3.1. Level of HMs in 32 Sewage Sludge Samples
The mean concentrations of Zn, Cu, Cr, Pb, As, Hg and Cd were in the following decreasing order: Zn > Cu > Cr > Pb > As > Hg > Cd, and mean value of each HMs individually was 281.31 mg/kg, 162.59 mg/kg, 152.26 mg/kg, 39.56 mg/kg, 14.83 mg/kg, 2.09 mg/kg and 1.97 mg/kg in
Table 3. Zn content was the most, while the Cd content was the least.
Because samples were from 32 WWTPs located in different regions, HM concentrations in sewage sludge all varied greatly. The concentration of Zn, Cr, Cu, Pb, Cd, As and Hg varied individually from 52.27 to 1613.50 mg/kg, 33.29 to 665.31 mg/kg, 56.15 to 520.53 mg/kg, 13.43 to 89.08 mg/kg, 0.33 to 17.23 mg/kg, 7.87 to 29.15 mg/kg and 0.15 to 7.99 mg/kg. It can be sorted by standard deviation in the following decreasing order: Zn > Cr > Cu > Pb > Cd > As > Hg (
Table 3). The values of standard deviation of Zn, Cr and Cu were much greater than that of Pb, Cd, As and Hg. This can be attributed to the variable anthropogenic sources in studying area [
42].
Table 3.
Statistical analysis of heavy metal (HM) concentrations (mg/kg).
Table 3.
Statistical analysis of heavy metal (HM) concentrations (mg/kg).
Metals | Minimum | Maximum | Mean | Range | Std. dev. | GB18918-2002 a |
---|
pH≥6.5 | pH<6.5 |
---|
Cu | 56.15 | 520.53 | 162.59 | 464.38 | 105.80 | 1500 | 800 |
Zn | 52.27 | 1613.50 | 281.31 | 1561.23 | 336.66 | 3000 | 2000 |
As | 7.87 | 29.15 | 14.83 | 21.27 | 4.96 | 75 | 75 |
Hg | 0.15 | 7.99 | 2.09 | 7.83 | 1.95 | 15 | 5 |
Pb | 13.43 | 89.08 | 39.56 | 75.66 | 19.81 | 1000 | 300 |
Cd | 0.33 | 17.23 | 1.97 | 16.90 | 3.55 | 20 | 5 |
Cr | 33.29 | 665.31 | 152.26 | 632.02 | 131.65 | 1000 | 600 |
Comparing with the threshold values of Chinese Discharge Standard of Pollutants for Municipal WWTP (GB 18918-2002) for agricultural use in
Table 3, the maximum concentrations of HMs were all within the content limits permitted by discharge standard.
Comparing HM contents in studying area with other regions in China (
Table 4) [
3,
4,
43], Cu, Zn, Hg, Pb, Cd and Cr were relatively lower and only As was practically equal to the value in north of China. The result indicated that HM pollution of sewage sludge in Shanxi was not as severe as other regions in China. Because of the wider range of sampling compared to other studies, ranges of HM concentrations in this study are all higher than other surveys on Shanxi [
1,
2,
3].
Table 4.
HM contents in different regions of China (mg/kg).
Table 4.
HM contents in different regions of China (mg/kg).
Region | Cu | Zn | As | Hg | Pb | Cd | Cr |
---|
North | 535.95 | 1220.88 | 14.20 | 5.81 | 113.11 | 7.33 | 242.71 |
South | 530.96 | 1301.63 | 18.03 | 2.99 | 116.22 | 7.09 | 208.70 |
East | 671.87 | 1446.23 | 15.94 | 3.92 | 140.47 | 7.32 | 209.36 |
West | 249.25 | 717.55 | 20.40 | 3.80 | 80.63 | 7.55 | 161.40 |
Mean values of Shanxi | 162.59 | 281.31 | 14.83 | 2.09 | 39.56 | 1.97 | 152.26 |
3.2. Contamination of HMs in Sewage Sludge, Shanxi
Contaminations of HMs in sewage sludge of 32 WWTPs were classified from class 0 to 5 according to
Igeo (
Table 5). Results depicted that Cu and Hg pollution were the highest; Cd and Cr pollution were moderately; Zn, As and Pb pollution were the least. Overall, HM pollution in sewage sludge in Shanxi was generally not significant. To better understand HM contamination in each site, type of process were presented in
Table 5.
