The annual amount of the sewage sludge generated in Japan has reached about 1.9 million DS-tons in FY 2001, and more than 7000 of the sludge was incinerated. Heavy metals contained slightly in the sludge take various kinds of chemical form in incineration process according to their thermo-chemical characteristics. Arsenic, selenium and most of their compounds can be vaporized easily in incineration process. It is known that the vaporized arsenic and selenium are liable to be condensed into the fly ash. Arsenic and selenium in the ash cause a problem to recycling sewage sludge, because of their solubility.
This study was conducted to develop the method to control their content in ash and promote the recycling of sewage sludge. For this purpose, we researched the behavior of arsenic and selenium in the fluidized-bed incinerator, which is widely used in Japan. In this study, we mainly focused on the vaporization of arsenic and selenium, and the transfer to ash. The present paper discusses the effect of temperature and coexisting sub-stances, based on experiments. The following findings were obtained.
1) The coagulant for dewatering and the temperature in ash collector strongly affected the behavior of selenium in incineration process. Most of the selenium in polymeric sludge was vaporized in incinerator. When the temperature in ash collector was 200°C or lower, most of the vaporized selenium was condensed into the ash. In case of the sludge treated with lime, selenium was not vaporized; it was estimated that calcium formed thermo-chemically stable compounds with selenium.
2) Vaporization of arsenic was not observed in our experiments, even in the experimental incinerator of reducing conditions, though it was predicted from the thermo-chemical characteristics of arsenic and from the results of chemical equilibrium simulation.
3) In order to estimate the history of selenium in ash, heating test of incinerated ash was conducted. This examination was also expected as a fundamental study to decrease the selenium in ash. As a result, the vapor-ization of selenium from the ash was not observed at 400°C or lower; this result was different from the temperature characteristics obtained in incineration of sludge. To remove selenium from the ash efficiently, it was necessary to heat the ash at 600°C or higher.

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