Acid drainage waters generated in the Curilo uranium deposit, Bulgaria, were treated by means of a pilot-scale multibarrier with a total volume of 23 m³ and consisting of an alkalizing limestone drain and an anoxic section for microbial dissimilatory sulphate reduction, biosorption and additional chemical neutralization. The waters had a pH in the range of about 2.5 – 4.0 and contained radionuclides (U, Ra), heavy metals (Cu, Zn, Cd, Pb, Ni, Co, Fe, Mn), arsenic and sulphates in concentrations usually much higher than the relevant permissible levels for waters intended for use in agriculture and/or industry. The water flow rate through the multibarrier varied in a large range (approximately from 1 to 17 m³/24 h), reflecting water residence times of about 300 – 18 hours. An efficient removal of pollutants was achieved by the multibarrier during different climatic seasons, even during cold winter days at external air and water temperatures close to 0^oC. However, some essential changes in the composition and properties of the multibarrier arised during the 28 months since the start of its operation.