Occurrence and emission of polycyclic aromatic hydrocarbons from water treatment plant sludge in Taiwan

ABSTRACT The concentrations level and distribution of 16 US EPA polycyclic aromatic hydrocarbon (PAHs) from the water treatment plant (WTP), sewage treatment plant (STP), and industrial water treatment plant (ITP) sludge in Taiwan were determined and then assessed the sources, and potential toxicity (carcinogenic polycyclic aromatic hydrocarbons [CPAHs] and toxic BaP equivalent [TEQ]). Results indicated that the total concentrations of PAHs ranged between 58 and 16,436 μg/kg dw. Among the 17 samples, the 2-4 ring of total PAHs were the predominant compound in three kinds of treatment plant (> 60%). Especially, ITP1 owns 95.8% of 2-4 ring of total PAHs and ITP3 owns 54% of five- and six-ring of total PAHs. The molecular indices and principal component analysis (PCA) were used to determine the source contributions, with the results showing that the contributions of combustion/grass, coal or wood combustion and combustion/ liquid (oil) fossil fuel combustion. A PAH toxicity indicated by TEQ was 2.5–506 μg TEQ/g dw. Although, the results indicated that these were not recommended for land applications, but analyses are beneficial to develop effective management strategies for controlling PAH discharge in treatment plants and establishing strategies for its reuse in managing pollutants. GRAPHICAL ABSTRACT


Introduction
Surface water treatment for potable supplies typically involves coagulation, flocculation, sedimentation, and filtration processes for removing colloidal as well as suspended solids from raw water. The water treatment plants (WTPs) produce waste/residue known as water treatment sludge during the purification of raw water [1]. PAH attracted the high interest of researchers and the public in general due to their persistence, bioaccumulation, and toxicity [2]. The PAHs have been investigated in many kinds of environment, such as sediment, sludge, and solid. The United States Environmental Protection Agency (US EPA) recommended that 16 PAHs which were on the priority list as photomutagenicity pollutants, including naphthalene (NA), acenaphthylene (ACY), acenaphthene (ACE), fluorene h,i]perylene (BP), due to their genotoxic, mutagenic, and carcinogenic properties as well as the toxicity to organisms [3]. The 16 PAHs compounds were classified into two groups for their number of aromatic cycles, LPAH as low molecular weight PAHs with two or three aromatic cycles, and HPAH as high molecular weight PAHs with at least four aromatic rings [4,5]. The HPAH is more persistent in the environment and has greater carcinogenicity and more complexity in degradation compared with those of LPAH [6]. Researchers have discussed PAH's presence in the sludge [7,8]. For example, Khillare et al. [9] showed the mean concentration of the sum of 16 PAHs in the sludge from five sewage treatment plants in Delhi. Torretta and Katsoyiannis [10] investigated the different stages of a wastewater treatment plant, and the result showed that the pyrene was continuously the most abundant PAHs and an average ΣPAHs concentration ranged between 2405 ng/g dw in the secondary sludge and 2645 ng/g dw in the final sludge. Reyes-Contreras et al. [11] also presented the effect of the treatment plant process on thermal and ultrasound hydrolysis processes. Lukic et al. [12] discussed the application of the landfarming treatment from a naturally contaminated soil. The result shows that process performance was monitored through chemical, microbiological and ecotoxicological analyses during 105 days of incubation with greater potential for PAH biodegradation. In Japan, the PAHs loading of a domestic sewer system and discharges from an STP are investigated and concentration or loading variations are discussed [13]. Suciu et al. [14] investigated the PAHs content of sewage sludge in Europe and its use as soil fertilizer for the European Commission has been planning limits for organic pollutants in sewage sludge. Meng et al. [15] reviewed the PAHs in sewage sludge before 2015 and found that there was no state-level data on PAH pollution in sludge. Sun et al. [16] also investigated the concentrations of PAHs in sewage sludge and estimated in influents and effluents in China. The result indicates the concentrations of 16 PAHs in sludge ranged from 565 to 280,000 ng/g dw which was at a moderate level in the world. Otherwise, in some industrial areas, concerns have been raised by the researchers about the PAH present in sludge. In many industrialized countries the application of sewage sludge to agricultural land is a major route for disposal. Wild et al. [17] investigated the history (the year 1942-1984) of the PAH contaminants in agricultural soil. Wu et al. [18] also evaluated the amount, source, composition, and risk of PAH pollutants from sewage sludge in a coal-producing region. Kipopoulou et al. [19] investigated that the PAH content was determined in the inner tissue of various vegetable species and their growing environment (soil and atmosphere) in the greater industrial area of northern Greece. On the other hand, the method of removal in aged contaminated soils for high potential for PAHs was also investigated [20].
At present, the recycling rate of sludge in Taiwan was limited. No legislation has been implemented by the Taiwan government for PAHs content in sludge reused in agricultural land. This study selected 17 typical and representative wastewater treatment plants in Taiwan. We have mainly studied the contents and spatial distribution of 16 PAHs specified by US EPA in sludge from different sources, as well as the sources of pollutants and pollution characteristics. This article presents an analysis and discussion on the way of sludge resource utilization in combination with the basic properties of sludge. We believe that this study can provide a reference for the reasonable selection of sludge treatment and disposal measures and the formulation of related policies. Figure S1 displayed the sampling site with three different sources of sludge sample. The three kinds of sludge were collected as water treatment plant (WTP), sewage treatment plant (STP), and industrial water treatment plant (ITP) and point out the location distribution. The location, water treatment volume (m 3 /day), pH value, total organic carbon (TOC%), fixed solids (FS%), volatile solids (VS%), sewage treatment, sludge digestion method, and dehydration method were performed in our previous study [21]. The sewage treatment, sludge digestion, and water source were shown in Table S1. The design capacity ranged from 150,000 to 700,000 m 3 /day (WTP), 500 to 130,000 m 3 /day (STP), and 5000 to 90,000 m 3 /day (ITP). The activated sludge was the popular sewage treatment method in the wastewater treatment plant and industrial treatment plant. The sludge digestion method was aerobic, anaerobic, aerobic/anaerobic, and organic sludge. The wastewater received by the WTP is diverse and complex (Table S1), especially ITP, which may lead to variations in the concentration of polycyclic aromatic hydrocarbons in the sludge.

