Large-scale Experiment On Biochemo- Hydro-mechanical Behaviors Of High-food- Waste-conetnt MSW And Applications

Currently, nearly 240 million tons of municipal solid wastes (MSW) were produced per year in China,with an increasing rate of 8?10%. Proper management of the increasing quantities of MSW would be a great challenge. The landfill method with processing principle of "harmless, reduction and recycling" is expected to be the preference in current and the foreseeable future in China, to meet the requirements of municipal environment security and sustainable development. The landfilled MSW is subjected to biochemical,hydraulic and mechanical processes. These connected processes are more prominent in the high food waste content (HFWC) MSW due to the rapid hydrolysis of food waste. The coupled processes are associated with many geo-environmental problems, i.e., slope instability, leachate leakage, landfill gas (LFG)diffusion. Therefore, an improved understanding of these connected behaviors of HFWC-MSW will be the key to prevent and control the geo-environmental problems.The research works in this paper are funded by the National Basic Key Research Program (973), via Grant No. 2012CB719802. In this work, both large-scale and small-scale experiments on HFWC-MSWs were carried out, the biochemo-hydro-mechanical behaviors were analyzed,the predicting models for moisture retention capacity (MRC) and permeability were estibilished, and the methods to estimate the leachate production, separate the gas-liquid connected effect on drainage, and control the disaster sources in the landfill stabilization process were proposed. The following findings were obtained:(1) Materials and methods for large-scale experiments. Two large-scale bioreactor experiments, CELL1 and CELL2, were presented to investigate the biochmo-hydro-mechanical behaviors of HFWC-MSW. The sizes of CELL1 and CELL2 were 5.0×5.0×7.5m and 5.0×4.0×7.5m, and durations of 1050 and 710 days, respectively.In the experiments, surcharge loadings were applied on waste surface to simulate the landfilling stress, leachate drawdown and refilling tests were conducted to measure the liquid permeability, air pumping tests were carried out to measure the gas permeability, and mature leachate refilling and warm leachate recirculation were performed to improve the degradation process. The leachate quantity, leachate level,leachate biochemistry (pH, COD, BOD, VFA, ammonia, alkalinity), gas composition(CH4, CO2, O2), waste temperature, earth pressure and waste settlement were measured. Besides, the liquid and gas existing patterns were monitored by time domain reflectometry (TDR) method, electrical resistivity tomography (ERT) method and pore pressure tubes. The results presented the signs of perched leachate in waste body while the leachate level was drawn down to the bottom, and revealed an accumulation-dissipation process of pore gas pressure in the waste underwater, and the peak pressures were 1-13 kPa larger than pore water pressure.(2) Self-released leachate from HFWC-MSW and estimation of leachate production at landfills. The large-scale experimental results showed that the leachate production rate (mass ratio between collected leachate and initial buried waste) was 26.5-29.4%in two months after waste filling, being 96% of the total amount collected in one year.The self-released leachate was estimated to contribute to over 55?70% of the leachate production at landfills in South China, which is often overlooked in the leaching coefficient method for the prediction of leachate production. The small-scale experiment found that the moisture retention capacity (MRC) of fresh HFWC-MSW decreased exponentially with degradation time under a sustained stress, and decreased linearly with a logarithmic increase of stress. The variation of MRC appeared to be independent of stress path in terms of stress and degradation time. Based on the test results, a time- and stress-dependent model was proposed for predicting the MRC of HFWC-MSW. The model was developed to evaluate the dewatering effect of the HFWC-MSW pile, and the strategy of combining the degradation-enhancing measures with stress-increasing measures is recommended in a rapid dewatering project. Finally, the model was further developed to predict the leachate production at HFWC-MSW landfills, in which the precipitation infiltration, the self-released leachate and the landfilling process were concerned. The calculated results of leachate production at Tianziling landfill (phase II) are in well agreement with the measured data.(3) Build-up of leachate leavel and its influence on effective stress. The large-scale experimental results showed that the top level of leachate mound was measured as 88%of the waste thickness after waste filling, and reached to over 100% after a surcharge loading of 32.2 kPa. The high leachate level was attributed to the great quantity of self-released leachate, significant compression of waste pile and large volume of gas entrapped underwater. The contribution from the entrapped gas was estimated to be 22?28% of the total leachate level at the stable methanogenesis stage. The self-weight effective stress of waste was observed to be close to zero under the condition of high leachate level after waste filling. Leachate drawdown led to a gain in the self-weight effective stress. A surcharge loading with a layer of dense materials on waste surface resulted in an increase of effective stress in waste.(4) Leachate-gas connected effect on drainage and separating method. The large-scale experimental results showed that the gas entrapped below the leachate level resulted in a reduction of hydraulic conductivity, decreasing by one order with an increase in gas content from 13% to 21%. Drawdown of leachate level resulted in an increase in the gas content underwater as a result of gas expansion. A permeability predicting model was proposed based on the Darcy' Law, vG-M model and Kozeny-Carman model. It was found that the leachate and gas block each other in the migration pathways in waste. On account of this, a three-dimensional drainage system is recommended to separate the leachate-gas connected effect on drainage, so as to efficiently drawdown the leachate level at HFWC-MSW landfills. The drainage system was successfully applied at Xiaping and Tianziling landfills, the leachate levels were reduced 5?10 m,resulting in an increase of gas collection efficiency and enhancement of slope stability.(5) Waste compression depended on stress and degradation. The large-scale experimental results showed that the waste compression developed rapidly within two months, with compression strains of 0.33?0.47,being over 94?96% of the strain recorded in one year. The primary compression ratio C'C was determined as 0.323?0.381 for fresh HFWC-MSW, while 0.215 for 2.5 years aged waste. The release of intra-particle water was concerned with the much higher C;C of fresh HFWC-MSW than those of fresh LFWC-MSW and aged HFWC-MSW. The accumulation of pore gas pressure would probably result in a rebound deformation in waste under a high leachate level. The small-scale experiment found that the compression strain of fresh HFWC-MSW decreased exponentially with degradation time under a sustained stress, and decreased linearly with a logarithmic increase of stress. The variation of compression strain appeared to be independent of stress path in terms of stress and degradation time.(6) Biodegradation process and degradation accelerating method. The large-scale experiments showed that the rapid accumulation of VFA and sustaining low value of pH occurred due to the rapid hydrolysis of the food waste contained in HFWC-MSW.This tends to result in a slow methanogenesis process, and the recirculation of mature leachate was able to relieve the inhibition of methanogenesis process. The drawdown of waste temperature in winter (typically below 20 ?) resulted in a slowdown of methanogenesis process, and warm leachate recirculation was able to mitigate the problem. The leachate ammonia concentration tended to increase during the whole experimental period, and the intermittent aerobic operation was able to efficiently reduce the ammonia. The rapid hydrolysis of the food waste contained in HFWC-MSW tends to result in a loss of LFG potential of 10?13% with the effluent leachate in the first two months after waste filling.(7) Stabilization process and staged controlling approaches. The Biochemo-Hydro-Mechanical coupled model was verified by the large-scale experimental results,and a typical set of parameters was obtained for analyzing the complex interactions at HFWC-MSW landfills. A framework of landfill stabilization evaluation indices,including solid mass loss, intra-particle water release, consolidation and LFG generation,was established and a stabilization analysis method combining field test and numerical simulation was proposed by the research group of 973 programe.Based on the variations of the above indices, the stabilization process of HFWC-MSW was divided into three stages, which were rapid degradation stage, slow degradation stage and post stabilization stage. The approaches to control the disaster sources during the landfill stabilization were suggested for each stage.