Study On Landfill And Durability Of Sewage Sludge Solidified With Skeleton Builders

Landfill is a major disposal process of sewage sludge in China. However, high water content and low strength restrict the landfill of dewatered sewage sludge. Thus, it is of great significance to improve strength of dewatered sewage sludge in an economic way, in order to meet the geotechnical mechanical requirements for landfill. In this study, raw sewage sludge with moisture content of about97wt%was dewatered with skeleton builders (fly ash and lime) and ferric chloride, and dewatered sludge with moisture content lower than60%was reached. The geotechnical properties and long-term stability of such dewatered sludge were discussed emphatically without any other additives, i.e. cement, asphalt. This dissertation focuses on the following parts:1. Dewatering experiments of sewage sludge conditioned with skeleton buildersBased on the results of previous studies, a series of dewatering experiments were conducted with water content of dewatered sludge and SRF as evaluation index. The water content of dewatered sludge can be reduced to60%when the optimum dosages of skeleton builders are2.0g·g-1DS in small scale dewatering experiment. Moreover, the experimental results were tested and verified in a pilot scale test.2. Geotechnical properties of the dewatered sludgeGeotechnical properties of raw sludge, dewatered sludge and clay were investigated respectively. And the physical properties of dewatered sludge containing different dosage of composite conditioners were tested. Comparative analysis of raw sludge, dewatered sludge and clay conditioned by composite conditioners were done. As experimental results shown, the plastic range, permeability coefficient and optimum water content of dewatered sludge are better than raw sludge. In addition, dewatered sludge and clay present obvious differences in physical properties.3. Geotechnical properties and long-term stability of dewatered sludgeThe geotechnical properties of the dewatered sludge specimens conditioned by the inorganic composite conditioners were investigated. The results demonstrated that the strength of the specimens increases with an increase of curing days. It can be attributed to the filling effect of ettringite and C-S-H generated by the hydration reaction and skeleton builders. The strength of specimens reached the peak value on90d and it showed a downward trend with an increase of curing days. The unconfined compression strength and shear strength of specimens are over400kPa and130kPa after curing for1a, which exhibited excellent physical properties.The results of organic matter degradation test show the porosity of dewater swage sludge increases because of organic matter degradation, which lead to the decrease of strength of dewatered sludge. And a linear relationship between organic matter degradation ratio and strength loss ratio is also indicated. Dry-wet circulation can result the strength loss ratio of dewatered sludge in the range of9.59-59.45%and a variation of volume below11%. In freeze-thaw circulation, aggregation particles in dewatered sludge become smaller and skeleton structure is also changed. All the phenomenon occurred in dry-wet and freeze-thaw circulations cause the strength decrease. However, the results of accelerated carbonation tests indicate that the strength of specimens increases with an increase of carbonization days.4. Prediction on long-term environmental safety of dewatered sludgeThe dynamic leaching behavior and long-term leaching characteristics of specimens were investigated by heavy metals and organic matter leaching tests. The results indicate that leaching percentage of heavy metals decreases with the increase of curing age. The column leaching test which is used to simulate landfill for20a, show leaching percentages of Cr, Cd, Pb, Zn and TOC are13.7%,2.9%,10.6%,11.4%and28.5%, respectively.In addition, considering the long-term environmental safety and based on the essential principle of geomechanics groundwater movement and transport equations, a pollution diffusing model of heavy metals is put forward, and numeric simulation of coupling model which is equivalent to landfill for20a is made by using Modflow. The results demonstrate that the plane and vertical pollution diffusion area of Zn is about140m and200m. Its environment pollution scope is limited and its diffusion is mainly affected by the convection.The results of this study can provide a theoretical basis for geotechnical properties and long-term safety of dewatered sludge specimens containing skeleton builders. The combined technology of sewage sludge dewatering with preparation for landfill disposal materials can support for the landfill disposal of other sludge.