Spatial Distribution Characteristics and Potential Risk Assessment of Heavy Metals in Sludge of Shanghai Sewage Treatment Plant: A Case Study

Abstract

Sewage treatment is an important indicator of the urban environment and an important method of sustainable development. In this paper, the spatial distribution characteristics of 8 heavy metals in sewage sludge from 44 sewage treatment plants in Shanghai were analyzed. The pollution degree and potential ecological risk of heavy metals in sludge were evaluated by the Nemerow comprehensive pollution index and Hakanson potential ecological risk index. First, the single factor pollution index of heavy metals was ranked. The Nemerow comprehensive pollution index showed that five sewage treatment plants in Nanxiang, Dongqu, Liantang, Songjiang west, and Xinhe were heavily polluted. Second, the single factor potential ecological risk index of each heavy metal was analyzed. Finally, it is concluded that in the monitoring and control of sewage treatment, Shanghai should pay attention to the sewage treatment plants with high potential ecological risks and gain an excellent performance in the reduction in the emissions of pollution sources, so as to improve environmental quality and reduce the potential ecological risks. This paper provides the basis for studying the characteristics of heavy metal pollution of sewage sludge.

1. Introduction

The concept of “green water and green mountains are golden mountains” [1] is the way to build an ecological civilization and ecological society and achieve high-quality development. The rapid development of urban construction and the industrial economy has increased the discharge of domestic sewage and industrial wastewater, which has brought enormous pressure to the ecological environment and environmental constraints to economic and social development. In order to ensure the water safety of urban residents, prevent the further pollution of water resources on which human beings depend, and achieve green development and a circular economy, urban wastewater treatment and recycling needs to be carried out.

In recent years, China’s water quality pollution problems have seriously affected people’s quality of life; the city bear the responsibility of improving the effectiveness and efficiency of the management of sewage as the basic unit of management of water pollution [2]. Additionally, the urban wastewater treatment plant is to treat the collected sewage so as to reduce the pollution of the environment by pollutants in the water. In municipal wastewater treatment, sludge, as an accompanying product of municipal wastewater, will generally account for 0.02% of the volume of sewage [3] and contains a large amount of organic matter and nutrients such as nitrogen, phosphorus, and potassium, which is a renewable resource that can be used in soil remediation and composting, feed processing, land use, and the production of building materials [4,5,6,7]. Since sewage sludge mainly originates from various industrial wastewater and domestic sewage, it inevitably contains heavy metals that are harmful to the environment and organisms [8,9]. Heavy metals are characterized by a strong latency, concealment, slow migration rate, serious pollution consequences, and complex ecological effects [10,11,12,13], which makes the resource utilization of sludge more difficult and poses a greater threat to human health through its impact on the ecological environment [14]; therefore, it is crucial to carry out research related to heavy metals in sewage sludge to strengthen urban governance and improve the ecological environment.

The most common heavy metals in sludge are copper (Cu), zinc (Zn), plumbum (Pb), cadmium (Cd), nickel (Ni), chromium (Cr), arsenic (As), and mercury (Hg) [15,16]. Due to the diversity of sewage sources, the types of heavy metals contained in different sources also show diversity, and the pollution of the environment by different types of heavy metal pollution varies and different disposal methods are used [17,18,19], which requires the development of a targeted sludge treatment policy and technology route selection [20]. Therefore, the study and analysis of the spatial distribution characteristics and potential ecological risks of heavy metals in urban sludge can help to develop reasonable sludge treatment measures for different situations.

At present, scholars have conducted relevant studies on the distribution characteristics of heavy metals in sludge from wastewater treatment plants and their ecological risk evaluation in other provinces and regions [13,15,19,21,22], but there are few studies involving the composition characteristics and influencing factors of the heavy metal content in sludge from Shanghai wastewater treatment plants. In this study, Shanghai was selected as the study area, and Kriging spatial interpolation [23]. It was performed on the sludge heavy metal monitoring data from various wastewater treatment plants in Shanghai to analyze the spatial distribution characteristics of different kinds of heavy metals. The Nemerow integrated pollution index [8] and Hakanson potential ecological risk index [24,25] were used to evaluate the pollution degree and potential ecological risk of various sludge heavy metals in different areas. The potential ecological risk was evaluated. For wastewater treatment plants with a high potential ecological risk, recommendations were made in terms of sludge treatment processes, with a view to providing some scientific basis for the rational disposal of sludge in various regions of Shanghai.

