Investigating the Efficiency of Wastewater Treatment Plant Sludge in Removing Azo Dye Reactive Red 195 from Synthetic Wastewater in Aerobic and Anaerobic Conditions

Background and purpose: Azo dyes are present in the wastewater of most textile factories, and if they enter water resources, they pose risks to the health of humans and the living environment due to the effects of toxicity, biological accumulation, and mutagenicity. This study aimed to determine the removal of azo dye by a biological method using wastewater treatment plant sludge in aerobic and anaerobic conditions and isolation and identification of bacteria. Materials and methods: This experimental study, was conducted in the faculty of health, at Mazandaran University of Medical Sciences. In this study, the removal of reactive red azo dye 195 in the wastewater of textile factories was investigated using the activated sludge of the wastewater treatment plant. Experiments were carried out in batch mode in both aerobic and anaerobic conditions with 192 samples. The concentrations of azo dye in the range available in the wastewater of textile factories were 50, 65, and 80 mg/liter. The culture medium used in this study was a mineral medium containing glucose. Bottles containing culture medium, sludge, and azo dye were kept at 30°C in an incubator in aerobic and anaerobic conditions for 8 days. The effect of factors including contact time, amount of sludge, and concentration of color was investigated. Sampling was done at the contact times of 1, 2, 4, and 8 days and after centrifuge, the color concentration was measured by spectrophotometric method at a wavelength of 545 nm. The total organic carbon (TOC) concentration of the samples was measured by a TOC analyzer. After removing the color, the bacteria in the bottles were identified by the differential diagnosis method and using special culture media, gram staining techniques, and biochemical tests. Results: In anaerobic conditions and contact time of 8 days, the removal of azo dye in concentrations of 50 and 80 mg/liter by using 5 ml of biomass was obtained by 93 and 82%, respectively. In the aerobic condition, the removal of the dye was achieved to more than 90% within two days, and after that, the removal efficiency reached 100% with a gently increasing slope up to 8 days. By increasing the concentration of dye from 50 to 80 mg/liter, the removal efficiency decreased by about 10%. Increasing the volume of sludge from 5 to 10 ml in both cases did not have a significant effect on increasing the amount of dye removal. The amount of azo dye removal was higher in aerobic than in anaerobic conditions. In aerob