reverse osmosis followed by electrochemical oxidation of the RO concentrate

Desalination 331 (2013) 26–34

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Removal of pharmaceuticals from a WWTP secondary effluent by ultrafiltration/reverse osmosis followed by electrochemical oxidation of the RO concentrate A.M. Urtiaga ⁎, G. Pérez, R. Ibáñez, I. Ortiz Departamento de Ingenierías Química y Biomolecular, Universidad de Cantabria, Av. Los Castros s/n, 39005 Santander, Spain

H I G H L I G H T S • Removal of emerging pollutants from WWTP secondary effluent by a tertiary treatment that combines UF and RO • Electrooxidation as means of mineralization of micropollutants contained in the RO concentrate

a r t i c l e

i n f o

Article history: Received 8 July 2013 Received in revised form 9 October 2013 Accepted 11 October 2013 Available online 1 November 2013 Keywords: Emerging contaminants Reverse osmosis Ultrafiltration Electrooxidation Municipal wastewaters

a b s t r a c t This study aims to assess the removal of emerging contaminants from municipal wastewaters using a pilot system that integrated ultrafiltration (UF), reverse osmosis (RO), and electrooxidation, which mineralized the RO concentrate. Initially, the study monitored 77 emerging contaminants in the influent and effluent of a wastewater treatment plant (WWTP). Most of the compounds were detected in significant amounts in the WWTP effluent. A group of 12 compounds that represent the most prevalent therapeutic pharmaceutical categories was selected to monitor their removal by UF/RO. For the majority of the micropollutants, the UF removal efficiency was less than 20%. Excellent removal efficiencies were achieved with the RO treatment. As a result, the concentrations of the emerging contaminants in the RO permeate varied between 44 ng/L for naproxen and 4 ng/L for ofloxacin, and furosemide, bezafibrate and fenofibric acid were not detected. After the RO treatment, electrooxidation of the RO concentrate with boron-doped diamond electrodes reduced the total micropollutant content in the RO concentrate from 149 μg/L to less than 10 μg/L. Increasing the intensity of the electrooxidation treatment is expected to further reduce the micropollutant concentrations. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The focus of environmental research has expanded to include both “classic” environmental pollutants and so-called “emerging contaminants”, which comprise pharmaceuticals and personal care products (PPCPs) [1–4]. Concern about their presence in aquatic environments has been increasing because their presence in small concentrations has been associated with chronic toxicity, endocrine disruption and the development of pathogen resistance [5–9]. Because of the observed concentrations of emerging pollutants in raw wastewaters and the limited effectiveness of secondary treatments, municipal wastewater treatment plant (WWTP) effluents are the main disposal pathway for pharmaceuticals and personal care products into the environment [10–12]. In general, the total concentration of emerging contaminants in WWTP effluents ranges from ng/L to μg/L ⁎ Corresponding author. Tel.: +34 942 20 15 87; fax: +34 942 20 15 91. E-mail address: [email protected] (A.M. Urtiaga). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.10.010

[13]. The technologies for removing emerging contaminants from WWTP effluents include ultra-violet (UV) radiation, granulated activated carbon, ion-exchange, membrane filtration and advanced oxidation processes such as ozonation, photocatalysis and the Fenton reaction [1,5,14–22]. Membrane processes are being increasingly implemented in water treatment because these technologies combine process stability with an excellent effluent quality [23–26]. However, widely used microfiltration and ultrafiltration technologies have been found to filter out only a few emerging organic contaminants [27–29]. In contrast, it has been shown that nanofiltration and reverse osmosis can separate out many of these compounds [5,21,30–32]. While the permeate water obtained from NF/RO treatments of WWTP effluents can be employed for industrial uses that demand high quality water [24,33–37], the pollutants are accumulated in a concentrate stream and an additional step is required to treat it [38–40]. In addition to conventional technologies, such as coagulation and activated carbon adsorption [41], advanced oxidation technologies, including ozonation,

