent conventional biological treatment.
[4]
[5]
[6] [7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Funding [16]
Authors thank the Spanish CICYT and Feder funds for the economic support through project CTQ2009/13459/ C05/05. Also, Ms. Espejo thanks Gobierno de Extremadura for providing her a FPI grant.
[17]
[18]
References [1] Espejo, A.; Aguinaco, A.; Amat, A.M.; Beltr an, F.J. Some ozone advanced oxidation processes to improve the biological removal of selected pharmaceutical contaminants from urban wastewater. J. Environ. Sci. Health, Part. A 2014, 49, 410–421. [2] Kasprzyk-Hordern, B.; Dinsdale, R.M.; Guwy, A.J. The removal of pharmaceuticals, personal care products, endocrine disruptors and illicit drugs during wastewater treatment and its impact on the quality of receiving waters. Water Res. 2009, 43, 363–380. [3] Fernandez, C.; Gonz alez-Doncel, M.; Pro, J.; Carbonell, G.; Tarazona, J.V. Occurrence of pharmaceutically active compounds in surface waters of the Henares-Jarama-Tajo river system (Madrid,
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Spain) and a potential risk characterization. Sci. Total Environ. 2010, 408, 543–551. Deblonde, T.; Cossu-Leguille, C.; Hartemann, P. Emerging pollutants in wastewater: a review of the literature. Int. J. Hyg. Envir. Heal. 2011, 214, 442–448. Snyder, S.A.; Westerhoff, P.; Yoon, Y.; Sedlak, D. Pharmaceuticals, personal care products, and endocrine disruptors in water: implications for the water industry. Environ. Eng. Sci. 2003, 20 (5), 449–469. Fent, K.; Weston, A.A.; Caminada, D. Ecotoxicology of human pharmaceuticals. Aquat. Toxicol. 2006, 76, 122–159. Sarria, V.; Parra, S.; Adler, N.; Peringer, P.; Benitez, N.; Pulgarin, C. Recent developments in the coupling of photoassisted and aerobic biological processes for the treatment of biorecalcitrant compounds. Catal. Today. 2002, 76, 301–315. Oller, I.; Malato, S.; S anchez-Perez J.A. Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination—A review. Sci. Total Environ. 2011, 409 (20), 4141–4166. Charpentier, J.C. Mass Transfer Rates in Gas Liquid Absorbers and Reactors, Advances in: Chemical Engineering. Academic Press: New York, 1981. Andreozzi, R.; Caprio, V.; Marotta, R.; Radovnikovic, A. Ozonation and H2O2/UV treatment of clofibric acid in water: a kinetic investigation. J. Hazard. Mater. 2003, 103 (3), 233–246. Rivas, J.; Sagasti, F.J.; Encinas, A.; Gimeno, O. Contaminants abatement by ozone in secondary effluents. Evaluation of secondorder rate constants. J. Chem. Technol. Biotechnol. 2011, 86, 1058–1066. Broseus, R.; Vincent, S.; Aboulfadl, K.; Daneshvar, A.; Sauve, S.; Barbeau, B.; Prevost, M. Ozone oxidation of pharmaceuticals, endocrine disruptors and pesticides during drinking water treatment. Water Res. 2009, 43, 4707–4717. Rosal, R.; Gonzalo, M.S.; Boltes, K.; Let on, P.; Vaquero, J.J.; Garcıa-Calvo, E. Identification of intermediates and assessment of ecotoxicity in the oxidation products generated during the ozonation of clofibric acid. J. Hazard. Mater. 2009, 172, 1061–1068. Huber, M.M.; Canonica, S.; Park, G.Y.; von Gunten, U. Oxidation of pharmaceuticals during ozonation and advanced oxidation processes. Environ. Sci. Technol. 2003, 37, 1016–1024. Real, F.J., Acero, J.L., Benitez, F.J., Rold an, G., Fern andez, L.C. Oxidation of hydrochlorothiazide by UV radiation, hydroxyl radicals and ozone: kinetics and elimination from water systems. Chem. Eng. J. 2010, 160, 72–78. Benner, J.; Salhi, E.; Ternes, T.; von Gunten, U. Ozonation of reverse osmosis concentrate: kinetics and efficiency of beta blocker oxidation. Water Res. 2008, 42, 3003–3012. Beltr an, F.J.; Aguinaco, A.; Garcıa-Araya, J.F. Mechanism and kinetics of sulfamethoxazole photocatalytic ozonation in water. Water Res. 2009, 43, 1359–1369. Johnson, P.N.; Davis, R.A. Diffusivity of ozone in water. J. Chem. Eng. Data, 1996, 41, 1485–1487. Feng, W.; Nansheng, D. Photochemistry of hydrolytic iron (III) species and photoinduced degradation of organic compounds. A minireview. Chemosphere 2000, 41, 1137–1147. Rodrıguez, E.M.; Fern andez, G.; Alvarez, P.M.; Hern andez, R.; Beltr an, F.J. Photocatalytic degradation of organics in water in the presence of iron oxides: effects of pH and light source. Appl. Catal. B-Environ. 2011, 102, 572–583. Nakamuro, K.; Ueno, H.; Nakao, M.; Sayato, Y. Formation of hydrogen peroxide by aqueous ozonation of humic acid and aromatic hydrocarbons, Chemosphere 1990, 20 (5), 525–531. Mazellier, P.; Jirkovsky, J.; Bolte, M. Degradation of diuron photoinduced by iron (III) in aqueous solution. Pestic. Sci. 1997, 49, 259–267.
