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Open AccessArticle

Effect of O3 Dose on the O3/UV Treatment Process for the Removal of Pharmaceuticals and Personal Care Products in Secondary Effluent

1
Department of Civil and Environmental Engineering, Idaho State University, 921 S. 8th Ave., Stop 8060, Pocatello, ID 83209, USA
2
Department of Construction Environment Engineering, Environmental and Plant Engineering Institute, Korea Institute of Civil Engineering and Building Technology, University of Science & Technology/Senior Researcher, (Daehwa-Dong 2311) 283, Goyangdae-ro Ilsanseo-gu, Goyang-si, Kyonggi-do 411-712, Korea
3
Research Center for Environmental Quality Management, Graduate School of Engineering, Kyoto University, 1-2 Yumihama, Otsu, Shiga 520-0811, Japan
4
Department of Civil and Environmental Engineering, Idaho State University, 921 S. 8th Ave., Stop 8060, Pocatello, ID 83209, USA
*
Author to whom correspondence should be addressed.
ChemEngineering 2019, 3(2), 53; https://doi.org/10.3390/chemengineering3020053
Received: 28 February 2019 / Revised: 10 May 2019 / Accepted: 27 May 2019 / Published: 1 June 2019

Abstract

A municipal wastewater treatment plant (WWTP) is a melting pot of numerous pharmaceuticals and personal care products (PPCPs) together with many other substances. The removal of PPCPs using advanced oxidation processes within a WWTP is one way to reduce the amount of PPCPs that potentially enter an aquatic environment. The aim of this study was to examine the effectiveness of the ozone (O3)/UV treatment process, especially, the effects of O3 dose and reaction time, on the removal of PPCPs in the secondary effluent of a WWTP. Experiments were conducted using a pilot-scale treatment process that consisted of two flow-through reactors connected in series. Each reactor was equipped with three 65 W lamps (UV65W). The experimental variables were ozone dosage (1, 2, 3, 4, and 6 mg L−1) and hydraulic retention time (HRT; 5 and 10 min). On the basis of the PPCP concentrations after O3/UV65W treatment and their limit of detection (LOD), 38 PPCPs detected in the secondary effluent were classified into 5 groups ranging from the category of “sensitive” to O3/UV65W or “unstable” in the O3/UV65W process to the category of “insensitive” to O3/UV65W or “very stable” in the O3/UV65W process.
Keywords: ozone; UV; pharmaceuticals; personal care products; wastewater; advanced oxidation process ozone; UV; pharmaceuticals; personal care products; wastewater; advanced oxidation process

