Treatment of waters polluted with crude oil and heavy metals by means of a natural wetland.

In the Dolni Dubnik oil deposit, Northern Bulgaria, waters polluted with crude oil and heavy metals (iron, manganese, zinc, cadmium, lead, copper) were treated by means of a natural wetland located in the deposit. The waters had a pH in the range of about 4.5-6.5 and contained about 1-5 mg/l oil. The concentrations of heavy metals usually were about 2-4 times higher than the relevant permissible levels for waters intended for use in the agriculture and industry. The watercourse through the wetland covered a distance of about 100 m and the water flow rate varied in the range of about 0.2-0.8 l/s. The wetland was characterized by an abundant water and emergent vegetation and a diverse microflora, including different oil-degrading bacteria and fungi. The treatment of the polluted waters by means of the above-mentioned wetland markedly depended on the temperature but was efficient during the different climatic seasons, even during the cold winter months at temperatures close to 0 degrees C. The oil content in the wetland effluents in most cases was decreased to less than 0.2 mg/l, and the concentrations of heavy metals were decreased below the relevant permissible levels. The removal of oil was connected with its microbial degradation. The removal of heavy metals was due to different processes but the microbial dissimilatory sulphate reduction and the sorption of metals on the organic matter and clay minerals present in the wetland played the main role.