A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Environment, Resources and Development

Thesis (M.Eng.) - Asian Institute of Technology, 1982

The purpose of this research work was to study the effect of trivalent and hexavalent chromium on the activated-sludge treatment of tannery waste. A complete characteristics analysis and treatability study of tannery waste was performed. Since Cr-III toxicity is a direct function of Cr-III in soluble form and solubility is a function of pH alkalinity and waste characteristics; particular emphasis was given on the study of the solubility of trivalent chromium. In case of Cr-VI, an effort was made to investigate the uptake of hexavalent chromium by activated sludge for consequent removal. Results of the research work showed that 81% of the COD can be removed from tannery waste by activated-sludge process in a detention time of 12 hrs. The trivalent chromium toxicity is due to its soluble form. At any influent Cr-III concentration, the available soluble Cr is less than 4 mg/t and the rest is retained with MLSS in the reactor in precipitated form (pH= 6.8 - 9.0). A daily addition of 100 mg/t of Cr-III for 10 days with a gross accumulation of 576 mg/t (28.8% of MLSS on dry wt. basis) of precipitated Cr-III and 4 mg/t of soluble Cr-III, the laboratory activated-sludge units showed no loss in efficiency. The hexavalent chromium toxicity is a function of only MLSS concentrations at and above F/M ratio of 0.25. A metal to biomass ratio, i.e. Cm/Cx; ratio of 0.005, is the safe limit for Cr-VI. Below F/M ratio of 0.25, the toxicity is a function of both substrate and MLSS concentrations and increases with the decrease of substrate and MLSS concentrations. A maximum uptake of 45 mg of Cr-VI is possible with 2 gm of MLVSS.