Until today, industrial effluents loaded with particular organic compounds, which are difficult to biodegrade, have frequently been mixed with other effluents and "treated" together in WWTPs. Over time, the disadvantages of this technology have become obvious:

• The chemical oxygen demand (COD) of the effluent is considerably higher than legal limits, resulting in high discharge fees or the requirement to supplement the process with activated carbon treatment.

• Stripping of a part of the organic volatile compounds leads to air pollution problems and the necessity to cover the aerated basins and to treat the waste air.

• Certain effluents are only diluted, such as those from chemical plants with a high concentration of non-biodegradable compounds.

• Effluents containing toxic compounds reduce the removal rate in activated sludge plants.

• Endocrine-disrupting compounds discharged into wastewater can pass through WWTPs and enter aquatic environments.

Many more reasons can be cited for the need to turn away from end-of-the-pipe technology. New approaches are necessary, especially when new production units are to be constructed or old units have to be modernized. Furthermore, local circumstances have to be taken into consideration. In industrialized countries, a large percentage of industrial effluent is discharged into municipal WWTPs. But a significant number of large cities around the world do not have any municipal WWTP, so that industrial wastewater treatment is absolutely necessary to protect ground water reservoirs and to guarantee a minimum quality of raw water for the production of clean drinking water. Since clean water is also an essential raw material for industry and agriculture, many national economies will be affected by the need for clean water in the near future.

This makes it necessary for industry to conserve water by reducing its consumption. In principle, there are essentially three ways to conserve water:

1. Design new production processes with lower water consumption.

2. Recycle cooling and process water.

3. Develop economical solutions for the treatment of effluents with different types and strengths of problematic and non-problematic compounds (see Chapter 13).

The treatment of wastewater and process water should be carried out in most cases in the immediate proximity of the production unit. Both production and waste management have to be designed and optimized by a single team; and both units have to be operated by a single team. Wastewater treatment must be integrated into the production process!

Physical, chemical and biological processes must all be considered to find the best solution. Biological and chemical oxidation and reduction processes are of special interest if almost complete transformation to compounds such as CO2, H2O, N2 and CH4 is possible. The pollution in the effluents from such optimized processes is no longer characterized by thousands of different chemicals, rather there are often only three or four single compounds which dominate; and a knowledge of stoichiometry as well as kinetic coefficients may be helpful for a better understanding and optimization of the biodegradation processes.

Before we pick up this topic in Chapter 13, this chapter will discuss groups of problematic dissolved materials, which are not easily mineralized by microorganisms.

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