Wastewater originating from the textile industry is one of the major sources of pollution for surface and groundwater bodies in countries where textiles and other dye-products are produced. Along with dyes, textile wastewaters also contain varying amounts of metals/metalloids, salts and organic pollutants. Moreover, these wastewaters have high temperatures and varying pH. Various physico-chemical and biological strategies have been devised to remove dye contaminants from such wastewaters. However, biotechnological approaches have attracted worldwide attention for their relative cost-effectiveness and environmentally friendly nature. Most biotechnological approaches rely on the use of microbes that have the potential to enzymatically degrade and decolorize dye-containing textile effluents. During recent years, several microbial cultures as well as microbial enzymes have been characterized and used for removal of dyes from simulated wastewaters having defined chemical compositions. However, there are still many challenges in scaling up microbial and enzymatic technologies for decolorization of raw textile wastewater that contain metals/metalloids, salts and other toxic compounds. The present review article summarizes the findings of recent studies conducted on decolorization of raw textile wastewaters. To the best of our knowledge, this is the only review reporting the biodegradation of azo dyes in raw textile effluents.

Microbial biotechnology for decolorization of textile wastewaters

The industrial dye-contaminated wastewater has been considered as the most complex and hazardous in terms of nature and composition of toxicants that can cause severe biotic risk. Reactive azo, anthroquinone and triphenylmethane dyes are mostly used in dyeing industries; thus, the unfixed hydrolysed molecules of these dyes are commonly found in wastewater. In this regard, bacterial species have been proved to be highly effective to treat wastewater containing reactive dyes and heavy metals. The bio-decolourisation of dye occurs either by adsorption or through degradation in bacterial metabolic pathways under optimised environmental conditions. The bacterial dye decolourisation rates vary with the type of bacteria, reactivity of dye and operational parameters such as temperature, pH, co-substrate, electron donor and dissolved oxygen concentration. The present paper reviews the efficiency of bacterial species (individual and consortia) to decolourise wastewater containing reactive azo, anthroquinone and triphenylmethane dyes either individually or mixed or with metal ions. It has been observed that bacteria Pseudomonas spp. are comparatively more effective to treat reactive dyes and metal-contaminated wastewater. In recent studies, either immobilised cell or isolated enzymes are being used to decolourise dye at a large scale of operations. However, it is required to investigate more potent bacterial species or consortia that could be used to treat wastewater containing mixed reactive dyes and heavy metals like chromium ions.

The efficacy of bacterial species to decolourise reactive azo, anthroquinone and triphenylmethane dyes from wastewater: a review

Humus constitutes a very abundant class of organic compounds that are chemically heterogeneous and widely distributed in terrestrial and aquatic environments. Evidence accumulated during the last decades indicating that humic substances play relevant roles on the transport, fate, and redox conversion of organic and inorganic compounds both in chemically and microbially driven reactions. The present review underlines the contribution of humus-reducing microorganisms in relevant environmental processes such as biodegradation of recalcitrant pollutants and mitigation of greenhouse gases emission in anoxic ecosystems, redox conversion of industrial contaminants in anaerobic wastewater treatment systems, and on the microbial production of nanocatalysts and alternative energy sources.

Humus-reducing microorganisms and their valuable contribution in environmental processes.

Comparison of microbial community in hydrolysis acidification reactor depending on different structure dyes by Illumina MiSeq sequencing

Decolorization and detoxification of Direct Blue 2B by indigenous bacterial consortium.

Decolorization of Orange I under alkaline and anaerobic conditions by a newly isolated humus-reducing bacterium, Planococcus sp. MC01.

A novel thermotolerant bacterium, designated SgZ-8T, was isolated from a compost sample. Cells were non-motile, endospore-forming, Gram-staining positive, oxidase-negative and catalase-positive. The isolate was able to grow at 20–65 °C (optimum 50 °C) and pH 6.0–9.0 (optimum 6.5–7.0), and tolerate up to 9.0 % NaCl (w/v) under aerobic conditions. Anaerobic growth occurred with anthraquinone-2,6-disulphonate (AQDS), fumarate and NO3
- as electron acceptors. Phylogenetic analysis based on the16S rRNA and gyrB genes grouped strain SgZ-8T into the genus 
Bacillus
, with the highest similarity to 
Bacillus badius
JCM 12228T (96.2 % for 16S rRNA gene sequence and 83.5 % for gyrB gene sequence) among all recognized species in the genus 
Bacillus
. The G+C content of the genomic DNA was 49.3 mol%. The major isoprenoid quinone was menaquinone 7 (MK-7) and the polar lipids consisted of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine and an unidentified phospholipid. The major cellular fatty acid was iso-C16 : 0. On the basis of its phenotypic and phylogenetic properties, chemotaxonomic analysis and the results of physiological and biochemical tests, strain SgZ-8T ( = CCTCC AB 2012108T = KACC 16706T) was designated the type strain of a novel species of the genus 
Bacillus
, for which the name 
Bacillus
thermotolerans sp. nov. is proposed.

Bacillus thermotolerans sp. nov., a thermophilic bacterium capable of reducing humus.

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Decolorization of Orange I under alkaline and anaerobic conditions by a newly isolated humus-reducing bacterium, Planococcus sp. MC01.

Bacillus thermotolerans sp. nov., a thermophilic bacterium capable of reducing humus.