| As the mode of municipal solid waste (MSW) management has been transformed from landfill-based to resource recovery-based, it is urgent to conduct an inquiry into the properties and pollution status of MSW. Study on the characteristics of waste is essential before scientific classification and disposal. However, for a particular city, MSW differs in characteristics depending on several factors, such as economic level, energy structure, natural conditions and living habits of the residents. It is important to do a deep analysis on the characteristics of MSW before choosing the most effective way to dispose MSW of a specific city. Thus, considering the development of the five major economic zones and varied terrains of Sichuan, MSW of six cities including Chengdu, Yibin, Nanchong, Dazhou, Xichang and Ma’erkang were collected in both summer and winter. Physicochemical properties and heavy metal pollution status of the whole waste and its main components were examined. The results of this study are as follows:(1)In general, the main components and their seasonal variation of MSW in Sichuan were similar, however, moisture content and calorific value of MSW varied from city to city though they showed the same seasonal variation. Whether summer or winter, kitchen waste, wrapping paper, toilet paper, plastic and dust were the main components which accounted for 78.25%～96.92% of the total MSW wet weight. The highest proportion was kitchen waste, followed by dust. Moisture content of MSW was slightly higher in summer (51.64%～64.84%) than that in winter (46.14%～60.05) and the moisture content of kitchen waste (60.05%～77.78%) was relatively higher than other components. The low heating values (LHV, wet base) of MSW were between 4795 kJ/kg and 6352 kJ/kg with higher values in winter. Except for the summer waste in Nanchong and Dazhou, MSW of other cities were adapted for incineration without adding auxiliary fuel.(2) The content of nutritional elements did not have obvious spatial variation but presented a similar seasonal variation, higher in summer than winter. Organic carbon content in the whole waste and its main components was toilet paper> kitchen waste> mixed MSW> dust. TN was kitchen waste> mixed MSW> toilet paper> dust. TP and TK were kitchen waste> mixed MSW> dust> toilet paper. The content of organic carbon, TN, TP and TK in the kitchen waste met the control standards for garbage used in agriculture and the carbon nitrogen ratio was in a proper range as well.(3) Cu (14.23～107.56mg/kg), Cd (0.07-0.63mg/kg), Zn (53.77-338.30mg/kg), Hg (0.01-0.41mg/kg) and Cr (35.39～179.63 mg/kg) in MSW should be controlled in waste disposal as they were beyond the soil environmental quality standard in different degrees. Due to the differences in industrial structure and urban construction activities in different cities, the total content of heavy metals showed significant spatial differences. However, the total contents of main heavy metals of all the cities were higher in summer than that in winter. The spatial distribution of the same heavy metal form was actually the same. Kitchen waste and dust were the main sources of Cd in Chengdu MSW in summer; dust was one of the main sources of Cu, Zn and Cr in Sichuan MSW and dust from coal burning was the main source of Hg in Xichang MSW in winter. Cd and Zn existing in MSW samples were present as acid soluble form. Cu existed mainly in oxidizable form while Cr existed mainly in residual form. Kitchen waste fit for compost when nutritional elements and total heavy metal had met the control standards for the wastes used in agriculture. If total heavy metal could not be ignored, composting of kitchen waste was still feasible when the bioavailable heavy metal met the control standards. But it is necessary to monitor the changes of bioavailable heavy metal content in the composting process, especially Cd.(4) Considering the properties and heavy metal pollution status in Sichuan MSW, it is suggested that the kitchen waste, dust, hazardous waste and other components are collected separately at source. Then kitchen waste is for compost, dust for landfill. Hazardous waste should be collected and treated by qualified companies. The remaining components (besides kitchen waste, dust, hazardous waste and recyclable material) accounted for only 30% of the total waste, and could be appropriately classified further on the basis of local MSW disposal facilities. For instance, in Chengdu, the remaining components can be classified as combustible components (toilet paper, unrecyclable wrapping paper, plastic, fabric and wood) for incineration and incombustible components (brick and ceramic) for landfill. The remaining components in other cities can be collected with dust for landfill. |
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