Determination of Polycyclic Aromatic Hydrocarbons and Their Methylated Derivatives in Sewage Sludge from Northeastern China: Occurrence, Profiles and Toxicity Evaluation

This paper assesses the occurrence, distribution, source, and toxicity of polycyclic aromatic hydrocarbons (PAHs), and their methylated form (Me-PAHs) in sewage sludge from 10 WWTPs in Northeastern China was noted. The concentrations of ∑PAHs, ∑Me-PAHs ranged from 567 to 5040 and 48.1 to 479 ng.g−1dw, which is greater than the safety limit for sludge in agriculture in China. High and low molecular weight 4 and 2-ring PAHs and Me-PAHs in sludge were prevalent. The flux of sludge PAHs and Me-PAHs released from ten WWTPs, in Heilongjiang province, was calculated to be over 100 kg/year. Principal component analysis (PCA), diagnostic ratios and positive matrix factorization (PMF) determined a similar mixed pyrogenic and petrogenic source of sewage sludge. The average values of Benzo[a]pyrene was below the safe value of 600 ng.g−1 dependent on an incremental lifetime cancer risk ILCR of 10−6. Sludge is an important source for the transfer of pollutants into the environment, such as PAHs and Me-PAHs. Consequently, greater consideration should be given to its widespread occurrence.


Introduction
Polycyclic aromatic hydrocarbons represent a group of aromatic hydrocarbons with two or even more fused benzene rings, which were some of the main classes of organic hydrophobic pollutants. PAHs were recognized as the primary component responsible for the impacts of the organism [1]. Some PAHs have a great concern because of their carcinogenic effects, including BaA, BbF, BkF, BaP, IcdP, and BahA [2]. The International Agency for Research Cancer (IARC, 2010) has long recommended that benzo[a]pyrene a level 1 carcinogen, based on sufficient evidence in humans and animals [3]. Furthermore, PAHs are present in the environment; they have been discovered in fossil fuels, wildfires, natural vegetation, and volcanoes [4]. Methylated polycyclic aromatic hydrocarbons (Me-PAHs), one among PAHs derivative groups, are widely distributed in the environment, such as unsubstituted PAHs. Additionally, Me-PAHs such as methyl naphthalene (Me-Nap), methyl

Sample Collection
The sewage sludge samples were collected from 10 WWTPs along the Songhua River in the Northeast of China, Heilongjiang province. The detailed information of sampling sites was presented in Table S1. Ten sludge samples were collected starting from June, July until October. Six sampling sites were chosen in the most populated and industrial area of Harbin (W5, W6, W7, W8, W9, and W10), two sampling sites from Jiamusi Eastern and Jiamusi Western (W3 and W4) and two sampling sites from Qiqihar and Mudanjiang (W1 and W2). All samples were gathered from an anoxic/oxic (A/O) biological tank and sludge dewatering tank from WWTP. All samples were gathered (i) tacked and saved in aluminium containers, (ii) then freeze-dried, and (iii) finally stored in darkness under −20 • C. All samples were transmitted in the International Joint Research Center for Persistent Toxic Pollutants (IJRC-PTS), Harbin Institute of Technology (HIT), Harbin, China.

Chemicals and Reagents
High-performance liquid chromatography (HPLC) exhibiting grade-quality was used as a solvent in the experiments described in the present study. Dichloromethane (DCM), methanol (MeOH). Additionally, toluene was obtained from Fisher Scientific, Fair Lawn-New Jersey-USA. A Milli-Q system, Millipore-Billerica-MA, was used to prepare Pure reagent water (>18 MΩ-cm R).

