Dyes, especially azo dyes contained in wastewaters released from textile, pigment, and leather industries, are entering into natural waterbodies. This results in environmental deterioration and serious health damages (for example carcinogenicity and mutagenesis) through food chains. Physiochemical, membrane processes, electrochemical technology, advanced oxidation processes, reverse osmosis, ion exchange, electrodialysis, electrolysis, and adsorption techniques are commonly used conventional treatment technologies. However, the limitations of most of these methods include the generation of toxic sludge, high operational and maintenance costs. Thus, technological advancements are in use to remediate dyes from effluents. Adsorption using the nonconventional biomass-based sorbents is the greatest attractive alternatives because of their low cost, sustainability, availability, and eco-friendly. We present and reviewed up-to-date publications on biomass-based sorbents used for dye removal. Conceptualization and synthesizing their state-of-the-art knowledge on their characteristics, experimental conditions used were also discussed. The merits and limitations of various biosorbents were also reflected. The maximum dye adsorption capacities of various biosorbents were reviewed and synthesized in the order of the biomass type (algae, agricultural, fungal, bacterial, activated carbon, yeast, and others). Surface chemistry, pH, initial dye concentration, temperature, contact time, and adsorbent dose as well as the ways of the preparations of materials affect the biosorption process. Based on the average dye adsorption capacity, those sorbents were arranged and prioritized. The best fit of the adsorption isotherms (for example Freundlich and Langmuir models) and basic operating parameters on the removal dyes were retrieved. Which biomass-based adsorbents have greater potential for dye removal based on their uptake nature, cost-effectiveness, bulk availability, and mono to multilayer adsorption behavior was discussed. The basic limitations including the desorption cycles of biomass-based adsorbent preparation and operation for the implementation of this technology were forwarded.

Textile industry wastewater has become a growing concern in recent years due to it has been characterized by a high load of organic dyes, suspended and dissolved solids, alkaline pH, and low biodegradability. As a result, environmental authorities necessitate textile industries to treat effluents before discharge into the environment. Tertiary filters, particularly membrane filtrations, are the most preferable process to recover good-quality water at the tertiary treatment phase, which feeds from secondary effluents, in wastewater treatment processes. However, fouling is still a challenge due to a higher load of suspended solids, colloids, organic matter, and a high level of bio-colloids (mostly from secondary effluents) in the textile wastewater treatment process. Bio-colloids are any colloidal entities of organic matter including microorganisms and their exudates. Hence, a coagulation/flocculation unit process, as a pretreatment option, is critical both at the primary treatment stage and after secondary (biological) effluents to prevent fouling problems at the tertiary filters. We reviewed identifying major foulants causing tertiary filter damage and the available pretreatment option for the removal of these foulants. We focus on and suggest the coagulation/flocculation process as a good pretreatment alternative to prevent filter fouling as it provides a reliable process to treat high water turbidity that arises from a high load of solids and colloids. Amongst different types of foulants, we focus on and present the colloidal solids and bio-colloidal foulants that could be major causes of fouling. These foulants are less understood and expected to be dominant in the textile industry wastewater, and established pretreatment alternatives are not well developed for the bio-foulants fed from the secondary effluent. Thus, these foulants need to be critically identified in the textile wastewater treatment plants to integrate suitable pretreatment options to prevent fouling potentiality. We proposed a coagulation/flocculation unit process as a pretreatment option to reduce colloidal and bio-colloidal fouling before the tertiary treatment stage, next to the secondary effluent, is critical.

Release of dye-containing textile wastewater into the environment causes severe pollution with serious consequences on aquatic life. Bioremediation of dyes using thermophilic microorganisms has recently attracted attention over conventional treatment techniques. Thermophiles have the natural ability to survive under extreme environmental conditions, including high dye concentration, because they possess stress response adaptation and regulation mechanisms. Therefore, dye detoxification by thermophiles could offer enormous opportunities for bioremediation at elevated temperatures. In addition, the processes of degradation generate reactive oxygen species (ROS) and subject cells to oxidative stress. However, thermophiles exhibit better adaptation to resist the effects of oxidative stress. Some of the major adaptation mechanisms of thermophiles include macromolecule repair system; enzymes such as superoxide dismutase, catalase, and glutathione peroxidase; and non-enzymatic antioxidants like extracellular polymeric substance (EPSs), polyhydroxyalkanoates (PHAs), etc. In addition, different bacteria also possess enzymes that are directly involved in dye degradation such as azoreductase, laccase, and peroxidase. Therefore, through these processes, dyes are first degraded into smaller intermediate products finally releasing products that are non-toxic or of low toxicity. In this review, we discuss the sources of oxidative stress in thermophiles, the adaptive response of thermophiles to redox stress and their roles in dye removal, and the regulation and crosstalk between responses to oxidative stress.

Dye degradation mediated by oxidoreductive enzymes has been well explored in textile wastewater treatment. Ligninolytic enzymes (LEs) play key roles in the biodegradation of textile dyes. This paper aimed to provide a comprehensive review of the potentiality of LEs in textile dye biodegradation. The sources, biodegradation mechanisms, and dye removal efficiencies of LEs were discussed. Moreover, factors affecting dye biodegradation efficiency, and applications and limitations of LEs were highlighted. LEs are extracellular protein complexes that have a synergetic effect on dye biodegradation and can remove up to 99% of the dye using one or more LEs over wide pH and temperature ranges. Laccase is a highly explored and examined enzyme compared to lignin peroxidase (LiP) and manganese peroxidase (MnP) for dye biodegradation. Although extensive research has been conducted on fungal laccases, it is worth noting that bacterial laccases exhibit unique properties, such as stability at elevated temperatures and alkaline pH, which are not typically observed in fungal laccases, making them an ideal candidate for the treatment of textile wastewater. Further research on the optimization of experimental conditions is required to improve the dye biodegradation efficiency of these enzymes. It is also crucial for improving biodegradation capabilities using recombinant DNA technology.