Ecology & Safety ISSN 1314-7234, Volume 11, 2017 Journal of International Scientific Publications www.scientific-publications.net Page 85 NITRITE IONS REDUCTION BY IMMOBILIZED MICROBIAL CELLS ON GRANULATED ACTIVATED CARBON Tsvetomila I. Parvanova-Mancheva, Evgenia Vasileva, Venko Beschkov Institute of Chemical Engineering, Bulgarian Academy of Sciences, str. “Acad. G. Bonchev” bl.103, 1113 Sofia, Bulgaria Abstract Nowadays pollution of natural waters, rivers and lakes by nitrate is one of the main problems related to the environment. Nitrate originated from agriculture is increasingly growing all over the world due to the extreme use of fertilizers. Nitrate salts reach the underground water as they percolate through the soil. There are various methods for treatment of water for nitrates, but the majority of them yield secondary pollutants. An exception is the biological denitrification, at which nitrates are reduced to harmless nitrogen gas, and side waste products practically do not occur. The potential of this method is high. The process of nitrate reduction is inhibited by nitrite accumulation being intermediate of microbial denitrification. In the present work the influence of nitrite ions on the process of microbial denitrification was investigated. Both free and immobilized microbial cells were tested. Granulated activated carbon was chosen as support for microbial cell immobilization. It was found out that granulated activated carbon enables microbial denitrification at very high concentrations of nitrite ions reaching 100 mg /l. Key words: wastewater, denitrification, nitrate/nitrite reduction, granulated activated carbon INTRODUCTION Water is of essential importance for life on earth. The mechanism of metabolism, in the cells of living organisms are closely related to the specific characteristics of water. The water resources are limited and only 2.66% of the total global water resources comprising groundwater, lakes and rivers, polar ice and glaciers are fresh water. Furthermore, only a small fraction, about 0.6%, is usable as drinking water. For this reason, water resources must be necessarily treated properly and wastewater treatment must be done efficiently (Ghafari et al., 2008) (Shimali et al., 2001). Nitrate and nitrite are ubiquitous within environmental, food, industrial and physiological systems, and although our understanding of their role within such matrices has increased, a substantial degree of uncertainty and speculation remains (Matthew et al. 2001) Nitrate and nitrite have become intertwined with domestic life, and it is effectively impossible to proceed in everyday activities without encountering these ions or the products of their use (Matthew et al. 2001). In recent years, nitrate pollution of water resources has become a serious environmental problem due to discharge of domestic and industrial wastewater and widespread use of nitrogen fertilizers (Darvishi, 2016). The traditional methods (coagulation, ion exchange, reverse osmosis and electro-dialysis UV, filtration, chlorination, ozonation) for waste water treatment are very expensive and not sufficiently useful for the elimination of nitrate ion from the water. Therefore, to remove nitrate ion, a supplementary method is necessary (Sumino et al., 2006). For this reason, biological denitrification is an important approach which actually removes nitrate from the drinking water sources. In biological denitrification nitrate successively reduced to nitrogen gas passing through nitrite, nitrous oxide and nitric oxide, according to the following scheme (Payne, 1973): 2 2 2 3 N O N NO NO NO      