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[23] Abrahamson, H.B.; Rezvani, A.B.; Brushmiller, J.G. Photochemical and spectroscopic studies of complexes of iron (III) with citric acid and other carboxilyc acids. Inorg. Chim. Acta 1994, 226, 117–127. [24] Prabhakaran, D.; Sukul, P.; Lamsh€ oft, M.; Maheswari, M.A.; Z€ uhlke, S.; Spiteller, M. Photolysis of difloxacin and sarafloxacin in aqueous systems. Chemosphere 2009, 77, 739–746. [25] Fatta-Kassinos, D.; Vasquez, M.I.; K€ ummerer, K. Transformation products of pharmaceuticals in surface waters and wastewater formed during photolysis and advanced oxidation processes - Degradation, elucidation of byproducts and assessment of their biological potency. Chemosphere 2011, 85, 693–709. [26] Randall, T.L.; Knopp, P.V. Detoxification of specific organic substances by wet oxidation. J. Water Pollut. Con. F. 1980, 52(8), 2117–2130.
Espejo et al. [27] LeBlanc, G.A. Acute toxicity of priority pollutants to water flea (Daphnia magna). Bull. Environ. Contam. Toxicol. 1980, 24, 684– 691. [28] Sanderson, H.; Thomsen, M. Comparative analysis of pharmaceuticals versus industrial chemicals acute aquatic toxicity classification according to the United Nations classification system for chemicals. Assessment of the (Q)SAR predictability of pharmaceuticals acute aquatic toxicity and their predominant acute toxic mode-ofaction. Toxicol. Lett. 2009, 187, 84–93. [29] Guerra, R. Ecotoxicological and chemical evaluation of phenolic compounds in industrial effluents. Chemosphere 2001, 44(8), 1737–1747. [30] European Commission. Council directive of 21 May concerning urban waste water treatment (91/271/EEC). Off. J. Euro. Commun. 1991, L135, 40.
Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lesa20
Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater a
a
a
Azahara Espejo , Almudena Aguinaco , J. F. García-Araya & Fernando J. Beltrán
a
a
Departamento de Ingeniería Química y Química Física, Universidad de Extremadura, Badajoz, Spain Published online: 05 May 2014.