Sample Pre-Treatment and Instrumental Analysis
Sludge samples were extracted by ultrasonic extraction. We carried out the following steps sample for pre-treatment instrumental analysis: (i) 0.5 g of sludge sample, 80% moisture content grinded with anhydrous Na 2 SO 4, (ii) 25 mL mixture solvent of MeOH-DCM (1:1 v/v) were added, (iii) then extracted for 30 min, (iv) and 5 min was centrifuged at 3000 rpm, (v) after that extraction has been repeated twice, (vi) and the extract was gathered into a flask, then (vii) approximately 1 mL of extract was concentrated, (viii) and re-dissolved to 500 mL purified water, finally (ix) the solution has been extracted by the solid-phase extract. HLB cartridges to (500 mg.6 cc −1 ) (Water Milford, MA, USA) were noticed of DCM with 5 mL and 5 mL of MeOH, accompanied by ultrapure water (5 mL) at a rate of approximately 1 mL min-1. After that, water samples (1 L) were loaded at a rate of about 5 mL.min −1 . Then, drying for 60 min with a gentle stream of N2, the SPE cartridges were fully clarified from the sorbent as follows: (i) into 15 mL tubes with 7 mL DCM and (ii) into 7 mL MeOH at a flow rate of 1 mL/min. A gentle stream of N2, around 1 mL, was used to extract the extracts, and the solvent was changed to 1 mL with toluene until being shifted to 1.5 mL. For sediment, ultrasonic extraction had been used. The sediment sample was dried and homogenized, whereas 3-5 g of MeOH-DCM (1:1 v/v) mixture solvent had been ultrasonically for 20 min extracted and for 5 min centrifuged at 3000 rpm, the supernatant then was picked, and the extraction was repeated several times.
The detection of PAHs and Me-PAHs was performed by use of Agilent 7890A-7000B gas chromatography-tandem triple-quadrupole mass spectrometry applied to an EI ion source (GC-EI-MS/MS) and (MRM) chromatogram. An agilent 19091J-433E (30 m × 250 µm × 0.25 µm) HP-5MS chromatographic column was employed in GC. All the parameters of transitions collision energy and retention time were listed in (Table S2) and (Figures S1 and S2).

Quality Assurance/Quality Control (QA/QC)
All of the data were established, including quality assurance (QA) as well as quality control (QC). A blank procedural and a matrix spike (20 ng/g dry weight with sludge samples and 100 ng/g for samples), and to verify the contaminants, an extra matrix spike was accurately checked, peak identification and measurement in each batch of 12 samples analyzed. Unchanging level of internal standard (100 ng/mL) with a sequence of injections of objective compounds at different concentrations was attained to figure out the system of a linear range. If each of the sample extracts reach the range, it would be diluted appropriately to get the reaction within the calibration range.

Positive Matrix Factorization and Statistical Analysis
The statistical analyses were performed using SPSS Statistic V25 software. The diagnostic ratio approach was utilized to determine potential sources of PAHs and Me-PAHs in several functional areas, and principal component analysis (PCA) was also chosen for discovering possible sources. Pearson correlation analysis was applied to check out the relationships between concentrations of PAHs and Me-PAHs in sewage sludge. The US EPA Positive matrix factorization (PMF) model V.5 software was utilized. PMF is a multivariate factor analysis tool that decomposes the data matrix into 2 matrices: factor contributions and factor profiles with a residual matrix [21]. The weighted sum of squares (Q) value is the difference between the data set input and PMF output [22], where the objective function (Q) is minimized in the PMF solution, as displayed in Equation (1).
whereas (f kj ) profiles a species of each source, (g ik ) is mass contributed by each factor to each sample, (p) is the number of factors, (i) is measured in the sample, and (U ij ) is the uncertainty estimate of the source (j). The uncertainty (U nc ) matrix values were calculated based on the PAHs method detection limits utilizing Equations (2) and (3) [23], as follows:

Health Risk Assessment
The potential cancer risk for PAHs was calculated in Equation (4) by multiplying the concentration of each compound by its corresponding TEF values. The total BaP equivalent concentrations (BaPeq) were calculated from the individual PAHs concentrations in each sample (Ci) and the toxicity equivalency factor (TEF) of target compounds from the flowing equation: where BaPeq is the carcinogenic potency of a congener evaluated based on BaP-equivalent concentration. TEF is the toxic equivalent factor provided [24].

