HAZARDOUS POLLUTANTS FROM SLUDGE DRYING AND INCINERATION IN CHINA

Sludge samples from four different wastewater treatment plants, with a mixture of municipal and industrial wastewater as influent in different proportions and from variable origins, were dried in a paddle dryer and then incinerated in a fluidized bed incinerator. SO2 and NH3, the main gas-phase compounds emitted during sludge drying process, and their discharge were tested. Results show that the concentration of SO2 varies from 2.32 to 6.32 mg m(-3) and that of NH3 from 0.72 to 7.96 mg m(-3). The emission of SO2 and NH3 shows a good linear correlation with the amount of sulphur and nitrogen content in raw sludge. In sludge incineration process, the flue gas concentrations of SO2, NOx, HCl and HF are higher than those typical for coal combustion. The concentration of SO2, NOx and HCl in flue gas largely depends on the sulphur, nitrogen and chlorine content of sludge. The metals Cd, Hg, Ni and Cu are easier to enrich in the fly ash. On the contrary, the content of Zn is largely enriched in the bottom ash. The PCDD/Fs emission factors from sludge incineration are 0.082, 0.030, 0.069 and 0.237 ng I-TEQ Nm(-3) for the four sludge sources, respectively. Tests presented in this study may assist in identifying problem areas among sludge drying and incineration.