Physicochemical Characterization of Singapore’ Municipal Solid Waste Air Pollution Control Residues and its Stabilization by VKI Treatment?

摘要

Vandkvalitets Institute (VKI) process is a treatment technology which employs the technique of extraction and chemical stabilization on waste, targeting an end pH of 7 to 10 using water, concentrated phosphoric acid (H3PO4) and carbon dioxide (CO2) as the main stabilizing materials. Different parameters for the ash stabilization were investigated. Based on the acid neutralizing capacity (ANC) and pH-dependent leaching results of Singapore’s air pollution control (APC) residues produced from a municipal solid waste (MSW) incineration plant, the main objective of this study was to examine the suitable end pH to be used to treat APC residues. After which, a two-step VKI treatment was carried out on APC residues, using CO2 and H3PO4 dosing as the two main stabilizing items. ANC results showed that there was a steep drop in pH from 10 to 7 at approximately 20.40 equivalent/kg of acid addition, while pH-dependent leaching results indicated that the most ideal pH range for the suppression of most heavy metal release was between 7 and 10. Subsequently, pH 7 was then selected as the end pH for the VKI treatment process. Under optimum conditions of 0.2 g and 0.5 g of H3PO4/100 g of APC residues (equivalent to 2 and 5 kg H3PO4/ton of residues) with a CO2 flow rate of 1500 cm3/min, the treated APC residues had an end pH of around 7 after 100 and 80 minutes of treatment, respectively. Toxicity characteristic leaching procedure (TCLP) was used to evaluate the leaching of heavy metals (Cd, Cr, Cu, Pb, Zn) before and after treatment. The results showed that the leaching of Pb was significantly reduced after treatment, and was able to meet Singapore’s acceptance criteria for landfill disposal. However, the leaching of Cd increased by two orders of magnitude after treatment, but was still below Singapore’s acceptance criteria for landfill disposal.