To cite this article: Azahara Espejo, Almudena Aguinaco, J. F. García-Araya & Fernando J. Beltrán (2014) Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater, Journal of Environmental Science and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:9, 1015-1022, DOI: 10.1080/10934529.2014.894845 To link to this article: http://dx.doi.org/10.1080/10934529.2014.894845
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Journal of Environmental Science and Health, Part A (2014) 49, 1015–1022 Copyright Ó Taylor & Francis Group, LLC ISSN: 1093-4529 (Print); 1532-4117 (Online) DOI: 10.1080/10934529.2014.894845
Sequential ozone advanced oxidation and biological oxidation processes to remove selected pharmaceutical contaminants from an urban wastewater AZAHARA ESPEJO, ALMUDENA AGUINACO, J.F. GARCIA-ARAYA and FERNANDO J. BELTRAN
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Departamento de Ingenierıa Quımica y Quımica Fısica, Universidad de Extremadura, Badajoz, Spain
Sequential treatments consisting in a chemical process followed by a conventional biological treatment, have been applied to remove mixtures of nine contaminants of pharmaceutical type spiked in a primary sedimentation effluent of a municipal wastewater. Combinations of ozone, UVA black light (BL) and Fe(III) or Fe3O4 catalysts constituted the chemical systems. Regardless of the Advanced Oxidation Process (AOP), the removal of pharmaceutical compounds was achieved in 1 h of reaction, while total organic carbon (TOC) only diminished between 3.4 and 6%. Among selected ozonation systems to be implemented before the biological treatment, the application of ozone alone in the pre-treatment stage is recommended due to the increase of the biodegradability observed. The application of ozone followed by the conventional biological treatment leads high TOC and COD removal rates, 60 and 61%, respectively, and allows the subsequent biological treatment works with shorter hydraulic residence time (HRT). Moreover, the influence of the application of AOPs before and after a conventional biological process was compared, concluding that the decision to take depends on the characterization of the initial wastewater with pharmaceutical compounds. Keywords: Aerobic processes, biodegradation, emerging contaminants, environmental preservation, ozone advanced oxidation processes, wastewater treatment.
Introduction In a previous article [1] the sequential treatments of conventional aerobic biological oxidation and advanced oxidation of a wastewater effluent from the primary sedimentation unit of the wastewater treatment plant of Badajoz (Spain) was studied to check the importance of this sequential treatments for the removal of nine pharmaceutical compounds previously doped into the wastewater. The compounds were acetaminophen (AAF), antipyrine (ANT), caffeine (CAF), carbamazepine (CRB), diclofenac (DCF), hydrochlorothiazide (HCT), ketorolac (KET), metoprolol (MET) and sulfamethoxazole (SMX). These compounds are used both for medical or social purposes and are commonly identified in urban wastewaters.[2-4] The removal of these contaminants, also called emergent contaminants (ECs), is necessary due to their potential
Address correspondence to Almudena Aguinaco, Departamento de Ingenierıa Quımica y Quımica Fısica, Universidad de Extremadura, Avenida de Elvas S/N, 06071 Badajoz, Spain; E-mail: [email protected] Received November 25, 2013.
hazardous effect some them have shown to disrupt endocrine systems of living beings.[5,6] Best technologies to remove ECs are membrane, activated carbon adsorption and advanced oxidation processes, that is, tertiary treatments methods. However, advanced oxidation processes (AOP) are the only technologies able to completely remove these contaminants from water. In the previous article,[1] after the aerobic biological oxidation, the action of some AOPs: combinations of ozone with UVA light and iron catalysts (Fe(III) and Fe3O4) was studied. However, in order to complete the study, the sequential combination of these two oxidation systems first applying the AOP and then the biological step was thought to be necessary. This sequential order of combination could be a good option in some cases. For example, the use of AOPs may be recommended in a pretreatment stage if they convert the initially persistent organic compounds into more biodegradable intermediates, which would then be treated in a biological oxidation process with a better efficiency and lower cost.[7,8] Thus, in this work, an urban wastewater doped with the nine ECs mentioned above, has been chemically and biologically treated in two sequential steps with different AOPs and activated sludge, respectively. Ozone alone (O3) and combined with UVA radiation (Black light: BL) and
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two iron catalysts (O3/BL/Fe(III), O3/BL/Fe3O4, photocatalytic ozonation) have been the AOPs applied. These two catalysts were selected knowing the effectiveness of iron oxides as catalysts for this type of AOPs. Fe(III) combined with radiation and ozone could be an alternative to photo-Fenton reaction and Fe3O4 is a mixed oxide of Fe (II) and Fe(III) with magnetic properties that has the appeal of being a solid easy to remove from the medium. Additionally, the ecotoxicity of the chemical and biologically treated samples and some kinetic aspects are also addressed.
Experimental
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Primary wastewater effluent and chemicals Chemicals, experimental conditions for AOPs, concentration of ECs spiked and the characteristics of the wastewater treated were those shown in the previous article.[1] The wastewater was collected from the primary treatment effluent of the Rinc on de Caya Wastewater Treatment Plant (WWTP) (Badajoz, Spain) that was spiked with nine selected ECs with an initial concentration of 200 mg L1 each one. Table 1 shows the main physicochemical properties of the wastewater used.
