Effect of moisture on sewage sludge combustion temperature profile and heavy metal leaching

Moisture is the most important parameter affecting sewage sludge treatment efficiency and cost. In this article, the effect of moisture on sewage sludge combustion temperature profile and heavy metal leaching behavior has been experimentally studied. The temperature profiles inside the sludge pellet were recorded, and the volume shrinkage was obtained by image processing method. Results revealed that combustion rate and burnout of sludge strongly depends on moisture content and there exists an optimal moisture content for sludge incineration with respect to best combustion performance and minimum heavy metal leaching. The enlarged pore size may account for this moisture rate, that is, 48% for the sludge studied. During combustion, the shrinkage rate varied from 15 to 25%, and it was found to be proportional with moisture content. The cross-section image of burning sludge pellet illustrated that during the drying and combustion stage, the moisture evaporation was running parallel with the decomposition of volatile that was burned out in the vicinity of the sludge surface. Measured temperature profiles indicate that effect of volatile combustion on inner temperature profile was ignorable. A mathematic model taking sludge volume shrinkage into consideration has been developed to predict sludge central and surface temperature during combustion.