Mechanism Of Leachate Generation, Transport And Mound In MSW Landfills And Control Of Leachate Level

Large leachate production and high leachate mounds are common in municipal solid waste (MSW) landfills of China, which can cause severe problems including instability of waste body, pollutant transport and landfill gas explosion. Based on systematic tests of hydraulic properties of landfilled wastes, the research works in this paper focused on the study and simulation of leachate generation, transport and mounds in municipal solid waste landfill as well as their control measures. The main research works and conclusions are as follows:(1) Based on available testing results, the average initial moisture content of landfilled wastes in western countries was found to be about 27%. However, large amount of organic matters in fresh wastes contributes to the much higher initial moisture content in Chinese MSWs with an average value of about 52%. After landfilling, leachate is gradually generated during the compression and biodegradation processes of wastes.(2) Field capacity tests were carried out on drilled MSW samples with different ages. It showed that field capacity is related to the age and stress level of landfilled wastes. Field capacity decreases with the increase of stress level and waste age. Soil waste characteristic curve (SWCC) of the drilled MSW samples was summarized.(3) On-site pumping test in Suzhou Qizishan Landfill were carried out to study the saturated permeabilities of MSWs in different depths. It was found that saturated permeabilities of MSWs ranges from 2.62×10-4cm/s to 5.76×10-4cm/s. The value tends to decrease with an increase of depth which can be simulated by the power function. Due to the complexity of waste composition, biodegradation and compression, saturated permeabilities from on-site tests reported by foreign studies have a very large range from 1×10-5cm/s to 1×10-1cm/s. Saturated permeabilities of wastes in Suzhou Qizishan Landfill are at the middle level of that large range.(4) Laboratory hydraulic tests were taken on the drilled MSW samples from Suzhou Qizishan Landfill and Chendu Chang’an Landfill. The results showed that saturated permeabilities decreases with an increase of effective stress. The value decreased from the order of 10-4cm/s under 20kPa to 10-6cm/s under 250kPa. Compared to laboratory testing results reported by foreign studies, Chinese MSWs showed a relatively lower value.(5) Based on the study of field capacity and leachate generation mechanism during biodegradation and compression processes, a leachate generation calculation model was proposed. The effect of initial moisture content on the leachate production of a valley landfill was studied by this model. With an increase of initial moisture content, it was found that leachate production increases. At the same time, leachate generated from the initial moisture content make up a larger part of the total leachate production. As initial moisture content of landfilled MSWs in most Chinese landfills can even reach 60%, the predicted leachate production will be much underestimated if the contribution from the initial moisture content is neglected.(6) Based on the above study of leachate generation process, a modified model was proposed for the leachate generation rate calculation based on the model in the standard of leachate treatment. The monitored leachate production data were simulated through the modified model. A secure tendency of the simulated results indicated that the model is fit for the design of leachate treatment in Chinese landfills.(7) Based on the tested values of hydraulic parameters and numerical model for the valley landfill, the influence of initial moisture content, precipitation, separating system of rainwater and sewage, waste permeability and clogging of leachate drainage system on leachate mounds processes were studied. It showed that the separating function tends to be worse with an increase of initial moisture content, after which leachate mound has more possibility to be formed due to the decrease of waste body permeability and the clogging of leachate drainage system.(8) Based on numerical study and on-site monitoring results, existence forms of the water in landfilled waste were studied. It was found that, if leachate drainage system is in good function and permeability of wastes at the bottom of the landfill is relatively low, leachate in leachate drainage system might be separated from the main leachate layer in the above waste body. Under this condition, three main forms of saturated water exist in a landfill including leachate in drainage layer, main leachate layer in waste body and partially blocked leachate mound, and the leachate head in the drainage layer may be low. If the leachate drainage system is clogged, leachate in the drainage system can be elevated until connecting the water in waste body. In this case, there are only two forms of saturated water region in a landfill which is main leachate layer and partially blocked leachate mound, and the leachate head in the drainage layer may be high.(9) Stereospecific drainage system composed of bottom leachate drainage layer, vertical drainage well and horizontal drainage trench in waste body was proposed to for the leachate control of Chinese landfills with very large leachate production. The effect of vertical drainage well in practical use was numerically studied. It showed that drainage effect is better with deeper vertical drainage wells. Wastes around the vertical drainage wells become less permeable during long-term leachate drainage process. However, the anisotropic property of permeability in layered wastes showed little influence on the drainage results of vertical drainage wells. For the practical application in a landfill, shallowly but densely (i.e. small spacing) installed vertical drainage wells can be applied in the part with large leachate production. Conversely, sparse (i.e. large spacing) but deeply installed vertical drainage wells can be used for the part with relatively lower leachate production.(10) The effect of horizontal drainage trench on leachate control was numerically studied. The simulating results showed that horizontal drainage trenches can decrease the main leachate level and reduce hydraulic head in waste body effectively. The anisotropic property of permeability in layered wastes showed little influence on the drainage results of horizontal drainage trenches. Intermediate liner system should not be installed beneath the horizontal drainage trenches.