Experimental setup Experimental setup, procedures and analytical determinations for both AOPs and biological oxidation runs were as those described in the previous article [1] with the exception that chemical and biological reactor volume was 5 L. Ferrioxalate actinometry was used to determine the incident flux of radiation on the chemical reactor, that was found to be 2.36 0.05 ´ 105 Einstein min1. Additionally, in the present work, the generation of carboxylic acids and inorganic anions was monitored by ion chromatography using a Metrohm 881 Compact IC provided with a Metrosep A Supp 7 column (150 4.0 mm, particle size 5 mm) (Metrohm, Herisau, Switzerland). The injection volume was 20 mL. The program used for anionic analysis comprised a 15 min isocratic step with 0.6 mM Na2CO3, followed by a 9-min gradient program to reach a 90/10 (v/v) of a mixture of 0.6 mM Na2CO3 and 35.6 mM Na2CO3, respectively, and finally, another 26 min gradient program to end with a 60/40 (v/v) mixture of 0.6 mM Na2CO3 and 35.6 mM Na2CO3, respectively. The mobile phase flow rate was always kept at 0.7 mL min1.
Results and discussion Activated sludge
Advanced oxidation processes
Conventional activated sludge used as biomass in the aerobic biological treatment was also collected from the returning pipe of the activated sludge in Rinc on de Caya WWTP (Badajoz, Spain). Table 1. Wastewater characterization before pharmaceuticals spiking. Parameter pH Turbidity (NTU) Conductivity, mS cm1 COD, mg L1 BOD5, mgO2 L1 Biodegradability index (BOD5/COD) Alkalinity, mg CaCO3 L1 TOC, mg L1 Cl, mg L1 F, mg L1 N-NH4þ, mg L1 N-NO3, mg L1 NTOT, mg L1 PO43, mg L1 SO42, mg L1 Acetic acid, mg L1 Piruvic acid, mg L1 Propionic acid, mg L1
Value 7–8 72.8 828 228 90 0.28 275 56 67.06 1.452 2.53 12.32 64.80 5.56 39,26 38.94 1.984 1,192
Removal of pharmaceutical compounds from wastewater First, a series of UVA radiation-oxidation experiments of the pharmaceutical compounds mixture was carried out in a primary sedimentation effluent of a municipal wastewater. Regarding specific pharmaceutical compound removal, application of ozonation and photocatalytic ozonation processes, lead to concentrations below the detection limit of the HPLC method applied (2 mg L1) in 1 h of reaction. On the other hand, as expected, direct photolysis of any of the 9 compounds did not lead to positive results since these compounds do not absorb radiation in the wavelength emitted by the UVA lamp used in this work. Also, photocatalytic oxidation with the iron catalysts used did not either lead to any pharmaceutical degradation. Figures 1 and 2 show, as examples, the time concentrations profiles of AAF and HCT from experiments of ozonation and photocatalytic ozonation. The nine compounds studied behave in two different ways when treated with ozone processes. Thus, as observed from Figure 1 for AAF there are no significant differences in the removal rates with any ozone process applied (similar results were obtained in the case of ANT, CRB, DCF, KET and SMX). For ozonation systems the results, a priori, suggest that direct ozonation is the main mechanism of AAF, ANT, CRB, DCF, KET and SMX
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Table 2. Hatta values for the ozone-pharmaceutical direct reactions.a
CAAF/CAAF0
0.8
Pharmaceutical Compound
0.6
AAF ANT CAF
0.4
CRB DCF HCT
0.2 0.0 0
10
20
30
40
50
KET MET
60
Fig. 1. Time evolution of the remaining concentration of AAF during the ozonation processes applied to pharmaceutical compounds mineralization in wastewater. Conditions: temperature: 20 C, pH 3, (buffered systems), gas flow rate: 35 L h1, inlet ozone gas concentration: 13 mg L1, Fe(III) concentration: 2.8 mg L1, Fe3O4 concentration: 150 mg L1. Systems: ^ O3, ~ O3/BL/Fe3O4, & O3/BL/Fe(III).
removal at these conditions. A contrary situation is noted in the case of HCT removal rates (Fig. 2) (similar results were obtained in the case of CAF and MET). Now, there are differences in the removal rates due to ozone processes. Then, for CAF, HCT and MET removal, indirect oxidation due to hydroxyl radicals seems to be, a priori, the main mechanism of oxidation which would support the fact that the photocatalytic ozonation system leads to the highest removal rate.
