ReviewThe migration and transformation behaviors of heavy metals during the hydrothermal treatment of sewage sludge
Introduction
Sewage sludge is the main by-product of wastewater treatment. It is considered as a very heterogeneous material and comprises a mixture of various compounds including microorganisms, undigested organics, inorganic materials and moisture (Fonts et al., 2012). Some components in it such as non-toxic organic matter and nutrients are worth to be recycled. However, it also carries undesirable components, such as heavy metals, synthetic organic compounds and pathogenic micro-organisms, which are not environmentally friendly. Thereby, the disposal of this by-product may lead to significant environmental impacts such as public health risks and the possibility of contaminating atmosphere, soil and water resources; thus, appropriate treatment, controlled disposal and careful management are, in general, of great importance (Manara and Zabaniotou, 2012).
Nowadays, the main approaches of disposing of sewage sludge can be classified into three categories: agricultural use, incineration, and landfill (Fonts et al., 2012). These conventional disposal methods are facing increasing pressure and protest from environmental authorities and the public (Tyagi and Lo, 2013). Looking into the future, it can be expected that sewage sludge management and research into innovative treatment methods will focus on three aspects: recovery and reuse of valuable products from sludge, a complete solution of the sludge problem, especially regarding the toxics, and acceptable costs. In this respect, it is expectable that the recovery of sustainable energy from sewage sludge will become more and more of interest (Rulkens, 2008, Rulkens and Bien, 2004).
Various methods of recovering energy from sewage sludge have been discussed in detail by several researchers (Fonts et al., 2012, Manara and Zabaniotou, 2012, Mills et al., 2014, Rulkens, 2008, Rulkens and Bien, 2004, Werle and Wilk, 2010). In summary, the various approaches for the recovery of energy from sewage sludge, or more precisely, from the organic compounds in the sludge, can be divided into nine groups: (i) anaerobic digestion, (ii) production of biofuels, (iii) direct production of electricity in microbial fuel cells, (iv) incineration for energy recovery, (v) co-incineration in coal-fired power plants, (vi) gasification and pyrolysis, (vii) use as an energy and raw material source in the production of Portland cement and building materials, (viii) supercritical wet oxidation, and (ix) hydrothermal treatment (Rulkens, 2008).
Hydrothermal treatment is a process in which sewage sludge is heated and converted into useful resources in water or other suitable solvents at certain temperatures and pressures. Recently, hydrothermal carbonization, liquefaction and sub/super-critical water gasification have been widely studied in the field of sewage sludge treatment. Hydrothermal carbonization is used to recover solid carbonaceous fuel, i.e. hydrochar, from sewage sludge (He et al., 2013, Parshetti et al., 2013). Treatment of sewage sludge by liquefaction aims at the production of liquid bio-oil (Huang et al., 2014, Wang et al., 2013). Sub/super-critical water gasification process is applied to the generation of hydrogen-rich gas from sewage sludge (Afif et al., 2011, Zhu et al., 2011). The differences and connections among the above three hydrothermal treatment processes are depicted in Fig. 1.
As listed in Table 1, researchers have also paid a lot of attention on the migration and transformation behaviors of heavy metals during the hydrothermal treatment of sewage sludge in recent years (Chen et al., 2014, Huang et al., 2011, Leng et al., 2014, Li et al., 2012, Shi et al., 2013a, Shi et al., 2013b, Yuan et al., 2011, Zhai et al., 2014). In general, there are mainly three aspects of work carried out in this field: (i) the distribution of heavy metals in the triphase products of sewage sludge treatment, especially the solid/liquid products; (ii) the transformation of the chemical speciation of heavy metals; (iii) the changes in the bio-availability and eco-toxicity of heavy metals. There are some distinct/competing results existing in different studies, especially for the changes in the contamination degree/risk of heavy metals, due to the different evaluation methods used.
This review will firstly summarize the assessment methods of heavy metals’ contamination level/risk and then discuss the effect of process parameters on the migration and transformation behaviors of heavy metals during the hydrothermal treatment of sewage sludge from the following aspects: (i) the effect of reaction temperature; (ii) the effect of additives (catalysts and other biomass); (iii) the effect of the type of liquefaction solvent; (iv) the effect of reaction time. The ultimate objective of this review is to provide references for the further study of the migration and transformation behaviors of heavy metals during the hydrothermal treatment of sewage sludge.
Section snippets
Relationships among the chemical speciation, eco-toxicity and bioavailability of heavy metals
It is widely accepted that the bioavailability and eco-toxicity of heavy metals in the environment mainly depend on their chemical speciation. The chemical speciation of heavy metals can be determined by the selective sequential extraction analysis. The two most widely used sequential extraction methods are Tessier (Tessier et al., 1979) and BCR (Rauret et al., 1999). The Tessier method selects five fractions including exchangeable fraction (F1), fraction bound to carbonates (F2), fraction
The migration and transformation behaviors of heavy metals
Since the hydrothermal treatments of sewage sludge are usually conducted at relatively low temperatures (<550 °C), it is assumed that only few heavy metals will be transferred to the gas phase products. Thus the content of heavy metals in gaseous products is usually not analyzed. During the hydrothermal treatment processes, both extraction and stabilization effects of heavy metals existed simultaneously, leading to the redistribution of heavy metals in the liquid and solid phases (Fig. 3).
As
Conclusions
In determining the effects of hydrothermal treatments on the contamination level/risk of heavy metals in sewage sludge, the total content indices must be modified to avoid conflict with the speciation indices. The increase of reaction temperature will increase the total content of heavy metals in bio-chars. The addition of proper catalysts could not only enhance the conversion efficiency but also further immobilize the heavy metals in bio-chars. Co-processing of sewage sludge with
Acknowledgements
The authors gratefully acknowledge the financial support provided by the Natural Science Foundation of Jiangxi, China (No. 20151BAB213024), the Scientific Research Fund of Jiangxi Provincial Education Department (GJJ14302) and the Hunan Province Innovation Foundation for Postgraduate (CX2012B139).
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