Textile wastewaters are rated among the most polluting of all industrial sectors, both in terms of discharged volumes and composition, but their most important environmental problems arise from the high organic loads and the presence of color . Azo dyes are the most common synthetic colorants released in textile wastewaters due to their ease of synthesis, stability and variety of colors. Due to the electron-withdrawing nature of azo bonds, azo dyes are easily reduced by bacteria under anaerobic conditions, resulting in color removal with the formation of aromatic amines . Key data on the health hazard associated with the majority of these aromatic amines are limited , but some of these azo dye breakdown products have been considered of higher concern than the original dye with respect to their possible (eco) toxicity and/or mutagenicity/carcinogenicity and Although these amines are generally not further degraded anaerobically, their potential for aerobic biodegradation has been demonstrated for simple molecular structures Thus, staged anaerobic/aerobic sequencing batch reactor (SBR) systems using floc-forming biomass have been proposed for complete azo dye biodegradation, including decolorization and further aromatic amine mineralization , and However, the optimization of flocculent sludge technology is hampered by intrinsic operational features, such as poor settling characteristics that require a large footprint and compromise the treatment efficiency Thus, its replacement is regarded as inevitable in the near future.
In light of the environmental problems raised by textile industry wastewater and of the limitations of the treatment processes currently used, there is an urgent need for effective, environmentally friendly and economically attractive technologies for textile wastewater treatment. In this context, the novel aerobic granular sludge (AGS) technology is a promising bioprocess for textile wastewater treatment. The formation and use of AGS began to be reported in the late 1990s [10] and has recently been applied with success in domestic full-scale wastewater treatment plants (WWTP) with significant reductions in footprint and energy consumption, being pointed as the next generation of wastewater treatment technologies [11] and
In addition to the general advantages of the staged SBR technology, AGS systems present several unique attributes [13]: excellent settling properties, allowing shorter settling times for good solid–liquid separation and requiring lower construction area; good biomass retention, allowing higher concentration in the SBR and consequently lower reaction time and/or reactor volume; ability to withstand toxicity and high organic loading rates, making them attractive for industrial wastewater treatment applications; aerobic and anoxic/anaerobic zones within the granules, allowing organic matter, nitrogen and phosphorus removal in the same system and potentially contributing to the process of azo dye mineralization. Given the particular attributes of aerobic granules, this compact technology has a great potential for the treatment of the highly variable textile wastewaters, including the biodecolorization of textile dyes that are generally resistant to aerobic biodegradation, like azo dyes.
Despite the increasing reported applications of AGS for municipal and industrial wastewater, textile wastewater treatment with this technology has scarcely been reported [14]. Thus, the objective of this work was to assess the applicability of the novel AGS technology in SBR as an alternative to the conventional flocculent activated sludge SBR technology for dye-laden textile wastewater treatment. For that, the performance of two anaerobic/aerobic SBR systems, one with activated sludge flocs and the other with aerobic granules, was evaluated in terms of chemical oxygen demand (COD) and color removal efficiencies in the treatment of a simulated textile wastewater. To our knowledge, this is the first time that such a comparison is performed. In parallel to color and COD removal profiles, the potential detoxification of the simulated textile wastewater during SBR operation was also examined by using yeast-based assays [15] and [16] with the eukaryotic model Saccharomyces cerevisiae. This is a simple, animal-alternative and relatively inexpensive experimental test system that can provide a fast preview of the potential toxicity of chemicals/effluents meaningful for other eukaryotes [15], [16] and [17].