Experimental Study on Treating Landfill Sludge by Freeze–Thaw Combined with Vacuum Preloading Using Prefabricated Horizontal Drains

Abstract

The traditional chemical vacuum preloading method for landfill sludge (LS) often has problems of pollution, deformation, and blockage of the drainage plate. Consequently, we proposed a freeze–thaw process with vacuum preloading via prefabricated horizontal drains (PHDs). First, the LS was pretreated by freeze–thaw. Then, a comparative model test of prefabricated vertical drains (PVDs) and PHDs was carried out to investigate the anticlogging mechanism and vacuum consolidation effect of PHDs from macro- and microaspects. The results showed that the water discharge of PHD was 13.4% higher than that of PVD, the settlement of PHD was 16% higher than that of PVD, the volume reduction ratio of PVD was 58.2%, and that of PHD was 67.5%. The initial water content of LS was 86%, the minimum for PVD was 61%, and the minimum for PHD was 56%. The particles of PVD were mainly transported in the radial direction, PHD was transported primarily in the vertical direction, and there were more small particles in the center of the PVD. PHD had a high degree of consolidation, mainly with micropores and mesoporous distribution, and the PVD consolidation degree was low, primarily the distribution of small pores. The study found that freezing, thawing, and vacuum preloading with PHDs may efficiently minimize the amount of LS. As a special biological colloid, landfill sludge (LS) has the characteristics of high moisture content, limited permeability, and a very low load-bearing capacity, resulting in low efficiency of vacuum drainage and consolidation, and construction personnel and heavy machinery are often unable to carry out direct operations, which greatly reduces the efficiency of vacuum preloading construction operations. In view of this, this study proposed a more environmentally friendly and efficient method of freeze–thaw combined with vacuum preloading using prefabricated horizontal drains (PHDs) for LS treatment. Using PHD can effectively improve the efficiency of drainage and consolidation and significantly improve construction efficiency. At the same time, the freeze–thaw method can effectively avoid the secondary pollution caused by the use of chemicals, reduce the cost of later sewage treatment, and improve the way of sludge resource utilization. The method of freeze–thaw combined with vacuum preloading using PHDs can effectively achieve in situ volume reduction treatment of LS, which is a guideline for the treatment of LS.