1.0
0.8 0.6
CHCT/CHCT0
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t, min
0.4 0.2 0.0 0
10
20
30
40
50
60
t, min
Fig. 2. Time evolution of the remaining concentration of HCT during the ozonation processes applied to pharmaceutical compounds mineralization in wastewater. Conditions and symbols as in Figure 1.
SMT
kO3, M1 s1
Ha
4.11106 1.92 6.15 105 (4.07 105)c 0.67 (0.54) 0.02 650d (0.25)e (4.1104) 0.42 3105 0.68 106 0.04 3.1103 (91.3)e (6.5103) 3.4 105 0.43 0.03 2 103 (330)e (0.01) 4.1 105 0.47 (3.17 105)e (0.41)
Reference [10] [11] [12] [13] [14] [14] [11] [11] [16] [17]
kL: 5 105 m s1, [9] DO3: 1.7 109 m2s1 [18] pH 7 unless indicated. Initial pharmaceutical concentration: 200 mg L1. c Rate constant at pH 2. d Rate constant at pH 8.1. e Rate constant at pH 3. (AAF), antipyrine (ANT), caffeine (CAF), carbamazepine (CRB), diclofenac (DCF), hydrochlorothiazide (HCT), ketorolac (KET), metoprolol (MET) and sulfamethoxazole (SMX). a
b
Kinetics aspects In this work, Hatta numbers of the reactions between ozone and the ECs were calculated. The Hatta number compares the relative importance of chemical reaction and mass transfer rates in a gas liquid reaction.[9] Thus, for an initial concentration of about 200 mg L1 the Hatta numbers of the ozone-pharmaceutical direct reactions were determined and results obtained are shown in Table 2. It should be noted that due to the lack of data found in literature on rate constants of direct ozone reactions at pH 3 most of Ha values have been determined with the rate constants at pH 7 and only a few of them at the pH of photocatalytic reactions, that is, at pH 3. As it can be seen from Table 2, Ha presents values higher than 0.3 except for the cases of CAF, HCT and MET ozone reactions. A value of Ha lower than 0.3 indicates low kinetic regime for the corresponding ozone direct reaction. Then, at these conditions, ozone could reach the bulk water or the catalyst surface. Thus, CAF, HCT and MET removal from water is due to two types of reactions: the ozone direct reactions and the hydroxyl radical oxidation. In the semibatch perfectly mixed reactor used in this work, the mass balance of these pharmaceuticals compounds can be expressed as follows:
dCM ¼ kD CO3 CM þ kHO COH CM dt
ð1Þ
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where CM is the concentration of pharmaceutical compound, CO3 is the dissolved ozone concentration in water, and kD and kHO are the rate constants of the reactions of ozone and hydroxyl radicals with the pharmaceutical compound, respectively. Accordingly, for these three pharmaceuticals, the percentage contribution of the direct ozone reaction (%DR) for their removal has been calculated using Eq. (2): kD CO3 CM %DR ¼ 100 dCM dt
ð2Þ
previous works.[19,20] pH¼3
hy
FeðIIIÞ þ H2 O ! FeðOHÞ2þ ! FeðIIÞ þ HO ð3Þ FeðIIÞ þ O2 !FeðIIIÞ þ O 2
ð4Þ
þ 2O 2 þ 2H !H2 O2 þ O2
ð5Þ
Ferrous ion formed trough reduction of ferric ion (see Eq. (3)) could react with ozone forming FeO2þ, which hydrolyzes generating more OH (Eqs. (6) and (7)).
