As activated carbon is able to adsorb heavy metals that have inhibitory or toxic effects on activated sludge microorganism, the addition of powdered activated carbon (PAC) to the aeration tank of conventional activated sludge system-PACAS system-shows to be able maintain and achieve a high quality effluent. There is little direct information regarding factors that control the effectiveness of PAC for the treatment of wastewaters containing hexavalent chromium and organic compounds. Accordingly, the objectives of this study are to evaluate the performance of PACAS system for the treatment of wastewater containing hexavalent chromium (Cr(VI)), and to quantify parameters that affect removal of Cr(VI) and organic compounds in PACAS system.
The effects of PAC on Cr(VI), chemical oxygen demand (COD), oxygen consumption rate by activated sludge microorganism were investigated in batch and continuous process. The initial adsorption rate Cr(VI) by PAC was greater than that of Cr(VI) by activated sludge floc, and equilibrium data for Cr(VI) was found to be described well by Freundlich isotherm equation in low concentration. The addition of PAC was an effective control method to allow continued biological operation in the presence of Cr(VI), and increased oxygen consumption rate, Cr(VI) removal efficiency, and the stability and reliability of COD removal efficiency in continuous flow process: removal efficiencies obtain were 95-97% (COD) and 38-44%(Cr(VI)) with PAC addition, and were 77-92% (COD) and 5-12%(Cr(VI)) without PAC addition.
The kinetic constants of biological oxidation were determined on the laboratory scale units of completely mixed activated sludge (CMAS) system and PACAS system fed with synthetic wastewater containing hexavalent chromium. Introduction of 15 mg/1 Cr(VI) to a wastewater increased overall microorganism growth rate and auto-oxidation factor. The biological substrate removal rate in the reactor with PAC addition was almost twice that in the reactor without PAC addition.