1. Introduction

In modern societies, numerous kinds of personal care products (PPCPs) are being consumed in large quantities [1], and the usage of PPCPs will likely continue to rise in the future. Although certain types of PPCPs are degraded via human/animal metabolism or decomposed quickly in the environment, other types of PPCPs are recalcitrant and maintain their biologically active properties after being excreted from a body and disposed of into a wastewater treatment system [2,3]. A wide range of PPCPs found in the aquatic environment are mainly originated from wastewater treatment plants (WWTPs) [4,5,6,7]. Because most public WWTPs are not equipped with processes to remove biorecalcitrant PPCPs [8,9,10,11], these compounds are likely discharged into receiving water bodies [8,9,10,12,13]. For example, sulfamethoxazole (antibiotic), carbamazepine (anticonvulsant), bezafibrate (lipid-modifying agent), clofibric acid (lipid-modifying agent) are commonly found in municipal wastewater effluent around the world [13,14,15]. Erythromycin is one of the most frequently detected antibiotics in the environment [13,14,15,16,17,18,19,20]. The removal of PPCPs at WWTPs has been considered as a way to reduce the amount of PPCPs that potentially enter the aquatic environment [21].
Advanced oxidation processes (AOPs) can produce highly reactive free radicals such as hydroxyl radicals (·OH); thus, AOPs have the potential to decompose biorecalcitratnt PPCPs present in water and wastewater [22]. In the AOPs, hydroxyl radicals are the primary oxidants that attack and decompose organic compounds (their characteristics and effectiveness are described elsewhere [23]). Among various AOPs, UV photolysis, ozonation, and their combination are potential technologies capable of destroying organic pollutants in water and wastewater [21,24,25,26,27].
Ozone (O3) has been successfully employed for the disinfection and decomposition of dissolved organic pollutants [21,24,25,26,27]. Several studies have shown that ozonation could effectively remove certain PPCPs [9,14,17,21,28,29,30]. The study by Park et al. [31], however, showed that a higher ozone dose (4–8 mg L−1) was required for PPCP removal as compared to the dose (1–1.5 mg L−1) applied for disinfection. Although UV light at λ = 254 nm has been applied for the disinfection of water and wastewater [32], it was shown to be ineffective for the direct photolysis of organic compounds that have a low quantum yield and small molar absorption coefficient [33]. In water and wastewater, UV photolysis of O3 generates ·OH [34,35,36], which can mineralize various pollutants to CO2, H2O, and inorganic compounds, or at least, more innocuous products [37]. Ozone has the molar absorption coefficient (ε) of 2952 mol−1 dm3 cm−1 at 254 nm UV [38], which is sufficient to generate ·OH in an aqueous solution. Therefore, the O3/UV method has the potential to enhance the generation of ·OH [39,40]) and subsequently degrade organic pollutants [33].
Past studies on the combination of O3 and UV focused mainly on the dosages of ozone and UV [11,41,42]. In the present study, the effects of reaction time (hydraulic retention time, HRT) in addition to the ozone dosage, were comparatively examined using a pilot-scale O3/UV reactor process. The results suggested that the reaction time, in addition to the O3 dosage, is an important factor to be considered in designing an O3/UV process and that the O3/UV treatment has the potential to remove a wide range of PPCPs from the secondary effluent of a WWTP. Furthermore, the PPCPs detected in secondary effluent were grouped on the basis of their degradability in the O3/UV process. These PPCPs were divided into five categories ranging from the group of “sensitive” to O3/UV or “unstable” in the O3/UV process to the group of “insensitive” to O3/UV or “very stable” in the O3/UV process. A pilot-scale process used in this study provided a better understanding of the effects of O3 on the O3/UV process for the degradation of PPCPs present in the secondary effluent of a municipal WWTP.

2. Methodology

2.1. Experimental Setting and Operational Conditions

A pilot-scale flow-through O3/UV process was designed in order to evaluate the removal of PPCPs found in the secondary effluent of a WWTP (Figure 1). The reactors (Metawater Co. Ltd., Tokyo, Japan) were constructed to model actual UV reactors. The experimental system consisted of a sand filtration unit (for pretreatment), feed units (a feed wastewater tank and a feed pump), two identical stainless steel O3/UV reactors (R1 and R2), an ozone generator, and a tank for receiving the treated wastewater. Two UV reactors were connected in series, each having an effective volume of 35 L (189.9 cm length, 15.5 cm I.D.). The UV source was a 65 W low-pressure mercury UV lamp (UV65W) with λ = 254 nm. The UV65W lamp exhibited the UV output of 21.8 W and the UV irradiance of 1.025 mW/cm2. Each UV reactor was equipped with three UV lamps. The wastewater flow rate was set at 7 L min−1 to give the hydraulic retention time (HRT) of 5 min per reactor, that was a total of 10 min with two UV reactors in series.
Ozone gas was continuously supplied from the bottom of each UV reactor at a flow rate of 0.5 L min−1. As is illustrated in Figure 1, the concentration of ozone in the inlet gas (CO3)in, was measured by an ozone monitor (③). The gas trapped in the top of the UV reactor was released through a pipeline (① and ② dotted line), and the concentrations of ozone (O3) in the exit gas, (CO3)1 and (CO3)2) were measured by an ozone monitor (④) located in front of an ozone absorber (⑤). The ozone dose, which was the amount of O3 directed from the ozone generator to the unit volume of water to be treated, was calculated using the following equation [30,43,44]:
[ Ozone   Dose ] = ( C O 3 ) in ( Q O 3 Q WW )
where [Ozone Dose] is the applied ozone dose (mg dm−3 or mg L−1), (CO3)in is the ozone concentration in the gas stream at the inlet or the feed gas, QO3 is the feed ozone flow rate (dm3min−1 or Lmin−1), and QWW is the wastewater flow rate (dm3min−1 or Lmin−1). For the two reactors in series, the total ozone dose was calculated by adding the ozone dose of the two reactors. Because the concentrations of ozone in the exit gas, (CO3)1 and (CO3)2, were about 1/10 of (CO3)in, the ozone losses in the process were considered minimal.

