Co-stabilization Of MSWI Bottom Ash And Municipal Sludge With An Emphasis On Geoenvironmental Behavior Of Heavy Metals

In recent years, the incineration capacity of Municipal Solid Waste(MSW) in China has increased rapidly. By 2013, 166 MSW incinerators have been built, having a incineration capacity of 5785 Million Tons per Year. If 20% of MSWI bottom ash generated erery tons of incinerated soild waste, then 1157 Million Tons of MSWI bottom ash was produced in 2013. MSWI bottom ash can be viewed as recyclable resources. At present, there are so many studies focusing on utilization of MSWI bottom ash. However, due to high alkalinity and relatively higher heavy metals content that may pose potential risks to surrounding, stabilization pretreatment is normally needed before utilization or disposal. In addition, the amount of municipal sludge has increased rapidly as a result of increasing treatment ratio of municipal sewage in China. The amount of wet municipal sludge(moisture content of 80%) in our country will be 3000 Million Tons per Year by 2013. As a solid waste, municipal sludge has a high moisture content and high organic content. Cost-efficient stabilization of municipal sludge has become a research focus in this research field.Based on the status mentioned above, this study puts forward a new method named co-stabilization of MSWI bottom ash and municipal sludge. On one hand, MSWI bottom ash uses CO2, generated from biodegradation of sludge, to accelerate carbonation. On the other hand, because of porous structure on MSWI bottom ash, once mixing together the sludge can contract with air, then sludge organic matter can be degraded fully for stabilization in aerobic condition. Consequently MSWI bottom ash and municipal sludge can be stabilized simultaneously. But MSWI bottom ash has higher quantity of heavy metals; it may be potential harm to environment. So geoenvironmental behaviors of heavy metals should be researched during co-stabilization of MSWI bottom ash and municipal sludge. The aim of this study is to explore a economic, safe, efficient technique for stabilization of MSWI bottom ash and sludge. The main research results are listed as follows:First of all, the basic properties of MSWI bottom ash and municipal sludge were analyzed. The results showed that the content of Ca O in bottom ash is 37.51%. The p H of bottom ash is 12.3. The content of organic matter in sludge is 40.57%. The standard content of heavy metals in bottom ash and sludge is more than soil environmental standard. TCLP of different heavy metals in bottom ash and sludge were not over standard. For the higher leaching concentration and ratio of Cu、Mn and Ni in sludge, it may be ascribed to the fact that considerable amount of Cu、Mn and Ni presented as exchangeable fraction and carbonate bound fraction in sludge.Four reaction columns of 1#, 2#, 3#and 4# were set up in the study for exploring the feasibility of co-stabilization of MSWI bottom ash with municipal sludge. The four columns have a mixing ratio of 0:10, 1:9, 2:8 and 3:7(sludge to bottom ash) respectively. All the reaction columns were controlled in aerobic condition. Water was distributed regularly to guarantee its moisture content. Soild samples as well as gas samples were analyzed every two weeks for monitoring the status of the stabilization process. The analysis results showed that p H of 1#, 2#, 3# and 4# drop from initial values of 12.37, 10.44, 10.21 and 9.74 to the end values of 10.80, 9.67, 8.43 and 8.25 respectively. The carbonation degree followed the sequence of 4#>3#>2#>1#. Degradation ratio of sludge organic matter for 2#, 3# and 4# were 11.38%, 14.99% and 35.57% respectively. Degradation ratio of sludge DOC for 2#, 3# and 4# were 22.77%, 38.13% and 44.94% respectively. Decrease of organic matter and DOC in sludge showed stabilization for sludge. The CO2 concentration in reaction columns of high sludge content 3# and 4# increased at first and then decreased, which showed that sludge in aerobic condition can produce much CO2, which can accelerate carbonation of MSWI bottom ash. The above showed that MSWI bottom ash co-stabilized with sludge at mixing ratio of 2:8 and 3:7.Geoenvironmental behavior of heavy metals is studied during the co-stabilization process of MSWI bottom ash and sludge. It showed that addition of sludge to bottom ash can promote mobility of heavy metals Cu, Mn and Ni, but only leaching concentration of heavy metals of Zn showed an increasing trend with stabilization time, Other six heavy metals all showed decreasing trend with stabilization time for the highest sludge adding reaction column. And the final leaching concentrations of heavy metals were lower than criterion limit. The chemical speciation of heavy metals showed that the exchangeable fraction of Cu and Mn for the initial mixing materials increased because of sludge adding, but their exchangeable fraction showed decreasing tread with stabilization time. The residual fraction of Pb and Cr were above 50% along the whole reaction period, which showed that heavy metals Pb and Cr have high stability. The exchangeable fraction of Ni and Cd were respectively about 10% and 20% along the whole reaction period. The also calculated the bioavailability of heavy metals in an effort to evaluate the environmental impact of the mixing material through bioavailability. The calculation results showed the bioavailability of heavy metal for Zn and Cd were increased with stabilization time, but the others had no increase.In conclusion, MSWI bottom ash can co-stabilized with with a proper mixing ratio, and most heavy metals showed no increasing negative effect during stabilization. Therefore co-stabilization of MSWI bottom ash and sludge can be viewed as feasible pre-treatment method before utilization or disposal. And during using this pretreatment method, special attention should be paied to heavy metals Cu、Zn and Cd.