Fluxes Calculations of PAHs and Me-PAHs
Daily and annual fluxes of PAHs and Me-PAHs discharged from WWTP sewage sludge were calculated using the following equation: where © represents the concentration of ∑Me-PAHs and ∑PAHs in the sludge ng.g −1 dw, (0.8) defines as the evaluated fraction of water in the sludge, and (M) indicates the quantity of sludge from the wastewater [16].

Occurrence of PAHs and Me-PAHs in Sludge
The concentrations of Me-PAHs and PAHs in sludge samples were summarized in Table 1. The concentrations of ∑PAHs varied from 567 to 5040 ng.g −1 dw, with an average 2030 ± 1340 ng.g −1 dw. ∑Me-PAHs varied from 48 to 479 ng.g −1 , with an average of 205 ± 139 ng.g −1 dw. Moreover, the concentrations in three sites (W9, W2, and W5), located in two large urban cities (Mudanjiang and Harbin), which were the very significant industrial centers along the Songhua River, were up to 2500 ng/g dw. At the same time, the lowest value was discovered in site W6 (625 ng.g −1 dw), which was situated in a relatively low-density urban area. The levels of PAHs were relatively lower at the less populated sampling sites than high populated sites, which indicated the exposure depend directly on human activity and populated area; vehicle exhaust was generally regarded as the essential pollution sources in the metropolitan region of China due to increasing vehicle usage and opposed atmospheric conditions. Appropriate composition modifications, domestic and industrial, could explain why the PAHs and Me-PAHs composition of sludge is different from one site to another.  (Table 1). The seven PAHs carcinogenic values vary from 213 to 1712 ng.g −1 dw, and the greatest concentration was detected in the sludge from site W5, which is located in Harbin city, the most populated and old industrial city [25]. Whereas the levels of ∑PAHscarc in this research were greater than five times than in Guangzhou, China, which was 201 to 308 ng.g −1 dw [26], and nine time than in Tunisia, which varied from 16.14 to 1366 ng.g −1 [14]. The outcome proved that the sludge was predominantly of urban origin in this research; this explains the higher PAHs content, which may contaminate sludge of industrial origin. Generally, PAHs concentration depends on the source position, including such industrialized areas or very highly populated regions [27]. In addition, 16 PAHs were lower in this study than in Zhuhai, China, which was between 53 and 25000 [9]. The characteristics of Me-PAHs, as well as PAHs, have been further investigated utilizing the statistical method, in particular, the correlation analysis ( Figure S3). There was a significant positive weak correlation appeared between ∑Me-PAHs and ∑PAHs was noticed among the sampling sites, suggesting that similar emission sources were identified for these two groups (R 2 = 0.02, p <0.01). Moreover, organic carbon and temperature were detected during the data measurement to explore their influence on the concentration of PAHs and Me-PAHs. The temperature varied from 8.7 to 24.8 • C, and the plot was comparing temperature and PAHs; Me-PAHs are displayed in Figure 1. Weak influences of temperature between PAHs, Me-PAHs and temperature were observed (R 2 = 0.16, p < 0.01) as well as (R 2 = 0.14, p < 0.01). Temperature is the main factor impacting the level of PAHs in sludge; previously, research stated that with an increase in temperature and increased PAHs removal rate in sludge. Meanwhile, the weak correlation observed with temperature suggested that the adsorption of PAHs increases at very low temperatures [28]. Moreover, as shown in Figure 1, positive weak linear correlations between the TOC and concentration of PAHs, Me-PAHs were also observed (R 2 = 0.14, p < 0.01) as well as (R 2 = 0.18, p < 0.01), indicating that TOC had a significant weak influence on PAH's and Me-PAH's distribution in sludge. A similar result showed that total organic carbon is one of the most important factors in detecting PAH sorption and immobilization [4].

