# Field Treatment of Storage Sludge and Stability Analysis of Overlying Municipal Waste Landfilling
## 3 Stratified Treatment of Storage Sludge  
The field sampling and laboratory test results reveal that the sludge pond at the Qizishan landfill is divided into a mud-water mixing layer, fluid-like sludge layer, and fluid-plastic sludge layer, from top to bottom. The water content test of the raw storage sludge showed a decreasing trend with the depth, and that the values maintained a high level. Correspondingly, the strength of the raw storage sludge increased with depth, and the values maintained an extremely low level. Simultaneously, considering the large volume and high buried depth of the sludge pond at the Qizishan landfill, the stratified treatment was adopted for the sludge pond (Figure 7). First, the sludge in the mud-water mixing layer was dehydrated using an ex situ geotextile tube. Second, due to the layered sedimentation of the raw sludge in the pond, the in situ solidification by stirring was applied in the shallow sludge, whose buried depth was less than 10 m. Finally, the high-pressure chemical churning pile was utilized in the deep sludge, which was located at buried depths of more than 10 m. This technique is quite effective in improving the shear strength of the foundation, which is formed of silt, mucky soil, liquid-plastic clayey soil, and soft-plastic clayey soil [26]. In-situ stirring solidification requires: (1) sampling of the sludge to analyze its physical properties and designing the solidifying agent formulation; (2) dividing the grid and measuring the depth according to the working area to calculate the dosage of the solidifying agent; (3) arranging the types and numbers of solidifying equipment and solidifying route. The high-pressure chemical churning pile applied in the sludge needs to test the pile-forming before construction, and the technical parameters, such as slurry ratio, injection pressure, and slurry volume, are determined according to the actual situation in the field. Where the calculation and design process need to use the physical properties of the sludge cannot be replaced by improving the method, while the actual treatment operation can improve the efficiency of the treatment by improving the process. The sludge from the mud-water mixing layer was pumped into the geotextile tube using a submersible mud pump. There were 20,000 m\({}^{3}\) of sludge treated by dewatering in the geotextile tube in 40 days. To improve the dehydration effect, a flocculant was  
Figure 7: Schematic description of the stratified treatment of the sludge pond. added to the sludge. It should be noted that the content of the admixture is defined as the wet mass ratio of the solidifying materials to the sludge. In the dehydration of sludge, the polyaluminium chloride (PAC) and polyacrylamide (PAM) are widely used for flocculation [27,28]. For each 1 m\({}^{3}\) of sludge, 1200 g of 1.7% PAC and 200 g of 0.05% PAM were added simultaneously. The mix was conveyed to the snake-like pipe, which was used for increasing the contact time of the sludge, as well as the flocculant, and enhancing the flocculation effect of the sludge, and then it was pumped into the geotextile tube for dehydration (Figure 8a). The strong pressure inside the geotextile tube effectively dewatered the sludge and held the solid particles [29]. In the shallow sludge, 110,000 m\({}^{3}\) of fluid-like sludge was solidified by in situ agitation in 131 days. The solidifying materials were pumped into the sludge pond using in situ curing stirring equipment. The addition of a chemical conditioner transformed part of the bound water into free water, allowing further dewatering. Thus, based on the pilot tests, the formula of the solidifying materials was optimized by adding chemical conditioners. The final formula is shown in Table 1. The in situ curing stirring equipment included an in situ stirring device, a solidifying material mixing device, and a pumping device (Figure 8b).