Water environment is mainly affected by the contaminants which have their origin in industry, agriculture, transport and households. Waste water which is not purified entirely, has got a very negative impact on the quality of water in recipients. That is the reason why it is necessarily important to put the emphasis on the quality of cleaning process leading into its harmlessness. The harmlessness of that water is done by means of comparison within ecotoxicological tests performed on the waste water samples. In addition, we take into consideration input and output of clean water. Deep studying of the living organisms influence might bring the new knowledge into the issue of polluting material influence on environment. The diploma thesis is focused on cleaning process quality comprehension. It includes some of the sewage treatment plants along the river Nitra and Hron with the help of ecotoxicity tests. Theoretical part of the work defines the problem of waste water, the possibility of toxicants´ establishment in water and experiences with test´s use. Experimental part describes the way how the samples of waste water were put into ecotoxicological trials. On the basis of results, the effectiveness of cleaning process was evaluated. It supports the fact that spreading of waste water is very risky for environment. In spite of the fact that physical and chemical indicators fulfill criterion of the clean water quality, it is highly recommended to be focused on the processes in the cleaning of waste water. Apart from that, it is essential to spread the knowledge about these processes and add the new ones.