Abstract
A sequential operation of lime treatment, carbonation and coagulation of wastewater from tunnel construction works including water glass and portland cement for grouting was designed. Through jar tests of model suspensions containing water glass, portland cement and kaolin, the operational characteristics were examined. Treated effluent was evaluated by S.SiO2, turbidity, Ca2+ and electric conductivity, and accute toxicity to rainbow trouts (Salmo gairdneri).
The results are summarized as follows : (1) The contact time requiered was about 60 min to remove S.SiO2 from some hundreds mg·l-1 to under 30 mg·l-1 with approximately equimoller dosing of lime slurry to water glass solution. With the increase of lime dosage, the required contact time become shorter. (2) Regardless of the S.SiO2 concentration of the wastewater, S.SiO2 and turbidity were removed to less than 30 mg·l-1 and less than several turbidity unit, respectively, only adjusting pH more than 11.9 (ca. 25°C). (3) Sufficient precipitation of CaCO3 was observed from the wastewater at pH9.5 10.5. The optimum pH of the precipitated CaCO3 by alum was about pH = 8.5. (4) It was difficult to decrease electric conductivity less than 0.3μS·cm-1 (25°C) by this treatment method. (5) Accute toxicity to rainbow trouts were not observed in the effluent treated by the lime treatment.
