Biostabilization of municipal solid waste

A mechanical-biological process for municipal solid waste (MSW) treatment was monitored for one year. Mechanical pre-treatment provided two fractions. The oversize fraction (Ø>50 mm) (yield of 600 gkg-1ww) (46 Mgday-1) was used for refuse derived fuel production, after undergoing a mechanical refining processes, because of low moisture content (200-250 gkg-1) and high calorific value (2500-2800 kcalkgww -1). The undersize fraction (Ø<50 mm) (yield 400 gkg -1ww) (30 Mgday-1) contained about 800 gkg-1 of the MSW organic matter. This fraction was biologically treated using an aerobic process with an organic waste fraction from separate collection (77 Mgday -1) and recycled stabilized material (62 Mgday-1) obtained from end-product sieve (Ø<20 mm) used as bulking agent. A retention time of three weeks was sufficient to obtain stabilized products in agreement with up-dated rules of the Lombardy Region (North Italy) regarding biostabilization and composting processes. Dynamic Respiration Index (DRI), such as required by both Lombardy Region rules and suggested by the European Community, was chosen in preference to other indices in order to assess the degree of biological stability of the end products. A mean DRI value of 1164 mg O2 kgSV-1h-1 was obtained and is in agreement with the proposed limit of 1000±200 mg O2 kgSV -1h-1. Self-heating test, potential biogas production and fermentable volatile solids were also used as parameters to describe the potential impact of treated waste, providing further useful information. Nevertheless, all of these methods revealed analytical or interpretative limits. A complete mass balance of the biological treatment section showed that, from a net input of 107 Mgday-1, only 250 gkg-1 (27 Mgday -1) of the waste needed to be landfilled, with 750 gkg-1 (80 Mgday-1) being lost as CO2 and H2O.