It is very seldom that any of these are produced in nature. Nevertheless, some bacteria and yeasts "learn" to use their carbon, nitrogen and/or energy for growth after going through mutations, which can occur within a relatively short time due to the brevity of each generation. However, the processes of mineralization are often very difficult; and several different strains must cooperate. In some cases, toxic compounds are produced which are more dangerous to animals and human beings than the original compounds.
Most of these compounds cannot be removed in activated sludge plants or trickling filters; and thus it is necessary to develop tailored processes for their treatment. For high removal efficiencies it is necessary to combine different processes.
One alternative is to utilize precipitation, adsorption and membrane separation to obtain concentrates which can be disposed by other methods, such as incineration. Another alternative is the use of biological and chemical oxidation processes which result in the production of biomass and CO2. We have shown some examples in which the combination of both can be successful. However, in nearly all cases the treatment costs are relatively high.
The best way to avoid these problems, therefore, is to use more and more natural dyes (Seweko 1988) and thus stop the production of water-polluting products by the chemicals industry which cannot be biodegraded, or only with great difficulty. The production of chlorinated organics was remarkably reduced in Europe over the past two decades. The production of azo dyes, however, is still increasing. We shall need much time to reach this goal, but we must start.
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