Efeito do período de estocagem, tempo de incubação e da temperatura de secagem na avaliação de parâmetros químicos e biológicos e na disponibilidade de metais de lodo de esgoto

Abstract

Agricultural use of sewage sludge has become, increasingly, a practice adopted by sanitation companies to solve partly the problem of final disposal of waste. As the sludge to be used generally need to be dry and that the drying process can alter the distribution of metals, the general aim of this study was to evaluate the effect of drying temperature and storage time on chemical parameters and the metal availability and distribution. The specific objectives were: (i) to study how the drying temperature (air, 40°C and 65°C) sludge, associated with the storage time (zero-fresh, and one year), influenced the availability of metals and the possible chemical changes induced in the sludge, (ii) to evaluate how the high content of iron present and the microbial activity affect the availability of metals in the sludge studied, as a function of temperature of drying, and (iii) to characterize the soluble organic fraction leached by infrared absorption spectrometry in samples of dried sludge at different temperatures and stored for one year. Fresh and stored for one year sludge were dried with forced air circulation, grounded and chemical analyzed. The BCR sequential extraction was applied to assess the availability and distribution of metals. The drying of sewage sludge in the air encouraged increasing availability of metals, however, where temperature increased it diminished due to binding of metals with more stable compounds. The reduction of total organic carbon due to volatilization and biodegradation was favored by increased temperature. The decrease in the levels of iron and zinc, with increasing time and temperature drying to constant weight and pH close to neutral, favored readsorption and/or precipitation of metals to less labile forms. The wet sewage sludge, stored in refrigerated conditions (4°C), was subjected to different times and drying temperatures in the greenhouse and, after being re-moistened been and incubated (28 ± 1ºC) for zero, one and three days. The methods of fumigation extraction and Mehlich3 were used to assess the microbial activity and availability of metals, respectively. Increased levels of soluble organic carbon and iron were directly related to the increase of incubation time and temperature drying. For leaching, the dried sludge’s were packed in PVC columns of 50 mm and related to the mass of wet sludge, on a dry basis. The increase in drying temperature of the sludge increased the organic carbon dissolved in the leachate extract, without, however, a corresponding increase in metal content in this extract.