A new strategy for composting has been developed, based on O2 feedback control. Experiments were carried out on composting the biodegradable fraction of municipal solid waste in a closed bioreactor, aerated by pressure ventilation. Ventilation was controlled in order to maintain the O2 level in the internal atmosphere of the composting mass between 15 and 20%. The new strategy seems to give satisfactory results in terms of process control, quality of end-product, low energy consumption, and hygienization of compost. These results were supported by analyses of: (1) the variation of the main microbial groups during composting; (2) the balance of material; (3) the gas present in the internal atmosphere (O2, NH3, CO2, H 2S); (4) phytotoxicity; and (5) pathogenic and indicator micro-organisms. The importance of carrying out these analyses to guarantee the quality of the end-product and to control the process is discussed. Finally, the new system for controlling composting is compared with the existing strategies (Beltsville and Rutgers systems).
Little information exists about nitrogen losses through microbial activity during treatment of solid urban waste (SUW) by processes such as composting. In the present study, in addition to evaluating the pattern of nitrogen losses by denitrification at different stages of the process, a comparison between the method of Pochon and Tardieux, and an improved gas chromatographic method for estimating denitrifying populations was undertaken. Though the MPN (Most Probable Number) enumerations were higher using the colorimetric method than the gas chromatographic one, the pattern of the two graphs showing numbers of denitrifiers during composting were the same. The highest numbers were revealed immediately after loading the reactor (107-108/g d.w.), lower numbers of denitrifiers were found in the second sampling corresponding to the thermophilic phase (103-104/g d.w.). These numbers increased gradually as the waste material stabilized (10th to 123rd day of composting) to again reach values of 107-108/g d.w
Humic acids obtained from a commercial source and isolated from mushroom wheatstraw compost inhibited nitrification by Nitrosomonas europea and Nitrobacter agilis in a continuous‐flow column at concentrations in excess of 100 μg cm−3. Lowering the pH of the input medium also reduced the rate of nitrification but in the presence of the compost, and to a lesser extent the commercial humic acid, pH‐inhibition was relieved. With the commercial humic acid, this effect can be explained in terms of the buffering capacity of the extract.
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