2.2. Experimental Procedure

Three different runs (Run 1, 2, and 3) were carried out with the injection O3 concentration (CO3)in of 14 mg L−1, 28 mg L−1, and 42 mg L−1, respectively, in each reactor. As presented in Table 1, the experiments were conducted with these values of the two variables: (i) applied ozone doses of 1, 2, 3, 4, and 6 mg L−1), and (ii) reaction time or HRT of 5 and 10 min. Note that the reactor 1 (R1) in run 2 and the reactor 2 (R2) in Run 1 gave the same ozone dose of 2 mg L−1.
The feed secondary effluent was obtained from a conventional activated sludge WWTP in Shiga, Japan, which had the following characteristics: pH ranging from 6.5 to 6.8, dissolved organic carbon (DOC) concentration from 2.7 to 3.4 mg L−1, and UV absorbance from 0.0514 to 0.0779 cm−1 at 254 nm (UV254). Other wastewater parameters were typically, biochemical oxygen demand (BOD) of 200 mg L−1 in influent and 1.1 mg L−1 in effluent (before sand filtration); chemical oxygen demand (COD) of 79 mg L−1 in influent, 5.4–10 mg L−1 before sand filtration, and 4.7–9.7 mg L−1 after sand filtration; suspended solids (SS) of 160 mg L−1 in influent, <1–8 mg L−1 before sand filtration, and <1–4 mg L−1 after sand filtration; and total organic carbon (TOC) of 83 mg L−1 in influent [45]. The concentrations of total nitrogen in influent and effluent were estimated to be 30 and 5.6 mg L−1, respectively, and those of total phosphorus in influent and effluent were estimated to be 3 and 0.06 mg L−1, respectively, from the work of Tainaka, 2008 [46]. For each experimental run, the system was operated for more than three HRT before the samples were taken to obtain steady-state samples.

2.3. Sample Preparation and Analyses

From a sampling port of each reactor, 1 L of sample was collected. The sample was immediately purged with N2 gas to remove residual O3 to stop the reactions between O3 and PPCPs and then analyzed for PPCPs using LC–MS/MS following the method described in the literature [47,48]. Dissolved O3 was analyzed by the indigo method [49], in which the absorbance was measured using a spectrophotometer (UV-16000, Shimazu) at 600 nm wavelength. Other water quality parameters (TOC, DOC) were measured according to Kim et al. (2009) [47].

3. Results and Discussion

In this study, 38 PPCPs were detected in the secondary effluent of a municipal WWTP. The limit of detection (LOD) values for these 38 PPCPs are presented in Table S1, as supplemental data. The PPCPs found in the feed wastewater and the effluent of the O3/UV reactors were examined here with respect to the ozone dose and the reaction time (HRT) in reference to their use groups. The concentrations of the PPCPs are shown on the basis of their use groups, i.e., antibiotics, analgesics, antiarrhythmic agents, anticonvulsants, vasodilators, lipid-modifying agents, and remaining PPCPs (anti-itch drugs, anti-psychotic drugs, insect repellents, bronchodilators, diuretics, peptic ulcer drugs, N-methyl-D-aspartate (NMDA) receptor antagonists, antifungal drugs, antimicrobial drugs, and antineoplastic agents) in Figure 2, Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7, respectively; (a), (b) and (c) in these figures refer to Runs 1, 2, and 3, respectively, and Ro, R1, and R2 refer to the feed secondary effluent, R1 reactor (HRT = 5 min) effluent, and R2 reactor (HRT = 10 min) effluent, respectively. The LOD of each PPCP compound is shown by the dotted line. The measured PPCP concentrations at or below their LOD (≤LOD) are not shown. The injected ozone concentrations and ozone dosages for these runs and reactors are summarized in Table 1.
The control run was carried out with UV65W in the absence of O3. In the control run, two analgesics (diclofenac and isopropylantipyrine), one antibiotic (ciprofloxacin), one lipid-modifying agent (clofibric acid), and one antimicrobial drug (chloramphenicol) were degraded to or below their LOD (≤LOD) in 5 min (R1 reactor). Naproxen (analgesic) and nalidixic acid (antibiotic) were degraded in 10 min (R2 reactor). These compounds appeared to be sensitive to UV.