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Fig. 1. Scheme of the WWTP treatment and sampling schedule — 1: WWTP influent; 2: Secondary WWTP effluent; 3: Filtered effluent of the UF unit; 4: RO permeate water; 5: RO rejection stream; 6: Output of the electrooxidation treatment. Several composite samples were obtained from points 1 (7) and 2 (8) and analyzed for 77 compounds; Two grab samples were obtained from points 3, 4 and 5 and analyzed for 12 compounds. The concentrations of 10 compounds were measured in 6 grab samples obtained from point 6.

photocatalysis, sonolysis, and electrochemical oxidation have been proposed for eliminating contaminants from the concentrate stream [42–48]. Electrochemical oxidation, in particular has several advantages. For example, it can be used to treat RO concentrate streams with moderate to high salinity [49], which ensures excellent electric conductivity and reduces the energy consumption, and with moderate chloride concentrations, which promote indirect oxidation and disinfection pathways [39,44,46–52]. A few studies have shown that electrochemical oxidation with boron-doped diamond (BDD) electrodes is effective at eliminating emerging contaminants with removal percentages higher than 90% for most compounds [53–55]. In this work, an advanced tertiary treatment that includes membrane technologies and electrooxidation was proposed to treat a secondary WWTP effluent and eliminate the removed pollutants to prevent their discharge into the environment. The occurrence of numerous emerging contaminants in the WWTP influent and effluent was assessed, and the removal efficiency of some of the most prevalent pharmaceuticals and stimulants was determined by using an on-site pilot-scale integrated membrane system (UF-RO). The electrooxidation of the RO concentrate stream with boron-doped diamond electrodes was proposed for the mineralization of the retained pharmaceuticals. 2. Methods and materials 2.1. Description of the applied tertiary treatment The experimental work was performed on-site using the secondary effluent of the WWTP located in Vuelta Ostrera (Cantabria, Spain) as the feed water. This WWTP currently operates at 85% capacity, treating an average flow-rate of 110,000 m3/day, and utilizes a secondary treatment based on activated sludge. In this work, the applied tertiary treatment consisted of pilot-scale ultrafiltration (UF) and reverse osmosis (RO) units combined with laboratory-scale electrooxidation (ELOX). A process diagram showing the locations of the sampling points used to monitor the concentrations of emerging contaminants is presented in Fig. 1. The characteristics of the membrane pilot units are summarized in Table 1. After particles larger than 130μm were removed from the WWTP secondary effluent by a ring filter, the effluent was fed into a dead-end UF unit at an average flow-rate of 2.5 m3/h. The UF permeate was collected in a dumping tank to feed into the RO unit and backwash cycles of the UF membranes. The RO pilot plant was

operated at a low pressure gradient (ΔP = 11 bar) and constant feed flow-rate (QRO,F = 2.4 m3/h). Different productivity ranges (Productivity = QRO, Permeate / QUF, Permeate × 100), were obtained by varying the recirculation rate of the RO rejection stream. The electrooxidation of the reverse osmosis concentrate was performed in a commercial DiaCell system (two 70 cm2 circular electrodes, borondoped diamond on silicon anode and stainless steel cathode, separated by an electrode gap of 5 mm) using a power supplier (Agilent 6654 A, maximum output of 9 A and 60 V), feed tank with a cooling jacket and recirculation pump. The feed temperature was maintained at 20 °C. All the experiments were performed in batch mode, with a recirculation flow-rate of 10 L/min, and working volume of 2 L. The applied current (J) ranged from 20 to 100 A/m2. Composite water samples were taken from the WWTP influent and effluent, while grab samples were withdrawn from the outlet streams of the tertiary treatment units. The sampling schedule is indicated in the caption of Fig. 1. All samples were collected using 1 L pre-rinsed amber glass bottles and shipped to the laboratory at Universidad de Almería (Spain) in boxes packed with ice. Upon reception, all samples were stored in the dark at 4 °C until analysis and were extracted within 48 h. An Agilent 1100 series HPLC equipped with a 3200 QTrap MS/MS detector (Applied Biosystems) and a C-18 analytical column, (L × I.D. − 250 mm × 3.0 mm, particle size 5 μm, ZORBAX SB, Agilent Technologies) was used to analyze the emerging pollutants in the samples. A detailed description of the sample preparation method and operational conditions for the HPLC/MS analysis in positive and negative modes is given in the supplementary material (Table S1). The precision of the method, which was evaluated based on the replicability and reproducibility was acceptable. The RSD values were less than 23% in all cases, ranging from 2 to 12% for the replicability and from 4 to 23% for the reproducibility [8]. The water samples were also analyzed to determine other general physicochemical parameters. The analytical procedures are summarized in the supplementary material (Table S2). 3. Results and discussion 3.1. Physicochemical characterization The physicochemical characteristics of the macro-contaminants and main ionic components of the WWTP effluent and tertiary unit effluents have been included in the supplementary material (see Table S3