Sample preparation and determination of PAHs
Dewater sludge was collected from seventeen WTP in southern Taiwan. One gram of dried and homogenized sludge sample was placed in a clean glass test tube with a 5 mL acetone/n-hexane (1:1), and 10 µL surrogate standard mixture solutions (4 mg/L) was then added to it. The mixture was then vortexed for 1 min and extracted by ultrasonic treatment for 15 min for PAH extraction. The mixed sludge and organic phase were separated by centrifugation at 3000 rpm for 10 min. The organic layer containing the extracted compound was collected into another clean glass tube employing the Pasteur pipette, and the residual sludge was re-extracted twice with 1:1 (v/v) acetone/n-hexane to have a final volume of 5 mL. Activated copper was added to the extract for desulphurization. The extract was then dried over anhydrous sodium sulphate and concentrated to 0.5 mL using a gentle stream of nitrogen. An internal standard mixture (phenanthrene-d 10 , and chrysene-d 12 ) solution was added to the extract to be analysed using gas chromatography with mass selective detection (GC/MSD).

Instrumentation
The PAHs concentration was determined using gas chromatography-mass spectrometry (GC/MS) (Agilent 7890B GC, Agilent Technologies, Santa Clara, CA, U.S.A.) equipped with Agilent 5977A mass selective detector (MSD) and Agilent 7693A autosampler (Agilent Technologies, Santa Clara, CA, U.S.A.). The separation column was 30 m long and 0.25 mm in (HP-5MS capillary column, Hewlett-Packard, Palo Alto, CA, U.S.A.) and coated with 5% phenyl-methylsiloxane (0.25 m film thickness). Helium was the carrier gas at a flow rate of 1 mL/ min with the injection temperature of 280°C (5°C/min, hold for 3 min). The temperature programme was set initially at 40°C for 1 min, ramped to 120°C at a rate of 35°C/min, to 160°C at a rate of 10°C/min, and again to 280°C by the rate of 5°C/min. The temperature was then held at 280°C for 1 min. The GC/MS conditions were shown in Table S2. Moreover, Figure S2 was a GC-MS total ion chromatogram of 16 PAHs for standard and sample. The Fourier transform infrared microspectroscopy (μFTIR) (Nicolet™ iN10, Thermo Fisher Scientific, U.S.A.) was employed to determine the sludge parameter. The μFTIR parameters were as follows: spectral range: 4000-600 cm −1 , resolution: 4 cm −1 , measurement time: 51 s, number of co-scans: 256. The transmission mode or attenuated total reflection (ATR) mode.

