Characterization of solid residues from municipal solid waste incinerator

Abstract

In China, the generation of municipal solid waste (MSW) incineration solid residues is expected to increase significantly in the future. Because of ever-increasing generation rates and the concentrations of potentially hazardous heavy metals, which may endanger the ambient environment, these solid residues are of particular concern. Thus, such issues have necessitated the study of the characterization of these solid residues. A detailed characterization of five samples of different type of ashes, collected from two types of MSW incinerators located in Shenzhen (southern China), was carried out in terms of chemical composition, morphology, mineralogy and leaching behavior. Results of chemical analysis and leaching tests suggest that the MSW incineration fly ash must be classified as hazardous waste and should be properly managed. Therefore, the results of this characterization would contribute to the development of adequate waste management strategies.

Introduction

In China, municipal population growth, economic development and living-standard improvement have brought about a soaring increment of about 8–10% in municipal solid waste (MSW) year after year. The ever-growing MSW load has a great impact on the ambient environment and people's health, such as malodors from MSW landfill sites, explosion resulting from combustible gases of MSW, and contamination of groundwater and soil by hazardous organics and heavy metals in MSW [1], [2], [3]. Consequently, disposal of these MSW has become a serious problem with which China is currently confronted.

Landfill accounts for more than 80% of the MSW in China, and compost ranks the second. Enhancement of people's environmental awareness, continuing increments of landfill costs, sharp scarcity of landfill sites and slowness of compost process have led China to take alternative disposal methods into account.

Incineration of MSW has many advantages such as significant volume reduction (about 90%) and mass reduction (about 70%), complete disinfection and energy recovery [4], [5], [6], [7]. Thus, incineration meets the requirements of detoxification, decrement and resource recovery. In addition, these methods have been technically proven as an effective waste treatment approach [8]. Therefore, many big- and medium-size cities in China have constructed, or are constructing and/or plan to install MSW incineration devices. For instance, Shenzhen, Beijing and Shanghai have constructed mass-burning incinerators and/or pyrolytic incinerators, while Wuhan, Guangzhou and Shenyang plan to construct incineration devices. The Japanese-built MSW incinerator in Shenzhen is the first MSW incineration device in China.

Incineration of MSW not only produces significant pollutant flue gases, but also gives rise to considerable amount of solid residues (bottom ash, grate siftings, heat recovery ash, fly ash and air pollution control (APC) residues) that are generated at different points in the process of MSW incineration. With improvements in APC systems, modern MSW incineration plants emit practically no pollutants into the atmosphere, which results in the World Health Organization's statement that it no longer considers the emissions from modern, well operated and maintained MSW incinerators to be a hazard to human health or the environment [9]. However, the hazardous fractions in MSW are concentrated in the solid residues. Indeed, pollutant elements such as As, Cd, Cu, Cr, Hg, Ni, Pb and Zn have been described in such residues [10], [11]. Release of such elements during storage will have a potentially negative impact on environmental quality, human health and groundwater as well as surface-water resources [5], [12].

With the incineration method being widely adopted to cope with MSW, the generation of MSW incineration solid residues is expected to increase in the future in China. The large quantity coupled with the potential leachability of high metal concentrations in the residues has necessitated the study of the chemical, morphological, mineralogical and leaching properties of these solid residues. Moreover, determining safe management alternatives for these types of solid residues requires extensive characterization. Many studies have shown the presence of such toxic metals as Cd, Pb, Cr, Zn and Hg, and have investigated the morphology and mineralogy, and also have studied the leaching behavior of these types of solid residues [5], [11], [13], [14], [15], [16], [17].

This paper describes a comprehensive study of five samples of MSW incineration solid residues generated by two different types of MSW incineration facilities in China, with the intention of elucidating their chemical, morphological and mineralogical properties as a tool for the prevention of their impact on the environment and evaluation of utilization possibilities. In addition, a leaching test to shed light on their potential toxicity is also presented.