6.3. Biological treatments For removal of the bulk of toxic pollutants from complex organic effluent, biological treatment is usually believed to be the most effective mean. In such processes, microorganisms usually bacteria(natural or regenerated) are used for the biodegradation of syn- thetic dyes and other dissolved substances. The biological process removes dissolved matter in a way similar to the self-cleaning and in a more efficient way than clariflocculation. Biological treatments can be classified according to the presence and absence of oxygen, respectively known as aerobic and anaerobic treatments(Babu et al., 2007) The aerobic treatments, due to their high efficiency and wide application, are the most preferable biological treatment. Aerobic biological treatment can purify the water with the help of aerobic and facultative bacteria in the aerobic environment, which is divided into two major catego ie. activated sludge process and biofilm process. The Activated sludge is more common aerobic process in which cleaning efficiency up to 90x can be possible. However, biological treatment of most of the textile dyes and chemicals by the activated sludge process does not always achieve a great success. Because most of the dyes and chemicals used in the textile industry have low or no biodegradability. Therefore, adsor- bents, such as activated carbon or clay, are added to biological treatment systems in order to eliminate non-biodegradable sub- stances produced by the textile industry.