Data analysis, pollution source analysis
For the data analysis, the statistical analyses were performed using SPSS software. The Spearman's correlation coefficients (r) were calculated to determine the relationships between the different pollutants and other parameters of the sludge samples. A similar analysis was also performed with the levels of risk, calculated for different PAHs in the sludge samples. Statistical analysis was performed with Microsoft Excel. For the pollution source analysis, the PCA was also using SPSS software to identify the possible sources of PAHs. The PCA is a measurable method that uses asymmetrical change to adjust an arrangement of perceptions of the potentially corresponded variable into an arrangement of estimations of direct uncorrelated factors called essential segments (PCs) [22]. The input variables were the 16 PAHs concentrations measured in six sludge samples. Varimax rotation with Kaiser normalization is used to identify the contribution of variables to the formation of the factorial axes. Only major principal components and significant value having factor loadings higher than 0.5 were extracted.

Potential ecological risk assessment
After analyzing the PAHs contents in the sludge samples, the data were compared with other WTPs abroad to determine the pollution level of the samples of the sludge. By analysing the spatial and temporal distribution of PAHs, and then combining the ratio method, the source of PAH pollution and the impact were determined. And the potential ecological risks of PAHs in sludge were also evaluated. Tsai et al. [23] mention that the toxicity was calculated as the summed products of individual PAH concentration. To assess the potential ecosystem risk of PAHs in the reuse of sludge for agriculture, PAHs levels were detected. The method for evaluating potential PAH toxicity in the sludge was based on the concentration of potentially carcinogenic PAHs (CPAHs; i.e. BaA, CH, BbF, BkF, BaP, IP, and DA). The potential toxicity of sludge was assessed using the total toxic BaP equivalent (TEQ carc ). The equation for calculating TEQ carc is as follows:
The concentrations of total PAHs in WTP sludge particulate ranged from 58 to 71 μg/kg dw (n = 3), with an average of 64 ± 6.6 μg/kg dw. The STP sludge concentration was ranged between 63 and 898 μg/kg dw (n = 8), with the average 396 ± 248 μg/kg dw. As for the ITP, the concentration was found as 282-16,436 μg/kg dw (n = 6), along with the average concentration 4165 ± 6494 μg/kg dw. The total average concentration of 16 PAHs from different sources can be aligned in the decreasing order as ITP (4165 ± 6494 μg/kg dw) > STP (396 ± 248 μg/kg dw) > WTP (64 ± 6.6 μg/kg dw). The ITP sludge ΣPAHs concentration was 65 and 10 times higher than the WTP and STP sludge, respectively ( Table 1). In comparison to the European Union suggested limit (6 mg/kg dw) for the sludge which is considered safe for agricultural application, all the PAHs content in all the samples of tested sludge was much lower than the above limit. The concentration of 16 PAHs in sewage sludge in Zhejiang, China, was 56.7 ± 18.5 mg/kg dw, which was found higher when compared with the PAHs level in our study [24].  (Table 1). In STP sludge, the STP5 has the highest ΣPAHs value, the reason may be that STP5 includes domestic sewage and polluted river water intercepting the Tainan Canal, and its water is subjected to industrial wastewater, surface runoff, and domestic sewage from the river bank. The ΣLPAHs/ΣHPAHs ratio was the important value to evaluate the PAH characteristic. For example, a ratio of ΣLPAHs/ΣHPAHs greater than one suggested petrogenic origin, whereas a ratio of less than one suggests the predominance of combustion-derived compounds. The ΣLPAHs/ΣHPAHs ratio ranged from 0.2 to 7.3. In the WTP, the ratio was 0.6 to 1.6. The STP group owns the slight fluctuation, 0.2-1.6. The ΣLPAHs/ΣHPAHs ratio is inconsistent especially ITP1 (ΣLPAHs/ΣHPAHs = 7.3) is greater than one and ITP 2-6 (ΣLPAHs/ΣHPAHs = 0.44, 0.00, 0.70, 0.86, and 0.14) is less than one ( Table 1). The ΣLPAHs/ΣHPAHs ratios were < 1 except for ITP 1. Figure 1(A) shows the box plot with sources and PAH rings. With the 2-and 3rings, the ITP group got the