2. Research Data Background

Shanghai is a large international city with a resident population of about 24,237,800; in 2018, the city’s gross domestic product totaled CNY 3,267,987 million, the annual industrial added value was CNY 869,495 million, and the total annual water supply was 3101 million cubic meters, of which the industrial water consumption and domestic water consumption were 453 million cubic meters and 1999 million cubic meters, respectively [26]. To meet the development needs of the economy and population, as of 2019, a total of 46 urban sewage treatment plants has been built and operated in Shanghai, 30 urban sewage treatment plant sludge treatment facilities have been built and operated, and 44 urban sewage treatment plants have been built and operated and monitored for sewage sludge. Among them, 9 in the central city (including Shidongkou sewage treatment plant, Zhuyuan first sewage treatment plant, Zhuyuan second sewage treatment plant, and Bailonggang sewage treatment plant, as well as 4 major plants and 5 other sewage treatment plants in the central city), 4 in Jiading District, 10 in Qingpu District, 5 in Jinshan District, 2 in Fengxian District, 2 in Pudong New Area, 7 in Songjiang District, and 5 in Chongming County.

The source of sludge heavy metal data for each wastewater treatment plant selected for this study was the Monthly Report on Monitoring of Sewage Sludge from Urban Wastewater Treatment Plants in Shanghai from January to June 2019, including data related to several categories of heavy metals such as Cd, Cr, Hg, Ni, Pb, As, Cu, and Zn. From the data, the four major plants dominate the city’s sludge generation, accounting for 64.5% of the city’s combined volume, of which the second Zhuyuan sewage treatment plant has a lower content of all pollutants. Compared with the central city wastewater treatment plant, the suburban wastewater treatment plant sludge contains more Ni, Cu, Zn, Cr, Cd, and As.

The sludge heavy metal monitoring data were analyzed using Origin 2019b software and ranked by the weighted average, and the sludge heavy metal contents of Shanghai wastewater treatment plants were ranked as follows: Zn > Cu > Cr > Ni > Pb > As > Hg > Cd, where the Zn content was ranked highest in the minimum, maximum, and weighted average values, as shown in

Table 1. Statistics of monitoring results of heavy metal content in sludge of urban sewage treatment plants in Shanghai (unit: mg/kg).

Minimum Heavy Metal	Minimum	Maximum	Weighted Average
Cd	0.55	22.24	1.59
Cr	25.40	357.50	110.60
Hg	0.70	21.88	3.12
Ni	10.36	381.33	51.25
Pb	16.87	190.80	41.26
As	9.69	239.28	21.02
Cu	79.48	1383.33	278.33
Zn	258.00	2970.00	709.70

.

3. Research Methods

3.1. Spatial Interpolation Method

Spatial interpolation is a method to infer data of unknown regions from data of known points, including Kriging interpolation, the inverse distance weight method (IDW), and the spline function method [22]. In this study, the Kriging interpolation method was used to estimate the spatial distribution of heavy metals in sludge.

$$Z\left({\chi }_0\right)={{\displaystyle \sum }}_{i=1}^{i=1}{\lambda }_{i}Z\left({\chi }_i\right)$$

(1)

where ${\chi }_0$ is the value of the point to be estimated, ${\lambda }_i$ is the weight value, and ${\chi }_i$ is the value of the known sample point.

3.2. Sludge Heavy Metal Pollution Index Method

The single factor pollution index method and Nemerow comprehensive pollution index method were used for the heavy metal pollution of sludge.

