A Thermogravimetric Study on Tannery Sewage Sludges

Two different tannery sewage sludges (Sludge 1 and Sludge 2) have been characterized by thermal analysis coupled with mass spectrometry of evolved gases (TG-DSC-MS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), Gas chromatography - mass spectrometry (GC-MS), Purge and trap GC-MS. Four main temperature ranges of weight loss were individually examined in air flow TG-DSC-MS experiments on Sludge 1. The first, below 200 degrees C, coupled to an endothermic signal, is related to water release. Between 200 and 350 degrees C, an exothermic signal accompanies the loss of sulphate as evolved SO2 organic substances, aromatics and saturated cyclic compounds. In the latter range, development of CO2 was also observed, this becomes prevalent in the range 350-600 degrees C. From 600 to 700 degrees C carbon dioxide from carbonate decomposition was observed. In Sludge 2, three main temperature ranges of weight loss are present, below 150 degrees C, between 150 and 350 degrees C and from 350 to 600 degrees C, attributed, respectively, to water, sulphate, CO2 plus aromatic and carbonyl compound releases. A larger amount of Cr and Al was found by ICP-AES in Sludge 2. The differences between the samples evidence the influence of the composition of the parent wastewater and of the depuration process. The influence of additives on combustion profiles has been also evaluated. Catalyst addition to the sludge results to reduce the emission of cyclic and aromatic substances, to lower temperature and to increase the oxidation rate of organic fraction combustion.