outliers with the element PH. The PH was present in dominant amounts in ITP, especially in the content of ITP1 (12,182 μg/kg dw) in industrial wastewater treatment plant sludge. The content of PH in sludge sample was fluctuated from 27 μg/kg dw (for ITP4) to 12,182 μg/kg dw (ITP1). The area of ITP1 was located in petrochemicals industrial park. Chang et al. [25] also mentioned that PH was present in a significant amount in petrochemical sludge. The trend of BaA, CH, FLU, and PY were the same. The pattern of 4-ring element was in the order PY > FLU > CH > BaA in ITP, STP, and WTP sludge irrespectively (Figure 1(B)). Figure 1(C) shows the 5-ring (BaP, BbF, BkF, and IP) PAH in the sludge. The mean concentration was shown at the same level. The outliers were shown in ITP, especially in BbF. The 6-ring was displayed in Figure 1(D). ITP 6 shows the high concentration in the BP. The BP was involved in industrial operations, such as production and processing of metals, paper and wood product processing and operations within the energy sector are important sources of PAHs. The source of the industries is listed in Table S2. Table 2 lists Spearman's correlation coefficients between analysis PAHs in the sludge. Spearman's correlation indicates that PAHs had a positive correlation except for NA and ACE. The 4-and 5-rings had a higher correlation and are significant at the 0.01 level. The 4-and 5-rings PAHs were from the same emission source. The ∑PAHs were higher (r > 0.9, p < 0.05) in FLU, PY, BaA, CH, and BbF which can find that PY is the indicator compound to ∑PAHs. As the source identification by PAHs diagnostic ratios, the FLU/PY ratio presented input characteristics. The result also shows the high positive correlation between ΣHPAHs. The improved correlations between ΣHPAHs were observed with the increased pattern within FLU, PY, BaA, CH, BbF, BkF, BaP, and IP.
Source apportionment of PAHs using isomeric and diagnostic ratios of PAHs to determine the PAH sources in environmental appreciation is the most commented method for determining which were introduced into the environment mainly via industrial discharge, fossil fuel combustion, petroleum spills, and automobile exhausts [26,27].   In ITP group, the FLU/(PY + FLU) value were large than > 0.33 vs. AN/(AN + PH) > 0.09. The ITP4 is also funded by an outlier of the ITP group. As Brandli et al. [28] mentioned that, the FLU/(PY + FLU) ratio at the same study was quite constant between the raw wastewater and the final sludge is 0.78. That different wastewater, with different pollutant compositions and sources, is mixed in a WTP, and that the final sludge is produced after mixing. Besides, the PH/AN index indicates that PH is more thermodynamically stable than AN. Due to their distinct physicochemical properties, they might behave differently in the environment with characteristic PH/ AN values for the identification of the PAH origin. Similarly, the FLU is less thermodynamically stable than its isomer pyrene; they often are associated with each other in natural matrices and a predominance of PLU over PY is characteristic of a pyrolytic process, whereas, in petroleum-derived PAHs, pyrene is more abundant than fluoranthene.
The determination of the probable source of PAHs in the environment, the PCA provided reliable results and has been widely applied for the determination of the probable source of PAHs in the environment. The result shows the PCA distribution of the normalized PAHs data of sludge PAHs. The two principal components selected can account for PC1 and PC2. The PCA analysis was conducted on PAH from the 17 WTPs (Figure 3(A)) and 6 industrial WTPs (Figure 3(B)). The results show that two principal components (PCs 1-2) were obtained to be identified to account for 93.6% of 17 sludge samples and 98.4% of 6 industrial sludge samples. The PC1 owns component loadings > 0.98 (BaA, BbF, IP, CH, BaP, and BP) which is different for PC2 with the component loadings > 0.98 of (AN, ACE, PH, DA, and FL). We also can observe that FLU and PY were in a similar pattern. Among the 17 sludge samples and 6 industrial sludge samples, the PC1 is highly weighted by PAHs with 5 to 6 rings. The observation was believed to be indicative of combustion sources.

