Assessing the health risk of reuse of bottom ash in road paving
Research highlights
► The health risks of bottom ash in road paving with four applications are compared. ► Cr is the crucial toxicant, because it moves easily in soil and groundwater. ► The ingestions of drinking water and vegetable are the key exposure pathways. ► Controlling the using duration of pavement can reduce the leaching of chemicals.
Introduction
The performance of waste management has been improved greatly recently in many countries. For example, the rate of proper waste management including incineration, recycling, and reuse is approaching 100% in Taiwan. As incineration is the major waste treatment method, increasing rates of proper waste management lead to increased amount of fly ash and bottom ash, which used to be landfilled. However, because of limited landfill sites on one hand and the presence of Si, Ca, Al, and Fe in bottom ash on the other hand, bottom ash is considered increasingly to be reused for construction. For example, reuse of bottom ash has become a common practice in Denmark, Belgium, and the Netherlands for road repairing, asphalt concrete, permeable pavement, and brick making in general. Above all, the reuse of bottom ash for road paving in the Netherlands has been up to 100% (Abbott et al., 2003, Hjelmar et al., 2007). In the literature, bottom ash has been shown to be quite similar to natural aggregate in terms of characteristics concerned for engineering applications. Road paving with bottom ash seems to be feasible (Forteza et al., 2004).
In the literature, even fly ash has been found to be potential partial replacement for cement for concrete mixes (Shi and Kan, 2009). In some countries like Japan and the USA, bottom ash and fly ash are sometimes mixed together as combined ash for reducing toxicity of fly ash that contains toxic and relatively leachable heavy metals and organic matters (Ferreira et al., 2003). In order to utilize the ash without increasing environmental impact, the ash must be subject to separation processes, solidification/stabilization, and thermal methods before being reused (Lam et al., 2010). Some other countries like Denmark, France, and Taiwan believe more in solidification and subsequent landfill of fly ash separately from bottom ash, which can be reused.
While the reuse of bottom ash is appealing, the Taiwan government is concerned with the potential environmental impact associated with the reuse. Since 2007, regulation of the reuse has been directed to set restrictions on the source of bottom ash, pretreatment (segregation, fracture, stabilization, aging, and washing), the standard of Toxicity Characteristic Leaching Procedure (TCLP), and the characteristic of application site and so on.
Studies of bottom ash reuse have focused on the property of bottom ash, for instance water content, size, proportion of heavy metal, and pH value. The characteristics of bottom ash vary greatly with incineration processes (Chang and Wey, 2006). TCLP is used to detect the leaching potential of hazardous substances in bottom ash, although the standards of TCLP vary with applications of bottom ash and countries. In addition, there are restrictions on the application of the reuse of bottom ash: the application area of pavement needs to be more than 30 m apart from water sources in France; the application area of pavement must lie more than 1 m above groundwater in Germany; the application area of pavement must be more than 20 m apart from rivers and above groundwater in Denmark; and in Taiwan, the application area of pavement must be more than 20 m from drinking water sources and wells and lie more than 1 m above groundwater. There are more detailed regulations of paving with bottom ash in some countries. For example: in Japan, the usage of bottom ash with asphalting is limited to 10 kg m−2, and mixing rate of bottom ash to nature aggregate must be 10%; in Denmark, if an application area is to be smaller than 1000 m2, the paving thickness of application must be less than 2 m, and it is 1 m on average; if application area is to be over 2000 m2, the paving thickness of application must be less than 0.3 m; in the Netherlands, road paving is the main application, and the ratio of bottom ash: nature aggregate: additive = 5:5:1, and the paving thickness is 25–30 cm (Industrial Technology Research Institute, 2007).
Leaching has been shown to be the main release mechanism from reuse of bottom ash (Birgisdóttir et al., 2006, Olsson et al., 2006). Risk assessment has also been conducted on road paving with bottom ash. In the USA, bottom ash has been used as additive of asphalt and cement and structure protection material, and in road paving and covering of landfill. The associated risk assessment estimated chemical concentrations in the soil from air deposition and surface runoff and in the groundwater, by use of ISCST3 and AT123D modeling (Jackman and Roffman, 1995). And some of the estimated risks were slightly greater than 1.0E−06. Additionally, a UK. study included risk assessment of road repairing with bottom ash and found the risks to human health and the environment are likely to be minimal in a typical UK situation (Abbott et al., 2003). Another study in Norway used the frameworks of human health and ecological risk assessment to assess the hazard of road paving with recycled materials, including bottom ash (Petkovic et al., 2004).
Although the reuse of bottom ash has been favored gradually, the associated risk is still an issue of great concern of the government and even more of the public. The purpose of this study was to quantify the health risk from multimedia and multi-pathway exposure to road paving reuse of bottom ash with consideration of various application scenarios. In particular, using duration of the pavement was taken into consideration because movement of chemicals in soils would affect the subsequent impact. The application on road paving was chosen as the subject of investigation because of its relatively large need and greater potential of risk. Groundwater transport modeling and food chain exposure modeling are performed of the various road paving scenarios with bottom ash, of which the paving structure has been determined by engineering practice. The results indicated the length of using duration was influential to risk and should be taken into account as an important factor of management and regulations. It is expected that such scenarios analysis would facilitate the management and regulation of reuse to reduce risk effectively.
Section snippets
Evaluation scenarios
The major metals that exist in the bottom ash, including As, Cd, Cr, Cu, Pb and Zn, are the primary concern of reuse of bottom ash. Based on TCLP and toxicity, As, Cd, Cr and Pb were chosen as target compounds, and Table 1 shows the range of these compounds’ concentrations in the bottom ash. As and Cr are known human carcinogens, and Cd and Pb are identified as probable human carcinogen. No formal carcinogenic and non-carcinogenic data have been developed from toxicity studies of Cu (
Leaching results
Through the simulation of soil–groundwater transport, the considered chemicals have been retained in the soil and have not infiltrated into the groundwater in 2 years, but in 20 years, different chemicals have exhibited different leaching and transport behaviors. In 20 years of duration, almost 94% of Cr reaches the groundwater; the concentration of As in the groundwater located 20 m away (which is the point of regulation) from the road is 1.86E−6 mg L−1. At the same time, 75% of As reaches the
Conclusion
Based on the risk assessment of various application scenarios, Cr has been identified as an essential risk contributor of the bottom ash reuse, and the ingestions of drinking water and vegetables are the major exposure pathways. Nevertheless, both the resulting risks and HQs from the various reuse scenarios are deemed acceptable in general (less than 1E−06 and 1, respectively). However, that the scenario C exhibited the greatest degree of risk because of its longer using duration suggests that
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