Municipal sludge is a sedimentation waste produced during the wastewater process in sewage treatment plants. Among recent studies, pilot and field tests showed that chemical conditioning combined with vacuum preloading can effectively treat municipal sludge. To further understand the drainage and consolidation characteristics of the conditioning sludge during vacuum preloading, a large deformation nonlinear numerical simulation model based on the equal strain condition was developed to simulate and analyze the pilot and field tests, whereas the simulation results were not satisfactory. The results of the numerical analysis of the pilot test showed that the predicted consolidation degree was greater than that measured by the field tests, which is attributed to the relatively low permeability layer formed during the preloading process of the prefabricated vertical drain. To better reflect the consolidation process of the conditioned sludge, a simplified analysis method considering the low permeability layer around the prefabricated vertical drain was proposed. The initial permeability coefficient of the low permeability layer is determined via numerical simulations using finite difference method. The predicted settlement curve was in good agreement with the measured results, which indicated that the numerical simulation based on the equal strain condition considering the relatively low permeability layer can better analyze the consolidation process of ferric chloride-conditioning sludge with vacuum preloading.

As the main accumulation reservoir of various pollutants, the sediment plays a very important carrier role in the process of migration and transformation of pollutants. In this paper, based on the test device such as the annular water tank developed, the pollutant release characteristics of the river sediment under still water and moving water conditions were studied. The experimental results show that the content of pollutants nitrogen and phosphorus in river sediments in Nanjing is higher and the content of heavy metals is lower. The static release of pollutants in the sediment reached an equilibrium state in about 60 hours, and the release was faster in the early stage, and the release rate in the later stage gradually slowed down and tended to the equilibrium state. The velocity of river water flow directly affects the release characteristics of sediments. As the water flow velocity increases continuously, the sediments have three states: “not started”, “small amount of movement” and “general movement”. The ratio of pollutants (total nitrogen, total phosphorus, heavy metals) release to total pollutants in the sediment of the river under still water conditions is 0.96%, 0.09%, 0.01-0.06%, about 18.46%, 15.43%, and 0.05-4.55% under hydrodynamic conditions, which are much higher than the amount of pollutants released under hydrostatic conditions, and the release amount of heavy metals Zn and total phosphorus is more affected by hydrodynamic conditions than other pollutants.

Vacuum preloading is one of the effective methods for in-situ dewatering and reduction treatment of municipal sludge landfill, but because the consolidation coefficient of municipal sludge is 1 or 2 orders of magnitude smaller than that of sludge, the drainage consolidation technology will face the problem of vacuum for too long. In order to improve the consolidation characteristics of sludge, combined with the chemical conditioning technology in the field of water treatment, firstly, the chemical conditioner suitable for landfill sludge was selected. The effects of different conditioning combinations on sludge dewatering characteristics were studied by microscopic testing techniques (zeta potential analysis and environmental scanning electron microscope analysis) and vacuum filtration test and consolidation test. The results show that the combination of ferric chloride and calcium oxide can significantly improve the dewatering characteristics of sludge. After conditioning with 10% ferric chloride and 10% calcium oxide (relative to sludge dry base), the specific resistance of sludge is reduced from 2 × 10¹³m/kg to 3 × 10¹²m / kg, and the consolidation coefficient is 3.5-10 times higher than that of original sludge, which lays a foundation for improving the efficiency of vacuum preloading treatment. In addition, from the results of vacuum filtration test and consolidation test of sludge, it can be found that there is a good negative linear relationship between specific resistance and consolidation coefficient of sludge. Compared with the consolidation test, the vacuum filtration test is simple and easy, so it can be used as an effective means to select sludge conditioner or test the conditioning effect.

The output of municipal sludge in large and medium-sized cities in China has increased year by year, which results in the dilemma of having no remaining land for sludge to filling. Therefore, it is urgent to reduce the amount of sludge storage in stock. "Chemical conditioning combined with vacuum preloading" is an effective in-situ treatment method for sludge reduction, but the properties of sludge from different sources and different sewage processes vary greatly. Besides, the existing research lacks systematic analysis of the consolidation and permeability properties of different types of municipal raw sludge and conditioned sludge. In this paper, different inorganic salts (Fe 3+ , Ca 2+ , Mg 2+ , Al 3+) were selected to carry out chemical conditioning from different sources, and the influence of chemical conditioning on the consolidation characteristics of municipal sludge from different sources was studied. The test results show that the sludge with different pre-treatment process has a great influence on the consolidation and drainage characteristics of sludge. Shanghai municipal sludge using first-level enhanced coagulation process has the worst performance in consolidation and permeability characteristics. On contrary, sludges with A2/O process have the best performance of them. Fe 3+ salt and Al 3+ salt could improve the dewatering characteristics of different types of sludge, while Mg 2+ salt and Ca 2+ salt had weak effects. In addition, the specific resistance of Shanghai sludge after 10% FeCl3 conditioned decreased from 1.24×10 11 m/kg to 2.13×10 9 m/kg, and the consolidation coefficient increased by 13.5-101times.