Recent developments in textile wastewater biotreatment: dye metabolite fate, aerobic granular sludge systems and engineered nanoparticles

Textile wastewater (TWW) represents a major source of pollution worldwide, carrying high organic loads, recalcitrant azo dyes and engineered nanoparticles (ENP), namely silver nanoparticles (AgNP). The development of cost-efficient, environmentally-friendly TWW treatment solutions is critical. Despite the successful biodecolorization of azo dyes under anaerobic conditions, clear evidence for subsequent aerobic biodegradation of the often toxic breakdown sulfonated aromatic amines is scarce. Moreover, the debatable AgNP toxicity mechanisms, and apparent AgNP retention in activated sludge have raised concerns regarding eventual negative impacts on wastewater treatment efficiency. The aerobic granular sludge (AGS) technology, which has recently been scaled-up and implemented for the treatment of domestic wastewater and some industrial wastewaters, seems highly promising for TWW treatment, due to the high biomass retention capacity, anaerobic/anoxic/aerobic microenvironments within granules and enhanced tolerance towards high organic loads and toxic compounds. A review of the existing literature on AGS application to TWW treatment is presented, with a focus on the removal of azo dyes and their metabolites and ENP. The applicability of AGS to dye-containing synthetic and real TWW has been tested in different SBR systems. Their hydrodynamic regimens and operational conditions have been optimized, namely regarding granulation, long-term stability, azo dye decolorization and biodegradation of aromatic amines. Although promising results have been published regarding AGS resistance towards ENP (particularly AgNP), their long-term effects on the physical stability, biochemical properties and microbial community of AGS deserve more investigation. Overall, this review provides relevant support for the application of AGS SBRs in TWW treatment as a potential sustainable alternative to avoid the pollution of natural water bodies with synthetic dyes and ENP.