Combustion Characteristics of Dried Sewage Sludge and Control of Trace-Metal Emission

While the volume of sludge is increasing, it is difficult to find new final disposal sites, especially in Japan. The incineration of sewage sludge is a suitable way to decrease the volume. However, one of the principal problems is the elevated content of trace metals in sewage sludge. These trace metals can escape post-combustion cleanup as a gaseous phase or enrichment in fine particles. In this paper, the combustion tests of dried sewage sludge are performed using a 2-m-tall, 40-mm-inner-diameter electrically heated drop tube furnace. Experimental procedures involve (i) the combustion characteristics of dried sewage sludge and (ii) baseline runs of sludge alone and several runs with the kaolin addition as a sorbent at 1073, 1148, and 1223 K. The sample is collected by an aerosol size fractionation system that is composed of a water-cooled rapid N2 quenching probe and a low-pressure impactor. The results show that the ignition temperature of dried sewage sludge is <900 K. Hollow cenospheres are formed during the sludge combustion. Without the sorbent addition, baseline measurements of chemical compositions in the particle size distributions indicate that most of the lead and cadmium is contained in the fine particles (<1 μm), whereas the variation in chromium concentration with particle size shows a trend similar to the total particle mass distribution of the ash particles. The addition of 5% dispersed kaolin powder in the sewage sludge results in a shift of lead and cadmium from fine particles to coarse particles, especially at a temperature of 1223 K during the combustion of sewage sludge. This shift is caused by the chemical reaction between kaolin particles and trace-metal vapors.