Degradation And Stabilization Process Of Refuse At Landfill

Refuse in landfills becomes stabilized and an aged refuse is thus formed, as the organic matters in the refuse degrades during its long-term stabilization process. Excavation of aged refuse has been practiced and the recoverable materials can be reclaimed and the resultant landfill space can be reused for the re-placement of fresh refuse. However, the prerequistic for the sustainable landfill of refuses is dependent on the stabilization extent.In this work, the refuse at Shanghai Refuse Landfill was sampled from the compartments closed from 1991 to 2004, with a placement age of 1-14 years. The composition of the refuse wase determined and their relationships with landfill ages were established. Furthermore, the environmental impact and technical and economical feasibility refuse excavation and recycling was assessed.The refuse stabilization process was formulated. Fourteen parameters were selected to characterize the stabilization extent of refuse. Humic composition and molecular weight of humic substance in the refuse sampled was used as stabilization indexes of refuse and closed landfill compartments. It was found that total extraction ratio of humic substance to organic matters (y₁) and HA/FA ratios (y₂) of humic substances linearly decreased and increased with landfill age (x, year) with equationsof y₁=0.0697·x + 1.3041 (R²=0.7416) and y₂ =-1.2545·x + 33.616 (R²=0.7282),respectively. Molecular-weight polydispersity index (PDI, y) of the refuse significantly increased with landfill age (x), with a relationshipof y = 25.786·exp(-0.1477·x) (R²=0.88). Based on these equations, the refuse with PDI of over 15 may be used an index of aged refuse and can be excavated.As the excavation of aged refuse destroys the final layer structure, the air pollutants will be released from compartments. To assess environmental impact, the air quality in the aged refuse excavation and separation sites was monitored. It was found that the concentrations of air pollutants at these sites were complied with the Integrated Emission Standard of Air Pollutants (GB16297-1996) and Hygienic Standards for the Design of Industrial Enterprises (GBZ 1-2002). Among the air pollutants monitored, the main pollutants at excavation site were total suspended particles (TSP) and inhalable paniculate matters (PM₁₀). However, the pollutants risks can be well controlled within the acceptable level when the personal protective equipments (PPE) were used. No pathogen and virus were detected in the aged refuse. To provide detail information for refuse recycling, all excavated refuses were sorted into eight fractions, finer refuse, plastic, fiber, paper wood, glass and metal. Finer aged refuse and mixed plastic was found to be the two main recoverables of the excavated aged refuse, with 14-year averaged contents of 50 percent and 22 percent. The contents of plastic and finer refuse among these fractions were found to increase and decreased with placement time repectively. A logarithmic relationship was established between plastic contents (y, %) and placement time (x, year),y = 10.269×ln(x-1990) + 7.03 (R²=0.90). Based on the equapition, the plasticcontent was predicted to supass finer refuse and will occupy half of refuse in 2007.The mechanical and thermal properties of mixed plastic were also tested and compared with wasted plastic in MSW. It was found that the properties of the mixed plastics in refuse were not significantly deteriorated with landfill age of below 8 year. However, weathering process of plastics was accelerated when the landfill age was over 8 years. The plastic at the landfill can be recycled using the technologies done in the conventional plastic processing.