Anaerobic Treatment

Anaerobic reduction of dyes using microbial sludges can be an effective and economic treatment process for removing color from dye house effluents. Previous studies have demonstrated the ability of anaerobic bacteria to reductively cleave

Fig. 12.2 A schematic diagram (a) and photo (b) of a laboratory scale upflow anaerobic sludge blanket reactor (UASB)

the azo linkages in reactive dyes (Chung et al. 1978; Brown and Laboureur 1983a; Brown and Hamburger 1987; Ganesh 1992; Loyd 1992; Razo-Flores et al. 1997; Chinwetkitvanich et al. 2000). Although this effectively alters the chromogen and destroys the observed color of the dye, many aromatic groups are not susceptible to anaerobic reduction. However, there is an evidence that some azo dye metabolites may be fully stabilized in the anaerobic environments (Razo-Flores et al. 1997).

Chung et al. (1978) conducted a study on the degradability of seven azo dyes using intestinal and other major anaerobes. The studies were carried out using isolated strains of bacteria in suspended cell mediums containing different azo dyes. Although the dyes studied were not fiber-reactive dyes, their findings showed that the reduction of azo compounds could be accomplished by intestinal and other major anaerobes. Furthermore, the presence of aromatic intermediates was also detected in measurable amounts for each dye. Toxicity tests were not conducted, but some of the intermediates had been previously reported to be mutagenic.

In a three-part research series, Brown and Laboureur (1983a, b) studied the degradability of various azo dyes in both anaerobic and aerobic systems. In the first study, Brown and Laboureur (1983b) investigated the anaerobic degradability of 22 commercial dyes. Of the dyes studied, four monoazo and six diazo dyes showed substantial biodegradation, while two polyazo dyes showed moderate to variable reductions.

Later, Brown and Hamburger (1987) conducted a study on 14 azo dyes subjected to anaerobic sludge digestion followed by aerobic treatment. This study focused on both the reduction of the dye molecules as well as the production and subsequent degradation of dye metabolites. Brown and Hamburger's results confirmed the findings of earlier research, showing decolorization of the azo dyes. Confirming the cleavage of azo linkages, the production of metabolites was also observed, at less than theoretical concentrations. Speculation was made as to why these concentrations were low, but no conclusive evidence was provided. They do indicate that based on the yield of metabolites, further dye reduction in anaerobic environments is in general, neither rapid nor appreciable.

Razo-Flores et al. (1997) investigated the fate of mordant orangel (MO1) and azodisalicylate (ADS) under methanogenic conditions using continuous UASB reactors. Their research focused on the reduction of by-products, 5-aminosalicylic acid (5-ASA) and 1,4-phenylenediamine. Co-substrates, volatile fatty acid (VFA) or glucose, were also fed to the reactors in order to supply the reducing equivalents needed for the reduction of the azo bonds. The results of this study demonstrated the ability of an anaerobic consortium to completely mineralize some azo dye compounds. The experiments were conducted using two reactor series. In the first reactor, only MO1 and VFA were fed for a 217-day period. Data from this period showed high decolorization, and high consent ratio of 5-ASA and 1,4-phenyl-enediamine. In the second reactor, MO1 was fed for 217 days, which was followed by ADS, with and without glucose, for a period of 340 days. Data from this reactor showed high decolorization throughout testing, but a lower concentration of 5-ASA. Razo-Flores et al. (1997) observed the complete mineralization of ADS with and without a co-substrate, indicating the possibility for aromatic amine destruction in methanogenic environments. The compound, 1,4-phenylenediamine was not observed to degrade in either test reactor, indicating the specificity of aromatic amine utilization by anaerobes.

The studies were conducted by Loyd (1992), Ganesh (1992) on the anaerobic reduction of textile mill effluents and the azo dyes Reactive Black 5 and Navy 106 were investigated, respectively. In both cases, laboratory scale anaerobic reactors were used for dye degradation. The results of Loyd (1992), Ganesh (1992) were similar; both observed good decolorization with minimal nutrient removal. These findings support the earlier studies found in the literature. While high decolor-ization of textile effluents was often achieved in the anaerobic environments, poor TOC and nitrogen removals were usually observed.

Chinwetkitvanich et al. (2000) performed a study on various reactive dye bath effluents. They examined the effect of co-substrate and initial color concentrations on fiber-reactive dye reduction efficiencies in UASB reactors. Five different experiments were conducted using a variation of red, blue, and black dye synthetic wastewaters and also real dye house effluents composed of red, blue, and black dyes. Their results showed that with the addition of a co-substrate, such as tapioca, increased reduction efficiencies could be achieved. However, at high level of tapioca addition, no enhancement was observed. Furthermore, Chinwetkitvanich et al. (2000) concluded that higher initial color concentrations might be deleterious to acid forming bacteria, resulting in a lower dye removal. Additionally, the authors suggest that sulfate-reducing bacteria might out-compete other anaerobic microorganisms for available organic carbon, but contribute minimally to decolorization. This could serve to limit the reduction equivalents necessary for dye degradation.

The interest in the use of anaerobic treatment process can be explained by considering the advantages and disadvantages of this process. The principal advantages and disadvantages of anaerobic treatment are listed as follows: Advantages

• Less energy requirement since aeration is not required.

• Less biological sludge production.

• Fewer nutrients required.

• Methane production, a potential energy source.

• Smaller reactor volume required.

• Elimination of off-gas air pollution.

• Rapid response to substrate addition after long periods without feeding. Disadvantages

• Longer start-up time to develop necessary biomass inventory.

• Requires alkalinity addition.

• Requires further treatment with an aerobic treatment process to meet discharge requirements.

• Biological nitrogen and phosphorus removal is not possible.

• Much more sensitive to the adverse effects of lower temperatures on reaction rates.

• More susceptible due to toxic substances.

• Potential for production of odor and corrosive gases.

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