A Study Of Coupled Temperature And Chemical Processes In Municiple Solid Waste's Landfill

The stabilization of waste in landfill is essentially a biodegradation process of theorganic component. Analyzing from the chemical reaction kinetics, temperatureexerts important influence upon degradation process of organic waste. On the otherhand, the thermal effect of chemical reaction process changes temperature fielddistribution in landfill body analyzed by thermodynamics method. Therefore thelandfill body is in an equilibrium system under the interaction of multipleenvironmental factors such as temperature and chemistry.According biochemical reaction mechanisms and the chemical expressions ofanaerobic biodegradation of organic component, the thermal effect of anaerobicdegradation reaction process of organic waste was analyzed. By the laboratorycolumns simulated experiment, the effects of temperature to anaerobic degradation oforganic waste were researched. Then the simulated distribution of temperature inlandfill body under coupling effects of temperature and chemistry was furtherresearched.The chemical components of MSW mainly are constitute by carbohydrates,proteins and fats, which could be corresponding expressed by typical and simplifiedchemical molecular: (CH₂O)₆, C₄₆H₇₇O₁₇N₁₂S and C₅₅H₁₀₄O₆.The anaerobic degradation process could be divided into three stages. Accordingto the degradation mechanism in each stages, the degradation process was expressedby chemical expressions which had been further simplified and quantized. Thequantitative analysis indicated that, During the whole degradation processes of refuse,about 0.34 kg·kg^-1 methane and 0.90 kg·kg^-1 carbon dioxide were generated fromprotein, 0.27 kg·kg^-1 methane and 0.73 kg·kg^-1 carbon dioxide were generated fromcarbohydrate, 0.72 kg·kg^-1 methane and 0.82 kg·kg^-1 carbon dioxide were generatedfrom fat.On the basis of MSW of Chengdu city as an example, it was studied the thermaleffect of the Degradation of organic waste during whole three stages. The resultsshowed that, During the whole degradation processes of refuse in landfill, it releases 168.97kJ·kg^-1 and it releases 14.37 kJ·mol^-1 energy of organic carbon degradationfrom the solid phase Transform into the gas phase, and it releases 26.08 kJ·mol^-1energy of methane produced during the whole three stages.It was shown from the simulated experimental study by conducting five wastecolumns simulated experiment under different temperatures that: the highertemperature was, the more advantageous to enter acidogenic and methanogenic stagein the range of 29～45℃, thus higher temperature accelerated stabilization. Methanecontent was the most stable in the methanogenic stage when the control temperaturewas 41℃, and it was beneficial for recycle of landfill gas.Using the relative thermodynamic theories, the influences of temperature on theequilibrium constant of the typical reaction in the methanogenic stage was analyzed.If taking no account of microbial effect, the higher temperature was, the moredisadvantageous to degradation of organic waste and transformation from solid phaseto liquid phase; whereas it was contrary for the methanogenic stage.Because the essence of the degradation of organic waste in anaerobic landfill is acomplicated biochemical reaction process, which involves several kinds of bacteriabiochemical and degradation phases, and temperature has important effects onmicroorganism and degradation reaction, the effects of temperature on anaerobicdegradation could be expressed by these two aspects. It was shown from thesimulated experimental study and analysis that, therein to, the effect of temperatureon microorganism played a leading role.Taking the attenuation coefficient of COD concentration from leachate as theembodiment of anaerobic degradation of organic waste, and the quantitative formulasfor effects of temperature on anaerobic degradation were established. The resultsindicated that there were two peak values among the optimum temperatures. Theattenuation coefficient of COD concentration from leachate achieved maximumvalue at 30.8℃and 39.7℃. When lower or higher than these two temperatures, thecoefficient showed evident downtrends. The first peak value reflected the promotionof temperature on hydrolytic and acidogenic stage, and the reason for this was theexothermic reaction which induced migration to low temperature. The second peak value reflected the promotion of temperature on methanogenic stage, and the reasonfor this was the endothermic reaction which induced migration to high temperature.Many experimental results could be explained by this conclusion.Based on above researches, the model of coupled temperature and chemicalprocesses in landfill was established. Then, the distribution of temperature in landfillbody was predicted by the model. The simulated results showed that: as temperatureincreased with the landfill time, temperature also increased with the landfill depth;temperature increased rapidly in the initial time with the maximum value of 55～60℃,and temperature increased slowly in the later time; however, the value of simulatedtemperature was higher than actual value due to neglecting the feedback effect of thethermal effect on biochemical reaction in degradation process. Therefore, thesimulation results considering coupling effect were more practical and reliable.