Study On Combustion Properties And Landfill Behaviour Of Mixed Msw Based On Mechanical-biological Pretreatment

China as a developing country has a relatively poor infrastructure and low level of MSW management. The MSW generated in most cities is collected without separation and contains a relatively high proportion of food waste. As a result, the MSW is typically characterized by high moisture content, low heating value and high organic portions. Pressure on limited land resources and long-term environmental pollution such as leachate and landfill gas emissions accompany mixed MSW landfills. The low heating value and high moisture content of mixed MSW affects the economic applicability of incineration and may cause serious pollution in the environment. Mechanical-biological pretreatment (MBP), which could enhance evaporation of moisture and improve heating value by air ventilation and heat generated by biodegradation, has been regarded as a good pretreatment prior to landfill and incineration. Currently MBP facilities are in operation in some developed countries.In this work, status of municipal solid waste management in typical cities of China was investigated systematically. Characteristics of raw MSW and typical components of MSW were analyzed. Influencing factors and characteristics of MBP process were identified. Combustion properties and landfill behaviour of MSW after MBP process were studied. A treatment technology based on MBP suitable for mixed MSW generated in China was put forward. The technology had been applied in Pulandian city and Pikou town. In this dissertation, the following several parts of work have been done:(1) The study investigated the status of MSW collection, transfer, transportation, treatment and final disposal in urban district of Dalian and its surrounding small and medium-sized towns. Landfill (sanitary landfill and simple landfill) was the main MSW diposal method in the whole area of Dalian. Simple landfill yard and open dump in small and medium-sized towns led to serious pollution problems such as leachate and flue gas produced by spontaneous combustion process. MSW samples were collected from waste containers of different regions. The physical chemistry analysis including bulk density, composition, moisture content, combustible and incombustible fractions, and lower heating value (LHV) was carried out. The high proportion of food waste (>50%) and moisture content (50.7%-56.9%) and low LHV (3310-4129kJ/kg) indicated the mixed MSW was not suitable for landfill or incineration directly. The ultimate analysis, proximate analysis, total volatile solids (TVS), total organic carbon (TOC), total Kjeldhal Nitrogen (TKN) and C/N ratio of typical components (plastic, rubber, textile, paper, wood, food waste, plant and ash) and mixed MSW were measured.(2) In this study, four MBP reactors were operated in order to specify the influence factors of MBP process. The results showed that combustible components (plastic, rubber, textile, wood, paper) improved porosity and reduced moisture of the food waste. The MBP process of mixed MSW had a minimum COD, compared to untreated MSW in landfills. The feasible air-inflow rates at different stages (earlier, middle, later) were2.7×10-3,0.081,3.6×10-3Nm3/min·m3respectively. The intial particle size of mixed MSW that was suitable for MBP process was5cm. Mechanical agitation could not only reduce the particle size of biodegradable components effectively, but also reduce the material temperature obviously. At middle stage, mechanical agitation should be stopped in order to maintain the heat generated by biodegradation. The height of MSW could be reduced gradually in the MBP process and the heat loss increased. The temperature increased rapidly accompanied by supplementary MSW material. Compared to compression and fermentation pretreatment, the MBP process parameters of mixed MSW had been systematically studied.(3) Combustion properties of mixed MSW, mechanical-biological pretreated MSW (MBP MSW) were studied. Compared to compression and fermentation pretreatment, the moisture content of MSW dropped from61.2%to11.3%within12days of MBP process. The LHV increased from4862kJ/kg to12843kJ/kg. The particle size of biodegradable components sharply reduced during the period of9days. The material particle size of less than7mm was characterized with high ash content (54.6%) and low LHV (5505kJ/kg) and was not suitable for incineration. The kinetic characteristics of non-isothermal thermo gravimetric analysis of initial MSW and pretreated MSW was studied and kinetic parameters were obtained by Coats-Redfern method. The analysis results of moisture content, LHV, particle size distribution, apparent activation energy, theoretical air volume, combustion flue gas volume, theoretical combustion temperature indicated that MBP had a significant influence on combustion properties of mixed MSW.(4) Landfill behaviours of mixed MSW, mechanical-biological pretreated MSW (MBP MSW) and compost were studied. The weights of MBP MSW and compost was much lower than mixed MSW. There was no leachate generated in the MBP MSW and compost landfill reactor at the closed landfill stage. The characteristics (COD, NH4+-N, pH, EC, colority) of leachate generated in different material landfill after precipitation were determined. Compared to leachate generated in untreated MSW landfills (COD:6,276-18,326mg/L; NH4+-N:332～621mg/L), the leachte generated in the pretreated MSW landfill had a minimum COD (1,757～9,874mg/L) and NH4+-N (22～154mg/L) value. (5) According to the characteristics of mixed MSW generated in China and the MBP experimental results, an integrated treatment technology of mixed MSW based on MBP was put forward. The technology had been applied to Pulandian MSW treatement plant and Pikou MSW reduction and resource transfer center.