| dc.description.abstract | Water-washing process is a common pre-treatment because of the advantage of washing out undesired substances from the surface of ash and aggregate for improving their applications. However, little information concerning the properties of derived wastewater from water-washing process has been found in literature. Therefore, the main objective of this study was to examine the characteristics of wastewater derived from water-washing municipal solid waste incinerator (MSWI) bottom ash. In addition, the treatability of the wastewater by sedimentation/filtration and the feasibility of recycle and reuse the treated wastewater were evaluated.
The results of this study indicated that the water-washing wastewater was high alkaline with pH greater than 11. The concentrations of SS, COD, and total heavy metals (e.g. Pb, Zn, Cu, Cd, and Cr) in wastewater were basically increased with the increase of water-washing time, and the maximum concentrations after 180 min of washing were 31429 mg/L, 3000 mg/L and hundreds of mg/L, respectively. Increasing the ratio of liquid-to-solid (L/S) for washing, the concentration dilution effect was significant for the derived wastewater. Also, the dilution ratio of wastewater concentrations was proportioned to L/S ratio, except for COD.
For wastewater derived from water-washing under the conditions of 25 min of washing at L/S ratio was 5, the removal efficiency of SS, COD, and total heavy metals by simple gravity sedimentation was 99%, 76%, and 96%, respectively. The additional removal was insignificant if followed by filtration. According to the mass balance calculation, about 8% of total heavy metals were distributed in the wastewater when MSWI bottom ash was washed. However, most of the total heavy metals in wastewater were removed and settled down as a part of the sludge after sedimentation. The main oxide forms of settled sludge were consisted of SiO2 and CaO. In addition, the results of sludge leaching test revealed that the heavy metals concentration in leachate could meet the TCLP limits of Taiwan.
When the supernatant of treated wastewater by sedimentation was recycled and reused as the water for bottom ash washing, it found that the concentrations of SS, COD, and heavy metals in the following derived wastewater and treated supernatant were accumulated gradually with the increase of the recycle number. The maximum concentrations of soluble COD and chloride in wastewater became two times higher than that in original; however, the concentration of heavy metals was only 10% higher. Nevertheless, the removal efficiency of sedimentation could still maintain a stable performance for treating the recycle-derived wastewater. Based on the unit weight of bottom ash, the amounts of TDS and chloride washed out were reduced with the increase of the recycle number, but the effect was insignificant for heavy metals. | en_US |