Experimental study on treating landfill sludge by preconditioning combined with vacuum preloading: Effects of freeze-thaw and FeCl3 preconditioning

The deep dewatering of landfill sludge (LS) mainly uses the methods of chemical preconditioning and mechanical dewatering, which is easy to cause environmental pollution and is not conducive to the subsequent recycling treatment of sludge. To find a more environment-friendly and efficient method for LS's deep dewatering and volume reduction, an in-situ sludge treatment method combining freeze-thaw (F/T) preconditioning and vacuum preloading was proposed. Firstly, the F/T test of LS was carried out to explore the optimum freezing temperature. FeCl3, the most widely used agent, was selected as the chemical preconditioning. Then carry out vacuum preloading model box test. The data were compared after the test. The mechanisms of the two different sludge preconditioning methods on the LS's consolidation were analyzed. The results show that: after freezing, the specific resistance of LS decreases obviously, the overall particle size increases, the content of small particles decreases. Too fast freezing rate is not conducive to the LS's dewatering. After preconditioning (F/T and FeCl3) combined with vacuum preloading, the volume reduction ratio was 57.1% and 41.1% respectively, the water content was reduced from 73.4% to 53.7% and 58.1%, and the unconfined compressive strength(UCS) was improved from 15.5 kPa to 50.9 kPa and 77.3 kPa. The total water discharge, drainage rate, volume reduction, and water content of freeze-thaw preconditioned LS are better than FeCl3 preconditioned, while FeCl3 preconditioned LS has higher UCS. F/T can aggregate small sludge particles but the acidification and hydrolysis of FeCl3 always produce small particle, which is not conducive to the consolidation of LS during vacuum preloading. [Display omitted] •Preconditioning is applied to reduce sludge with vacuum preloading innovatively.•Effects of Freeze-thaw and FeCl3 preconditioning on dewatering are compared.•Optimum freezing temperature is −11 °C, too fast freezing rate limits the treatment.•Freeze-thaw plays aggregation, FeCl3 plays acidification and leads to dispersion.•Vacuum preloading with freeze-thaw can reduce the landfill sludge effectively.