With the development of urbanization and industrialization, water consumption and wastewater production are increasing annually in China. As a result, more and more sewage treatment facilities have been built to meet the increasing demand for wastewater treatment. According to statistics (GOMEP, 2016), a total of 3809 wastewater treatment plants (WWTPs) have been established in China in 2016, which has doubled in the past decade (Xu et al., 2019; Zhou et al., 2019). Similarly, the sewage treatment capacity (i.e., the wastewater treatment rate) of WWTPs has rapidly increased from 25.6 billion m3/a (70.2%) to 46.5 billion m3/a (94.5%) (MOHURD, 2017; Zhou et al., 2019). With the increase of sewage treatment capacity, the total sewage sludge production increased significantly. China’s sewage sludge production (80% moisture content) increased from 30 million tonnes in 2012 to 34 million tonnes in 2015, and it is expected to exceed 60 million tonnes in 2020, with an annual growth rate of 15% (Chen et al., 2019; Feng et al., 2015). The accumulation of sewage sludge will pose growing environmental problems and secondary pollution if treated improperly (Shao et al., 2015).
The environmental impact of industrial sewage sludge is significantly greater than that of municipal sewage sludge because of its high content of toxic and harmful pollutants, such as heavy metals, organic contaminants and pharmaceuticals. Moreover, the yield of industrial sewage sludge is obviously greater than that of municipal sewage sludge. Generally, treatment of 10,000 tonnes of domestic wastewater produces 5−8 tonnes of sewage sludge with 80% water content, while for industrial wastewater, it produces 10−30 tonnes of sewage sludge. Therefore, the disposal of industrial sewage sludge is the key and difficult point in the effective treatment of sewage sludge. However, in China, the combined treatment of municipal wastewater and industrial wastewater brings obstacles to the disposal of sewage sludge (Feng et al., 2015). For example, industrial wastewater contains high concentrations of heavy metals and persistent organic pollutants (POPs), resulting in the sewage sludge produced by the combined system cannot be applied to land as fertilizer or soil conditioner. As high as 35% of industrial wastewater enters WWTPs in China, while only 20% of sewage sludge is treated and disposed safely (Feng et al., 2015).
The Chinese government is actively addressing the issue of sewage sludge treatment and disposal, and has proposed that the non-hazardous treatment and disposal efficiency of sewage sludge is expected to be 90% by 2020 (NDRC and MOHURD, 2017). To achieve this goal, China is actively deploying and building sewage treatment facilities to meet the requirement of the separate treatment of municipal wastewater and industrial wastewater. However, implementation of this proposal will remain a challenge. Many industrial enterprises in China are scattered and not integrated into industrial parks for unified management. Meanwhile, most industrial parks are not equipped with independent wastewater treatment facilities. Furthermore, different from developed countries, China’s investment in sewage sludge treatment and disposal (29.4 billion CNY/year) is much lower than that in wastewater treatment (193.8 billion CNY/year) (NDRC and MOHURD, 2017). Low capital investment and marketization operation make enterprises bear most of the pressure of sewage sludge treatment and disposal. Enterprises involved in wastewater discharge are required to set up wastewater treatment facilities or concentrate their wastewater into the wastewater treatment facilities in industrial park for centralized treatment. The sewage sludge produced by the sewage treatment facilities should be safely treated and disposed by enterprises with relevant qualifications, the cost of which is relatively high (~ 400 yuan per tonnes). To save these costs, some enterprises may dump or bury sewage sludge privately. However, it is difficult for regulators to identify the companies responsible for these violations. One of the feasible methods is to develop an effective and promising identification technology based on the characteristic fingerprints of sewage sludge that can be used to trace the enterprises discharging sewage sludge. The key is to select indicators that can represent different industries. Compared with organic pollutants and pathogens, heavy metals are ubiquitous in industrial sewage sludge, and they cannot be degraded under natural conditions (Dai et al., 2019; Marchenko et al., 2018; Xu et al., 2017). Common treatment methods, e.g. composting, aerobic/anaerobic digestion, cannot remove heavy metals from sewage sludge (Chipasa, 2003; Paulsrud and Nedland, 1997). Likewise, the sludge disposal measures of land application, incineration and landfill can effectively degrade organic pollutants, however, these methods cannot remove heavy metals, leading to their accumulation in the sewage sludge (Cai et al., 2007; Williams, 2005). Moreover, the concentration and composition of heavy metals in sewage sludge discharged by industrial enterprises are quite different due to different production processes and raw and auxiliary materials. Therefore, heavy metals possess the properties of characteristic fingerprints in industrial sewage sludge and we predict heavy metal profiles can be used to identify sewage sludge between different industries, which may be a potential method for tracing the source of illegally discharged industrial sewage sludge.
