Treatment and reuse of tunnel construction wastewater by coagulation-flocculation process was studied. Several commonly used recipe of grouting system were adopted to synthesize tunnel construction wastewater. Representative tunnel construction wastewater studied in current work was synthesized by mixing 100 g of cement (Portland cement, Chi-Mei) and 352 mL of water in a polyethylene bottle (reagent A), and 192 g of sodium metasilicate (Na2SiO3, Asia Chem. Co) and 192 mL of water in another polyethylene bottle, respectively (reagent B). Reagents A and B were then mixed and stirred rapidly for 16 s and poured to a plastic container full of 15 L water. After stirring for 3 min, the suspension was let to settle for 15 min, and the supernatant was decanted and stored as wastewater sample for experiments. The wastewater was high in soluble silica (1,500-2,500 mg/L) and in alkalinity. Three types of chemicals, calcium chloride (CaCl2), magnesium chloride (MgCl2), and polyaluminum chloride (PACl), were used and compared. Experimental results from jar test revealed that coagulation by adding CaCl2 effectively removed silica and the optimum pH was 12. Major reaction reaction mechanisms of silica removal was the formation of calcium silicate precipitant (C-S-H) and its subsequent adsorption onto calcium hydroxide precipitate (Ca(OH)2). The residual turbidity was very low, implying that removal of suspended solids was relatively easy. At pH higher than 10, MgCl2 was also an effective coagulant and the mechanisms involved precipitation of magnesium silicate (Mg2SiO4) and co-precipitation, depending on pH and dose. It was found that PACl showed higher removal efficiency for silica and the optimum pH was between 9-10. The possible reuse treated effluent that contained ca. 100 -140 mg/L soluble silica and turbidity lower than 4 NTU was evaluated. It was suggested for on-site recycle and reuse in grouting.

Treatment and reuse of tunnel construction wastewater by coagulation-flocculation process

2019 Fall National Meeting

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