Research On The Feasibility Of Fly Ash/Dry Sludge Modified Clay As Bottom Liner

The use of sanitary landfills is a final treatment option for MSW (municipal solid waste), and is still important in all treatment options. At present, landfill is one of primary treatment options, which is used in most countries, especially in developing countries. Sanitary landfills of the MSW can pollute the surrounding environment and the main problem is that the leachate can easily pollute the surrounding soil and groundwater. Once the leachate enter the environment, it can cause directly harm to people. So, the design of the bottom liner is crucial to the landfill's construction. But the construction of a sanitary landfill leakage control system is a difficult work, e.g. the problem of material source. Due to economy, facile, convenient construction, natural clay liner is known as a good leakage control material for sanitary landfill. And it has been general employed by the landfill construction project. But, the demand of clay to construct a sanitary landfill is stupendous. So, at this dissertation, we think over capitalize on other materials to modify the clay liner. The two materials (fly ash and dry sludge) which we choose to modify the clay liner are both solid waste in need of burying. If we can use them as modified materials to clay liner, that is to say, we can both reuse fly ash/dry sludge and save the consumption of clay soil. We can make effectivity depression in the cost of engineering project, especially in the area which lack of clay soil.This dissertation based on several key issue of landfill liner, such as permeability, attenuation effect, adsorption capacity, strength et al. Overall research on the feasibility of fly ash/dry sludge modified clay as bottom liner. Geotechnical, adsorption and environment characteristics of clay and mixed soils are investigated in the following aspects:(1) In order to examine whether the fly ash and dry sludge has the effect on the Geotechnology of clay liner, especially the effect on the shear strength, and to choose likely mix ratio schemes in diversified mix ratios, liquid and plastic limits tests have been done. We tested the clay and mixed soils liquid and plastic limits. And in order to test the shear strength of clay and mixed soils, fast direct shear tests have been done. In order to confirm the relationship between dry density and water content ratio, compaction tests have been done. It demonstrates that the plasticity of fly ash modified clay soil was reducing. So the mix ratio of fly ash shouldn't be too high. In contrast, the plasticity of dry sludge modified clay soil was ascending. But due to high plastic index is also not good for leakagecontrol effect. so the mix ratio of dry sludge shouldn't be too high, too Generally the shear strength of mixed soils are higher than the natural clay soil. With the mix ratio of fly ash raised, shear strength of mix soils also raised. The main reason for such raise of shear strength is that the friction angelφis increased. By the compaction tests, we can confirm the relationship between the water content ratio and the dry density which is the important parameter of examination of soil column.(2)For the sake of confirming the adsorptive capacity of clay soil and mixed soils, we did the batch-sorption tests.First, sorption tests have been done to confirm the adsorption balance time of COD_Cr and NH4-N by the 4 types of soils. The adsorption balance time of COD_Cr and NH4-N are both 4h by the clay soil. The adsorption balance time of COD_Cr is 4h and NH4-N is 2h by the 5% fly ash modified clay soil. The adsorption balance time of COD_Cr and NH4-N are both 2h by the 20% fly ash modified clay soil. The adsorption balance time of COD_Cr and NH4-N are both 2h by the 15% dry sludge modified clay soil.Secondly, by 4 types of soils and a series of diluted concentration leachate, we process batch-sorption tests. Based on the experimental data, we carry through the mathematical fitting. We can conclude that the initial adsorption effect of COD_Cr is 5% fly ash > 20% fly ash > clay soil > 15% dry sludge. Based on the Langmuir isothermal equation and Freundlich isothermal equation, we carry through the mathematical fitting, it conclude that the adsorption effect of COD_Cr is 20% fly ash > 5% fly ash > clay soil > 15% dry sludge. The reason for 15% dry sludge modified clay soil adsorption effect of COD_Cr is the most weak is that organic substance in dry sludge is high and part of oganic released from the soil. We can conclude that the initial adsorption effect of NH4-N is 5% fly ash > 20% fly ash > clay soil =15% dry sludge. Based on the Langmuir isothermal equation and Freundlich isothermal equation, we carry through the mathematical fitting, it conclude that the adsorption effect of NH4-N is 20% fly ash >5% fly ash > clay soil > 15% dry sludge. The reason for 15% dry sludge modified clay soil adsorption effect of NH4-N is the most weak is that more organic substance contained in the dry sludge, thought from similarity can dissolve each other, it adsorptive ability of NH4-N is the weakest.(3) For comfirming clay and mixed soils actual leakage control ability to leachate, and attenuation ability to the contaminated substance, we carry through the examination of one dimension simulation of soil column.First, based on the water yield per day, it can calculate the hydraulic conductivity of 4 types of clay soils liner. From hydraulic conductivity(K) of the four, leakage control ability from good to bad is clay soil > 5% fly ash > 15% dry sludge > 20% fly ash. After modified by fly ash and dry sludge, hydraulic conductivity of liner increased, especially fly ash, with mixtrue ratio rose, hydraulic conductivity increased fast. But, with the experiment went deep, we can find that hydraulic conductivity of fly ash modified clay liner has a decline tendency. So, for achieving the call for K < 10^-7, the mixed ratio of fly ash shouldn'd be too high. The ratio of fly ash in mixture shouldn't higher than 20%.Secondly, from the attenuation ability to COD_Cr, natural clay soil are best of all. The ability from good to bad is clay soil > 20% fly ash > 5% fly ash > 15% dry sludge. The clay soil modified by the dry sludge which attenuation ablility to COD_Cr is the worst, the reason is that the organic substance of dry sludge is high, part of the organic substance releasing when leachate penetrate through the liner. That's why 15% dry sludge modified clay liner adsorption to COD_Cr is low.Third, from average attenuation ability to BOD₅ is 20% fly ash > clay liner> 5% fly ash >15% dry sludge, the average figure is 69.91%, 59.55%, 48.81%, 24.8%. The reason for 15% dry sludge adsorption to BOD₅ is low is that the time of the experiment is too short, the activity of microbe hasn't bring up yet. Meanwhile, part of the organic substance releasing is also a reason.Fourth, 4 types of soils attenuation to NH₄-N are all very well. But along with adsorption carry through continually, liner's adsorption capacity achieve to saturation. And attenuation ability also become weak. The main part of ammonia,nitrite and nitrate is ammonia. So, the three-N's change tendency are as the same as ammonia. The attenuation ability to 3N is 5% flly ash > 20% fly ash> clay lienr > 15% dry sludge. The reason why 15% dry sludge modified soil attenuation ratio is only 56.1% is that dry sludge's organic content is high, the sorption ability to inorganic NH₄⁺ is not high, and this is the same as the adsorption tests before.Fifth, 4 types soils attenuation ability to Fe is very good, because clay soil itself has a good adsorption ability to Fe.Sixth, combine with the value of pH, we can consider that Ca, Mg are likely to be adsorping in soil liners with complexation form. That is the same reason why fly ash modified clay soil's hydraulic conductivity become low in the later period of experiment.