Abstract
Allowable concentration of soluble silicate (S. SiO2) in treated waste water containing water glass in rainbow trouts (Salmo gairdneri) rearing was examined with acute toxicity tests and histopathological examinations. The polymerization rate of soluble silicate or water glass at pH 7.27.8 in a rearing water and the time course change of electric charge of colloidal silica were measured. These measurements make clear the states of silicate, the mechanism of acute toxicity occurrence and the histopathological phenomena.
The results of this research are summerized as follows.
(1) LC50 (48h) value of rainbow trout (four to five month old) was estimated to be 302±27 mg E l-1 with initial S. SiO2 concentration.
(2) Samples which have heigher concentration than 300 mg·l-1 decreased to 160 mg ± l-1 through polymerization at pH 7.27.8.
(3) At heigher S. SiO2 concentration as 350 mg·l-1, the negative charge of colloidal silica increased with aging time at neutral zone.
(4) The death of rainbow trout were considered to be caused by necrosis of the gill filaments with the colloidal silica.
(5) The allowable S. SiO2 concentration of a treated waste water containing water glass would be 150 mg·l-1 in order to avoid sol formation. Thence, 100 mg·l-1 of S. SiO2 concentration could practically be set as an allowable concentration of the treated effluent.
