Multiple stage anaerobic treatment for reductive decolorization of azo dye in the presence of sulfate

Reductive decolorization is widely used for removal of color from azo dye textile wastewater. Although efficient, this process has some drawbacks regarding the interference caused by sulfate ions, which may cause a competition mechanism for reducing equivalents. We propose the use of multiple stage anaerobic treatment to eliminate issues related to this competition. Results showed that electrons were driven to preferentially reduce the azo dye in the 1st-stage reactor (R1), where methanogenesis and sulfidogenesis were inhibited by the operating conditions applied, i.e. low pH and high organic loading rate. Reductive decolorization was found to be co-metabolic and mainly associated with hydrogen-producing pathways. The 2nd-stage reactor (R2) achieved nearly complete organic matter and sulfate removals, and also kept the overall decolorization efficiencies around 90% even when R1 presented decreased performance. Acidogens from R1 were observed to be sensitive and undergo selection upon exposure to azo dyes, whereas microbial communities in R2 are exposed to lower levels of dye and therefore appear less sensitive. To further elucidate the azo reduction mechanism, a Lactococcus lactis strain that was enriched throughout the operation was isolated from R1 and investigated using proteomics analysis. The mechanism involved biosorption by glycoconjugates, particularly exopolysaccharides and rhamnolipids, as proteins from the LPS O-antigen metabolism were statistically more abundant in cells challenged with the target compound. Electrons were transferred through the biofilm matrix by hopping, in a mechanism that involved a SDR family oxidoreductase and riboflavin carriers. The results show that enzymes with broad substrate specificity are involved in the co-metabolic degradation of azo dyes by syntrophic microbial communities. The proposed configuration was proven effective in the removal of color, organic matter, and sulfate from azo dye textile wastewaters and showed feasibility for industrial application due to its stable performance and higher azo dye load capacity when compared to singlestage systems. (AU)