The objectives of this report are: (a) the study of the mercury contaminated wastewater input effects at the activated sludge process in an activated sludge pilot plant and (b) the estimation of the removal ability of mercury's input, at different concentrations, from the activated sludge pilot plant.The concentration of inorganic mercury at the inlet of the plant varied from 0 to 100 µg/l HgCl 2 . The most important physical, chemical and biological parameters of the pilot plant were measured at least 4 times a day. Consequently, it was possible to describe, in a more detailed way, the effect that mercury's input had on the activated sludge procedure and to point at the most critical fluctuations of mercury's removal ability from the pilot plant.