① The single factor pollution index method is a method to evaluate the pollution degree of a certain heavy metal in sludge. The specific calculation method is shown in Formula (2), which is a relatively simple and general method [25].

$$P_i=\frac{C_i}{S_i}$$

(2)

$P_i$ is the environmental quality index of heavy metal $i$ in sludge, $C_i$ is the measured content of heavy metal $i$ in sludge (mg/kg), and $S_i$ is the environmental quality standard of heavy metal mine in sludge (mg/kg), which mainly uses the Standard for Control of Pollutants in Agricultural Sludge (GB 4284-2018), as shown in

Table 2. Environmental quality standards for heavy metal pollution (Unit: mg/kg).

	Ni	Cu	Zn	Cr	Pb	Cd	Hg	As
Environmental quality	100	500	1200	500	300	3	3	30

.

② The Nemerow comprehensive pollution index method is a comprehensive reflection of the average pollution level of various heavy metals in sludge, but also highlights the harm caused by the most serious heavy metals to the environment. The calculation method is shown in Formula (3).

$$P_t=\sqrt{\frac{{\left(P_{i max}\right)}^2+{\left(P_{i ave}\right)}^2}{2}}$$

(3)

$P_t$ is the comprehensive pollution index of sewage treatment plant, $P_{i max}$ is the maximum value of the single factor pollution index of heavy metal $i$, and $P_{i ave}$ is the average value of the single factor pollution index of heavy metal $i$.

According to the single factor pollution index method and Nemerow comprehensive pollution index method, heavy metal pollution can be divided into 5 levels, as shown in

Table 3. Nemerow pollution index classification criteria.

Grade Classification	Pollution Index	Comprehensive Pollution Index	Pollution Grade
1	$P_i\le 0.7$	$P_t\le 0.7$	Security
2	$0.7<P_i\le 1$	$0.7<P_t\le 1$	Alert level
3	$1<P_i\le 2$	$1<P_t\le 2$	Light pollution
4	$2<P_i\le 3$	$2<P_t\le 3$	Moderate pollution
5	$P_i>3$	$P_t>3$	Severe pollution

.

3.3. Potential Ecological Risk Assessment Method of Heavy Metals in Sludge

The potential ecological risk assessment of heavy metals in sludge mainly adopts the potential ecological hazard index, which was proposed by Swedish scientist Hakanson (1980) and is also an important method for heavy metal pollution hazard assessment [24]. Specific calculation methods and steps are shown in Formulas (4)–(6).

$$C_r^i=C_i/C_n^i$$

(4)

$$E_r^i=T_r^i\times C_r^i$$

(5)

$$RI={{\displaystyle \sum }}_{i=1}^n{E}_r^i={{\displaystyle \sum }}_{i=1}^n\left(T_r^i\times C_r^i\right)$$

(6)

$C_r^i$ is the pollution coefficient of the relative parameter ratio of heavy metal i, $C_i$ is the measured content of heavy metal $i$, $C_n^i$ is the evaluation parameter ratio of heavy metal $i$, $E_r^i$ is the single factor potential ecological risk index of heavy metal $i$, $T_r^i$ is the toxicity response coefficient of heavy metal $i$, and RI is the comprehensive potential ecological risk index of all heavy metals. Among them, the heavy metal toxicity response coefficient and evaluation parameter ratio are shown in

Table 4. Heavy metal pollution evaluation parameter table (unit: mg/kg).

	Ni	Cu	Zn	Cr	Pb	Cd	Hg	As
Heavy metal toxicity response factor	5	5	1	2	5	30	40	10
Heavy metal evaluation reference values	40	30	80	60	25	0.5	0.25	15

.

According to the calculation of the potential ecological hazard index, the potential ecological risk levels of heavy metals in sludge were divided into five types by the single factor potential ecological risk index $E_r^i$ and the comprehensive potential ecological risk index RI, as shown in

Table 5. Potential ecological risk level of heavy metals in sludge.

${\mathit{E}}_{\mathit{r}}^{\mathit{i}}$	$\mathit{R}\mathit{I}$	Degree of Potential Ecological Risk
$E_r^i<40$	$RI<150$	Low
$40\le E_r^i<80$	$150\le RI<300$	Mid
$80\le E_r^i<160$	$300\le RI<600$	High
$160\le E_r^i<320$	$600\le RI<1200$	Very high
$E_r^i\ge 320$	$RI\ge 1200$	Strongly high

[24,25].

