Study On Solidification And Landfill Of Wastewater Sludge

The present environmental regulations in China require sludge, a kind of waste from wastewater plant, to be disposed in environmentally sound manner. To develop an effective and economic method for sludge monofill, studies include laboratory tests, theoretical analysis and numerical simulations were conducted in the method of geotechnical and geoenvironment engineering.Different kinds of sludge disposal method at present were compared firstly. Then both advantages and disadvantages of these methods were summarized. In these methods, landfill was considered to be more suitable for China. In order to solve the technical problems in landfill of sludge, a method, in which the sludge will be disposed by monofill after being solidified, was put forward in this paper.Laboratory tests were conducted to determine the feasibility of solidifying sludge using fly ash, lime, and silty clay at varied mixing ratios. Variable head permeability test, torsional vane shear tester and pocket penetrometer were utilized to study engineering properties of solidified sludge. Significant increases in sludge strength and permeability was achieved to match suitability for monofill. It was revealed that the strength of solidified sludge increased with the curing period and the decrement of moisture content. The adding of lime can improve the volume stability and permeability of solidified sludge. Based on orthogonal experiment, the solidifying effectiveness of silty clay was found to be more notable than fly ash. This can be resulted from the special environment of sludge in which the activities of fly ash was not excited. Based on these tests, a suitable solidifying method was put forward: m (lime) : m (silty clay) : m (sludge)=1 : 4 : 5.Freeze-drying technology and scanning electron microscope (SEM) were utilized to study the microstructure of solidified sludge. It was found density of sludge was improved after solidification. The hydration effect of solidifying materials and sludge can be the reason of improvement in strength and complication in microstructure. Applying theory of fractal geometry, quantitative analysis was carried out on the SEM images. Fractal dimension increased with the adding of solidifying material and curing time. The analysis also revealed a considerable positive correlation between fractal dimension and strength.Extraction tests were conducted to study the leachate properties. The column fill with lime and sludge performs the highest permeability, the largestmount of leachate, the highest PH and the highest COD because of the adding of lime. Lime performed a double influence on heavy metals in sludge. It can cause the instability of copper ion in sludge but make the zinc ion more stable. The influence of silty clay and fly ash was not significant.The potential pollution of landfill site's leachate to groundwater was studied by numerical simulation. Basic theory and governing equations of groundwater flow and contaminant transport were firstly introduced. Then using finite difference method, a numerical model of contaminant transport in groundwater was developed. With this model, numerical studies were carried out on the contaminant transport in groundwater owning to the leak of leachate from landfill site. It was revealed that in natural state, the pollution zone was ellipse and this zone developed with time both in horizontal and vertical dimension. When groundwater was exploited, the pollution zone can be significantly expanped.