An Evaluation of Anoxic/Aerobic Treatment for the Removal of Chemical Oxygen Demand and Fiber Reactive Azo Dye Color

Abstract

Textile dye effluent is mainly characterized by salts, organic matter, and color and the color of water discharged by manufacturing locations may be regulated. Industry is constantly searching for more effective and economical methods for meeting these regulations. The standard method of treating textile wastes uses aerobic microorganisms to cost effectively decompose organic waste. This process is ineffective in removing color related to azo dyes. Fiber reactive azo dyes, which represent a significant market portion, were chosen for investigation in this study and four hydrolyzed dyestuffs were utilized, containing C. I. Reactive Red 198, Yellow 86, Black 5, and Violet 5. This research investigated the effectiveness of a sequential anoxic/aerobic treatment process for the removal of chemical oxygen demand (COD) and fiber reactive azo dye color from wastewater using a bench-scale treatment system. The performance of the anoxic/aerobic process was compared to a bench-scale aerobic system, which represented a conventional treatment system. A viable anoxic/aerobic biomass was developed and acclimated to a synthetic influent. Using fully acclimated biomass, kinetic rate studies were performed to determine the percent and rates of COD and color removal by the anoxic/aerobic process and the aerobic control. The rate of COD removal under aerobic conditions was twice the rate under anoxic conditions. The percent COD removal by the anoxic/aerobic process was 95% vs. 97% removal by the aerobic control. The rate of color removal was highest for Reactive Violet 5, followed by Reactive Black 5, Reactive Red 198, and Reactive Yellow 86. For each dye, the degradation rate during the anoxic phase was over ten times the rate during the aerobic phase. The percent color removal by the anoxic phase was five times the removal by the aerobic phase and by the aerobic control. Reactive Yellow 86 exhibited lower color removal and certain structural differences, as compared to the other three dyes studied. This information indicates that certain structural features prevent degradation of a dye under both anoxic and aerobic conditions. In terms of process design, the kinetic rate studies for COD and color removal indicate that the majority of the time in an anoxic/aerobic cycle should be devoted to the anoxic phase, in order to maximize color removal. Since certain products of dye degradation have been identified as toxic, the toxicity removal of the anoxic/aerobic system must be investigated before specific design recommendations can be made.