Although HM pollution was overall not serious, it was notable in some samples. Hg pollution class varied mostly ranging from class 0 to 4. For Hg, 40.63% samples were in class 0 and class 1; 25% samples were in class 2 and 34.37% samples were in class 3 to 4. Cu was another serious contaminated metal: 15% samples were in class 0 and 1, while the others were all in class 2 to 3. It was noting that contamination of Cu and Hg generally synchronously varied with the severest pollution of Cu and Hg at the same sites WWPT 8, 10, 11, 18, 25, 29, 30, 31 and 32. This phenomenon suggested that Cu and Hg might have the same anthropogenic sources.
Table 5.
Classes of contamination by Igeo of sewage sludge for different WWTPs in Shanxi.
Table 5.
Classes of contamination by Igeo of sewage sludge for different WWTPs in Shanxi.
WWTP | Cu | Zn | As | Hg | Pb | Cd | Cr | Type of Process |
---|
1 | 0 | 2 | 0 | 1 | 0 | 1 | 1 | CarrouselOxidationditch |
2 | 0 | 2 | 0 | 1 | 0 | 1 | 1 | CarrouselOxidationditch |
3 | 0 | 2 | 0 | 0 | 0 | 0 | 1 | Anaerobic/Anoxic/Oxic |
4 | 0 | 1 | 1 | 0 | 0 | 1 | 2 | CarrouselOxidationditch |
5 | 0 | 1 | 0 | 0 | 0 | 0 | 2 | SequencingBatchReactor |
6 | 2 | 3 | 0 | 0 | 0 | 1 | 2 | Anaerobic/Anoxic/Oxic |
7 | 0 | 1 | 0 | 0 | 0 | 1 | 1 | Anaerobic-Oxicprocess |
8 | 2 | 0 | 0 | 3 | 1 | 0 | 1 | CarrouselOxidationditch |
9 | 1 | 0 | 0 | 2 | 0 | 0 | 0 | Activatedsludgeprocess |
10 | 2 | 0 | 0 | 3 | 1 | 1 | 1 | Anaerobic/Anoxic/Oxic |
11 | 2 | 0 | 0 | 3 | 0 | 1 | 0 | Activatedsludgeprocess |
12 | 2 | 0 | 0 | 2 | 1 | 1 | 0 | Activatedsludgeprocess |
13 | 2 | 0 | 1 | 2 | 0 | 0 | 1 | Biological contact oxidation process |
14 | 1 | 0 | 0 | 4 | 0 | 0 | 0 | SequencingBatchReactor |
15 | 2 | 0 | 0 | 3 | 0 | 1 | 3 | Anaerobic/Anoxic/Oxic |
16 | 2 | 0 | 0 | 3 | 0 | 5 | 0 | Biological contact oxidation process |
17 | 1 | 0 | 0 | 4 | 0 | 0 | 0 | Biological contact oxidation process |
18 | 2 | 0 | 0 | 3 | 0 | 1 | 0 | Activatedsludgeprocess |
19 | 2 | 0 | 0 | 2 | 1 | 1 | 0 | Anaerobic/Anoxic/Oxic |
20 | 2 | 0 | 0 | 2 | 0 | 0 | 0 | Anaerobic/Anoxic/Oxic |
21 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | Anaerobic-Oxicprocess |
22 | 1 | 3 | 1 | 1 | 0 | 5 | 1 | Anaerobic/Anoxic/Oxic |
23 | 1 | 2 | 0 | 0 | 0 | 2 | 1 | Biological contact oxidation process |
24 | 1 | 4 | 1 | 2 | 1 | 4 | 1 | Anaerobic/Anoxic/Oxic |
25 | 2 | 0 | 0 | 4 | 0 | 2 | 0 | Biomembrane process |
26 | 1 | 2 | 0 | 0 | 0 | 2 | 2 | CarrouselOxidationditch |
27 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | Biomembrane process |
28 | 1 | 2 | 0 | 0 | 0 | 1 | 2 | Anaerobic-Oxicprocess |
29 | 3 | 0 | 0 | 4 | 1 | 2 | 1 | Biomembrane process |
30 | 3 | 0 | 1 | 4 | 1 | 2 | 3 | Anaerobic-Oxicprocess |
31 | 2 | 0 | 0 | 2 | 1 | 1 | 0 | Biomembrane process |
32 | 3 | 0 | 0 | 2 | 1 | 2 | 1 | Biomembrane process |
Mean | 1.38 | 0.78 | 0.16 | 1.81 | 0.28 | 1.22 | 0.91 | — |
Class contamination of Cd ranged from class 0 to 5; 71.88% samples were in class 0 and 1. Only one was in class 4 and only two were in class 5, the rest 18.75% were all in class 2. It indicated that most samples were free of Cd contamination, and the heavily contaminated ones as WWTP 16, 22 and 24 were polluted by special conditions in these places. The contamination of Zn: 71.88% samples were in class 0 and 1; 18.75% were in class 2 and 9.38% were in class 3 and 4. Zn had the same variation as Cd with the most seriously polluted samples in WWPT 22 and 24. This pollution may be due to the zinc smelting industry nearby WWPT 22 and 24. For Cr, 6.25% samples were classified into class 3, 15.63% were in class 2 and the rest 78.12% were all in class 0 and 1. The pollution of Cr was moderately. Pb was almost unpolluted, 84.38% samples were in class 0, and the rest were all in class 1.
