Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative

Removal of synthetic dyes from wastewaters: a review.

The removal of colour from textile wastewater using whole bacterial cells: a review

Review paper on current technologies for decolourisation of textile wastewaters: Perspectives for anaerobic biotechnology

Azo dyes are the most important group of synthetic colorants. They are generally considered as xenobiotic compounds that are very recalcitrant against biodegradative processes. Nevertheless, during the last few years it has been demonstrated that several microorganisms are able, under certain environmental conditions, to transform azo dyes to non-colored products or even to completely mineralize them. Thus, various lignolytic fungi were shown to decolorize azo dyes using ligninases, manganese peroxidases or laccases. For some model dyes, the degradative pathways have been investigated and a true mineralization to carbon dioxide has been shown. The bacterial metabolism of azo dyes is initiated in most cases by a reductive cleavage of the azo bond, which results in the formation of (usually colorless) amines. These reductive processes have been described for some aerobic bacteria, which can grow with (rather simple) azo compounds. These specifically adapted microorganisms synthesize true azoreductases, which reductively cleave the azo group in the presence of molecular oxygen. Much more common is the reductive cleavage of azo dyes under anaerobic conditions. These reactions usually occur with rather low specific activities but are extremely unspecific with regard to the organisms involved and the dyes converted. In these unspecific anaerobic processes, low-molecular weight redox mediators (e.g. flavins or quinones) which are enzymatically reduced by the cells (or chemically by bulk reductants in the environment) are very often involved. These reduced mediator compounds reduce the azo group in a purely chemical reaction. The (sulfonated) amines that are formed in the course of these reactions may be degraded aerobically. Therefore, several (laboratory-scale) continuous anaerobic/aerobic processes for the treatment of wastewaters containing azo dyes have recently been described.

Basic and applied aspects in the microbial degradation of azo dyes

Azo dyes account for 60 to 70% of all textile dyestuffs produced and are the most common chromophore of reactive textile dyes. Colouration of textile effluents usually can be linked to the presence of water-soluble (reactive) azo dyes. Under anaerobic conditions azo dyes can be used as terminal electron acceptors during microbial respiration, being reduced and decolourised concurrently with re-oxidation of reduced flavin nucleotides. Thus, the process of anaerobic digestion is under investigation as an effluent treatment option for the textile industry. The ability of anaerobic microbial consortia to decolourise azo dyes is well established; however, an additional factor that requires consideration is the high concentration of salts present in reactive dyeing effluent. Nitrate, sulphate, chloride and carbonate salts can all be used during reactive dyeing. This paper presents results of an investigation into the effect of nitrate and sulphate on the decolourisation of a reactive azo dye, Reactive Red 141. ...

The Effect of Salts Used in Textile Dyeing on Microbial Decolourisation of a Reactive Azo Dye

Reactive dyes are difficult to remove from textile waste water due to their solubility and they pass through conventional aerobic biological sewage treatment systems and enter the receiving water body. Investigations into the use of anaerobic digestion to decolourise reactive azo dyes have been successful on a laboratory scale and the investigation was extended to full-scale trials. Exhausted reactive dyebath effluent (3 kl/d) was discharged into a primary digester (1.34 Ml) on weekdays for a 151-d period. On average, 48 kl/d of sludge was fed to the experimental and control digesters. The overflow was monitored for colour, sodium and sulphide concentrations. A laboratory digester was also set up to simulate the full-scale conditions but was operated at twice the exhausted dyebath loading recipe. No visual difference in colour was noted between the overflow of the primary or laboratory digester and the control digester, but elevated levels of sodium and sulphide were obtained due to the high concentration of sodium sulphate used in the reactive dyeing process. The laboratory digester became unstable at sulphide concentrations of 400 mg/l. However, the sulphide concentrations in the primary digester never increased such that it threatened digester stability.

Treatment of exhausted reactive dyebath effluent using anaerobic digestion : Laboratory and full-scale trials

Reactive dyes have been identified as problematic compounds in textile effluents as they are water-soluble and not removed by traditional aerobic biological waste-water treatment systems. The use of anaerobic digestion for the decolorisation of selected reactive dyes was investigated. It was found that 80 % of the dyes studied were decolorised and, based on the results from a detailed study into C.I. Reactive Red 141 decolorisation, it was proposed that this occurred via a reduction mechanism. This was supported by the tentative chemical identification of the dye degradation products

S. J. Barclay

Papers

The Effect of Salts Used in Textile Dyeing on Microbial Decolourisation of a Reactive Azo Dye

Treatment of exhausted reactive dyebath effluent using anaerobic digestion : Laboratory and full-scale trials

Anaerobic decolorisation of reactive dyes in conventional sewage treatment processes: short communication

Management of water resources in South Africa with respect to the textile industry

Promoting sustainable industry through waste minimisation clubs.