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k¼8:2105 M 1 s1
Table 3 shows the values of%DR corresponding to the removal of CAF, HCT and MET. As it can be seen, free radical reactions are also responsible of oxidation, so photocatalytic ozonation processes are recommended to eliminate CAF, HCT and MET. On the other hand, for both ozone and O3/BL/Fe3O4 processes, the increase of reaction time leads to an increase of direct ozone reaction contribution. However, a different trend is observed when O3/BL/Fe(III) is applied. In this latter case, initially, the importance of the direct reaction contribution increases, but after 30 min of reaction it decreases. As it is known, formation of hydroxyl radicals has different possible ways. Thus, photolysis of these iron catalysts leads to hydroxyl radicals as has been shown in
Table 3. Relative importance of the direct reaction, %. O3 Process CO3, M
CAF
HCT
MET
4 10 20 30 45 60
5 107 2 106 4 106 5.7 106 6.75 106 7 106
0.02 0.07 0.12 0.14 0.43 0.07
3.0 14.5 22.3 19.7 100 100
15.2 67.3 100 100 100 100
4 10 20 30 45 60
O3/BL/Fe3O4 Process 0.004 5 107 0.011 1.05 106 0.014 2 106 0.010 2.7 106 6.91 3 106 71.6 3.3 106
1.5 3.3 6.0 4.6 100 100
7.6 18.7 30.6 26.7 100 100
4 10 20 30 45 60
O3/BL/Fe(III) Process 0.007 5 107 0.02 1.5 106 0.016 3 106 0.007 4.05 106 0 4.5 106 0 4.8106
0.01 5.2 5.0 3.2 0.92 0.17
9.9 27.0 23.8 19.9 11.4 0.45
t. min
FeðIIÞ þ O3 ! FeO2þ þ O2
ð6Þ
k¼0:013s1
FeO2þ þ H2 O ! FeðIIIÞ þ HO þ HO 2 ð7Þ Another possible contribution to form hydroxyl radicals is the ozone-hydrogen peroxide (ionic form) reaction. In the aqueous ozonation of unsaturated organics such as ECs studied in this work, hydrogen peroxide is likely formed as a result of aromatic ring and/or double carbon bond breaking in the ozone direct reactions with ECs.[21] In fact, in this work, hydrogen peroxide was experimentally detected in all oxidising systems studied reaching concentrations up to 1.2 104 M (4.1 mgL1). An important and fundamental aspect of AOPs is the concentration of hydroxyl radicals. In this work, this concentration was determined by applying Eq. (1) and hydroxyl radical reaction rate constants for CAF, HCT and MET, already reported in other investigation works.[12,15,16] Table 4 shows some of the results obtained. As can be seen from Table 4, concentration of hydroxyl radicals increases when ozone is combined with black light and iron photocatalysts as it could be expected according to Eqs. (6) and (7). Furthermore, HO radical concentration increases with time when Fe(III) is used. However, when O3/BL/Fe3O4 process is applied, hydroxyl radical concentration decreases after 30 min of reaction. These results justify the evolution of the relative importance of the ozone direct reaction shown in Table 3. Table 4. Hydroxyl radical concentration (M). Oxidation process t, min 4 10 20 30 45 60
O3/BL/Fe3O4
O3 13
1.29 10 1.69 1013 1.95 1013 2,94 1013 6.61 1014 4.05 1013
kOH-CAF: 5.9 109 M1 s1 kOH -HCT: 5.07 109 M1 s1 kOH-MET: 7.3 109 M1 s1.
13
3.84 10 4.90 1013 5.89 1013 1.11 1012 1.71 1015 6.83 1017
O3/BL/Fe(III) 2.53 1013 3.46 1013 9.18 1013 2.26 1012 1.02 1011 5.94 1011
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Ozone advanced and biological oxidation for pharmaceutical removal Table 5. Carboxylic acid intermediates identified after the application of chemical treatments.a Component Acetic acid Formic acid Oxalic acid Piruvic acid Propionic acid Succinic acid
O3
O3/BL/Fe(III)
O3/BL/Fe3O4
19.23 5.14 0.64 1.01 0.00 0.77
16.59 5.31 0.61 0.00 0.74 0.00
3.56 4.18 0.89 1.41 0.59 0.71
Units are mg L1.
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a
The increase of OH radical concentration when Fe(III) photocatalytic ozonation is applied could be due to the formation of ferricomplexes with carboxylic acids of low molecular weight such as oxalic acid. Photolysis of these ferricomplexes to yield OH radicals, especially the ferrioxalate complex, has quantum yields higher than that of Fe (III) photolysis,[22,23] so that, their presence improves the process rate. Ferricomplex formation due to the presence of different carboxylic acids has been observed in this work as shown in Table 5 and also explains the decrease observed on total iron concentration with time as presented in Figure 3. Thus, from Figure 3, two main observations can be made: 1. Fe(II) concentration is very low and stays practically constant with time so that most of total iron concentration is mainly due to Fe(III) and 2: The decrease of total iron concentration, that is, Fe(III) concentration with time, especially from 30 min reaction, confirms the formation of ferricomplexes with carboxylic acids, main end products of ozonation processes. TOC, COD and biodegradability O3, O3/BL/Fe3O4 and O3/BL/Fe(III) systems only lead to 3.4, 4.3 and 6% mineralization in 1 h, respectively.
