| With the rapid development of China's economy, rapid urbanization and improvement of living standards have brought about an unprecedented increase in the amount of municipal solid waste(MSW), causing a lot of environmental pollutions in cities. Therefore, proper waste treatment is an urgent and important task for the continued development of cities. However, China's MSW classification and management are not sound, which makes MSW to be collected in a mixing way and reclamated in difficultly. Meanwhile, the traditional landfill, incineration, anaerobic digestion, composting and other waste treatment technology have poor efficiency on China's MSW since its high organic content, high moisture content, low heating value and complex components. It mainly presents in inadequate waste resources recovery and quantitative reduction, secondary contamination problems and so on. High pressure extrusion pretreatment is considered as an effective separation technology of mixed MSW without previously classification at collection period, which separates mixed MSW into dry and wet fraction. As a result, the heating value of dry faction is increased and convenient to incineration due to its low moisture content. While the wet faction in MSW has the highly degradable organic content that can be used for the anaerobic digestion. The technology can eventually achieve the purpose to mixed MSW resources recovery and quantitative reduction.In this paper, the efficiency of dry and wet fraction separation was tested with small scale high pressure extrusion apparatus for mixed MSW. Properties of dry and wet fraction were also examined in this experiments. These results show that the volume reduction rate of mixed MSW at different pressure were more than 64% after high pressure extrusion pretreatment. It got dry fraction of 41.4% and wet faction of 51.5% under 20 MPa pressure,which had the better separation efficiency than that of other pressure. At the same time, the largest low heating value of dry fraction was 14503 k J/kg as the ratio of volatile solid content(VS) and total solid(TS)(VS/TS) were more than 90% and the moisture content were less than 30% at different pressure. While the wet fraction had good biodegradability since all the moisture content and the value of VS/TS in different wet fraction were more than 84% and 76.7%, respectively, and all the C/N value were between 15 and 30. However, when the pressure was increased, dry and wet separation efficiency become drop, including the decreasing of the heating value for the loss of organics in dry fraction and decreasing of methane production performance in anaerobic digestion process for the entry of plastic and paper into the wet fraction.In addition, the experiments were enlarged for the original mixed MSW from the small scale test to pilot scale test. The results showed that the pilot test had the similar efficiency with small scale test. The minimum moisture content of dry fraction was 28% at different pressure. But the largest low heating value was 15883 k J/kg at the pressure of 22 MPa. The wet fraction also had good biodegradability since all the value of VS/TS in different wet fraction were more than 55.6% and all the C/N value in different wet fraction were between 20 and 24. Moreover, the largest methane yield was 521 m L/g VS for the wet fraction at pressure of 22 MPa.On the other hand, gas production performance of anaerobic digestion and the operation stability of anaerobic digestion system were investigated by semi-continuous anaerobic digestion and 24 h cycle experiments with different organic loads for wet fraction. The experiments were tested for 96 days. The results showed that gas production at different organic loads were relatively stable since methane content of biogas keeped at about 63% and the daily biogas yield of per VS maintained between 0.63 L/(g VS·d) and 0.70 L/(g VS·d). However, the removal rate of VS had a slightly decreased but keeping at 78%, which had the better effect than theoretical calculation value. Additional, the alkalinity and concentration of volatile organic acids(VFA) maintained at the normal range of anaerobic digestion that could maintain the p H value at about 7.3, which showed that the system runs in stability. It can be seen that the organic loads could still be improved for this anaerobic digestion process of wet fraction. At the same time, 24 h cycle experiments also showed that all the value of V(CH4)/V(CO2) in biogas were almost consistent with the classical theoretical value of 1.39, which illustrated that the biogas production of this anaerobic digestion process were in good condition.This study has important demonstration value for the development of mixed MSW disposal technology, which contributes to the transformation of China's mixed MSW disposal mode. |
PDF Full Text Request