Study On Control Technics Of N₂O Emission In Aerobic Denitrification System

N₂O is an important green house gas in the atmosphere. The N₂O control has been a research focus because of the continuous increase of N₂O concentration in the atmosphere and its environmental problems such as global greenhouse effect. Research has suggested that biological nitrification and denitrification of wastewater is one of the important sources of N₂O. Some effective methods should be applied to control the N₂O emission in order to reduce the second pollution to air while more and more new denitrification technologies are dealing with wastewater with high nitrogen concentration and serious rich nutrition in water.At present, most of the research about N₂O emission during denitrification was limited macroscopically and seldom concerned about bacteria, and nitrification and aerobic denitrification were often confused. Directing to these problems, laboratory-scale experiments were conducted in aerobic denitrification activated sludge system which was cultivated maturely with NO3- as basis and high DO concentration in Sequencing Batch Reactor(SBR). The emission of harmful gaseous products such as NO,N₂O and NO2 during aerobic denitrification was studied deeply in this paper. And the relation of the N₂O emission and operating parameters(DO, COD/N, pH, T) was analyzed here. The results indicated that DO, COD/N, pH, and temperature all made impacts on the emission of NO, NO2 and N₂O. Among them, COD/N and pH made greater impacts than others and were the major control factors about the N₂O emission. The rate of total denitrification decreased and the amount of harmful gaseous products increased quickly when COD/N in inflow reduced from 18 to 6 gradually. The amount of harmful gases decreased greatly if COD/N>12. Alkalescence and acidity were not favourable for aerobic denitrification between 6.0 and 8.2 in pH. The amount of harmful gases emission increased apparently on the condition of acidity. The denitrification rate was the fastest at the point of 7.2 in pH and the amount of the N₂O emission was a little. The rise of DO resulted in the increase of harmful gases emission and the increase of temperature benefited to control the harmful gases emission. Both of them were not considered as major factors about the N₂O emission because of a bit impacts.The amount of N₂O emission was far greater than the amount of NO and NO2 in all experiments, which indicated that N₂O was the most major harmful gaseous product in aerobic denitrification system. Environmental factors influenced the amount of N₂O emission much more greatly than the other two gases for the possibility of N₂O enzyme activity. The amount of N₂O emission was greatly consistent with the accumulation of NO2--N, especially on the condition of very slow deoxidization rate of NO2- which was almost 100% converted to N₂O in the experiment. So we can conclude that the accumulation of NO2--N is the major reason of N₂O production and there is NO NO2 because of the N₂O production accompanied with NO and NO2.The denitrification pathway of aerobic denitrification was typical two-grade deoxidization reaction which was NO3--N→NO2--N→gaseous N. And the NO and NO2 peaks both appeared before NO2--N, so there was probably a pathway that NO3--N could directly be deoxidized to NO and NO2. The aerobic denitrifying bacterial strain in the system was filtrated and seven bacterial strains were successfully identified through 16S rDNA sequence. The ratio of nitrogen removal was above 70% by the measurement of the aerobic denitrification performance.The method of measuring N₂O emitted in wastewater denitrification was erected in the paper. Varian 3800 gas chromatography, 10mci63N electron capture detector(ECD) and the following operating parameters such as detector temperature(340℃), separate column temperature(50℃)and flow rate of carrier gas(20ml/min)were applied here. This method had high sensitivity, accuracy and fine recurrence(C.V=1.05%,n=23), and detection limit reached up to 0.175ppm. On the above conditions, work curves about N₂O measurement were made. Meanwhile, the air sampling bags were proved to be able to preserve N₂O samples effectively for a short time.