Aerobic pretreatment of mechanical screened MSW: Effects on Landfill anaerobic biodegradation

The Municipal Solid Waste (MSW) resulting from Source Segregated (SS) collection follows different fate depending on the waste management system implemented in the several European States. The EU27 mean data shows that about 38% w/w of the 255.000.000 tonnes per year of waste produced is directly disposed in landfill without any pretreatment. This makes landfill one of the most important sources of anthropogenic GHG emission, contributing for many EU states from 3 to 13%. Further, landfill produces also other pollutant substances, as leachate, that has to be correctly managed for avoiding environmental threats. In that areas that lacks of incineration facilities, the Mechanical Biological Treatment (MBT) of the MSW represents a possible solution for trying to increase the amount of recovered materials and reducing both the mass and the reactivity of the waste disposed in landfill. Combining the MBT with the management of the landfill in bioreactor mode, instead of traditional one, can represent a sustainable solution for waste management in many European areas. Inside bioreactor landfill, biological aerobic and/or anaerobic processes are enhanced for achieving waste stabilization and environmental recover of the site, in a narrowed period (i.e. 10-15 years instead of 40-50 years). The attention of this work has been focused mainly on the MBT treatment before landfilling. Starting from the Waste Organic Fraction (WOF) arising from the mechanical sorting section of an existing MBT facility, different analysis have been performed on the effect of aerobic pretreatment length both on WOF stability and on its behaviour during anaerobic degradation process. Aerobic treatment has been performed on 500 kg of WOF by two aerobic bins of about 800 litres of volume each one. The Landfill Gas production (LFG) has been evaluated by a modified incubation test, treating about 5 kg of WOF sample with no inoculums. The stability achieved by the WOF after different days of aerobic pretreatment has been evaluated both by a self developed Dynamic Respiration Index (DRI) apparatus and by a fermentation test for the evaluation of the GB parameter. Both the WOF as produced by the MBT and the WOF samples with different days of aerobic pretreatment have been characterized by Moisture Content (MC) and Volatile Solids (VS) determination. As expected, longer is the aerobic pretreatment length lower is the humidity of the sample. After the 30 days both the MC and the VS concentration remains quite constant. The most suitable GB parameter for comparing the different WOF samples suitable is the BG at 42 days, as a consequence of the long lag phase occurring for the WOF with 0 days of aerobic pretreatment. GB at 42 days decreases as the days of pretreatment rises. The DRI confirms substantially these results eve if the trend is less regular. The maximum DRI potential is achieved for the sample with 0 days of pretreatment, about 5,000 mgO2/kgVSh, and the minimum is achieved for the sample with 120 days, about 330 mgO2/kgVSh. The aerobic pretreatment increases the rapidity of the methanogenic phase, related to one of the WOF with 0 days, achieving its maximum value for the sample with 30 days. In this case the LFG production related to the VS content of the WOF as withdrawn from the MBT is of about 85 Nl/kgVS0 with a mean methane concentration of about 45% up to 55% v/v. These results shows the positive effect that aerobic pretreatment can have on the anaerobic biodegradation process of the biodegradable fraction of MSW, aspect that can lead to further benefits if combined with the management of the landfill in sustainable mode.