Experimental analysis of the aerobic stabilization efficiency of an exisintg MBT plant

The management of the Municipal Solid Waste (MSW) has been strongly improved by the last Waste Framework Directive 2008/98/CE, by increasing the amount of waste materials that have to be reutilized, prepared for reutilization and/or recycled. In any case, even if these goals will be achieved, a large amount of residual MSW from the Source Segregated collection (SS) will need to be properly managed. Incineration is a quite suitable way for managing these materials even if it is costly and not homogenously spread in the different European Areas. Another diffused solution is represented by Mechanical Biological Treatment (MBT) facilities that can lead to some advantages as producing and high quality Solid Recovered Fuels (SRF) or reducing and stabilizing the mass of the waste to be disposed off. In particular, this last solution was the first exploited in MSW management since many years ago. Currently many of these plants operate in quite different conditions related to the design one. The difference arises from the variation both in waste management strategies and in the waste rates treated. These differences can influence both the mechanical and the Biological Treatment (BT) sections of the facility leading tom some modifications in plant efficiency. The present study analyses the BT section of an existing MBT facility operating the aerobic stabilization of the Waste Organic Fraction (WOF) produced by the mechanical screening of the MSW resulting from SS collection. This BT section is a continuous flow composting plant with an aerated flow through which electrical fans provides the process air. The continuous flow is performed by a crane bridge with screw that moves on the basin area. The screws provide to stir and move ahead the WOF from the inlet to the outlet section. The analysis has been focused on the evaluation of the main WOF physical and biological activity features during different days of the treatment period. Different sampling points have been fixed on the basin area corresponding to specific mean days of treatment. The stability achieved by the WOF in the different sampling points has been evaluated by a Dynamic Respirometer Index (DRI) apparatus developed by the LARn of the University of Perugia, able to evaluate the oxygen uptake rate (mgO2/kgVSh) of the material. Main results show that the screws has a relevant influence on some process parameters as temperature, Volatile Solids and Dynamic Respirometer Index potential. The continuous mixing effect combined with the continuous WOF introduction in the basin, produces a temperature profile that in some cases is quite constant during the whole basin length. Similarly for the VS concentration that seems not to vary significantly from the inlet to the outlet section. The high temperature level profiles combined with the amount of air injected beneath the WOF bed, causes a rapid humidity reduction in the first days of the process that can lead to an inhibition of the aerobic bacteria activity. This is confirmed by the VS concentration but also by the DRI values. The DRI potential of the WOF at the basin inlet is practically reduced in the first 25-50% meters of the basin length, remaining quite constant in the remaining part. Some improvement concerning process air regulation along with the possibility of increasing the WOF humidity if becomes to low, can lead to improve the stabilization process efficiency together with a reduction of the global impact of the MBT process.