| Nowadays there are many sludge lagoons existed around cities of China。Each year the amounts of disposed sludge reach up to million tons, and many landfills are now confronting with the shortage of storage capacity. If the volume of sludge in the lagoons can be reduced in situ, storage efficiency in the landfill can increase greatly, and the potential risk can be reduced. A new method, "vacuum preloading combined with chemical conditioning" is proposed in this paper to treat sludge lagoon. Because the method is proposed for the first time, no design methodology and engineering cases can be followed.Laboratory element test, model test, field test, and numerical simulation were carried out to investigate following issues associated with vacuum preloading on conditioned sewage sludge, the water distribution of sewage sludge and its transformation, the consolidation properties of conditioned sewage sludge, the variation of settlement and shear strength, the vacuum load transfer characteristic, and the countermeasures of low permeability column formed around PVD. The design method and technical craft for vacuum preloading on conditioned sludge were proposed. The main research works and conclusions are as follow.(1) A new method, establishing water distribution of sewage sludge based on soil-water potential was proposed. In this method, the relation of water-solid bond strength versus water content of sewage sludge could be obtained based on soil water characteristic curve (SWCC) measured by osmotic method and relative humidity method. In camparion with the traditional dilatometric method and combined thermal gravimetry-differential thermal method, for the proposed method the structure of sludge sample changed little during the measurement, and the measured results could cover the range exhibiting water-solid bond strength less than 1kJ/kg. Based on the water form division characterized with soil-water potential feature points proposed in soil science, the amounts of tightly bound water, loosely bound water, capillary water, free water in the sewage sludge were respectively 73%,167%,400%, and 220% of dry solid. After consolidation with a pressure of 2.5kPa, the amount of tightly bound water in sludge changed little, the amounts of loosely bound water, capillary water, and free water in sludge decreased in different degrees, in which the decrease on amount of capillary water was largest. After chemical conditioning, the amount of free water in sludge increased, and the amount of tightly bound water in sludge basically changed little.(2) Based on laboratory consolidaton test and filtration test, the consolidation properties and vacuum filtration properties of disposed sewage sludge conditioned by different chemical schemes were investigated. The test results reveald that the chemical conditioner FeCl3 could effectively improve the consolidation properties of four typical disposed sewage sludge in China. The coefficient of consolidation of unconditioned sludge was in the order of 10-5cm2/s, and its coefficient of consolidation increased significantly after chemical conditioning. When sludge was conditioned with 10% FeCl3, its coefficient of consolidation increased by 3-5.3 times, and the value increased to 3.9-5.3×10-5cm2/s. Based on consolidation test results and vacuum filtration test results, the relation of coefficient of consolidation (cv) and specific resistance to filtration (SRF) was established. After chemical conditioning with FeCl3, zeta potential of sludge decreased, the bound strength of sludge floes and water.decreased, therefore the permeability of chemical conditioned sludge was improved. The environmental SEM analysis results further revealed that the floe structure of sludge became dense after chemical conditioning.(3) Model tests were carried out to investigate the feasibility and effect of vacuum preloading treatment on unconditioned sewage sludge. The test results revealed that the vacuum preloading treatment could achieve the volume reduction and the formation of initial strength of sewage sludge. However the consolidation properties of sewage sludge was poor, the treatment effect still need to be improved. During the vacuum preloading process, the water content and permeability of sludge around PVD decreased sharply in early stage, which retarded the water drainage of sludge far away from the PVD. Therefore, reducing the spacing of PVD and re-installation of PVD could improve the treatment effect.(4) A two staged method, "vacuum preloading combined with chemical conditioning" was proposed to treat the sewage sludge. To verify the feasibility of the proposed method, pilot test and field test were carried out. The material and technical craft parameters, the development of settlement, and the vacuum load transfer characteristics associated with the vacuum preloading on conditioned sludge were investigated. The test results revealed that the proposed method could effectively achieve the volume reduction of sludge. In the pilot test (with a depth of 3 m and an area of 18 m2), the volume of conditioned sludge was reduced by 47.5% after 68 days of vacuum preloading. In the field test (with an average depth of 7 m and an area of 4300 m2), the volume of conditioned sludge was reduced by 40% after 114 days of vacuum preloading. In the field test, no attenuation was observed for the distribution of vacuum load along the PVDs, even when the deformation of the PVD was as high as 2.91 m. The pore water pressures in the sludge did not dissipate with respect to the hydrostatic water pressures. This may be related to the special consolidation properties of sewage sludge or soil with high water content. The integrated PVD showed a good performance during vacuum preloading of conditioned sludge. The core of the PVD kept intact under large settlement, the filter of PVD could resist chemical corrosion, and the PVD could effectively transfer vacuum load to bottom of sludge lagoon with a depth of 8m.(5) A numerical code for radial consolidation considering non-linearity and large strain of sludge was complied. The numerical code was used to back analyze results of model test, pilot test and field test of vacuum preloading on sewage sludge, and the numerical results based on uniform strain condition, uniform stress condition were compared with the measured results. The comparison revealed that the numerical results based on uniform strain condition and uniform stress condition were not satisfactory. The settlement based on uniform stress condition is much less than the measured settlement, and the settlement based on uniform strain condition is slightly higher than the measured settlement. Thereafter a simplified methd considering low permeability sludge column was proposed. It assumed that a low permeability sludge column formed instantly around PVD, the radius rs of the sludge column was about 3 times of the equivalent radius rw of PVD, and initial permeability of sludge in the column was about 1/10-1/40 of sludge outside of the column. Based on the simplified method, the numerical results of settlement coincided well with the measured settlemnt. During the vacuum preloading on sewage sludge, the degree of consolidation Up in terms of pore pressure is significantly lower than the degree of consolidation Us in terms of settlement.The effective consolidation stress method was proposed to calculate the shear strength growth of sludge during vacuum preloading. In this method, the growth of effective consolidation stress of sludge was obtained based on settlement of sludge layer (apparent water content of sludge). This method could elucidate the non-linear increase of shear strength versus its axial strain during vacuum preloading on sludge.(6) Vacuum filtration column tests were used to investigate the inhibition effect of poze size of filter and the chemical conditioners of sludge at vicinity area of filter to the formation of the low permeability sludge column. The test results revealed that utilization of the chemical schemes of ferric chloride, calcium oxide, and fly ash at vicinity area of PVD could effectively inhibit the formation of the low permeability column. Increasing of pore size of filter was not a good measure to inhibit the formation of the low permeability column of contioned sludge. |
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