Composition Profile
The composition profiles of individual and ring-numbers of PAHs and Me-PAHs in sludge over the sampling sites are presented in (Figures 2 and S3). It was shown that the most abundant PAHs were Phe (27%) in W10, followed by Nap (24%) in W9, and Fluo (16%) in W5, based on overall average samples. The same results were observed that Phe was Principally generated from fuel oil, diesel, gasoline, particularly petroleum products. The higher concentrations of Nap and Phe could, therefore, be attributed to industrial

Composition Profile
The composition profiles of individual and ring-numbers of PAHs and Me-PAHs in sludge over the sampling sites are presented in (Figure 2 and Figure S3). It was shown that the most abundant PAHs were Phe (27%) in W10, followed by Nap (24%) in W9, and Fluo (16%) in W5, based on overall average samples. The same results were observed that Phe was Principally generated from fuel oil, diesel, gasoline, particularly petroleum products. The higher concentrations of Nap and Phe could, therefore, be attributed to industrial wastewaters [29]. Other research stated that low molecular weight compounds (e.g., Phe, NaP) discovered in this research was found in manufacturing areas as well as wastewater from household [14]. For Me-PAHs, the most abundant were 9-MANT (20%) W10, followed by 2-MNAP (16%), W9, and 5, 8-DMBcPH (11%) W6, as shown in Figure S3. Additionally, researchers reported benzene ring numbers were used to determine PAH physical and chemical characteristics [4]. According to the number of benzene rings which given PAHs contains, the Me-PAHs and PAHs were classified into (6-ring, 5-ring, 4-ring, 3-ring, and 2-ring) Figure 2. We found the PAHs homologs in sludge, rings number HMW PAHs (4-ring) were much dominated accounted for (42.5%) at site W5, followed by 3-ring (41.1%) at site W10, 2-ring (25.6%) at site W9, 5-ring (20.8) at site W3, and 6-ring (20.1%) at site W3, respectively, Figure 2. For Me-PAHs, low molecular weight PAHs (2-ring) were much dominated accounted for (55%) at site W1, followed by 3-ring (46.9%) at site W10, 4-ring (42.9%) at site W6, and 5-ring (3.2%), at site W10. Generally, the PAHs and Me-PAHs of sewage sludge in this research were dominated by HMW and LMW 4 and 2-ring; HMW PAHs occupied higher percentages; his might result in very highly hydrophobic characteristics of high molecular weight PAHs [3], whereas low molecular weight Me-PAHs may characterize by higher resistance to microbial decomposition and greater susceptibility to biological degradation [4]. This was in contrast with the previous finding as follows: sewage sludge from Tunisia was reported to be dominated by 4-ring [14]; similarly, in sewage sludge from Korea, high molecular weight 4-ring such as pyrene was the most abundant [30]. Four-ring PAHs were reported to be the most abundant compounds and accounted for 86% of ∑PAHs from Guangdong Province, China [31], and four-ring PAHs were prevalent in sewage sludge found in southwestern Taiwan, China [11].
3-ring, and 2-ring) Figure 2. We found the PAHs homologs in sludge, rings number HMW PAHs (4-ring) were much dominated accounted for (42.5%) at site W5, followed by 3-ring (41.1%) at site W10, 2-ring (25.6%) at site W9, 5-ring (20.8) at site W3, and 6-ring (20.1%) at site W3, respectively, Figure 2. For Me-PAHs, low molecular weight PAHs (2-ring) were much dominated accounted for (55%) at site W1, followed by 3-ring (46.9%) at site W10, 4-ring (42.9%) at site W6, and 5-ring (3.2%), at site W10. Generally, the PAHs and Me-PAHs of sewage sludge in this research were dominated by HMW and LMW 4 and 2-ring; HMW PAHs occupied higher percentages; 7his might result in very highly hydrophobic characteristics of high molecular weight PAHs [3], whereas low molecular weight Me-PAHs may characterize by higher resistance to microbial decomposition and greater susceptibility to biological degradation [4]. This was in contrast with the previous finding as follows: sewage sludge from Tunisia was reported to be dominated by 4-ring [14]; similarly, in sewage sludge from Korea, high molecular weight 4-ring such as pyrene was the most abundant [30]. Four-ring PAHs were reported to be the most abundant compounds and accounted for 86% of ∑PAHs from Guangdong Province, China [31], and four-ring PAHs were prevalent in sewage sludge found in southwestern Taiwan, China [11].