As in other regions in China, HM pollution in sewage sludge was not significant [
1,
2,
3]. However, Hg pollution was higher in this study. That might be caused by typical industries which consume many coals distributed in Shanxi [
15,
22].
3.4. Potential Sources of HMs
The corresponding factors and loadings of variance were presented in
Table 7 and
Table 8. Three significant factors whose initial eigenvalues were greater than one had been obtained through PCA analysis, three factors explained about 80.9% of the total variance which were distinguished in the data set [
43].
Table 7.
Total variance and component matrixes for HMs in sewage sludge.
Table 7.
Total variance and component matrixes for HMs in sewage sludge.
Component | Initial Eigenvalues | Extraction Sumsof Squared Loadings | Rotation Sums of Loadings |
---|
Total | % of Variance | Cumulative% | Total | % of Variance | Cumulative% | Total | % of Variance | Cumulative% |
---|
1 | 2.57 | 36.66 | 36.66 | 2.57 | 36.66 | 36.66 | 2.50 | 35.65 | 35.65 |
2 | 2.04 | 29.17 | 65.82 | 2.04 | 29.17 | 65.82 | 2.03 | 29.04 | 64.69 |
3 | 1.05 | 15.06 | 80.88 | 1.05 | 15.06 | 80.88 | 1.13 | 16.18 | 80.88 |
Table 8.
Factor loading matrix of component matrix and rotated component matrix.
Table 8.
Factor loading matrix of component matrix and rotated component matrix.
Element | Component Matrix | Rotated Component Matrix |
---|
Factor 1 | Factor 2 | Factor 3 | Factor 1 | Factor 2 | Factor 3 |
---|
Cu | 0.897 | –0.117 | 0.055 | 0.874 | –0.051 | 0.237 |
Zn | –0.235 | 0.840 | 0.099 | –0.333 | 0.803 | 0.120 |
As | 0.347 | 0.786 | 0.088 | 0.241 | 0.796 | 0.232 |
Hg | 0.888 | –0.201 | –0.065 | 0.898 | –0.122 | 0.111 |
Pb | 0.827 | 0.120 | –0.326 | 0.861 | 0.219 | –0.127 |
Cd | 0.080 | 0.795 | –0.306 | 0.063 | 0.827 | –0.209 |
Cr | 0.324 | 0.136 | 0.911 | 0.113 | 0.060 | 0.968 |
Factor 1, with the highest eigenvalue of 2.50 accounting for 35.7% of the total variance, was the most charging factor. In this factor, elements Cu (0.874), Hg (0.898) and Pb (0.861) provided with significant loadings. High correlation coefficients with Cu, Hg and Pb revealed that they might be derived from the similar pollution sources. According to high values of
Igeo of Cu and Hg, factor 1 can be considered as anthropogenic component [
15].
As Shanxi is a heavy-industry province with abundant mineral resources, many large industries like metallurgy and coking are distributed in the studying area. Metallurgy and coking can give rise to local contamination of Cu, Hg and Pb. When metal-processing factories are in operation, metal particles and wastewater containing Cu, Hg and Pb are generated and eventually, enter into the municipal waste water drain. At a coal-fired power plant, these metals enter the wastewater with a similar route [
44]. As WWTP 14, 16, 18, 19, 25, 29 and 32 are nearby smelting and coking plants, the concentrations of Cu, Hg and Pb were higher than other samples. Level of Cu, Hg and Pb were also high for sample of WWTP 17, which was nearby a coking plant. Therefore, metallurgical industry and combustion of fossil fuels could be defined as the first sources of HM contamination in sewage sludge, and it was also certified by high relations of Cu-Hg-Pb and the greater
Igeo of Cu, Hg and Pb at the same sites [
33,
45,
46].