Thus, depending on the ozone processes previously applied, the initial TOC fed to the biological treatment was 55, 54.5 or 53.6 mg L1. On the other hand, COD degradation percentages were higher. Specifically, about 53, 38 and 36% COD reductions were achieved when O3, O3/BL/Fe3O4 and O3/BL/Fe(III) were applied, respectively. On the other hand, in order to quantitatively establish the oxidation capacity of the systems studied the average oxidation state of carbon (AOSC) and the increment observed in AOSC during the oxidation process (D[AOSC]) were also calculated as in the previous article.[1] Sarria et al.[7] reported that an increment of AOSC to 1.5 is characteristic of highly oxidized aliphatic compounds and readily degradable biocompatible organic substances such oxalic acid that can be treated in a conventional biological treatment. For the photocatalytic ozonation processes, D[AOSC] was 1 or 1.3 when Fe3O4 or Fe(III) was used as catalyst, respectively. Hence, ozonation is the process that leads to the highest oxidation state, reaching a value of 3.1 after 1-h reaction. It should be remembered, however, that these parameters and also the partial oxidation yield have to be taken with caution, as their values may also be due to the presence of inorganic substances that contribute to increased COD. Therefore, the biodegradability of the water treated has also been determined as the BOD5/COD ratio, which gives an estimation of how easy or difficult a subsequent aerobic biological treatment would perform (see Table 6). As it can be seen from Table 6 when photocatalytic processes are applied a slight increase in the biodegradability of treated wastewater was observed, while a 115% increase of this parameter is reached after the ozonation process. The low efficiency of photocatalytic ozonation treatments was not previously expected because the high hydroxyl radical concentration generated in these AOPs (see Table 4). This low efficiency can be due to the presence of several chromophores species (humic acids)[24] and OH scavengers like carbonates, bicarbonates and nitrates.[25] Thus, a priori, the recommended treatment in a pretreatment AOP stage would be ozonation alone, since this
Table 6. BOD5, COD and biodegradability as BOD5/COD for the secondary wastewater used before and after the chemical experiments.a System
Fig. 3. Time evolution of the concentration of total iron (lines and empty symbols) and ferrous ion (filled symbols) during photocatalytic ozonations applied. Conditions and symbols as in Figure 1.
Raw secondary wastewater with pharmaceuticals O3b O3/BL/Fe3O4b O3/BL/Fe(III)b
BOD5
COD
BOD5/COD
90
228
0.40
92 77 70
107 180 170
0.86 0.43 0.41
Units in mgO2 L1 for BOD5 and COD. Conditions as in Figure 1 after 1 h treatment.
a
b
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chemical process is able to convert the initially persistent organic compounds into more biodegradable intermediates, which would then be more easily removed in a subsequent biological oxidation process.
Table 7. Ecotoxicity results of Daphnia Magna immobilisation tests. Effect (%) Chemical pre-treatment
In a second series of experiments, a biological treatment was applied to the non-treated and chemically treated urban wastewater. In Figure 4, the changes observed on TOC with time corresponding to the biological process are presented. COD also showed similar results. As it can be seen, about 54% TOC reduction after 7-h treatment was reached when non-pretreated water was used, against about 58% TOC reduction achieved after 3 h biological treatment when the wastewater was pretreated with ozone. On the other hand, regarding photocatalytic ozonation processes, after 1 h of reaction time results are similar to those obtained for the biological treated raw wastewater spiked with pharmaceuticals. However, as can be inferred from Figure 4, when photocatalytic ozonation is applied in a previous stage to biological treatment, TOC removal rate is stopped after a 1-h reaction time in the activated sludge process. This may be due to the generation of toxic intermediates, as can be deduced from ecotoxicity data (see later). In fact, no swimming inhibition in Daphnia Magna was observed when raw wastewater with pharmaceuticals was exposed, while, as can be seen in Table 7, the application of the aforementioned oxidation processes leads to intermediates which show high toxicity. It should be noticed that concentrations of carboxylic acids detected in this work were below the EC50.[26–29] Thus, as it was previously stated,[1] toxicity to D. Magna 1.0 0.8 TOC/TOC0
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Biological treatment
0.6
0.4 0.2 0.0 0
60
120
180
240
300
360
420
t, min
Fig. 4. Changes of TOC with time during biological oxidation of non-treated and chemically treated urban wastewater. Conditions: temperature: 20 C, pH: 7.5, Dissolved oxygen concentraNontion: 3.0 mg L1, Initial MLVSS: 1.0 g L1. Symbols: treated wastewater. Pretreatments: ^ O3, ~ O3 /BL/Fe3O4, & O3/BL/Fe(III).
