The transformation and toxicity of anthraquinone dyes during thermophilic (55°C) and mesophilic (30°C) anaerobic treatments

We studied in batch assays the transformation and toxicity of anthraquinone dyes during incubations with anaerobic granular sludge under mesophilic (30degreesC) and thermophilic (55degreesC) conditions. Additionally, the electron shuttling capacity of the redox mediator anthraquinone-2-sulfonic acid (AQS) and subsequent increase on decolourisation rates was investigated on anthraquinone dyes. Compared with incubations at 30degreesC, serum bottles at 55degreesC presented distinctly higher decolourisation rates not only with an industrial wastewater containing anthraquinone dyes, but also with model compounds. Compared with batch assays at 30degreesC, the first-order rate constant "k" of the Reactive Blue 5 (RB5) was enhanced 11-fold and 6-fold for bottles at 55degreesC supplemented and free of AQS, respectively. However, the anthraquinone dye Reactive Blue 19 (RB19) demonstrated a very strong toxic effect on volatile fatty acids (VFA) degradation and methanogenesis at both 30degreesC and 55degreesC. The apparent inhibitory concentrations of RB19 exerting 50% reduction in methanogenic activity (IC50-value) were 55 mgl(-1) at 30degreesC and 45 mgl(-1) at 55degreesC. Further experiments at both temperatures revealed that RB19 was mainly toxic to methanogens, because the glucose oxidizers including acetogens, propionate-forming, butyrate-forming and ethanol-forming microorganisms were not affected by the dye toxicity. (C) 2004 Elsevier B.V. All rights reserved.