4. Simulation and Results Analysis

4.1. Spatial Distribution Characteristics of Heavy Metals in Sludge

The spatial distribution of heavy metals in sewage sludge is also different due to different treatment areas of sewage treatment plants. By using the Arcgis10.2 software and the Kriging spatial interpolation method, the spatial distribution characteristics of heavy metals in sludge can be visually displayed (Figure 1).

As can be seen from Figure 1, the spatial distribution of various heavy metal elements in the study area is obviously different, roughly in island-like distribution, among which Cd, Cr, As, Cu, and Zn appear in high content areas. The Cd content in sewage sludge in southwest Shanghai was generally high and showed a decreasing trend from the southwest to northeast. The heavy metal Cr content is the highest in Nanhui seashore and Fengxian west, followed by Lingang new city, Jiading and Songjiang districts, and the lowest in Chongming. The heavy metal Hg content in Fengxian east is the highest, and the content in Jinshan and Chongming is the lowest. Heavy metal Ni showed the characteristics of non-point source distribution, among which, the total Ni content of sludge in Jiading, Fengxian, and Jinshan was high, while the total Ni content of sludge in Chongming gradually decreased from west to east. The content of heavy metal Pb in sewage sludge is higher in Nanxiang and Jiading, lies in Jinshan District, and the total Pb content in Chongming decreases gradually from west to east. The content of heavy metal As in sewage sludge peaked in Chongming, and Chongming was higher than other regions. The content of heavy metal Cu in sludge is the highest in Songjiang District, and radiates in three sewage treatment plants in Songjiang, Songjiang east, and Songjiang west, while it is lower in Baoshan District, Pudong New Area, and Chongming. The content of the Zn element in the sludge was the highest in the interface area of Qingpu and Jiading, followed by Fengxian east.

As can be seen from Figure 2, from the perspective of administrative districts, the content of heavy metal elements such as Pb, Hg, and Zn in sewage sludge in downtown Shanghai is relatively high. The overall situation of Chongming Island is better, but the total As content of sludge is higher. The content of Ni and Cr elements is high in the west of Jiading, while the content of heavy metals such as As, Zn, Cr, and Ni is high in Jiading. The content of Zn, Cu, Ni, and Cr is high in the north of Qingpu, the content of Cu, Zn, and Cr is high in the middle of Qingpu, and the content of heavy metal Cd is high in the south of Qingpu. In addition to the low content of As and Hg, the content of other heavy metals is high in Songjiang area. Among them, the content of Cr is higher in northern Songjiang areas, and the content of Cu, Cr, Ni, Cd, and Zn is higher in the central and southern Songjiang area. The Zn and Cr content in Fengxian east is high, while the Cr and Ni content in Fengxian west is high; the heavy metal Cr content in the Nanhui seashore is high. Lingang new city has a high Cu and Zn content.

4.2. Analysis of Heavy Metal Pollution of Sludge

Through Matlab2016a, the sludge of 44 sewage treatment plants in the study area was treated and analyzed for heavy metals. According to the heavy metal Ni pollution index, Jiading north district had the highest index of 3.81, which belonged to severe pollution. In terms of the pollution index of heavy metal Cu, the highest index of Yexie is 2.77, followed by Songjiang west with 2.32, which belongs to moderate pollution. In terms of the heavy metal Zn pollution index, the highest pollution index of white crane was 2.48, which belonged to moderate pollution. In terms of the heavy metal Cr pollution index ranking, the highest pollution index of the seaside is 0.72, which belongs to light pollution. In terms of the heavy metal Pb pollution index, the highest pollution index in the Eastern District was 0.64, while the pollution index in other places was lower than 0.3, belonging to the safety level. In terms of the heavy metal Cd pollution index ranking, the highest index of Liantang is 7.41, followed by Songxi with 6.69, which is classified as heavy pollution. In terms of the heavy metal Hg pollution index, Eastern District has the highest pollution index of 7.29, which is classified as heavy pollution.

