Estimation and control of nitrous oxide production during biological denitrification process

生物脱氮工艺是防治水体富营养化问题的一种有效方法。然而，一氧化二氮是一种强度很大、寿命很长的温室气体，在反硝化过程中会由细菌产生。进行了实验室规模的实验，以明确在反硝化过程中产生一氧化二氮的条件。基质中含有909 mg/L(以COD计)的醋酸盐和91 mg/L的酵母膏，不同浓度的硝酸盐在20℃下连续进料到混流反应器中。改变固体停留时间和pH值，在稳态条件下测定氧化亚氮的生成量、硝酸盐氮、亚硝酸盐和COD。当COD/NO_3-N为1.5或2.5时，当污泥停留时间小于10天时，COD/NO_3-N比和SRT：N_2O_2的影响。这些COD/NO_3-N意味着反硝化有机供应不足。在底物COD3.54.5…的条件下，污泥停留时间缩短至0.5d，才能产生一氧化二氮更多或5.5。氮气中一氧化二氮的最大含量为12%。当COD/NO_3-N为1.5或2.5，生长系数为0.17~0.18(COD/NO_3-N)；当COD/NO_3-N为3.5或4.5时，生长系数为0.27~0.31。PH从7.5降至6.5时，氮气产量显著增加，氮气中氮气含量达到16.7%。考虑溶解氧化亚氮的氮素平衡表明，根据条件的不同，进水中10%~20%的氮被转化为亚硝酸盐氧化物。溶解的一氧化二氮是观测到的氧化亚氮产生量的2到4倍。在连续的实验中观察到了氧化亚氮的产生和亚硝酸盐的积累。间歇实验表明，亚硝酸盐的积累导致了氧化亚氮的生成，因此，低COD/NO_3-N、较短的SRT和较低的pH是产生氧化亚氮的最佳条件。避免这些运行条件有助于防止反硝化过程中产生一氧化二氮。较少

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