Table 1 Characteristics of the UF and RO membrane units.

Commercial reference Module configuration Membrane material Membrane area Configuration Other parameters

UF

RO

AquaFlex™ (Norit) Hollow fiber Polyethersulfone 80 m2 (two modules of 40 m2 each, placed in parallel) Dead-end filtration Average membrane pore size: 0.2 μm

LCF1-4040 (Hydranautics) Spiral wound Polyamide 15.8 m2 (two modules of 7.9 m2 each, placed in series) Tangential flow ΔP = 11 bar

28

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Fig. 2. Summary of the removal of macrocontaminants during the a) UF and b) RO treatments.

of the supplementary data). Because the WWTP effluent exhibited a high variability, the minimum and maximum values are given for each parameter, and the mean value of all the analyzed samples is given in brackets. Fig. 2 shows the results expressed as removal percentages of the main macrocontaminants. Ultrafiltration membranes remove approximately 100% of total suspended solids and microorganisms. Thus, the main objective of the UF pilot plant was to remove suspended solids and provide a disinfection barrier by retaining bacteria and viruses. Macromolecules, such as extracellular proteins and polysaccharides, were removed with an efficiency of approximately 60%, while all small molecules and colloidal matter were not significantly retained by the ultrafiltration treatment. The dense membranes used in the reverse osmosis unit allowed for the retention of small organic compounds and ions. Obviously, larger components would also be retained, but they were previously removed during the UF pre-treatment because they might have caused severe fouling problems in the RO unit [40]. Fig. 2 shows the conductivity parameters. More than 95% of the TDS, anions and cations were rejected by the RO membrane. Thus, the RO treatment resulted in high quality water that meets the quality standards for being industrially re-used according to Spanish legislation [56]. 3.2. WWTP micropollutants The WWTP influent and effluent were analyzed for 77 emerging contaminants encompassing several chemical classification groups (pharmaceuticals, stimulants, personal care products and metabolites). Eight samplings over all seasons were performed. The number of positive samplings, concentration range and mean concentration for each compound are given in Table 2. Additionally, a detailed list of monitored compounds that were not detected in any of the samples is given in Table 3. Most of the micropollutants found in the WWTP influent were pharmaceuticals and stimulants. The most prevalent pollutant was acetaminophen, an analgesic commonly used for the relief of headaches, colds and flu, with an average concentration of 23.8 μg/L, followed by gemfibrozil, caffeine and hydrochlorothiazide, with average concentrations of 18.5, 17.7 and 16.6 μg/L, respectively. Large amounts of paraxanthine, a metabolite of caffeine, were also detected (mean value concentration of 11.8 μg/L). Hydrochlorothiazide is a compound used to treat the heart failure and was usually detected in the analyzed samples. Kaspryk-Horden et al. [57] studied the fate of 55 emerging pollutants in two WWTPs in South Wales (UK) and reported an average acetaminophen concentration greater than 180 μg/L over a period of 5 months demonstrating that the micropollutant concentrations were correlated with their usage/consumption patterns. Yang et al.

