# Field Treatment of Storage Sludge and Stability Analysis of Overlying Municipal Waste Landfilling
## 2 Raw Storage Sludge Characterization
### Physical Properties  
In this study, water content is defined as the ratio of the water's mass to the dry solids' mass. It was determined using the oven-drying method, according to ASTM procedure D2974-14 [23]. To prevent the carbonization and oxidation of the organic matter in the sludge during the drying process, the drying temperature was controlled at 65 \({}^{\circ}\)C. Figure 4 illustrates the variance of the water content, which was measured from the raw storage sludge samples taken from the sludge pond, as well as the depth. The water content ranged from 172% to 7270%, with an average of about 1397%. The water content of the raw storage sludge within the same depth presented several different values. The reason for this is that, among the whole pond, the historical origin and conditions of the raw sludge were different. The average water content showed a fluctuating downward trend along with the depth. The reason for this is the sludge pond had an irregular depth, with a deep center and a shallow edge, rather than a constant depth. Related to the aforesaid appearance stratification, the water content showed a general decreasing trend along with the depth. This is because the large particles settled to the bottom of the pond while the slower sedimentation velocities of the fine particles kept them in the upper regions, resulting in the increase in the solid content of the sludge as the depth increased [20]. The variation of water content and its depth in the sludge pond is first discussed in two zones, namely the watery sludge zone (ZK10-ZK11, ZK16-ZK22) and the muddy sludge zone (ZK02-ZK09, ZK12-ZK15), which is shown in Figure 2. The large difference in the water content between the two zones was related to the topography of the bottom of the sludge pond and the location of the sludge dumping. The water content in the muddy sludge zone was almost all less than 1000%. This is due to the location of the sludge dumping in the watery sludge zone and the earlier deposit of sludge in zones I and II. The water content of ZK02-ZK06 in zone I varied less with the depth, while the water content of ZK08, ZK09, and ZK12-ZK15 in zone II decreases significantly with increasing depth. This is because  
Figure 3: The three layers of the raw sludge in the sludge pond. zone I was shallower than zone II. The water content of the sludge in the muddy sludge zone was generally high, and the water content of ZK10, ZK11, ZK18, and ZK19 within zone III varied less with the depth, while the water content of the remaining boreholes within zone IV varied significantly with the depth. This zoning situation correlated with the sludge dumping location at the edge of zone IV. The apparatus used for the CPT was a double bridge cone penetrometer, the probe was equipped with a cone-shaped tip (an apex angle of 60\({}^{\circ}\)), and the penetration rate of the probe was fixed at 1.20 m/min. The CPT procedure was according to ASTM D3441 [24]. Figure 5 shows the tip resistance and sleeve friction plots obtained from the CPT (C1-C5). The maximum values of the tip resistance and sleeve resistance of the raw storage sludge were both less than 50 kPa and much lower than that of the ordinary soft clay, whose tip resistance and friction resistance typically ranges from 100-700 kPa [3]. Moreover, the CPT readings of the three layers in the sludge pond were related to the stratified appearance and the stratified rule was different in each test position. Firstly, at depths of 0-8 m in C1 and C3, 0-17.4 m in C2, in 0-7.4 m in C4, 0-11 m in C5, the CPT values of the sleeve resistance and the tip resistance were essentially close to zero. This conforms to the appearance of mud-water mixing. Secondly, between the depths of 8 m and 10 m in C1 and C3, the depths of 17 m and 19 m in C2, the depths of 7.4 m and 15.3 m in C4, and the depths of 11 m and 12.5 m in C5, the CPT readings of the tip resistance and sleeve resistance slightly increased with the depth. This indicates that this layer was a fluid-like sludge layer.