Introduction
The presence of trace levels of antibiotics in the environmental system has concerned to developed regions as well as international organizations. These bioactive compounds have been found in various types of environmental matrices, such as soil, surface and ground water . They can enter to the environment after excreted by animals and accidentally released during manufacturing and discarding. In general, veterinary medicines cannot be fully metabolized by physiological pathways in animals.For several veterinary antibiotics, metabolite rates are very low(