[58] also reported acetaminophen and caffeine concentrations in the order of 100 μg/L, in an advanced wastewater reclamation plant in the U.S.A. They found high ibuprofen (~10 μg/L) and carbamazepine (~1 μg/L) concentrations as well. Other authors [59–62] also reported concentrations within the ranges detected in this work where the average concentrations of acetaminophen, caffeine, gemfibrozil and hydrochlorothiazide were 23.2, 22.8, 3.5 and 2.5 μg/L, respectively. N-Acetyl-4-amino-antipyrine (4-AAA) is a metabolite of metamizol, which has analgesic, anti-inflammatory and antipyretic properties, and was found in intermediate concentrations between 1 and 10 μg/L. Ibuprofen and naproxen, two common anti-inflammatories, were also detected within that concentration range. According to the literature, these compounds are two of the most abundant in the WWTP influents [12,63]. Ibuprofen and naproxen concentrations between 2.6 and 5.7 μg/L and 1.8 and 4.6 μg/L, respectively were also observed in a WWTP located in the north western area of Spain [4], indicating a similar consumption pattern in that area as in the region where the wastewaters used in this study were collected. The concentration of the rest of the analyzed compounds was less than 1 μg/L. As expected, the effectiveness of the WWTP secondary treatment at removing microcontaminants varied for each of the analyzed compounds. Although the removal efficiencies of some of the pollutants, such as acetaminophen and nicotine, were high, (99.5 and 98.1%, respectively), some of the emerging pollutants were still easily detected in the treated wastewater samples. As shown in Table 2, the caffeine concentration in the WWTP effluent was 5.8 μg/L, which was the highest concentration observed for the analyzed micropollutants. Similar observations for caffeine and nicotine were previously reported in the literature [11,64]. High removal yields, ranging between 80 and 90% were observed for gemfibrozil, hydrochlorothiazide, ibuprofen and paraxanthine. Similar or better ibuprofen removal rates have been previously reported by other authors [3,4,58,59,65]. In the case of hydrochlorothiazide, Radjenović et al. [59] and Castiglioni et al. [66] reported no elimination of this compound during conventional secondary treatment. Similarly, discrepancies about the elimination of diclofenac exist; Clara et al. [3] reported rejection values of approximately 70%, while Lindqvist et al. [63] achieved a removal rate of only approximately 20%. In this study, the observed average concentration in the effluent was larger than that in the influent, due to the large variability in the diclofenac concentration in both influent and effluent samples. The disparities between the different reports have been attributed to differences in the WWTP secondary treatment, such as the sludge composition and age, wastewater composition, and wastewater retention time in the treatment facility [8,57,59,67,68]. In any case, most of the compounds, including 4-AAA, furosemide and ofloxacin, were only partially removed and were detected in significant amounts in the WWTP effluent in this study. Other compounds, such as

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Table 2 Concentration of emerging pollutants in the WWTP influent and effluent. WWTP influent

WWTP effluent

% R WWTP

Positive samples

Min. value

Max. value

Mean value

Positive samples

Min. value

Max. value

Mean value

Pharmaceuticals and stimulants (ng/L) 4-amino-antipyrine (4-AA) 4MAA Acetaminophen Atenolol Bezafibrate Caffeine Carbamazepine Carb, epoxide Ciprofloxacin Clofibric acid Codeine Diazepam Diclofenac Fenofibric acid Fluoxetine Furosemide Gemfibrozil Hydrochlorothiazide Ibuprofen Indomethacin Ketoprofen Mepivacaine Metoprolol Metronidazole N-Acetyl-4-amino-antiyirine (4-AAA) Naproxen N-Formyl-4-amino- antiyirine (4-FAA) Nicotine Ofloxacin Omeprazole Paraxanthine Propranolol hydrochloride Ranitidine Salbutamol Sotalol Sulfamethoxazole Trimethoprim Clarithromycin 1 Cotinine Phenacethin Pravastatin Salicylic acid Sulfapyridine

5 6 4 7 6 7 4 1 5 2 5 2 6 1 1 6 6 6 5 2 5 3 1 5 7 6 6 7 5 1 6 2 4 3 2 5 6 2 2 2 2 2 1

