Experimental Study On Physical And Mechanical Characteristics Of Solidified Municipal Sludge Under Complicated Environment

The harmless treatment and resource utilization of municipal sludge has become a hot research topic in the field of geotechnical engineering.The mechanical properties of the landfill can be effectively improved after sludge being solidified,the safety performance and the treatment efficiency of the landfill can be improved at the same time.In the process of sludge treatment,solidified sludge will suffer environmental factors(such as temperature,water,etc.).The cognition of its internal mechanism and variation characteristics is lack of systematicness,and it needs to be improved urgently.Therefore,based on the physical and chemical characteristics of sludge and modified theory,combined with the indoor unit and model tests,a municipal sludge solidication technology using municipal solid waste incineration bottom ash as skeleton matrix was put forward,and microstructure and mechanical properties tests were carried out under different environmental factors(water immersion and freeze-thaw cycle and dry-wet cycle).The change mechanism of pore structure and strength of solidified sludge affected by environmental factors were analyzed and the inner mechanism was revealed.Above results provide basic datas for the feasibility and safety operation evaluation of the solidified sludge landfill disposal.The main research contents and results of this paper are as follows:(1)A municipal sludge solidication technology using municipal solid waste incineration bottom ash as skeleton matrix was put forward,and the physical and mechanical properties of solidified sludge were got.The contens,particle size and gradation of the municipal solid waste incineration were considered to study the solidified sludge.The variation law of physical and mechanical properties of the solidified sludge were studied,such as water and organic content,pH value,expansion ratio,unconfined compressive strength,shearing strength and permeability coefficient,etc.The.results show that the slag as skeleton builder co-solidified sludge,played a good skeleton role in the solidified body.The skeleton function increases with the increasing strength of the solidified sludge.And large size particles and good gradation of municipal solid waste incineration bottom ash are more likely to form a stable spatial framework,and to play a more effective role in the compressive strength of solidified sludge.(2)The model tests on solidified sludge landfilling were carried out,and the evolution properties of the solidified sludge was revealed.Contrast with sludge column solidified by commonly used curing agent,monitoring the temperature,water and organic content,pH value of curing sludge landfill column in a long time,regularly collecting the gas and leachate of the landfill column.The results showed that there was no obvious biochemical reactions in the solidified sludge,the variation of the temperature and water content caused by biochemical reaction curing body was not obvious,which had positive effect for stable operation of the landfill,but the ambient temperature affected the solidified sludge more obviously,so more attention should be payed to the ambient temperature.The pH of the solidified sludge had nothing to do with the time,but mainly related to the selected curing material and technology.Therefore,the curing agent and technology caused significant influence on the long-term stability of solidified sludge.(3)Long-term mechanical and deformation properties of solidified sludge immersed in water were studied.The long-term mechanical properties of solidified sludge which was immersed in internal or external water were studied,the influence of immersing occurrence time and duration time on the mechanical properties of solidified sludge was studied.The results showed that the effect of external water on characteristics of the solidified sludge was stronger than internal water,but it's difficult to change the mechanical strength properties of solidified sludge in a short term,and long-term immersion(120 to 150 days in the tests)can cause softening of the solidified sludge,which leads slowly decreasing of the mechanical strength.(4)Mechanical and microstructure properties of solidified sludge suffering freezing and thawing cycles(FTC)were studied.The influence of different freezing occurrence time,different freezing duration time and the number of cycles on the mechanical and permeability characteristics of the solidified sludge was studied,and analyzed through microstructure.The results showed that,the larger the strength of the solidified sludge was,the smaller the destruction due to FTC was,the 1st FTC made the greatest impact on the solidified sludge which decreased the strength by 30%and increased the permeability by 80%respectively.The strength gradually decreased with an increasing number of FTC,and then almost kept constant after 5-6 FTC.The permeability coefficient gradually increased with an increasing number of FTC,and then almost kept constant after 6-7 FTC.The variation of the permeability lags weakly behind that of strength.(5)Microstructure and mechanical properties of solidified sludge suffering drying and wetting cycles were studied.The influence of the initial water content of solidified sludge on the pore and mechanical properties were studied,and the law of the strength and permeability characteristics of the solidified sludge suffering drying and wetting cycles was studied.The results showed that,the 1st drying and wetting cycles made the greatest impact on the volume,unconfined compressive strength and permeability characteristics of the solidified sludge.Then the volume and permeability weakly decreased,but the unconfined compressive strength greatly decreased after the 2nd drying and wetting cycles.Municipal solid waster incineration bottom ash as the skeleton builder had a effectively action on resisting the variation of the volume,but the solidified sludge was easy to crack.