Nitrate Removal From Groundwater By Hydrogen Autotrophic Denitrification Using Bicarbonate As Carbon Source

Nitrate is very soluble in water and is simply transported to groundwater and surface water through uncontrolled discharge of nitrate containing sources such as industrial or domestic wastes, landfill leachate, animal wastes, agricultural drainage, and feedlot runoff. Contamination of water by nitrate is increasing worldwide and nitrate level in natural waters became an important indicator of water quality. It is potentially converted to nitrite in the gastrointestinal tract or to nitrous nitrogen compounds through reduction.Diverse methods are reported to remove nitrate from groundwater, such as reverse osmosis, ion exchange, electro-dialysis, which almost fail to entirely treat nitrate, except biological denitrification. Biological denitrification is normally conducted by facultative anaerobes which are in essential need for some food and energy sources which are organic or inorganic. This fact classifies denitrifiers into two main groups of heterotrophs and autotrophs. Heterotrophs are microorganisms that require organic substrates to get their carbon for growth and development. They also obtain energy from carbohydrates and other organic material. In contrast, autotrophs are microorganisms that use inorganic substances as energy source and carbon dioxide as a carbon source. Heterotrophic denitrification has been applied in recirculating systems and traditional wastewater treatment plants, whereas autotrophic denitrification is lately notified within the last two decades. These substrates do not always exist in the water or wastewater which is to be treated and should be supplemented, while, hydrogen autotrophic denitrification method attracts wide concern because of its clean, environmental friendly and economic.In this study, carbon dioxide and bicarbonate were selected as carbon source. Tests were performed to compare their effectiveness on denitrification under different conditions. The results showed that bicarbonate as the only inorganic carbon sources led to faster acclimation, and the stabilization of pH, which was better than that only with carbon dioxide or carbon dioxide combinated with bicarbonate as carbon source. The concentration of 2.0 g/L bicarbonate was sufficient for the growth of denitrifying bacteria if only bicarbonate was added as carbon source. The performance of denitrification increased with higher nitrate when its concentration was below 135.6 mg N/L. But it would inhibite denitrification with higher nitrate load. The nitrate degradation could be achieved when pH ranged from 6.0~9.0, but the best pH was 7.0~8.0. A lower (<6.0) or higher (>9.0) pH resulted in stagnation of denitrification. The best denitrification efficiency 2.83 mg/(Lh) was achieved with a temperature at 35℃. The accumulation of nitrite occurred at 15℃.From the results of this study, a noval carbon source can be found and will be utiliaed in the future.