Toxicity of chromium to activated sludge

The work deals with the toxicity of hexavalent and trivalent chromium to ac tivated sludge. Cr(VI) was added as K2Cr2O7, and Cr(III) as C-2(SO4)(3). To xicity is expressed as the inhibition of respiratory activity of microorgan isms present in activated sludge. Respiration was measured by two methods, by means of an oximeter with an oxygen electrode and by means of a manometr ic apparatus permitting continuous observation of biological oxygen demand (BOD). The equation of the normal distribution function was used to constru ct inhibition curves and to calculate effective concentration (EC50). The i nfluence of exposure time was observed as was the influence of concentratio n of activated sludge on the value of EC50. The l-h EC50 for Cr(VI) by the ISO method was determined to be in the range of 40-90 mg/liter. A greater s catter of measured values, 46-201 mg/liter, was found for the first hour of exposure by the manometric method. In the case of Cr(III) only combined in hibition by chromium ions and pH was determined. The 0.5-h EC50 was 49 mg/l iter. (C) 1999 Academic Press.