Stoyan N. Groudev, Department of Engineering Geoecology, University of Mining and Geology,  Studentski grad – Durvenitza, Sofia 1700, Bulgaria, Telephone: +359 2 8687396, Fax: + 359 2 8687396, Email: groudev@mgu.bg
Marina V. Nicolova, Department of Engineering Geoecology, University of Mining and Geology, Studentski grad – Durvenitza, Sofia 1700, Bulgaria, Telephone: +359 2 8687396, Fax: + 359 2 8687396, Email: mnikolova@mgu.bg
Plamen S. Georgiev, Department of Engineering Geoecology, University of Mining and Geology, Studentski grad – Durvenitza, Sofia 1700, Bulgaria, Telephone: +359 2 8687396, Fax: + 359 2 8687396, Email: ps_georgiev@mgu.bg
Irena I. Spasova, Department of Engineering Geoecology, University of Mining and Geology, Studentski grad – Durvenitza, Sofia 1700, Bulgaria, Telephone: +359 2 8687396, Fax: + 359 2 8687396, E-mail: spasova@mgu.bg
Ludo Diels, VITO, 2400 Mol, Belgium, Hoofd Milieu- en Procestechnologie, Vlaamse instelling voor technologisch onderzoek (Vito), Boeretang 200, B - 2400 Mol, België, Tel. + 32 (0)14 33 51 00, Fax. + 32 (0)14 58 05 23, Email: ludo.diels@vito.be

Acid drainage waters generated in the uranium deposit Curilo , Bulgaria, since the summer of 2004 are efficiently treated by means of a multibarrier consisting of an alkalizing limestone drain and a section intended for microbial dissimilatory sulphate reduction, biosorption and additional chemical production of alkalinity. This section was filled by a mixture of solid biodegradable organic substrates (cow manure, plant compost, straw) and crushed limestone and was inhibited by a microbial community consisting mainly of sulphate-reducing bacteria and other metabolically interdependent microorganisms. The waters had a pH in the range of about 2.5 – 4.2 and contained radionuclides (uranium, radium), toxic heavy metals (copper, zinc, cadmium, lead, cobalt, nickel, iron, manganese), arsenic and sulphates in concentrations usually much higher than the relevant permissible levels for water intended for use in the agriculture and/or industry. The water flow rate through the multibarrier usually varied in the range of about 5 – 15 m3/24 h, reflecting water residence times of about 70 to 23 hours. An efficient removal of pollutants was achieved by the multibarrier during the different climatic seasons, even during the cold winter days (in December - February) at water and ambient temperatures close to 0 ^oC. The removal was due to different processes but the microbial dissimilatory sulphate reduction and the sorption of pollutants by the living and dead plant biomass played the main role during the warmer months of the year. During the cold winter periods, when the plant and microbial growth and activity were markedly or even completely inhibited, the sorption by the dead plant biomass and the chemical neutralization by the limestone were the prevalent mechanisms in the pollutants removal. The effluents from the multibarrier usually were enriched in dissolved organic compounds and sometimes contained manganese and iron in concentrations higher than the relevant permissible levels. However, these residual pollutants were removed by means of natural and/or constructed wetlands located near the multibarrier.

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