O3/Fe(III)/BLa O3/Fe3O4/BLa
Time of treatment (min)
24 h
48 h
60 60
76.5 95
94 100
a
Conditions as in Figure 1.
must be due to formation of ferrioxalate complex in Fe (III) photocatalytic ozonation runs. On the other hand, when Fe3O4 photocatalytic ozonation is applied, the percentage of D. Magna immobilization should likely be due to other unknown intermediates, because neither iron leaching nor decrease of total iron concentration was observed.
Comparison of combined methods To determine the most recommended combined method to treat a secondary wastewater spiked with the nine pharmaceuticals, the influence of the application of AOPs before and after a conventional biological process was compared. However, no differences are observed in the removal of the nine pharmaceutical compounds by the application of the sequence of biological plus chemical or chemical plus biological treatments. Nonetheless, TOC and COD removal rates and the ecotoxicity were different depending on the type of sequential treatment applied. First, from results reported in the previous work,[1] amongst chemical processes applied after the biological treatment, similar evolution of TOC and COD with time was observed for photocatalytic ozonation treatments. Specifically, these processes led to about 69 and 71% TOC reduction after 480 min of sequential treatment and about 86 and 87% COD reductions were achieved with the Fe3O4 and Fe(III) photocatalytic ozonation processes, respectively. However, if ecotoxicity at the end of the sequential treatment is compared, only the wastewater treated with Fe(III) photocatalytic ozonation resulted in a given degree of ecotoxicity. When the sequential treatment consists of a chemical process (O3, O3/BL/Fe(III) or O3/BL/Fe3O4) followed by a conventional biological treatment (this work), the application of ozone alone in the pre-treatment stage is recommended since ozonation alone leads high TOC and COD removal rates, 60 and 61%, respectively, and allows the subsequent biological treatment works with shorter HRT. Moreover, it should be highlighted that using the biological treatment as the final step, no swimming inhibition of D. Magna was observed, regardless of the previous AOP applied.
Ozone advanced and biological oxidation for pharmaceutical removal Therefore, the selected combined method could be the application of a biological treatment followed by a photocatalytic ozonation process using Fe3O4 as catalyst or a previous step using ozone alone and a subsequent conventional biological treatment. The decision depends on the characterization of the initial wastewater with pharmaceutical compounds according to Council Directive 91/271/ EEC.[30]
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Conclusions From this work, the following conclusions can be derived: Application of any ozone process removes pharmaceuticals in a 1-h reaction. CAF, HCT and MET removal from water is due to two types of reactions: the ozone direct reactions and the hydroxyl radical oxidation. Photocatalytic processes only lead to a slight increase in the biodegradability of treated wastewater, while a 115% increase of this parameter is reached after the ozonation process. Aerobic biological oxidation of the non-pretreated urban wastewater spiked with the nine pharmaceuticals studied lead about 54% TOC reduction, against about 58% TOC reduction achieved after 3 h biological treatment when the wastewater was pretreated with ozone. TOC removal rate is stopped after a 1-h reaction in the activated sludge process when photocatalytic ozonation processes were applied in a previous stage to biological treatment, due to the generation of toxic intermediates. After the comparison of the influence of the application of AOPs before and after a conventional biological process, it can be concluded that the selected combined method could be the application of a biological treatment followed by a photocatalytic ozonation process using Fe3O4 as catalyst or a previous step using ozone alone and a subsequent conventional biological treatment.
[4]
[5]
[6] [7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Funding [16]
Authors thank the Spanish CICYT and Feder funds for the economic support through project CTQ2009/13459/ C05/05. Also, Ms. Espejo thanks Gobierno de Extremadura for providing her a FPI grant.
[17]
[18]
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