191 45 8162 48 76 93 10 49 32 23 19 6 198 139 230 425 652 16628 1128 31 152 4 21 36 357 1406 109 23 13 15 56 9 14 12 22 386 7 5 20 289 31 96 60

427 2935 47,680 1872 442 31,980 118 49 381 30 629 11 1935 139 230 2690 99,574 59,565 4080 39 589 16 21 176 20,200 2672 2374 26,694 181 15 32,627 20 523 16 32 1319 196 25 688 339 108 172 60

294 1094 23,781 778 208 17,725 56 49 163 27 422 8 560 139 230 1302 18,504 16,628 2514 35 439 9 21 95 7364 2672 1327 10,954 94 15 11786 15 233 14 27 672 145 15 354 314 69 134 60

6 7 1 8 8 8 7 4 7 3 7 7 7 2 0 8 8 8 3 7 7 6 6 7 8 8 7 8 8 2 7 7 5 5 6 7 7 2 2 2 2 2 1

74 5 126 314 62 128 26 15 65 5 63 4 205 151 n.d. 330 447 1000 209 19 319 3 12 17 534 206 186 53 9 6 116 10 n.d. 7 12 161 56 13 43 412 19 31 77

288 850 126 3494 926 8285 141 33 120 6 580 11 7043 277 n.d. 2825 12697 7864 915 34 471 11 21 102 6609 2518 1326 755 85 8 5699 23 466 14 38 543 257 13 392 437 184 52 77

213 369 126 882 207 5781 86 24 97 8 347 7 1281 135 n.d. 981 3567 2398 581 29 419 6 16 53 4015 2949 1128 205 81 7 1195 16 260 11 27 276 158 13 218 425 102 42 77

Personal care products (ng/L) Benzophenone-3 Celestolide Ethylhexyl methoxycinnamate Galaxolide Octocrylene Tonalide Traseolide Triclosan BHT Musk xylene

4 1 3 4 3 4 1 4 1 1

15 5 52 1848 338 183 9 290 169 425

314 5 333 5154 771 635 9 2570 169 425

165 5 154 3653 509 332 9 1070 169 425

3 0 3 3 2 3 0 3 3 1

n.d. n.d. 9 1259 20 56 n.d. 105 107 59

n.d. n.d. 44 3303 54 158 n.d. 177 147 59

n.d. n.d. 22 2277 37 108 n.d. 133 124 59

naproxen, atenolol and fenofibric acid, were not removed at all during the conventional treatment. To study the effectiveness of the different tertiary treatment steps, a smaller representative group of compounds was selected. Of the

Diatrizoate Traseolide Erythromycin Azithromycin Iopamidol Nadolol Fluoxethine

0.58 67.4 51.3 41.0 71.2 17.8 15.2

100 24.7 80.7 85.6 76.9 17.0 4.6 30.1 25.1 44.2 45.5 15.0 98.1 38.4 52.8 89.9

22.0 1.2 58.9 13.3 38.4

68.7

47.3 100 85.7 37.7 92.7 67.5 100 87.6 26.6 86.1

different criterion that can be used to select the most representative emerging pollutants [2,69], the concentrations were employed in this work; the compounds with the highest concentration for each therapeutic category were selected. The 12 compounds that were

Table 3 List of compounds not detected in any of the influent and effluent samples. Cefotaxime Ketorolac Fenoprofen Antipyrine Ifosfamide Sulfamethazine

27.4 66.2 99.5

Fenofibrate Urbason Terbutaline Citalopram Iopromide lincomycin 4-Dimethylaminoantipyrine (4-DAA) Mefenamic acid

Loratidine Paroxethine Amitriptyline Clomipramine

30

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Table 4 List of emerging pollutants monitored during the tertiary treatment. Compound

Chemical structure

Formula

Caffeine

C8H10N4O2

N-Acetyl-4-aminoantipyrine (4-AAA)

