# Field Treatment of Storage Sludge and Stability Analysis of Overlying Municipal Waste Landfilling
## 1 Introduction  
Sludge is the semisolid residue that is produced as a by-product during the sewage treatment of industrial or municipal wastewater [1]. The total sewage treatment in the urban cities and towns of China had an average annual growth of 10% from 2001 to 2015, and the treated sewage in 2015 was 428.83 \(\times\) 10\({}^{8}\) m\({}^{3}\). The early sewage treatment pays more attention to the sewage rather than the sludge, which leads to the sludge being simply piled or buried in the environment and becoming storage sludge [2]. There is a large amount of storage sludge in China. For example, the Laogang landfill in Shanghai received 1.28 million tons of sludge from 2012 to 2015; the Xiaping landfill in Shenzhen accommodated about 1.21 million tons of municipal sewage sludge; 1.5 million tons of sludge generated by the Shenyang sewage treatment plants was piled in Zhujia Town, Shenyang [3]. For the early treatment of the storage sludge before landfilling, the treatment method most frequently used is limited to conventional mechanical dehydration [4]. The conventional mechanical dehydration merely removes the free water as well as some of the interstitial water from the sludge, which results in a remaining water content of over 400% in the treated sludge. Due to the high water content, organic matter content, and plasticity, storage sludge presents low strength [5]. This contributes to the low bearing capacity of the foundation formed by the raw sludge and can cause the instability of the overlying landfill slope, thereby endangering its operation [6, 7]. Thus, the landfills with high storage sludgestockpiles pose potential safety hazards in capacity expansion. Therefore, it is urgent that the raw storage sludge is treated in order to enhance its strength and ensure the stability of the waste slopes above. There are several kinds of treatment available for enhancing the raw sludge at present. They include vacuum preloading dehydration, chemical conditioning combined with vacuum preloading, and solidification. Vacuum preloading decreases the water content and improves the strength of the sludge by setting vertical drainage channels and covering the sand cushion above the sludge to form a sealing film [7; 8]. [PERSON] [9] explored the sludge dehydration via vacuum preloading model tests and found that the average shear strength of the dewatered sludge increased from 0.3 kPa to 2-4 kPa. The chemical preconditioning of the sludge transforms part of the bound water to free water, which promotes dehydration [10; 11; 12; 13]. [PERSON] [9] used FeCl\({}_{3}\) with a dosage of 10% as the conditioner and drainage plates with a spacing of 0.4 m for combined dehydration. Results showed that the undrained shear strength of the sludge with a water content of 142% increased to 10 kPa. For sludge solidification, the most common solidifying materials are cement, lime, and fly ash. It is widely acknowledged that solidification decreases the water content of the sludge and enhances its strength by constructing a skeleton structure [7; 14; 15; 16; 17; 18]. [PERSON] et al. [19] solidified an industrial sludge lagoon and tested the in situ solidified sludge and found that the unconfined compressive strength of the solidified sludge with a water content of 150% reached 206.80 kPa by mixing it with 22% of Portland cement. From a throughout analysis of the different sludge treatment methods, the vacuum preloading reduced the water content of sludge but did not obviously improve its strength. Similarly, the chemical conditioning combined with vacuum preloading had more dramatic reductions in water content, but the strength improvement was not significant either. The solidification produces hydration products that consume the water and connect the soil particles to the sludge, and as a result, the solidified body has a significant strength enhancement. In conclusion, solidification is comprehensively the best method in the strengthening and decrease in water content of the sludge.