| Nowadays,landfill is the main disposal way of dewatered sludge in China.Each year nearly million tons of sludge is dumped into many sludge lagoons across the country,which has occupied lots of landfill storage capacity to restrict land use.Besides,these sludge lagoons are in need of treatment because they have brought a lot of environmental problems,such as waste mass instability,odor diffusion and seepage of leachate polluting the groundwater.In situ disposal of sludge not only improves the landfills storage efficiency,but also can eliminate potential safety hazards.Therefore,the key problem is to seek for an efficient sludge in-situ disposal method and technology.We proposed a new method named"heating and chemical conditioning combined with vacuum preloading" for reduction of landfill sludge based on previous method "chemical conditioning combined with vacuum preloading",which had some shortages like too long disposal time,jams around the PVDs and the unclear drainage consolidation mechanism of sludge.Laboratory element test,model test,numerical simulation and process design were carried out to investigate these issues associated with the sludge chemical conditioning dewatering effect,the temperature effect on sludge permeability,the drainage and strength growth of conditioned sludge under the effect of themperature and vacuum preloading and its numerical coupling analysis.The scheme of prefabricated vertical thermo-drains(PVTD)was designed.The construction-site method and technical craft for thermo with vacuum preloading on conditioned sludge was proposed.The main researches and conclusions of this paper are as follows.(1)The results of conditioned sludge filtration tests showed that the efficiency of combined chemical conditioning was significantly better than that of single conditioner.The tests also revealed the "10%iron chloride with 10%calcium oxide" and "0.5%chitosan with 10%ferric chloride" had the best conditioning effect,reducing the sludge water content by half.(2)Vacuum filtration tests were designed to investigate the inhibiting effect of the chemical conditioning on low permeability sewage sludge layer under conditions of vacuum preloading.The test results showed that the permeability of' sludge can be increased by ferric chloride conditioning and the formation of low permeability layer of sludge can be postponed on the basis of adding other conditioners.The combination of iron chloride,calcium oxide and fly ash had the best overall drainage effect,and the best control effect on the low permeability layer.(3)Variable head permeability test and GDS triaxial permeability test under temperature control were used to investigate the effect of the heat on the permeability coefficient of sludge in different areas.The test results showed that heating can significantly improve the permeability of sludge,which at 70? increased up to about 25?60 times those at 20?.Sludge permeability had temperature hysteresis effect that hydraulic conductivities was about 7 times as the initial permeability of sludge when temperature dropped from 70? to 20? because of heating causing a decrease in organic content,sludge viscosity.We also found that consolidation pressure had little influence on the sludge permeability.(4)A method named "heating and chemical conditioning combined with vacuum preloading" was proposed.And a model test was used to verify its effect on sludge dewatering.The test results revealed that the test period was shortened by half compared with the unheated disposal method,the sludge volume was reduced by 42%after 33 days of vacuum preloading,the water content decreased from 480%to 120%?370%,and the strength of the vane increased to 5-22kPa.In the test,the use of the PVTDs improved the permeability of the radial channel of the drainage,which caused a similar pore pressure at the same depth.(5)A multi-field coupling numerical model was established to simulate the model test,Temperature-Hydro-Mechanical coupling analysis results showed that the simulated temperature transfer and measured temperature had similar laws to,simulated settlement was greater than the actual settlement of 150mm and the simulated pore pressure was greater than the measured values because of the differences between the constitutive model of sludge and the actual sludge parameters of the case.(6)The electric and hot-water prefabricated vertical thermo-drains were improved.And the construction technology of the thermal vacuum preloading of conditioning sludge was proposed and optimized.Electric PVTD was suitable for a small space,short treatment period,and low economic demand situation while hot-water PVTD was suitable for a large area,long treatment period,safety and economic situation.Thermal vacuum preloading construction technology included drainage plate inserting,drainage pipeline and the hot water pipe connecting,the laying of the sealing system and the on-the-spot testing of the vacuum load and hot water. |
PDF Full Text Request