Study On The Generation Process And Mechanism Of Leachate From Stabilized Fiy Ash Landfills

With the continuous development of social economy,the amount of domestic waste in China is increasing year by year.Incineration is widely used in domestic waste treatment due to its advantages such as harmlessness,energy recovery,good capacity reduction effect and small footprint.However,a certain amount of fly ash and slag will be generated after incineration.Fly ash,a kind of hazardous waste,accounts for about3%to 5%of the waste into furnace.Currently,the main treatment method of fly ash is solidification/stabilization pretreatment.After the pretreatment reaches the"Pollution Control Standards for Domestic Waste Landfill Sites"（GB 16889-2008）,fly ash will be transported to the special fly ash landfills or to the domestic waste landfills and buried according to zoning rules.At present,there is no design specification and leachate calculation method for fly ash landfills,and there are significant differences in hydraulic characteristics between fly ash and domestic waste.Therefore,it is urgent to study the generation process and mechanism of leachate in fly ash landfills.In this paper,through a set of indoor column leaching device,the movement and change law of moisture in fly ash were studied,the generation process and mechanism of leachate were explored,and the water quality characteristics of leachate and its relationship with physicochemical characteristics of fly ash were analyzed.The toxic leaching results of raw ash showed that Pb exceeded the entry limit stipulated by GB 16889-2008,and it could enter the landfills only after the pretreatment reached the standard.The experimental results of pretreatment showed that 30%humidified water,2%chelating agent and 3 days curing time could make the fly ash meet the entry requirements.Three fly ash landfill columns with height of 30 cm,50 cm and 70 cm using deionized water as humidified water and one fly ash landfill column with height of 50cm using membrane concentrate as humidified water were established.3～6 water content monitoring points were set for each landfill column,and a 150-day leaching test was carried out.The results showed that with the extension of landfill age,water content exhibited a decreasing-rising-relatively stable trend;the generation time of leachate was proportional to the height of landfill column,and at the same height,the landfill column using more viscous membrane concentrate as humidified water required longer time;the output of leachate increased first and then stabilized,and the final output was close to the supply.The p H,conductivity and concentrations of COD,Cl^-,ammonia nitrogen and heavy metals in leachate of four fly ash landfill columns were continuously monitored.The results showed that the p H values of leachate from four columns were always maintained at about 10 due to the large amount of alkaline substances such as Ca Cl OH and Ca（OH）₂ in the fly ash;both conductivity and Cl^-concentration showed a trend of stabilization first and then gradual decline,and the smaller the landfill height,the earlier the time of decline appeared;affected by the quality of the membrane concentrate,the concentrations of COD and ammonia nitrogen in the leachate were higher when the membrane concentrate was used as humidifying water than when the deionized water was used as humidifying water;the leaching of heavy metals was mainly concentrated in the early stage of leachate generation,and except for the 70cm fly ash landfill column,the concentrations of heavy metals in the leachate of the other three columns reached a stable level by 150^th day.The author monitored the contents of soluble salt and heavy metals in fly ash from different layers.The results showed that the salt content in the upper layer of landfill column,such as 10cm and 20cm,gradually decreased with the extension of leaching time,while the salt content in the lower layer revealed a trend of increasing first and then decreasing;the content changes of heavy metals in the fly ash of four columns showed similar trends,taking the location at 10cm as an example,the contents of heavy metals increased rapidly first and then decreased slowly.By detecting the phase and morphology of fly ash before landfill and after leaching for 150 days,it was found that the contents of Cl,Na₂O and K₂O in fly ash decreased significantly after leaching,the surface of fly ash particles became compact and smooth,and the proportion of unstable state of some heavy metals such as Pb,Cu,Cr and Ni increased.