C13H15N3O2

Gemfibrozil

C15H22O3

Naproxen

C14H1403

Hydrochlorothiazide

C7H8ClN3O4S2

Atenolol

C14H22N2O3

Furosemide

C12H11ClN2O5S

Ibuprofen

C13H18O2

Bezafibrate

C13H20ClNO4

Nicotine

C10H14N2

Fenofibric acid

C17H15ClO4

Ofloxacin

C18H20FN3O4

concentrations of some compounds were slightly higher than the feed concentrations, which was attributed to the analytical variance [29]. Snyder et al. [21] observed rejection values as low as those found in this work; for instance, the retention of caffeine and ibuprofen was only 7%, that of naproxen was 1.2% and the gemfibrozil concentration was higher in the UF permeate than in the WWTP effluent. This behavior was also observed in this study for compounds such as caffeine, N-acetyl-4-amino-antipyrine and fenofibric acid, although it was mostly attributed to the grab nature of the samples used to characterize the UF system. Therefore, in this work the majority of the compounds were not eliminated by the UF membrane unit, as shown in Fig. 3. According to the literature, RO and NF exhibit excellent rejection rates for many analytes. Consequently, RO or NF wastewater treatments are adequate for the effective removal of a variety of micropollutants such as pharmaceuticals, personal care products and hormones [70]. One of the main drawbacks of these technologies is membrane fouling, which can be prevented by pretreating the water. In this study, the UF treatment largely prevented membrane biofouling. In the RO treatment, the retention of a given compound is dependent on many factors such as the chemical structure of the compound, membrane characteristics, water matrix and adsorption interactions between the compound and the membrane [71]. In this study, the main characteristics of the commercial LCF1-4040 RO membrane and the operational mode are summarized in Table 1. The RO unit was operated at low pressure (ΔP = 11 bar), and thus in a low-energy consumption regime, and at two productivity values, 50% and 70%, as defined in Section 2. The productivity was controlled by varying the amount of recycling as shown in Fig. 1. The concentrations of the selected micropollutants were measured throughout the RO treatment (RO feed, RO permeate and RO concentrate) and are given in Table 5. Moreover, the corresponding mean concentrations of those analyses are also presented in Fig. 3 and demonstrate the effectiveness of the RO treatment for eliminating emerging pollutants in the RO permeate water. The rejection of the compounds by the RO unit was determined using the following equation:   Rð% Þ ¼ 100 C fc –C p =C fc

selected are listed in Table 4 and represent the following groups: highly consumed analgesic/anti-inflammatories (naproxen, ibuprofen), antibiotics (ofloxacin), diuretics (furosemide, hydrochlorotiazide), lipid regulators (gemfibrozil, benzafibrate and fenofibric acid, metabolite of fenofibrate), β-blockers (atenolol), antipyretics (4-AAA, a metabolite of dipyrone) and stimulants (caffeine, nicotine). 3.3. Removal of emerging contaminants using the advanced UF-RO tertiary treatment Fig. 3 shows the concentrations of the 12 selected emerging pollutants in the feed and permeate streams of the UF unit. It was observed, that the removal efficiencies of gemfibrozil (71%) and nicotine (63%) were the highest. The bezafibrate, furosemide and naproxen reduced by 21, 17 and 12%, respectively. UF did not remove any of the other compounds. These results are in agreement with those reported by Sui et al. [1] who also found that UF treatment only eliminated a small amount of the investigated compounds. Yoon et al. [29] published UF rejection values of typically less than 40% for a wide range of endocrine-disrupting compounds and pharmaceutical and personal care products. For the four compounds that were also studied in this work, gemfibrozil, ibuprofen, naproxen and caffeine, the retention was less than 10%. In this study, the UF permeate