3.1. Antibiotics

Of the 38 PPCPs detected, 11 were antibiotics (Figure 2). The mean and standard error were calculated using the feed solution (Ro) in three runs (Runs 1, 2, and 3). The results showed that 8 (ciprofloxacin, sulfadimethoxine, lincomycin, nalidixic acid, sulfamethoxazole, levofloxacin, erythtromycin, and trimethoprim) out of the 11 antibiotics were degraded to or below their LOD (≤LOD) at the ozone dose of 6 mg L−1 in 10 min. In this O3/UV65W process, ciprofloxacin was degraded (≤LOD) at the ozone dose of 1 mg L−1 in 5 min in the O3/UV65W process. Note that ciprofloxacin was degraded (≤LOD) in 5 min in the control (UV only) run. In the past ozone treatment studies, ciprofloxacin was degraded to >99% at the ozone dose of <6.2 mg L−1 [13] and to 83% at 3 mg L−1 with the contact time of 10 min [50].
In the present study, clarithromycin, azithromycin, and roxithromycin were shown to be insensitive to O3/UV65W and very stable in the O3/UV65W process. Note that these PPCPs were not degraded (≤LOD) in the control run. With the O3-only (without UV) treatment, clarithromycin required the ozone dose of 6 mg L−1 and HRT of 10 min to degrade (≤LOD) [30]. In the past ozone treatment studies, roxithromycin was removed up to 73% and 84% with the ozone doses of 4 and 8 mg L−1, respectively [51], and >63% removal at the ozone dose of 5 mg L−1 [52]. In the present study with the O3/UV65W process, roxithromycin was not degraded (≤LOD) at the O3 dose of 6 mg L−1.
Sulfadimethoxine, lincomycin, nalidixic acid, and sulfamethoxazole required the ozone doses of 2–6 mg L−1 to degrade to their LOD level; thus, it can be said that these compounds are slightly sensitive to O3/UV65W. In the ozone-alone study by Rosal et al. (2010) [13], lyncomycin was removed by >88% at the ozone dose of <2.4 mg L−1. Since, in the O3/UV65W process, lyncomycin required the ozone dose of 2–3 mg L−1 to degrade (≤LOD), UV showed little effect on lyncomycin degradation by O3. The removal of sulfamethoxazole by O3 alone has been studied by several researchers. Their results showed sulfamethoxazole removal >92% at the ozone dose of 5 mg L−1 [14], >99% at 7 mg L−1 [53], 47% at 2 mg L−1, 99% at 4 mg L−1, and 100% removal at 8 mg L−1 [51]. Because sulfamethoxazole was degraded (≤LOD) at the ozone dose of 4 mg L−1 in the O3/UV65W process, UV appears to enhance the ozone treatment of sulfamethoxazole. Nalidixic acid was not degraded to ≤LOD in 5 min with or without O3 but was degraded (≤LOD) in 10 min with or without O3, indicating that the reaction time (not O3) is important for the degradation of nalidixic acid.
Erythromycin and trimethoprim required the ozone dose of 6 mg L−1 in the R2 reactor (HRT of 10 min) to be degraded (≤LOD); thus, they are relatively insensitive to O3/UV65W and stable in the O3/UV65W process. In the ozone treatment studies, erythromycin was degraded by 73% at the ozone dose of 4 mg L−1 [51], >92% at 5 mg L−1 [14], and >99% at 7 mg L−1 [53]. Past studies with ozone alone showed trimethoprim removal >85% at the ozone dose of 5 mg L−1 [14] and >99% at 7 mg L−1 [53]. In the present study, trimethoprim was degraded (≤LOD) at the ozone dose of 6 mg L−1 in the O3/UV65W process but it was not degraded in the control run, suggesting that O3 affected the photo-degradation of trimethoprim.

