Estimation and control of nitrous oxide production during biological denitrification process

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Biological denitrification process is widely used as an effective method to prevent eutrophication problems in water bodies. However, nitrous oxide, which is very strong and long-life green house gas, can be produced by bacteria during the denitrification process. Laboratory-scale experiments were conducted to clarify the conditions causing nitrous oxide production in denitrification. Substrate containing 909 mg/L(as COD)of acetate and 91 mg/L of yeast extract, and varied concentration of nitrate was continuously fed to mixed flow reactor at 20^OC. Solid retention time and PH were also varied and nitrous-oxide production, nitrate, nitrite and COD were determined in steady state condition.Effect of COD/NO_3-N ratio and SRT : Nitrous oxide was produced at SRT shorter than 10 days when COD/NO_3-N was 1.5 or 2.5. These COD/NO_3-N implies insufficient organic supply for denitrification. Nitrous oxide was not produced unless SRT was shortened to 0.5 day with substrate COD/NO_3-N of 3.5, 4.5 … More or 5.5. Maximum content of -nitrous oxide in nitrogenous gas was ag-out 12%. Growth yield coefficient was in the range of 0.17-0.18(COD/COD)with COD/NO_3-N of 1.5 or 2.5, and 0.27-0.31 with COD/NO_3-N of 3.5 or 4.5.Effect of pH on nitrous oxide production was significant. Decrease of pH from 7.5 to 6.5 caused remarkable increase of nitrous gas production whose content in nitrogenous gas reached to 16.7%.Nitrogen balance in which dissolved nitrous oxide was also taken into account shows that 10 to 20% of influent nitrogen was converted to nitrite oxide depending on the condition. Dissolved nitrous oxide was 2 to 4 times greater than the observed production of nitrous oxide.Fairly good correlation between. nitrous oxide production and nitrite accumulation was observed in the continuous experiments. Batch experiment suggests that build up of nitrite causes nitrous oxide production.In summary, favorite condition for nitrous oxide production are low COD/NO_3-N, short SRT and low pH. Avoiding these operational condition helps prevention of nitrous oxide production during denitrification. Less