where Cfc is the average of the feed and retentate concentrations (ng/L) and Cp is the permeate concentration (ng/L). Fenofibric acid, furosemide and bezafibrate were completely rejected, and greater than 99.8% of ibuprofen, caffeine, and hydrochlorothiazide were also rejected. More than 99.5% of 4-AAA, gemfibrozil, atenolol and ofloxacin and approximately 99.3% of nicotine and naproxen were rejected. Removal efficiencies of 97% were reported for compounds such as gemfibrozil, ibuprofen and acetaminophen when real wastewaters were treated in a full-scale MF and RO membrane facility, resulting in a product water concentration of less than 100 ng/L [72]. Rejection values as high as 95%, were also reported by Radjenović et al. [73] of pharmaceuticals, including ketoprofen, diclofenac and sulfamethoxazole, and more than 85% of carbamazepine and hydrochlorothiazide were rejected. In the same study, lower rejection values were obtained for acetaminophen due to its small molecular size. Similarly, Drewes et al. [74] did not detect any emerging contaminants in RO permeate samples, except caffeine which was detected in low concentrations in all the samples. Snyder et al. [21] confirmed that NF and RO membrane effectively retained a significant portion of these compounds, although they were detected in trace levels in the permeate. Similarly to this work they performed their study using UF unit followed by a RO unit and unspiked secondary effluent feed water. In fact, the majority of the concentrations were less than 1 ng/L, except for those of caffeine, galaxolide, musk ketone and tris-(2-chloroethyl)-phosphate (TCEP), which were less than 10 ng/L. The same behavior was also observed by other authors using synthetic waters; rejection values exceeding 95% were obtained

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31

50000

Concentration (ng/L)

50000

2949 2583 44 9223 658

43

451

4015 4472

124

33

10000

5781 6286

20000

463

15569

30000

3567 1035 12 9868

40000

0

Caffeine

Gemfibrozil

Naproxen

27000

25000

21250

20000

6400

581 574 18

268

882 884 4 2779

5000

981 811 n.d. 2522 105

10000

1344

15000

2398 n.d. 13

Concentration (ng/L)

30000

4-AAA

0

Hydroch.

Furosemide

Ibuprofen

5683

6000 5000

2575

4000

74

n.d.

32

135 194 n.d. d

205 75 18

1000

n.d.

2000

81 87 4

1480

3000

207 164 n.d. 583

Concentration (ng/L)

Atenolol

0

Bezafibrate

Nicotine

Fenofibric Ac.

Fig. 3. Concentrations (ng/L) of emerging pollutants at different stages of the advanced treatment at the Vuelta Ostrera WWTP. Concentrate; ELOX effluent. n.d.: not detected.

[75]. Recent studies [27] reported similar behavior for six antibiotics, three pharmaceuticals, a natural hormone and an industrial product. During the RO stages, the removal efficiencies were greater than 99% for the macrolides, pharmaceuticals, natural hormone and industrial product, and between 93 and 97% for the rest of the tested compounds. Thus, it was confirmed that RO membranes can greatly reduce the concentrations of emerging contaminants. The main drawbacks of using RO membrane processes are the costly disposal or treatment of the resulting RO concentrate, and the potential environmental threat to the receiving aquatic ecosystems [38]. As expected, the compounds rejected during the RO treatment, were concentrated to different degrees in the RO concentrate stream depending on the: i) concentration in the feed water; ii) operational conditions of the RO pilot unit and iii) interactions between each compound and the RO membrane. As a result, the total concentrations of the major micropollutants were 88 μg/L and 148.5 μg/L when the productivities of the RO pilot unit were 50% and 70%, respectively. Thus, after the RO treatment, an additional step is required to condition the

Ofloxacin WWTP effluent;

UF permeate;

RO Permeate;

RO

concentrate stream and avoid the discharge of emerging contaminants into nearby water bodies. Electrooxidation is a promising technology for removing emerging contaminants from the RO concentrate streams as shown in Fig. 3. High removal percentages were obtained for most of the selected compounds. At the electrooxidation conditions used in this study (BDD anodic area 70 cm2, applied current density 100 A/m2, volume treated 2 L and treatment time 60 min), more than 97% of hydrochlorothiazide, nicotine, atenolol, furosemide and bezafibrate were removed, while 94% of ofloxacin, fenofibric acid, 4-AAA, naproxen and gemfibrozil were eliminated. These removal values are comparable to those reported by Menapace et al. [76] who proposed combining electrooxidation and ozonation treatments to achieve elimination values of up to 99% for the group of studied pharmaceuticals. Nearly 100% of bisphenol A was degraded by electrooxidation with BDD electrodes in the study of Murugananthan et al. [54]. According to our results, ibuprofen was the most resistant to electrooxidation with only, 70% removed after 1 h of ELOX at 100 A/m2. After the

