Chemical and Fuel Characteristics of Anaerobically Stabilized Sewage Sludge and Its Ash

Abstract

Samples of the anaerobically stabilized sewage sludge from four large wastewater treatment plants and their ash were subjected to detailed chemical, fuel, and mineralogical examinations. Owing to their very high water contents, mechanically dewatered sewage sludge cannot be incinerated. Peculiar characteristics of the dried sewage sludge include very high proportions of volatile matter, high contents of the fuel-bound nitrogen and ash, which can affect their combustion. Drying and devolatilization of sewage sludge were quite rapid even at moderate temperatures. Up to 80 % of the sludge carbon can be released with the volatiles. Due to nitrogen, phosphorus, potassium, and magnesium contents, municipal sludge exhibits good fertilizer properties. However, the presence of heavy metals such as As, Cd, Hg, Cr, Ni, Pb, Cu and Zn in sludge is a serious concern. The ash from sewage sludge consists of quartz, hematite, spar, mica, apatite, and anhydrite. It does not soften or becomes sticky at temperatures up to 950 °C.