In terms of the heavy metal pollution of sludge from the Shanghai sewage treatment plant, the highest single factor pollution index was As (7.98) > Cd (7.41) > Hg (7.29) > Ni (3.81) > Cu (2.77) > Zn (2.48) > Cr (0.72) > Pb (0.64). The average value of the single factor pollution index was As (1.29) > Hg (0.95) > Cd (0.82) > Cu (0.77) > Zn (0.76) > Ni (0.7) > Cr (0.27) > Pb (0.14).

According to the pollution index, the As, Cd, Hg pollution index is high, indicating serious pollution; the pollution index of Cu, Zn, and Ni ranked in the middle, between light pollution and moderate pollution; and the pollution index of Cr and Pb is low, and the pollution situation is in the safe area.

From the perspective of Nemerow’s comprehensive pollution index, the overall distribution is shown in Figure 3.

As shown in

Table 6. Pollution classification of sewage treatment plants in Shanghai.

Sewage Treatment Plant	Comprehensive Pollution Index	Pollution Level
Langxia, Zhuyuan 2, Zhujiajiao, Fengting, Hongqiao, Fengjing, Qingpu, Xinjiang, Songshen, and Xingta.	$P_t\le 0.7$	Safety
Jinze, Jiadingdazhong, Shangta, Changqiao, Minhang, Bailonggang, Zhuyuan 1, Xicen	$0.7<P_t\le 1$	Alert level
Baihe, Changxing, Songjiang east, Shidongkou, Fengxian west, Lingang new city, Qingpu 2, Xujing, Liugang, Anting, Xinhua, Songjiang, and Wusong.	$1<P_t\le 2$	Mild pollution
Jiading north, Baozhen, Chenjiazhen, Fengxian east, Nanhui seashore, Yexie, Xinbang, Chengqiao	$2<P_t\le 3$	Moderate pollution
Nanxiang, Dongqu, Liantang, Songjiang west, and Xinhe.	$P_t>3$	Heavy pollution

, the pollution index of the Nanxiang sewage treatment plant was 5.72, which was the highest, including Dongdong, Liantang, Songjiang west, and Xinhe. The comprehensive pollution index $P_t$ > 3 was severe pollution. The comprehensive pollution index $2<P_t\le 3$ of 8 sewage treatment plants, including the Jiading North District, belongs to moderate pollution; the comprehensive pollution index $1<P_t\le 2$ of 13 sewage treatment plants, including White Crane, belongs to light pollution; the comprehensive pollution index $0.7<P_t\le 1$ of 8 sewage treatment plants, including Kanazawa, belongs to the warning level; and the composite pollution index $P_t$ of 10 sewage treatment plants, including Xingta, is $P_t$ ≤ 0.7, which belongs to the safety level.

4.3. Potential Ecological Risk Assessment of Heavy Metals in Sludge

According to Formulas (4)–(6), Matlab2016a was adopted to calculate the potential ecological risk index of heavy metals in sewage treatment plant sludge.

In terms of the potential ecological risk index ranking, the highest potential ecological risk index of Ni in Jiading north is 47.67, and the degree of potential ecological risk is medium. The potential ecological risk index of heavy metal Cu in Yexie is 230.56, and the potential ecological risk degree is very high. The potential ecological risk index of heavy metal Zn for Baihe is 37.13, and the potential ecological risk degree is low. The potential ecological risk index of heavy metal Cr in the Nanhui seashore is 11.92, and the potential ecological risk degree is low. The highest potential ecological risk index of heavy metal Pb in Dongqu is 38.16, and the degree of potential ecological risk is low. The highest potential ecological risk index of Cd in Liantang is 1334.4, and the potential ecological risk degree is very high. The highest ecological risk index of heavy metal Hg in Dongqu is 3500.8, and the degree of potential ecological risk is very high. The potential ecological risk index of heavy metal As in Nanxiang is 159.52, and the potential ecological risk degree is high.