PAHs and Me-PAHs in Sludge Worldwide
The comparison of the PAHs and Me-PAHs mean concentrations in sludge discovered from other researches around the world was shown in Table 2. The mean concentration of PAHs in this study was 2030 . dw, much greater than four sludge sites in Taiwan China (750 ng/g dw) [11], twelves sewage sludge from Beijing, China (1551 ng/g dw) [29], nineteen sewage sludge from Guangdong Province China (1276 ng/g dw) [7], but lower than the level in four sewage sludge from Harbin, Northeast China (8200 ng/g) [32], six sludge from Guangdong, China (3466 ng/g dw) [33], six Wastewater sludge from Korea (10,400 ng/g dw) [30], nineteen sludge from Madrid (5118 ng/g dw) [34], nine sewage sludge from northern and central Tunisia (11,216 ng/g dw) [14], and eleven

PAHs and Me-PAHs in Sludge Worldwide
The comparison of the PAHs and Me-PAHs mean concentrations in sludge discovered from other researches around the world was shown in Table 2. The mean concentration of PAHs in this study was 2030 ng.g −1 dw, much greater than four sludge sites in Taiwan China (750 ng/g dw) [11], twelves sewage sludge from Beijing, China (1551 ng/g dw) [29], nineteen sewage sludge from Guangdong Province China (1276 ng/g dw) [7], but lower than the level in four sewage sludge from Harbin, Northeast China (8200 ng/g) [32], six sludge from Guangdong, China (3466 ng/g dw) [33], six Wastewater sludge from Korea (10,400 ng/g dw) [30], nineteen sludge from Madrid (5118 ng/g dw) [34], nine sewage sludge from northern and central Tunisia (11,216 ng/g dw) [14], and eleven sewage sludge from the mainland and Hong Kong, China (30,000 ng/g dw) [9]. However, the level in this study was very close to three sewage sludge from Paris (2518 ng/g dw) [35]. The European Union has proposed a limited fixed value of ten PAHs of 6000 ng.g −1 for the implementation of sludge to agricultural fertilizer that includes Flua, BaP, IcdP, Ace, BghiP, BkF, Pyr, Phe, Flu, and BbF [29]. In our study, the values of 10 PAHs did not exceed the maximum limit. Therefore, these outcomes were at relatively low risk related to the European Union, with their agricultural land use. Nevertheless, corresponding to the disposal of sludge used as agricultural in China, from municipal wastewater treatment plants (5000 ng/g dw) [16], the concentration of ∑PAHs in this research was greater than the upper safety limit, with a comparatively high risk.

The PAHs and Me-PAHs Loads from Sewage Sludge Discharged from 10 WWTPs
Production of sludge in China was growing at an annual average rate of (13%), which was estimated at over 3000 tons of sludge in 2013 [36]. The estimated fluxes from the 10 WWTPs are presented in Table S4. The PAHs and Me-PAHs daily fluxes in the ten sewage sludge sites were ranged from 18.8 to 300 g/d and from 90 to 425 g/d, respectively. The annual quantity of flux varied from 6.99 to 111 kg/year and 33.3 to 157 Kg/year, respectively. Among sampling sites, W5 was dominated in both PAHs and Me-PAHs. The individual flux decreased in the followed orders: Fluo > Phe > Pyr > BbF, and 9-MANT > 2-MNAP > 7,9-MBaA > 1,6-DMNAP, respectively. The total flux in this study was much higher than the flux calculated from five sludge in Paris, France, which ranged from 0.53 to 316 g/d [37]. Moreover, more incredible than the quantity of released sludge from 15 WWTP in the Shanxi province ranged from 0.2 to 22.7 kg·year −1 [16]. Regarding the evaluation of the daily and yearly quantity of PAHs and Me-PAHs fluxes, sludge displayed the highest contamination level from the industrial and domestic areas in Northeast China compared to other quantities of flux worldwide.