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Table 5 Concentrations (ng/L) of the 12 emerging contaminants monitored during the RO treatment. Compound

UF permeate = RO feed

RO permeate

RO concentrate

Productivity

Atenolol Bezafibrate Caffeine Fenofibric acid Furosemide Gemfibrozil Hydrochlorothiazide Ibuprofen 4-AAA Naproxen Nicotine Ofloxacin Total

1044 164 6288 194 811 1035 239 574 4472 2583 75 87 17,566

electrooxidation, the concentrations of the selected compounds were less than 1 μg/L, except in the case of ibuprofen, which had an average concentration of 6 μg/L. The total concentration of the emerging contaminants was 9.9 μg/L.

4. Conclusions A wastewater treatment scheme that integrates activated sludge, ultrafiltration, reverse osmosis and electrooxidation was used to remove emerging contaminants from municipal wastewaters. The concentrations of 77 pharmaceuticals, stimulants, personal care products and metabolites were monitored in the raw municipal wastewater and secondary treatment effluent at a WWTP in the northern Spain over a period of two years. The amount of micropollutants removed during the secondary treatment varied widely for all the analyzed compounds. The most abundant compound in the secondary effluent was caffeine, with a concentration of 5.7 μg/L, and the total load of emerging pollutants was reduced from 128 μg/L in the feed to 32 μg/L in the effluent. A group of 12 compounds that are representative of the most prevalent stimulants (caffeine, nicotine) and various therapeutic pharmaceutical categories (naproxen, ibuprofen, ofloxacin, furosemide, hydrochlorothiazide, gemfibrozil, bezafibrate, fenofibric acid, atenolol and 4-AAA), was selected to study the effectiveness of a tertiary treatment consisting of on-site, pilot scale ultrafiltration and reverse osmosis units for removing contaminants. The UF removal efficiency for the different compounds varied significantly, although it was less than 20% for the majority of the studied contaminants. Excellent removal percentages were achieved by the reverse osmosis treatment. The RO membranes were subjected to a low pressure gradient (ΔP = 11 bar) and therefore operated in a low-energy consumption regime. They rejected more than 99% of all the target compounds. When the RO pilot plant was operated at a 70% recovery rate, the concentrations of the emerging contaminants in the osmotized water varied between 44 ng/L of naproxen and 4 ng/L for ofloxacin, while furosemide, bezafibrate and fenofibric acid were not detected in the RO permeate. However, the total concentration of the major micropollutants in the concentrate stream averaged 88.7 and 148.5 μg/L for RO productivities of 50% and 70%, respectively. Thus, after the RO treatment, an additional step was required to condition the concentrate stream and prevent the discharge of emerging contaminants into nearby water bodies. Electrooxidation with boron-doped diamond electrodes removed more than 95% of most of the studied compounds from the RO effluent, demonstrating its excellent efficiency for the mineralization of emerging contaminants in the waste concentrate stream generated during the reverse osmosis treatment of WWTP secondary effluents.

50%

70%

50%

70%

3 n.d. 3 n.d. n.d. 11 9 13 20 15 13 4

4 n.d. 33 n.d. n.d. 12 13 18 43 44 18 4

1452 500 33,939 800 – 5921 18,750 10,416 11,847 4161 912 – 88,698

2779 583 50,000 1480 2522 9868 27,000 21,250 15,569 9223 5683 2575 148,532

Acknowledgments Support from the CTQ2008-0690, 062/SGTB/2007/3.1, and CONSOLIDER CSD2006-44 projects and Greentech (New Indigo ERANet Programme) is gratefully acknowledged. Special thanks are given to Prof. Amadeo Fernandez–Alba and his research team (Universidad de Almeria) for the analysis of the emerging contaminants.

Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.desal.2013.10.010.

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