Abstract
Experiments with the composting of municipal sewage sludge were made to examine the effect of different bulking agents and of the oxygen supply on the process. The bulking agents have been chopped straw, milled bark and sawdust as organic materials and Styropor—foamed polystyrene as a synthetic material. The bulking agents have been dried at 105°C and then mixed with the same quantity of digested municipal sewage sludge. Because of the different water capacity of each material different structures were reached. i.e. the free air space in the mixture differed, because each material extracted a different quantity of water out of the sludge. This effect can be seen in Table 1, while a scheme of the experiment is given in Figures 1 and 2.
In a second run the oxygen content was changed from 21 Vol-% to 50 Vol-% in dry air at the same rate of 17 1/h for 3 min in each hour. In both runs the composting process was followed by measuring the temperatures and the CO2-rate in the exit air. For comparison two parameters have been calculated, the energy production and the carbon loss which are shown in Table 2. These results show that bark was the best bulking agent (2608 kJ and 45.4 g carbon loss), while the synthetic material produced almost no activity (326 kJ and 4.2 g carbon loss). The effect of the higher oxygen content can be seen by comparing the curves of the temperatures in Figs 3 and 4 showing the characteristic form. After reaching the maximum, the temperatures stayed for a longer time on a higher level during the second run, marking a higher activity of the process. The curves of the CO2-rate given in the Figs 5 and 6 show the same effect.
Overall it is evident, that the structure of the mixture, i.e. the bulking agent has a greater effect on the composting process than the increased oxygen content.
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