Aerobic Treatment

Conventional activated sludge treatment of wastes is often an effective and highly economic system for reducing organic pollutants in wastewater. A fair amount of research has been conducted assessing the viability of using activated sludge to treat textile effluents (Zissi et al. 1997). However, aerobic treatment of azo dye wastes has been proven ineffective in most cases, hence, a typical method of treatment is used today. Because aerobic microbes cannot reduce azo linkages, their ability to destroy dye chromogens is less than anaerobic bacterium. However, aerobic sludges have been successfully used to stabilize dye metabolites (Brown and Laboureur 1983a).

Pagga and Brown (1986) conducted a study on 87 commercial dye stuffs. Some of the dye stuffs were in a technically pure form, while others were in a sales form containing organic substances such as wetting agents. The exact class of each dye was not given. The tests were performed in a reactor designed to simulate the conditions of adapted activated sludge wastewater treatment plant. The samples were tested for color and dissolve organic carbon (DOC) removal following 42 days of treatment. Pagga and Brown (1986) concluded that as expected from their structures and function, dyestuffs are most unlikely to biodegrade in short-term aerobic tests. They further indicated that the primary mechanism for removal of dyes in activated sludge systems might occur by adsorption onto the cell walls. Also, they concluded that DOC removal was possible in an aerobic environment, but did not always correlate with decolorization. Degradation of non-dye molecules in the dye solution may be responsible for this reduction, as destruction of the chromogen is not generally observed.

In an earlier study by Brown and Laboureur (1983b), the aerobic biodegrad-ability of aniline, o-toludine, p-anisidine, p-phenetidine, o-dianisidine, and 3,3'-dichlorobenzidine, was investigated. These compounds are all lipophilic aromatic amines and possible by-products of azo dyes. Because many aromatic compounds are non-biodegradable in anaerobic environments and are not hydrophilic, they can accumulate in the adipose tissues of organisms. Many aromatics have been identified as possible carcinogens, which find their release into the environment. Brown and Laboureur (1983a) indicated that azo dyes might be broken down to their intermediate structures in the reductive environment, but were not amenable to further degradation by anaerobes.

Brown and Laboureur (1983b) concluded that aniline, p-anisidine, p-pheneti-dine and o-toludine were readily biodegradable by aerobes, while o-dianisidine and 3,3-dichlorobenzidine were inherently non-biodegradable. They suggested that these compounds could be stabilized if released into the environment or directly from a dyehouse into a conventional wastewater treatment plant.

Shaul et al. (1991) conducted a study on 18 dyes to determine their fate in the activated sludge process. Of these dyes, 15 were acid azo dyes and three were direct azo dyes. The dyes were spiked into pilot-scale treatment systems, and effluent and sludge samples were collected. High performance liquid chromatog-raphy (HPLC) was used to analyze the samples. Mass balance calculations were preformed to determine the amount of the dye in the sludge and in the effluent. Eleven of the dyes passed through the activated sludge system substantially untreated, four were significantly absorbed onto the sludge, and three were apparently biodegraded.

Loyd (1992) also performed activated sludge treatment tests on two textile wastewaters. The first was a textile dyeing and finishing process water that contained reactive navy 106, and the second was a municipal wastewater consisting predominately of textile effluents. Both effluents were fed to laboratory -scale activated sludge reactors. The reactor effluents were analyzed for ADMI color and total organic carbon (TOC) removal, and also for toxicity. He concluded that aerobic treatment of the azo dye wastewaters caused significant biodegradation with minimal decolorization, but the biodegradation did not include the azo dyestuffs. Again one would presume that the removal of organic compounds, such as wetting agents and other process additives resulted in the TOC loss. Toxicity tests indicated LC50 values falling outside of the 100% effluent concentration standard. However, he indicated that toxicity was slightly reduced following the aerobic treatment.

Furthermore, Zissi and Lyberatos (2000) investigated the biological oxidation of p-aminoazobenzene (pAAB) by Bacillus subtilis. This was carried out in batch experiments using a suspension medium supplemented with glucose, ammonium chloride, and pAAB under sterile conditions. Cellular growth rates and inhibition, glucose utilization, pAAB degradation, and by -product formation were critically observed. The results indicated that B. subtilis could co-metabolize pAAB in the presence of glucose, breaking the N=N double bond and produced aniline and p-phenylenediamine. However, an evidence was found that suggested pAAB was inhibitory to microbial growth, and that glucose was the growth-limiting substrate. The degradation of the dye was the direct result of an oxygen-insensitive azo reductase enzyme found to be present in the soluble fraction of the biomass. This enzyme was also synthesized independently of the presence of pAAB.

The previous researchers have suggested that aerobic biodegradation of most azo dyes is not very effective. Hence, conventional activated sludge systems are not adequate for treating azo dye wastewaters. Evidence on the contrary, the aerobic biodegradation of azo dye intermediates is possible and is perhaps an effective treatment process for stabilizing these compounds after anaerobic reduction.

An important property of textile dyes that they are resistant to oxidation. A garment saturated with water or perspiration and well inoculated with microorganisms is an excellent culture medium. The above criterion clarifies why many commercial dyes are recalcitrant to oxidative microbial breakdown and, therefore, existing forms of aerobic wastewater treatment are not usually effective against dye house effluent.

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