The objective of this study was to analyze the degree of decomposition of municipal solid waste (MSW) placed in a landfill site and, based on this analysis, to propose an investigatory procedure to be used to assess the energetic potential of any site.
A sampling technique was developed in order to reduce the size of the MSW sample to 25-30 kg with an acceptable error of 20 %. Profiles of the strata were established which have shown a well ordered structure in horizontal layers corresponding to different MSW ages. The location of the two drill holes were chosen consequently. A sample per MSW age was carried out for each drill hole. Their degradation states were measured using an accelerated biodegradability test which also gives the methane potential.
For all samples, the evolution of four parameters, temperature, % CH4, % VSS and methane potential, were analysed as a function of their age. Anaerobic microbial activity was detected in the top layer of the landfill, showing that refuse starts as soon as deposited to undergo anaerobic transformation. Indeed, the % CH4 reaches its maximum value of 60 % between 5 and 10 m. Temperature increases from about 26°C to 50°C in the first 15 m and stays at this maximum value. That shows that municipal wastes are degraded under mesophilic and thermophilic conditions.
Two relations between waste age and % VSS, and waste age and the energetic potential have been demonstrated. The % VSS represents about 65 % of the dry mass for the fresh wastes and is quite low for the 6 year-old refuse. This rapid waste transformation rate could be due to optimal conditions for anaerobic activity existing in the landfill. The methane potential is proportional to waste age, almost zero for the 6 year-old refuse and greater than 45 1.kg.−1 for fresh wastes. These results show that a simple and rapid measurement of the % VSS of an unknown sample could be a good indication of its age and its degradation state.
As a result of this study, a two-step procedure has been developed that enables an evaluation of the energetic potential of a landfill site. The first step is to collect all the general information on the site, thus providing an initial estimate of the decomposition degree within the mass of the waste. The second step is then to confirm this estimation by sampling and analysing a few samples taken at shallow depths.