In addition, emission of HMs from traffic including gasoline and break lining also enters into wastewater through runoff water, suspension as particulate material and dispersal in the atmosphere away the road [
47,
48]. Pb pollution in sewage sludge from traffic was originated from gasoline [
1,
2,
44]. In recent years, Pb-free gasoline has been required for use. However Pb-free does not mean that it contains no Pb. Moreover, in some places gasoline containing Pb is still in use [
3,
4,
43]. Meanwhile, Cu can arise from traffic due to vehicle’s break lining [
19,
47]. The sites with high concentration of Cu and Pb like WWPT8, 10, 12, 25, 29, 30, 31 and 32 represent economic and industry-developed regions. The contaminations of Pb and Cu that happened in these places were probably due to increasing amounts of vehicular traffic.
Factor 2, accounting for 29.0% of the total variance, Zn, As and Cd, which significantly correlated with each other, had high loadings. This factor was an anthropogenic source as factor 1.
Use of galvanized pipe is the major cause of Zn contamination [
1,
22,
47]. Distribution of galvanized pipe for the water supply had been forbidden in China since 2000, but lots of galvanized pipes are in current use [
1]. When galvanized pipes were replaced and water supply pipes made of other materials were installed, Zn pollution would decrease. The level of Zn in sewage sludge from most sites was less than the national average. Only a couple showed higher levels and these were associated with nearby industries
i.e., WWTP 6 was near a wood industry and WWTP 24 was near azinc smelting industry. Contaminations of Zn in those sites were considered as moderate to heavy. Besides this, another source of Zn contamination was car washing, which also correlated to Cd pollution in sewage sludge [
47].
Low pollution classes for Cd and As indicated that origins of these two metals do not generate great deal of Cd and As. High correlation coefficient (
r = 0.511) pointed out that they had the homologous sources. Household was the dominating source for Cd and As in sewage sludge [
18,
40]. Apart from household origin, inhabitants at work, in school and so on in their residences were also the source of Cd [
18]. The principal source of As was the usage of arsenic containing detergent [
1]. The content of As in general household detergent is approximately 3.6 × 10
−2 mg/kg and about 1.8 mg/kg in toilet cleaner [
49]. Thus, the detergents and cleaners in household are major source of As pollutants.
Factor 3, accounting for 16.2% of the total variance, only had strong positive loading only on Cr, which had poor relations with other metals. The contamination of Cr could be caused by leather tanning industries, textile manufacturing and printing and dyeing industry, as well as other industries like painting, coating and chemical engineering [
50,
51]. Only a few samples contamination class of Cr were in class 2 and 3. Samples of WWPT 4, 5, 6, 15, 26 and 30 were highly contaminated with Cr, and the sites of WWPT 4 and 5 are the location of the biggest tanning industry base in China; Sites of WWPT 6 and 30 are developing textile manufacturing and sites of WWPT 15 and 26 are famous of chemical engineering. This revealed that the sources of Cr contamination in Shanxi area could be attributed to the special industry as leather tanning industry, textile manufacturing or chemical engineering.
The dominant sources of HM contamination in sewage sludge were the metallurgy and coking industry, which are widely distributed in Shanxi Province. Traffic, water supply system, households and some special industries such as leather tanning industry, textile manufacturing and chemical engineering were also HM contamination sources in sewage sludge. Charging of HM pollution in sewage sludge, industry was much higher than domestic [
3]. In metal and steel industries, fossil fuels have been reported to be sources of Cu, Hg and Pb pollution [
38,
39]. Many kinds of literature indicate that leather tanning industry and textile manufacturing can cause Cr pollution [
50,
51]. 9%–11% of Pb pollution was from traffic, promotion of lead-free petrol can decrease that [
20]. General household detergent containing great deal of As and Cd in China will lead to great pollution in waste water [
1,
3]. Detergent reducing As and Cd can mitigate contamination of it in sewage sludge.
Source apportionment of HMs in sewage sludge can control HM contamination through suggesting improvements in government policies and industrial processes; also can promote the utilization of sewage sludge in agricultural land as an effective inexpensive fertilizer.