Treatment and reuse of tunnel construction wastewater

This study investigated chemical reaction and chemical reaction followed by microfiltration processes for treating synthetic tunnel construction wastewater, which was characterized as high in total suspended solids (2.53 ± 0.36 g/L) and in soluble silica (1.41 ± 0.20 g/L). The optimal chemical reaction conditions were assessed through jar test apparatus. Experimental results indicated that calcium chloride (CaCl2) could reduce soluble silica to 187.1 mg/L at pH 8.0 and dosage of 1000 mg/L. The treatment efficiency deteriorated as pH increased, while increasing CaCl2 dose at pH 8.0 did not enhance removal of silica. Magnesium chloride (MgCl2) could lower soluble silica concentration to 174.0 mg/L at pH of 8 and 9, and soluble silica concentration decreased further as MgCl2 dosage increased. Wastewater pretreated with CaCl2 and MgCl2 was then subject to microfiltration. Pretreatment by CaCl2 resulted in higher permeate flux, and moderate flux decline, while pretreatment by MgCl2 resulted in significant membrane fouling. Increasing transmembrane pressure (TMP) could induce higher initial flux and steady flux. Crossflow velocity did not show significant effect on the permeate flux. The potential reuse of treated effluent is assessed as well.

Silicate polymerization and formation of colloidal silica resulted in very low flux for raw tunnel construction wastewater. Both initial flux and final flux significantly increased when wastewater was pretreated with CaCl2 dosage of 1000, 2000, and 3000 mg/L, respectively. It is apparent that chemical pretreatment markedly enhanced MF of wastewater.Figure optionsDownload as PowerPoint slideHighlights
► We characterize tunnel construction wastewater as high in turbidity and soluble silica.
► CaCl2 and MgCl2 show good removal efficiency.
► Microfiltration following CaCl2 pretreatment yields higher steady flux.
► The treated effluent needs dilution or polishing before its reuse in concrete mixing.