Field implementation of FeCl₃-conditioning and vacuum preloading for sewage sludge disposed in a sludge lagoon: a case study

ABSTRACT: In China, sewage sludge from wastewater treatment plants is usually disposed in sludge lagoons in municipal solid waste landfills. The increasing amount of sewage sludge generated has become a serious environmental problem. A two-stage method called ‘vacuum preloading combined with chemical conditioning’ is proposed to treat the disposed sludge. This study presents a case study of full-scale field implementation of the method in a sludge lagoon at the Changan landfill, China. In the first stage, the chemical conditioner was injected into the sludge to improve its consolidation property. The effect of chemical conditioning on dewaterability of sludge was evaluated based on the vacuum filtration test. In the second stage, prefabricated vertical drains (PVDs) were installed in the sludge lagoon as a drainage path, and vacuum pressure was exerted on the conditioned sludge through the PVDs. After 4 months of vacuum preloading, the settlement of the 7 m thick sludge lagoon was approximately 2.91 m, and the volume of the sludge was reduced by 40%. The measurements of pore pressures in the PVDs showed that no attenuation took place for the distribution of vacuum pressure along the PVDs during the test, even when the axial strain of the PVD was 40%. The measurements of pore pressures away from the PVDs showed that pore water pressures in the sludge did not dissipate with respect to the hydrostatic pore water pressures. During the vacuum preloading of conditioned sludge, the degree of consolidation U_p based on pore pressure was significantly lower than U_s based on settlement.