Source Apportionment by Diagnostic Ratios
Several concentration ratios of various parent PAHs compounds are commonly used to assess the sources of PAHs [27]. In the present study, the following diagnostic ratios were selected to assess PAHs and Me-PAHs source: BaA/(BaA + Chr) vs. (MPhe/Phe) and Flu/(Flu +Pyr) vs. InP/(InP + BghiP) (Figure 4). Since (PAHs) are released into the environment from a variety of sources, and their profiles can change due to their reactions, these ratios must be assessed with caution [21]. The result indicated that ratios of BaA/(BaA + Chr) vs. (MPhe/Phe) were ranged from 0.20 to 0.35, 0.35 to 0.50, and 0.00 to 0.008, respectively, indicated that dominated of pyrogenic origin was a primary source in the sewage sludge (Figure 4a), and diagnostic ratios of Flu/(Flu +Pyr) vs. InP/(InP + BghiP) were varied from 0.42 to 0.50, 0.50 to 0.58, 0.40 to 0.50 and 0.50 to 0.75, respectively, the PAHs at second group sites, as shown in (Figure 4b) were the highly petrogenic source in

Source Apportionment by Diagnostic Ratios
Several concentration ratios of various parent PAHs compounds are commonly used to assess the sources of PAHs [27]. In the present study, the following diagnostic ratios were selected to assess PAHs and Me-PAHs source: BaA/(BaA + Chr) vs. (MPhe/Phe) and Flu/(Flu +Pyr) vs. InP/(InP + BghiP) (Figure 4). Since (PAHs) are released into the environment from a variety of sources, and their profiles can change due to their reactions, these ratios must be assessed with caution [21]. The result indicated that ratios of BaA/(BaA + Chr) vs. (MPhe/Phe) were ranged from 0.20 to 0.35, 0.35 to 0.50, and 0.00 to 0.008, respectively, indicated that dominated of pyrogenic origin was a primary source in the sewage sludge (Figure 4a), and diagnostic ratios of Flu/(Flu +Pyr) vs. InP/(InP + BghiP) were varied from 0.42 to 0.50, 0.50 to 0.58, 0.40 to 0.50 and 0.50 to 0.75, respectively, the PAHs at second group sites, as shown in (Figure 4b) were the highly petrogenic source in the sewage sludge. According to diagnostic ratios, it was proved that the sources of PAHs and Me-PAHs in sewage sludge were pyrogenic and petrogenic. the sewage sludge. According to diagnostic ratios, it was proved that the sources of PAHs and Me-PAHs in sewage sludge were pyrogenic and petrogenic.

Toxicity Evaluation and Risk Assessment in Sewage Sludge
To evaluate the PAHs carcinogenic potency in sludge from 10 WWTPs, (BaPeq) benzo [a] pyrene equivalent concentrations were estimated by (TEFs) benzo [a] pyrene toxic equivalency factors. The U.S. EPA has suggested seven carcinogenic Polycyclic aromatic hydrocarbons, including (B(a)P, Chr, IcdP, B(a)A, BghiP, B(k)F, and B(b)F). The BaPeq concentrations of ∑16 PAHs in sludge were dominant in W5 with a total of 406 and meant concentration 25 ng/g, followed by W3, with a total of 197 and mean concentration 12.3 ng/g and W2 with a total 185 and mean concentration 11.6 ng/g, respectively. The PAH carcinogenic potencies, as well as toxic equivalency factors which are detected in sewage sludge, were displayed in Table 3, individual PAHs, the concentration of (BaPeq) toxic equivalency factor, values arranged in the following descending order: BaP ˃ DahA ˃ BbF ˃ BaA in all the 10 WWTPs sewage sludge, which were more abundant than other compounds. Sludge used in France as agricultural fertilizer is only allowed when: BaP, BbF, and Flu are below 2000 . , 2500 . , 5000 . dw, respectively, regarding the USA legislation is 4600 . dw, calculated as the sum of (Ant, Chr, DahA, BkF, IcdP BaP, BkF, and BaA), but BaP concentration was not allowed to be greater than 1000 . [32]. In the Canadian soil quality guideline, dependent on (incremental lifetime cancer risk ILCR of 10 −6 ), a safe benzo[a]pyrene equivalent value is 600 . , for seven carcinogenic polycyclic aromatic hydrocarbons, such as BbF, Chr, BaP, BkF, InP, DBA, and BaA [31], while the concentrations of benzo[a]pyrene equivalent to seven carcinogenic PAHs in sewage sludge among 10 WWTP in this study ranged from 52.3 to 399 .
, with a mean concentration of 144 . dw. The result proved that the concentrations in 10 WWTP sludge samples were below the safe value of 600 . (dependent on ILCR of 10 −6 ).