3.2. Analgesics

Seven analgesics (diclofenac, mefenamic acid, naproxen, indomethacin, isopropylantipyrine, ethenzamide, and ketoprofen) were detected in the secondary effluent (Figure 3). Diclofenac, mefenamic acid, naproxen, and isopropylantipyrine were degraded (≤LOD) at the ozone dose of 1 mg L−1 in 5 min (R1 reactor) in the O3/UV65W process. In the control run, diclofenac and isopropylantipyrine were degraded (≤LOD) in 5 min, whereas mefenamic acid, naproxen, indomethacin, ethenzamide, and ketoprofen were not degraded. The results indicated that photodegradation of mefenamic acid, naproxen, indomethacin, ethenzamide, and ketoprofen were positively affected by O3. Naproxen was degraded in 10 min in the control run, whereas it was degraded in 5 min at the O3 dose of 1 mg L−1, showing that the photodegradation of naproxen was enhanced by O3 in the O3/UV65W process. The high degradability of naproxen by O3/UV was previously reported by Giri et al. [54]. Diclofenac was found to be highly susceptible to photodegradation [55,56]. Several past studies showed that diclofenac and naproxen degrade easily with O3. Diclofenac was removed by >96% at the ozone dose of 5 mg L−1 [14], >99% at 2.1 mg L−1 [17], >98% at 7 mg L−1 [53], and >94% at 5 mg L−1 [57]. With ozone doses similar to those of the present O3/UV65W work, naproxen was removed by >96% [17,53], and similar results were reported with ozone doses of 2, 4, and 8 mg L−1 [51]. Relatively high degradability of isopropylantipyrine by O3/UV has been reported by Giri et al. [54].
Indomethacin was degraded (≤LOD) at the O3 dose of 2 mg L−1 in 10 min but not degraded in 10 min in the control run, indicating that O3 enhanced the photodegradation of indomethacin. The result suggests that indomethacin is relatively sensitive to O3/UV65W. The reported degradability of indomethacin by ozone varies. Past studies showed that indomethacin was degraded by >50% at the ozone dose of 5 mg L−1 [14] and >97% at <2.4 mg L−1 [13]. The high degradability of indomethacin by O3/UV was reported by Giri et al. [54]. Ethenzamide was degraded (≤LOD) at the ozone dose of 4 mg L−1 in 10 min in the O3/UV65W process, whereas it was not degraded in the control run, suggesting that O3 affected the photodegradation of ethenzamide. The results indicated ethenzamide is slightly sensitive to O3/UV65W. Ketoprofen was not degraded (≤LOD) in the control and in the O3/UV65W run with a dose as high as 6 mg L−1 in 10 min. The result suggested that ketoprofen is insensitive to UV65W and O3/UV65W. In the studies with ozone, ketoprofen was removed by 11% at the ozone dose of 2 mg L−1 [51], 73% at 3 mg L−1 [58], and up to 98% at 16.3 mg L−1 [13]. It seems that considerably high doses of UV and O3 are required to completely destroy ketoprofen in wastewater.

3.3. Antiarrhythmic Agents

Four antiarrhythmic agents (propranolol, disopyramide, atenolol, and metoprolol) were detected in the secondary effluent (Figure 4). These PPCPs were not degraded (≤LOD) in the control run. In the O3/UV65W process, propranolol was degraded (≤LOD) at the O3 dose of 2 mg L−1 in 10 min, showing it is relatively sensitive to O3/UV65W. Disopyramide required the O3 dose of 2–6 mg L−1 to be degraded (≤LOD), indicating it is slightly sensitive to O3/UV65W. On the other hand, atenolol and metoprolol required the O3 dose of 6 mg L−1 and the HRT of 10 min to be degraded (≤LOD), suggesting they are relatively insensitive to O3/UV65W.

