Impact of moisture on volatility of heavy metals in municipal solid waste incinerated in a laboratory scale simulated incinerator
Introduction
Incineration can substantially reduce the volume of municipal solid wastes (MSW) incinerated and kill all the bacteria, but cannot eliminate any heavy metals at all (Jakob et al., 1995,Jakob, 1998; Amalandu and Dennis, 1989). Heavy metals in MSW may distribute among gaseous phase (exhaust gas), bottom ash, boiler ash, and filter ash during incineration in an incinerator (Hasan, 1994, Hasan, 1998; Zhang et al., 2000). Usually, the boiler ash and filter ash are combined together and referred to as fly ash.
Flue gas generated in incinerator of MSW can be properly treated by modern facilities installed in situ in the incineration plant (Hasan, 1994, Hasan, 1998). Hence, the remaining potential pollution sources in the MSW incineration process may be the bottom and fly ashes if these ashes are not properly treated before discharging into the environment.
The most significant pollutants in the ashes are heavy metals, whose contents and their distribution among flue gases and bottom ash vary with the composition and moisture in the MSW incinerated, incineration temperature and time, etc. (Cheng-Fong and Huang, 1990; Serge Biollaz et al., 2001). Typically, 10–25% of MSW will change into bottom ash and 0.5–2% into fly ash in a modern incinerator, with much less quantity of fly ash than that of bottom ash (Youcai et al., 2002a; Stucki and Jakob, 1997).
In general, the bottom ash may be considered non-hazardous and non-toxic, as the heavy metal contents in the ash may be lower than the regulated standard values in terms of leaching toxicity or absolute values (Tay, 1987, Tay and Goh, 1991). However, the contents of heavy metals in bottom ash seemingly increase in the developed nations as the MSW incinerated may contains higher ratios of wastes with heavy metals, such as paints, metal articles and goods, etc. (Jakob et al., 1995,Jakob, 1998). In some countries, such as Switzerland, the environmental requirements have become stricter and the heavy metal contents in the bottom ash have been found to exceed the constraints set by the relevant environmental regulations (Serge Biollaz et al., 2001). In this case, the heavy metals in the bottom ash should be removed before it is reused as construction materials or placed in a landfill.
However, the quantity of bottom ash is relatively great and the heavy metals contents in it are quite low. It will be cost- and energy-intensive when trying to reduce the heavy metal content in the bottom ash by any methods such evaporation at high temperature or separation of heavy metals before the waste is incinerated. Transferring heavy metals in MSW as much as possible into the flue gas so that their content in the bottom ash can be reduced to the regulated level may solve this dilemma. The corresponding content of heavy metals in fly ash collected from flue gas and boiler may thus increase to a level possibly comparable with ores. Much effort has been done in this innovative concept (Jorg and Stuki, 1999; Jorg et al., 1998).
Though the incineration process of MSW has been studied intensively, the effect of moisture on the volatility of heavy metals, nevertheless, has not been studied. According to the discharging standards set in most nations, the heavy metals in the bottom ash are always within the limits of standards. As the environmental constraints increase, more attentions are being drawn to the pollution arisen from the bottom ash because the heavy metals in the ash may have exceeded the newly regulated standards.
Many developing countries, such as China, have used incineration technologies to treat their increasing quantity of MSW. The moisture in the MSW in these countries may be quite high, usually as high as 40–65%, depending on their living customs and levels, as well as the functions of collection facilities. Take China as an example. Contents of paper, plastic, glass, etc., may be low, as these recoverable wastes have been collected by scavengers, while those of high-moisture food origin wastes may be high, in MSW. Moreover, most collection tools are not waterproof. As a result, the moisture in MSW in the dry and arid north part of China may be lower than that in the rainy, hot and humid south part of China, especially in the rainy summer season (Youcai et al., 2000, Youcai et al., 2001, Youcai et al., 2002a, Youcai et al., 2002b).
The changes of moisture in MSW may lead to transformation of chemical forms of heavy metals and even the incineration mechanism in the incineration process, and thus influence the volatilities of these metals. Consequently, the distribution of heavy metals in gaseous phase, bottom and filter ashes will be correspondingly changed. It can be reasonably predicted that different incineration parameters should be applied for different MSW with varying moisture, meaning that the incineration process should be adjusted when the technologies used in the developed nations are transferred to the developing ones. The objectives of this work are to explore the impacts of moisture in MSW on the volatility of heavy metals so that the distribution of heavy metals in three media can be roughly predicted.
Section snippets
Simulated laboratory scale incinerator
A scaled-down version of an MSW grate incinerator that would fulfil detailed geometric similarity is difficult to realize. Accepting, however, some simplifications, it is possible to build a simple model in the form of a tubular furnace reactor: the grate is then replaced by a ceramic crucible, as a sample container and means of transport of the sample in the furnace. The incineration process is carried out as a batch experiment: the temperature profile that a waste sample experiences in a
Temperature profile used
Fig. 2, Fig. 3 show the typical temperature profiles obtained with 5% and 59% moisture in the samples. 0.5 min is spent for the samples to be pushed from the entrance point to the highest temperature zone and then stay at the zone for 9.5 min before pulling out in 1.5 min, which is used for all experiments unless otherwise indicated. The temperature of sample with 5% moisture can reach to the peak value and ignite for incineration in 2.5 min, while for the sample with 59% moisture at least 5
Conclusions
Increase of moistures in MSW will reduce the volatilization of all chemical forms of lead and zinc and oxides of copper, while slightly increase the volatilization of chlorides and metallic form of copper, when the wastes are incinerated at a limited time of 10 min in a laboratory scale simulated incinerator. The chemical mechanisms governing the volatilization behaviors of different forms in the incineration process are proposed. Hydrolysis, dewatering of hydrolyzed species, sublimation,
Acknowledgements
This work was partially supported by Swiss National Science Foundation and China National Natural Science Foundation (No. 59778016), China Outstanding Young Scientist Program, Shanghai Key Research Program, Doctoral Research Program of China Education Ministry, and Shanghai Science and Technology Commission as part of Sino–Swiss Bilateral Cooperation Program. The authors want to thank Mr. Albert and Mr. Lutz for their work on the analysis of heavy metals.
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