| The production of dewatered sewage sludge in China is very large.Because the sludge is high in water content and low in shear strength,it should be solidified appropriately before landfilling disposal.Experimental study on the solidification of sewage sludge was carried out by using municipal solid waste incinerator(MSWI)bottom ash as skeleton material.The optimal solidification mateirials proportion was determined according to the solidification effect.The strength sources of solidified sludge were analysed.The influence of wetting-drying cycles and organic degradation on strength and physical stability of solidified sludge were measured.The stability analysis of solidified sludge landfill was carried out,and presented a reasonable kind of landfilling method.The mainly contents are as follow:(1)The sewage sludge's grain size is less than 1 mm,the water content is high and strength is low.The MSWI bottom ash's grain size is between 0 mm and 10 mm,could be used as skeleton material.The water absorption ability of MSWI bottom ash is strong and has volcano ash activity.Co-solidification of sewage sludge and MSWI bottom ash is an environmental friendly way of waste disposal.(2)Experimental study on the solidification of sewage sludge was carried out by using cement,lime and gypsum as solidification materials.Unconfined compressive strength(UCS),water resistance capacity,volume change ratio,and leaching test were performed to investigate the solidification effect.The experimental results indicated that the optimum solidification materials were cement and gypsum.(3)The optimal material proportion for the sludge solidification is 25%cement,25%gypsum,and 100%MSWI bottom ash of dry sludge.The UCS of the solidified siuge on 28 days is 110 kPa,water content is 62%,and the volume change ratio is 0.9.The laeching ratio of Cu,Zn and Pb decreased 56%,94%and 99%respectively than before solidified.The COD decreased 77%and the pH is close to 7.The gain in the strength of solidified sludge at the early stage is resulted from the skeleton effect and water absorption of MSWI bottom ash,and that at the later stage is from the hydration effect of solidification materials and the volcano ash effect of MSWI bottom ash.(4)Air drying and capillary water absorption process was carried on the solidified sludge to simulate the wetting-drying cycles.The experimental results showed that the UCS of the solidified sludge was between 24 kPa and 126 kPa,increasing during the first cycle,but then decreasing significantly during the following cycles.The internal friction angle measured from the direct shear tests was between 25°and 33°,and the cohesion calculated from the measured UCS according to the limit equilibrium equation was between 8 kPa and 34 kPa.The development of cracks and weakening of structure were happened during the wetting-drying cycles.Bioreactors were using to accelerat the degradation rate in order to find out the organic degradation process of the solidified sludge.The UCS of solidified sludge will be decreased about 60 kPa in 6 months since begining.(5)Stability analysis of the solidified sludge landfill was carried out by the Geo-studio,include the landfilling stage,wetting-drying cycles and rainfall before the landfill closed,and the long-term seepage after closed.The safety factor of the landfill will be decreased 45%after 4 times of wetting-drying cycles,and will be decreased 20%further if it suffered rainfall.The long-term seepage process will resulted in the safety factor decreased about 43%.(6)According to the stability analysis results,a method of solidified sludge landfilling was presented.At a rate of 1 m per 7 days,the 1:4 slope solidified sludge landfill could reach to 30 metres.In order to ensure the safety of sludge landfill,should use rough geomembrane as liner,set gas exhaust system,and the solidified sludge landfill should be closed in time after landfilling. |
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