The influence of thermal processing of sewage sludge on the usage properties of the formed ash

The influence of sewage sludge incineration temperature on the formed ash constitution was examined. The comparative extraction tests of two differently prepared ashes (laboratory and industrial) were carried out in order to verify if the parameters of sewage sludge incineration influence the extraction selectivity of phosphorus compounds. The laboratory ash (A_lab) were prepared from sewage sludge incinerated at 950°C on a laboratory scale while the industrial ash (A_ind) comes from thermal utilization system of the sewage sludge at the Gdynia Sewage Treatment Plant, which uses fluid-bed furnace incineration at 850 - 900°C. It was found that the temperature and the conditions of the sewage sludge incineration process have an effect on the usage properties of the formed ash. Despite the twofold lower Fe content in the industrial ash than that of the laboratory one, its content in extracts after phosphoric acid leaching is 4.7 times higher. The lower values of PO₄^3- leaching degree from the industrial ash than the laboratory ash were observed, as well as a decrease of extraction productivity.