3.4. Anticonvulsants

Carbamazepine and primidone are the only two anticonvulsants detected in the secondary effluent (Figure 5). Neither PPCP was degraded (≤LOD) in the control run. In the O3/UV65W process, carbamazepine was degraded (<LOD) at the ozone dose of 3 mg L−1 in 5 min, showing that O3 enhanced the photodegradation of carbamazepine. In the studies with ozone alone, carbamazepine was degraded by >98% at the ozone dose of 5 mg L−1 [14,57], >99% at 7 mg L−1 [53], and 99% at 4 and 8 mg L−1 [51]. It is noteworthy that Giri et al. [54] reported that the degradability of carbamazepine decreased in their O3/UV process as compared to the O3-only process due to a negative impact of UV. Primidone required the ozone dose of 6 mg L−1 and the HRT of 10 min to be degraded (≤LOD), indicating primidone is relatively insensitive to O3/UV65W.

3.5. Vasodilators

Two vasodilators (dipyridamole and diltiazem) were detected in the secondary effluent (Figure 6). Neither PPCP was degraded (≤LOD) in the control run. In the O3/UV65W process, dipyridamole and diltiazem were degraded (≤LOD) at the O3 dose of 1 mg L−1 in 5 min, showing that the photodegradation of these PPCPs was enhanced by O3.

3.6. Lipid-Modifying Agents

Two lipid-modifying agents (bezafibrate and clofibric acid) were detected (Figure 7).
Bezafibrate is relatively insensitive to O3/UV65W, requiring the O3 dose of 6 mg L−1 and the HRT of 10 min to be degraded (≤LOD). Bezafibrate was not degraded (≤LOD) in the control run. In the past studies with ozone alone, bezafibrate removal efficiencies of 49% and >99% were observed at the ozone doses of 2 and 5 mg L−1, respectively [21], up to 87% at 7.5 mg L−1 [52], and 97% at 16.3 mg L−1 [13]. Clofibric acid was degraded (≤LOD) at the ozone dose of 2 mg L−1 in 5 min in the O3/UV65W process. Note that clofibric acid was degraded (≤LOD) in the control run but it was detected at the level very close to its LOD in the effluent of the R1 reactor at the O3 dose of 1 mg L−1 in Run 1 (O3/UV65W run). In an ozone-alone study, clofibric acid was removed by 50% at the O3 dose of 5 mg L−1 [14]. Clofibric acid was shown to be relatively sensitive to UV and O3/UV65W.

3.7. Other PPCPs

The 10 remaining PPCPs found in the secondary effluent were crotamiton, sulpiride, N,N-diethyl-3-methylbenzamide (DEET), theophylline, furosemide, pirenzepine, ifenprodil, griseofulvin, chloramphenicol, and cyclophosphamide, which are an anti-itch drug, anti-psychotic drug, insect repellent, bronchodilator, diuretic, peptic ulcer drug, NMDA receptor antagonist, antifungal drug, antimicrobial drug, and antineoplastic agent, respectively (Figure 8). All of these PPCPs, except for chloramphenicol, were not degraded (≤LOD) in the control run. Cloramphenicol was degraded (≤LOD) in the control run and also in the O3/UV65W run at the ozone dose of 1 mg L−1 in 5 min, showing that chloramphenicol is very susceptible to UV and O3/UV65W. Furosemide was degraded (≤LOD) at the ozone dose of 2 mgL−1 in 10 min (R2 reactor), indicating that furosemide is relatively sensitive to O3/UV65W. Pirenzepine, ifenprodil, and griseofulvin required the O3 dose of 2–6 mgL−1 to be degraded (≤LOD), showing these PPCPs are slightly sensitive to O3/UV65W. Crotamiton and cyclophosphamide were degraded (≤LOD) at 6 mg L−1 in 10 min (R2 reactor), indicating that these compounds are relatively insensitive to O3/UV65W. On the other hand, sulpiride, DEET, and theophylline resisted to O3/UV65W. These compounds were not degraded (≤LOD) in 10 min at the ozone dose of 6 mgL−1, the largest dose applied in this study.

