Biodegradation is confirmed as an important mechanism of organic chemicals removal in natural systems. Estimation of biodegradability of chemicals which reach the aquatic environment in significant or even negligible quantities is necessary in assessing the entire hazard associated with their use. The aim of our study was to compare the standardized ready biodegradability assessment test (closed bottle test) and its modifications, employing the basic agreements on test principles of simulation of biodegradation in surface waters, yet no official standard method has been adopted. The standard test was modified using various natural river waters and a variety of nutrient conditions and microbial species trying to simulate the natural environment in the simplified way. Tests were performed with two compounds with different ability to degrade. Experiments indicate that predictability of biodegradation obtained by ready biodegradability assessment tests for surface waters in many cases is not reliable, because of different conditions, which may prevail in surface waters over time and affect biodegradation. However, the use of natural waters in screening tests assures additional information on fate of chemicals in rivers and lakes.

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