Sludge potential toxicity based on carcinogenic (CPAHs and TEQ carc )
The concentration of ΣCPAHs (sum of 7 carcinogenic PAHs) varied in the range of 14 (WTP) to 3436 (ITP) μg/ kg dw ( Table 1). The percentage of ΣCPAHs accounts for about 5.1-52.1% of PAHs in sludge (Figure 4). In Figure 4, WTP1 and 2 show the same pattern. In the STP sludge sample, STP4 displayed the different compositions of 7 carcinogenic PAHs. The percentage of BaP in STP2 and STP4 was higher than other sources of the treatment plant. The phenomenon was also apparent in the investigation of Dai et al. that level of BaP from various sewage treatment plants ranging from 0 to 8 mg/kg [29]. Liu et al. [30] also investigated that BaP, the most ubiquitous PAHs found in sludge, can impact the composting processes of sewage sludge as well as the quality of compost produced. The toxic equivalency factors (TEFs) for PAHs were calculated to characterize more precisely the carcinogenic properties of PAH mixtures. The US EPA separated PAHs into two subclasses consisting of carcinogenic and noncarcinogenic compounds. The BaP was used as a reference compound (US EPA). The Soil Standard in the Netherlands rec-  [32]. Therefore, the comparison for sludge of our study (STP and ITP) to soil standard in the Netherlands was not recommended for land applications. To evaluate the reuse in agriculture, nine of sludge (STP and ITP) were exceeded the BaP (16 μg/kg) level. The ΣTEQ of sludge is between 2.5 (WTP) and 928 (ITP) μg/kg TEQ/kg dw. The six industry sludge samples displayed the ΣCPAH concentration from 20 to 3436 μg/g dw and TEQ carc of 2.5 to 525 μg TEQ/g dw. ITP 3 CH and BbF are the major contributors of the ΣCPAH concentration. Followed by ITP 1 of all the seven CPAHs, the BbF, and BDA. On the other hand, the TEQ carc values in this study were higher as ITP3 (525 μg  TEQ/g dw) and 1 (506 μg TEQ/g dw). The TEQ carc average of WTP was 2.9 μg/kg TEQ/kg dw, STP (60 μg/kg TEQ/kg dw), and ITP (259 μg/kg TEQ/kg dw). The ITP TEQ carc PAH concentration was much higher than WTP (89 times) and STP sludge (20 times) (Figure 4).

FTIR characteristic of sludge
FTIR spectra technique was used to qualitatively determine the WTP sludge. Figure 5 displayed the main absorbance in the FT-IR spectra of sewage sludge (WTP, STP, and ITP). The WTP pattern is different from STP and ITP  sludge. The result shows that the band of 1033 cm −1 is assigned to C-O stretching of polysaccharides or polysaccharide-like substances. In the characteristic carbohydrate region with maximum at 1056 cm −1 [33], the band of 1420 cm −1 was defined as lignin [34]. The position of amide II is exactly at 1538 cm −1 [34], and  the band of 1538 cm −1 was defended as protein origin. The band is found in nitrogen-rich composts but not found in composted sewage sludge samples [35]. The band of exposed β-strands was located at 1619 cm −1 was also observed [36]. The O-H stretching vibrations were observed at 3286 cm −1 [37,38]. For the PAHs compound, phenanthrene, the dominant vibrational activity is observed in a more compact region of 3045-3065 cm −1 , and perylene is quite compact and dominated by a group of bands located in 3056-3070 cm −1 with the most intense transition occurring at 3063.8 cm −1 [39]. The above-mentioned bands were in regards to the location which have been found in the samples of sludge.

Conclusions
The chemical analysis of 16 PAHs was evaluated in sludge samples. The results were displayed as the total concentrations of PAHs. The highest concentrations of PAHs were found in ITP 1. The diagnostic ratios and the results of PCA indicated that the PAHs in the sludge were mainly contributed from sources related to coal, transportation, and oil (ITP 4,5,6)/petroleum combustion (ITP 2,3). As compared with the US EPA and Soil Standard in the Netherlands, the STP and ITP sludge were not recommended for land applications.
The results of the ITP sludge study provide insight into its characteristics. These results are beneficial for developing effective management strategies for controlling PAHs discharge in treatment plants and establishing strategies for its reuse in managing pollutants.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The author(s) reported there is no funding associated with the work featured in this article.