Anaerobic biodegradation of mixed azo dyes in thermophilic and mesophilic conditions

Abstract

The optimal solutions to treat dyeing and textile wastewater to contribute to the global goal of "Net Zero" emissions have attracted attention worldwide. The anaerobic digestion process has some advantages, including its low sludge production, effective removal of color, high COD reduction, and the recovery of CH₄ gas, which helps to mitigate greenhouse gas emissions. This study applied a batch-scale anaerobic digestion system at mesophilic conditions (∼35 °C) and thermophilic conditions (∼55 °C) to treat dyeing and textile wastewater containing a mixture of reactive dyes, including Reactive Yellow 145 (RY145), Reactive Red 198 (RR198), and Reactive Black 5 (RB5). The results indicated that the methane gas production from the thermophilic anaerobic digestion process was 1.41 times higher than that from the mesophilic process. Additionally, the specific methanogenic activity of the thermophilic process also outperforms the mesophilic one, with a specific organic load of 0.284 kgCOD/kgVSS.d for sludge in the thermophilic process compared to 0.195 kgCOD/kgVSS.d for the mesophilic process. The removal of color in the thermophilic process was 6.7% higher than that of mesophilic one. In addition, the first-order kinetic rate constant (k) for thermophilic anaerobic digestion (0.32 1/d) was 1.46 times higher than the mesophilic process (0.22 1/d). Further, an interesting observation is that color removal from real dyeing and textile wastewater was better than that from synthetic wastewater, with rates of 6.92 1/d and 0.32 1/d, respectively.