**4. Impact of dyes on natural environment**

Wastewaters containing dyes are troublesome in purification processes due to a complex structure of dye molecules. Even small amounts of dyes (of a few ppm order) are undesirable; they colour water, making it look unaesthetic and disturb life processes in water. Most dyes do not undergo biodegradation, deteriorate penetration of light into water and inhibit photosynthesis processes, increase chemical and biological demand for oxygen. Some dyes exhibit toxic and even cancerogenic as well as mutagenic action towards living organisms and therefore they should be carefully removed [5, 17]. Most dyes have a harmful effect (directly or indirectly) on fish. Direct activity consist in colouring of water and changing its composition, which significantly deteriorates the living conditions of fish and plankton, but indirect activity consist in poisonous properties of many dyes. Studies of dyes toxicity for fish seem to be particularly interesting not only with respect for estimation of water purity but also for the fact that they are a valuable source of food for people. Based on the ETAD (Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers) tests made for 3000 commonly used dyes in 27 cases, the registered LC50 (Lethal Dose50) values were of the order 0.05 mg/L (compared with, e.g. for DDT [dichlorodiphenyltrichloroethane] the LC50 value was 0.006 mg/L) [22]. It proved that 98% of studied dyes reveal LC50 toxicity over 1 mg/L. High toxicity was found in C.I. Basic Violet 1 with LC50 equal to 0.05 mg/L and C.I. Basic Yellow 37 with LC50 equal to 0.8 mg/L [22].
