Heavy metals behavior during monocombustion and co-combustion of sewage sludge

This paper presents the study of the combustion of granular dry sewage sludge performed on a pilot fluidized bed system. The results of monocombustion of sludge and co-combustion with coal were compared with those of coal combustion for ash partitioning, the formation of gaseous pollutants, and heavy metals behavior. It was found that the mineral matter of sludge was essentially retained as bottom ash. The production of fine ash was small during the monocombustion but was high during co-combustion due to the tendency of coal to produce fine ash, which also contained unburned char. The degree of heavy metal volatilization was found to be slightly higher during co-combustion than in monocombustion; however, most of the metals were retained in the ash and their emissions were found to be below the regulated levels. Hg was completely volatilized. However, during combustion trials involving coal, Hg was captured in the cyclone ash at temperatures below 300degreesC. During sludge monocombustion the retention of Hg in cyclone ash containing low loss on ignition (LOI) was not enough to decrease emissions below the EU regulated levels; hence, it is necessary to install dedicated flue gas treatment for Hg removal. The leachability and ecotoxicity of sludge and ash were compared with the new regulatory limits for landfill disposal in the European Union (EU). It was found that the release of organic matter and heavy metals, found in the sludge, was low from granular bed ash. Hence, except for sulphate release, bed ash was converted into inert and non-ecotoxic materials. Ash from tests with limestone and cyclone ash seemed to be more problematic because of pH effects and contamination with steel corrosion products. The recovery and reutilization of sludge bed ash could, therefore, be possible, as long as the release of sulphate does not interfere with the process.