3.8. Relative Degradability

Figure 9 shows the relative degradability of 38 PPCPs detected in the secondary effluent. In this figure, the relative degradability was grouped on the basis of the compounds’ concentrations after the O3/UV65W treatment and their corresponding LOD, i.e., above or below their LOD. The shaded area indicates that the PPCP was degraded to or below its LOD (≤LOD). The degradability of the PPCPs was classified into five main groups: (i) “sensitive” to O3/UV65W or “unstable” in the O3/UV65W process, (ii) “relatively sensitive” to O3/UV65W or “relatively unstable” in the O3/UV65W process, (iii) “slightly sensitive” to O3/UV65W or “relatively stable” in the O3/UV65W process, iv) “relatively insensitive” to O3/UV65W or “stable” in the O3/UV65W process, and v) “insensitive” to O3/UV65W or “very stable” in the O3/UV65W process [30]. Of 38 PPCPs detected in the secondary effluent, 8 PPCPs were classified into the “sensitive/unstable” group, including dipyridamole, diclofenac, mefenamic acid, diltiazem, ciprofloxacin, chloramphenicol, naproxen, and isopropylantipyrine. Of the eight “sensitive/unstable” PPCPs, four (diclofenac, mefenamic acid, naproxen, and isopropylantipyrine) were analgesics, and two (dipyridamole and diltiazem) were vasodilators. These PPCPs were degraded (≤LOD) at the O3 dose of 1 mg L−1 in 5 min.
Furosemide, indomethacin, clofibric acid, and propranolol were classified into the group of “relatively sensitive” to O3/UV65W and “relatively unstable” in the O3/UV65W process. These PPCPs were degraded (≤LOD) at the O3 dose of 2 mg L−1 in 10 min. The PPCPs in the group of “relatively insensitive” to O3/UV65W and “stable” in the O3/UV65W process were crotamiton, bezafibrate, levofloxacin, erythromycin, atenolol, trimethoprim, primidone, metoprolol, and cyclophosphamide. These PPCPs required the ozone dose of 6 mg L−1 to be degraded (≤LOD). The PPCPs classified into the group of “insensitive” to O3/UV65W and “very stable” in the O3/UV65W process were clarithromycin, sulpiride, DEET, ketoprofen, theophylline, azithromycin, and roxithromycin. These PPCPs were not degraded (≤LOD) in 10 min at the ozone dose as high as 6 mg L−1.
The group of “slightly sensitive” to O3/UV65W or “relatively stable” in the O3/UV65W process included all the remaining PPCPs. These PPCPs required the ozone doses of 2–6 mg L−1 to be degraded (≤LOD).
Noting that the ozone dose of 2 mg L−1 occurred in the R2 reactor (HRT = 10 min) in Run 1 and also in the R1 reactor (HRT = 5 min) in Run 2 (Table 1), a comparison was made in terms of the effects of the reaction time (HRT) on the degradability of the PPCPs. At the same O3 dosage (2 mg L−1), sulfadimethoxine, lincomycin, ifenprodil, and nalidixic acid were not degraded in 5 min but were degraded (≤LOD) in 10 min, showing that sulfadimethoxine, lincomycin, ifenprodil, and nalidixic required a longer reaction time (>5 min).
The AOP treatment efficiency relies heavily on the chemical properties of contaminants and on the operating conditions [59]. The decomposition of PPCPs by AOPs depends on a number of factors, for example, the oxidant dosage, the concentrations of PPCPs, the mode of the AOP operation, and water quality. Natural organic compounds such as carbonate, bicarbonate, chlorine ions are known to act as radical scavengers. Because these compounds compete with target pollutants for hydroxyl radicals, their presence increases oxidant demands and lowers treatment efficiency [60]. Although the O3/UV65W treatment is a promising technology for the removal of a wide range of PPCPs from secondary effluent, further comprehensive understanding is needed in the areas of the reaction kinetics, decomposition pathways, formation of byproducts, and toxicity of wastewater treated by the O3/UV65W process.