Toxicity Evaluation and Risk Assessment in Sewage Sludge
To evaluate the PAHs carcinogenic potency in sludge from 10 WWTPs, (BaPeq) benzo [a] pyrene equivalent concentrations were estimated by (TEFs) benzo [a] pyrene toxic equivalency factors. The U.S. EPA has suggested seven carcinogenic Polycyclic aromatic hydrocarbons, including (B(a)P, Chr, IcdP, B(a)A, BghiP, B(k)F, and B(b)F). The BaPeq concentrations of ∑16 PAHs in sludge were dominant in W5 with a total of 406 and meant concentration 25 ng/g, followed by W3, with a total of 197 and mean concentration 12.3 ng/g and W2 with a total 185 and mean concentration 11.6 ng/g, respectively. The PAH carcinogenic potencies, as well as toxic equivalency factors which are detected in sewage sludge, were displayed in Table 3, individual PAHs, the concentration of (BaP eq ) toxic equivalency factor, values arranged in the following descending order: BaP > DahA > BbF > BaA in all the 10 WWTPs sewage sludge, which were more abundant than other compounds. Sludge used in France as agricultural fertilizer is only allowed when: BaP, BbF, and Flu are below 2000 ng.g −1 , 2500 ng.g −1 , 5000 ng.g −1 dw, respectively, regarding the USA legislation is 4600 ng.g −1 dw, calculated as the sum of (Ant, Chr, DahA, BkF, IcdP BaP, BkF, and BaA), but BaP concentration was not allowed to be greater than 1000 ng.g −1 [32]. In the Canadian soil quality guideline, dependent on (incremental lifetime cancer risk ILCR of 10 −6 ), a safe benzo[a]pyrene equivalent value is 600 ng.g −1 , for seven carcinogenic polycyclic aromatic hydrocarbons, such as BbF, Chr, BaP, BkF, InP, DBA, and BaA [31], while the concentrations of benzo[a]pyrene equivalent to seven carcinogenic PAHs in sewage sludge among 10 WWTP in this study ranged from 52.3 to 399 ng.g −1 , with a mean concentration of 144 ng.g −1 dw. The result proved that the concentrations in 10 WWTP sludge samples were below the safe value of 600 ng.g −1 (dependent on ILCR of 10 −6 ).

Conclusion
In this research, the level of PAHs and Me-PAHs were investigated in sewage sludge. The most abundant PAHs compounds in sludge were Phenanthrene and Naphthalene, while Me-PAHs were 9-Methylanthracene and 2-Methylnaphthalene. The flux of sludge discharged from the 10 WWTPs, were estimated to be greater than 100 kg·year −1 . The sources were analyzed by positive matrix factorization, principal component analysis, and diagnostic ratios, were identified similar source, analytical results of PMF explained 4 Factor, PAHs source includes, gasoline and diesel fuel (35%), coke production (33%), wood and biomass (19%) and average diesel and natural gas combustion, fore Me-PAHs, wood combustion (34%), coke production (28%) wood-burning stoves (22%) and average diesel fuel vehicle (16%). Temperature and total organic carbon were reported to have significant weak PAHs and Me-PAHs influencing distribution in sludge. The occurrence of PAHs in sludge confirms that there is a high risk for sludge applied to agriculture, according to China's safety limit. Sludge acts as an important source for the transfer of pollutants such as PAHs and Me-PAHs into the environment; therefore, greater attention should be paid to its occurrence and distribution.