Solidification/Stabilization (S/S) Methods
Solidification/Stabilization (S/S) treatment technology is one of the international main methods at present, refers
to those technologies that use additive or binder to immobilize the hazardous content present in MSWI fly ash before
landfill disposal or utilization. Generally, the solubility, leachability and toxicity of MSWI fly ash will reduce after
S/S treatment. According to “Standard for Pollution Control on the Landfill Site of Municipal Solid Waste” (GB
16889-2008), MSWI fly ash can go to sanitary landfills after it is treated and meets the heavy metals leaching toxicity
criteria.
x Cement solidification
Among the different available S/S technologies, cement-based processes are still most widely used in hazardous
waste treatment due to the good treatment effect, relatively low treatment costs and easy to application. During cement
solidification process, in order to reduce leachability and its environmental impact in the final disposal, MSWI fly ash
and cement are mixed with water in an appropriate proportion, hazardous content like heavy metals in MSWI fly ash
will be immobilized after cement hydration reaction. [6-8] According to the character of MSWI fly ash, the addition
of different type hydraulic binders can improve the cement solidification effect. [9-10]
USEPA has regarded cement solidification technology as the best technology of hazardous waste treatment.
However, the drawback of this process is that the volume of fly ash will be increased (almost doubled) using this
method. Also, the adverse effects caused by heavy metals and soluble salts (such as chlorides and sulphates) in fly ash
will disturb the normal hydration process of cement and reduce the strength of cement solidified waste form. Slow
release of heavy metals from the treated fly ash was found in a wet environment, thus making it still hazardous to the
environment.
x Chemical stabilization
Through adding chemical stabilizers, chemical stabilization technology can change the physical and chemical
properties of MSWI fly ash. The existing forms of heavy metals in MSWI fly ash are changed by pH control,
precipitation and adsorption, in order to reduce bioavailability and mobility of metals, and realize stabilization.
Compared with cement solidification, the mass and volume of MSWI fly ash will not change a lot through chemical
stabilization. Currently there are many types of chemical stabilizers for MSWI fly ash, which can divided into two
groups: organic and inorganic. The most commonly used stabilizing agents are: gypsum, phosphate, bleach, sulfides
(sodium thiosulfate, sodium sulfide) and polymer organic stabilizers.