4. Conclusions

The effects of O3 dose and reaction time on the O3/UV65W treatment process were investigated for the removal of PPCPs in the secondary effluent of a WWTP. The experimental variables were the injected O3 concentration (14, 28, and 42 mg L−1), the ozone dosage (1, 2, 3, 4, and 6 mg L−1), and the reaction time (HRT, 5 and 10 min). Among 38 PPCPs detected in the secondary effluent, 31 PPCPs were degraded to or below their LOD (≤LOD) at the O3 dosage of 6 mg L−1 in 10 min. Of these, eight PPCPs, including dipyridamole, diclofenac, mefenamic acid, diltiazem, ciprofloxacin, chloramphenicol, naproxen, and isopropylantipyrine, were readily degrade (≤LOD) at the O3 dose of 1 mg L−1 in 5 min, and seven PPCPs, including clarithromycin, sulpiride, DEET, ketoprofen, theophylline, azithromycin, and roxithromycin, were not degraded (≤LOD) at the ozone dosage of 6 mg L−1 with the HRT of 10 min.
The 38 PPCPs detected in the secondary effluent were classified into 5 groups according to their degradability: (i) “sensitive” to O3/UV65W or “unstable” in the O3/UV65W process; (ii) “relatively sensitive” to O3/UV65W or “relatively unstable” in the O3/UV65W process; (iii) “slightly sensitive” to O3/UV65W or “relatively stable” in the O3/UV65W process; (iv) “relatively insensitive” to O3/UV65W or “stable” in the O3/UV65W process; and (v) “insensitive” to O3/UV65W or “very stable” in the O3/UV65W process. The results from this study suggest that O3/UV65W treatment has the potential to remove a wide range of PPCPs from the secondary effluent of a WWTP.

Supplementary Materials

The following is available online at https://www.mdpi.com/2305-7084/3/2/53/s1, Table S1: Use and name of PPCPs detected in tested water.

Author Contributions

N.E.P. was responsible for analyzing data and writing the manuscript; I.K. for carrying out the experiments; H.T. and C.S. for advising, coordinating, and editing the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic diagram of the experimental process.
Figure 1. Schematic diagram of the experimental process.
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Figure 2. Concentrations of 11 antibiotics in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3. PPCP: personal care products.
Figure 2. Concentrations of 11 antibiotics in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3. PPCP: personal care products.
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Figure 3. Concentrations of seven analgesics in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 3. Concentrations of seven analgesics in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 4. Concentrations of four antiarrhythmic agents in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 4. Concentrations of four antiarrhythmic agents in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 5. Concentrations of two anticonvulsants in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mgL−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 5. Concentrations of two anticonvulsants in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mgL−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 6. Concentrations of two vasodilators in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 6. Concentrations of two vasodilators in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 7. Concentrations of two lipid-modifying agents in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 7. Concentrations of two lipid-modifying agents in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 8. Concentrations of 10 other PPCPs in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
Figure 8. Concentrations of 10 other PPCPs in the feed secondary effluent (Ro) and effluent of the R1 and R2 reactors at the O3 doses of: (a) 1 and 2 mg L−1, respectively, in Run 1; (b) 2 and 4 mg L−1, respectively, in Run 2; and (c) 3 and 6 mg L−1, respectively, in Run 3.
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Figure 9. Degradability of 38 PPCPs in the two flow-through O3/UV65W reactors in series (R1, R2).
Figure 9. Degradability of 38 PPCPs in the two flow-through O3/UV65W reactors in series (R1, R2).
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Table 1. Ozone dose applied to the O3/UV65W reactors.
Table 1. Ozone dose applied to the O3/UV65W reactors.
RunInjected O3 Concentration (mg L−1)O3 Dose per UV Reactor (mg L−1)
R1
HRT (R1) = 5 min
R2
HRT (R1 + R2) = 10 min
11412
22824
34236
R1: Reactor 1; R2: Reactor 2.
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