In recent years, with the popularity of fast fashion, the fashion industry has achieved unprecedented development. According to the Ellen MacArthur Foundation (EMF) (EMF, 2017), fashion brands produced almost twice as much clothing in 2016 as when the fast fashion phenomenon began (Remy et al., 2016), and the annual growth rate of clothing production is approximately 2% (Fletcher, 2016). However, the environmental problems caused by the fashion industry have attracted more and more attention and criticism (Niinimäki et al., 2020). At each stage of the industrial chains, the fashion industry has an impact on the environment, such as water pollution in the production and manufacture of textiles, and CO2 emissions in the distribution and consumption of clothing. The fashion industry produces 4−5 billion tonnes of CO2 annually, accounting for 8−10% of global CO2 emissions (Quantis, 2018; UNCC, 2018), while it consumes 79 trillion litres of water every year (GFA and BCG, 2017), of which textile dyeing contributes 20% of industrial water pollution (Kant, 2012). However, the globalization of the fashion industry has led to an uneven distribution of these environmental consequences (Niinimäki et al., 2020). Due to the competitive advantages of developing countries in manufacturing and labour costs, textile production and manufacturing has shifted to the developing countries, resulting in the aggravation of environmental pollution in these nations. China has the largest textile production and manufacturing market, ranking first in terms of exports of textiles and clothing, with an annual export value of $109.9 billion of textiles and $158.4 billion of clothing (Sheng, 2018). As an important part of textile manufacturing, printing and dyeing industry is a traditional pillar of light industry in China, which is mainly located in the eastern coastal areas (including Zhejiang, Jiangsu, Guangdong, Shandong, Fujian and Shanghai). Printing and dyeing wastewater generally has a high concentration of contaminants and a complex chemical composition (including heavy metals, organic pollutants and dyes), and it is difficult to be removed by conventional water treatment technologies (Liang et al., 2013). The major treatment technologies of printing and dyeing wastewater contain physicochemical treatment (such as coagulation or flocculation) and biodegradation, both of which produce a large amount of sewage sludge (Gotvajn and Zagorc-Koncan, 2004). Studies indicated that 1000 tonnes of printing and dyeing wastewater can produce 10 tonnes of printing and dyeing sludge (80% moisture content) (Zhang et al., 2018). According to China Statistical Yearbook on Environment 2016 (NBS, 2016), China produced 20.2 million tonnes of printing and dyeing sludge from 2.02 billion tonnes of printing and dyeing wastewater in 2015 (Ran et al., 2019). The production of a large amount of printing and dyeing sludge is easy to cause environmental risk because of the weak legal consciousness of enterprises and the high cost of sludge treatment (most printing and dyeing enterprises are small and medium-sized enterprises). Therefore, the development of traceability technology of sewage sludge is conducive to the emergency response and accountability for environmental risks caused by illegal disposal of printing and dyeing sludge.
In this study, the heavy metal profiles of industrial and municipal sewage sludge were analyzed to evaluate the application of this technique in identification of printing and dyeing sludge. The aim of the present study was to establish a new and potential method for tracing the industrial origins of sewage sludge and provide technical support for the government to supervise the illegal disposal of sewage sludge.