According to the single factor maximum potential ecological risk index, Hg (3500.8) > Cd (1334.4) > Cu (230.56) > As (159.52) > Ni (47.67) > Pb (38.16) > Zn (37.13) > Cr (11.92); according to the average value of single factor potential ecological risk index, Hg (456.44) > Cd (147.41) > Cu (63.72) > As (25.78) > Zn (11.40) > Nil (8.72) > Pb (8.61) > Cr (4.48).

The potential ecological risk degree of heavy metals Hg and Cd is very high, the potential ecological risk degree of Cu and As is high, and the potential ecological risk degree of Ni, Pb, Zn, and Cr is low.

From the perspective of the RI comprehensive ecological risk index, as shown in

Table 7. Potential ecological risk classification of heavy metals in sludge of Shanghai sewage treatment plant.

Sewage Treatment Plant	$\mathit{R}\mathit{I}$	Pollution Level
	$RI<150$	Low
Shangta, Xingta	$150\le RI<300$	Mid
Xujing, Huaxin, Xicen, Xinhe, Songjiang, Zhuyuan 2, Nanxiang, Baihe, Nanhui seashore, Chenjiazhen, Fengting, Qingpu, Baozhen, Jiadingdazhong, Zhujiajiao, Hongqiao, Langxia, Xinjiang, Chengqiao, Fengjing, and Songshen.	$300\le RI<600$	High
Xinbang, Lingang new city, Wusong, Jiading north, Yexie, Anting, Jinze, Changqiao, Qingpu 2, Fengxian west, Songjiang east, Zhuyuan 1, Bailonggang, Minhang, Liugang, and Changxing.	$600\le RI<1200$	Very high
Dongqu, Songjiang west, Liantang, Fengxian east, and Shidongkou.	$RI\ge 1200$	Strongly high

, the highest ecological risk index of Dongqu is 3942.1, including the ecological risk index RI ≥ 1200 of five sewage treatment plants in Dognqu, Songjiang west, Liantang, Fengxian east, and Shidongkou, indicating a very high degree of potential ecological risk. The ecological risk index of 16 sewage treatment plants, including Xinbang, is 600 ≤ RI < 1200, indicating a high potential ecological risk. The ecological risk index of 21 sewage treatment plants, including Zhuyuan 2, is 300 ≤ RI < 600, indicating a high potential ecological risk; and the ecological risk index of Shangta and Xingta sewage treatment plants is 150 ≤ RI < 300, indicating that the potential ecological risk degree is medium.

5. Conclusions

The sewage sludge of the Shanghai sewage treatment plant mainly contains eight heavy metal elements, including Ni, Cu, Zn, Pb, Cr, Cd, Hg, and As, and presents diversified spatial distribution characteristics. In this paper, the spatial distribution characteristics of 8 heavy metals in sewage sludge from 44 sewage treatment plants in Shanghai were analyzed. The pollution degree and potential ecological risk of heavy metals in sludge were evaluated by the Nemerow comprehensive pollution index and Hakanson potential ecological risk index. The following conclusions can be drawn. (1) The pollution of heavy metals As, Cd, and Hg in sewage treatment plant sludge in Shanghai is the most serious; (2). Cu, Zn, and Ni pollution is in the middle degree; and Cr and Pb pollution is low. According to the Nemerow comprehensive pollution index, 10 sewage treatment plants belong to the safety level, 8 to the warning level, 13 to light pollution, 8 to moderate pollution, and 5 to severe pollution, among which the Nanxiang sewage treatment plant index is the highest. (3) The single potential ecological risk analysis of heavy metals in sludge shows that the potential ecological risk degree of Hg and Cd is very high, the potential ecological risk degree of Cu and As is high, and the potential ecological risk degree of Ni, Pb, Zn, and Cr is low. In Fengxian east sewage treatment plants, which has a high potential ecological risk, in addition to adopting curing and thermal desorption technologies, electric remediation can also be considered to remove Cr elements from the soil. Since the soil of the cave entrance also contains high Cr and Hg, the above-mentioned techniques are also recommended for soil remediation.