Study On Nitrous Oxide Emission Characteristics In A Wastewater Treatment Plant

Nitrous oxide（N₂O）is a kind of powerful greenhouse gas.Recently,N2 O concentration in the atmosphere has increased with the rapid growth of human activities,and wastewater treatment plants（WWTPs）were recognized as one of the important N2 O emission sources.Therefore,it is necessary to study N2 O emission from WWTPs.This study aims at clarifying N2 O emission characteristics in a WWTP,and field investigations were carried out at Takasu WWTP in Japan.Anoxic/Oxic activated sludge process（process 6）at the WWTP was targeted,and N2 O emission from both wastewater and sludge treatment systems were observed.As for the wastewater treatment system,influent to the WWTP was mixed with the supernate from sludge treatment system and then flowed into a primary sedimentation tank（PST）.Biological reactors were divided into four sections（Anoxic/Oxic-1/Oxic-2/Oxic-3）,and treated wastewater and activated sludge were separated in a final sedimentation tank（FST）.Three different kinds of sludge were treated in the sludge treatment system: raw sludge from PST,excess sludge from FST,and mixture of raw and excess sludge from other WWTP.Firstly,special variations of dissolved nitrous oxide（D-N₂O）concentration and gasous nitrous oxide（G-N₂O）emission were clarified though the investigations of wastewater treatment system.Results showed that G-N2 O emission from biochemical reaction tanks were always higher than that of other tanks,and the highest G-N2 O emission was detected at Oxic-3 tank.The D-N2 O concentration of PST was the lowest and the highest D-N2 O concentrations of biochemical reaction tanks and others were detected at Oxic-3 tank and influent,respectively.The peaks of D-N2 O concentrations of Oxic-3 and influent was always observed in the afternoon（around 15:00）.Secondly,daily variations of D-N2 O concentration and G-N2 O emission from Oxic-2 and Oxic-3 were investigated.N2 O emission from Oxic-3 was always higher than that from Oxic-2 and the N2 O emission peaks of Oxic-2 and Oixc-3 were often observed at 15:00 during a day.The highest G-N2 O emission rates of two tanks were 27.14μg N/L/h and28.39μg N/L/h,respectively and the highest D-N2 O emission rates of two tanks were 1.05μg N/L/h and 5.20μg N/L/h,respectively（2016.2.23）.G-N2 O emission played more important role in total N2 O emission of two tanks and it was 69%-97% of total N2 O emission.With the data of inorganic nitrogen,dissolved nitrogen（DN）and D-CODCr,it was clarified that nitrification was the main factor which caused N2 O emission from Oxic-2 and Oixc-3 and dissolved oxygen（DO）played an important role in D-N2 O emission characteristics of Oxic-2 and Oxic-3.Thirdly,the source of high D-N2 O loading to the PST was investigated.Results showed that G-N2 O emission from wastewater from city was higher that of influent and that the highest D-N2 O concentration was detected in supernate of sludge treatment system.D-N2 O played more important role in total N2 O emission.N2 O loadings and DN loadings of wastewater from city,supernate of sludge treatment system and influent were calculated and the calculations showed that D-N2 O loading of the supernate from sludge treatment system was the main source of high D-N2 O loading to the PST.At last,N₂O emission from the sludge treatment system at Takasu WWTP was investigated.Results showed that D-N2 O concentrations of three kinds of sludge were individually low with low variation,while extremely higher D-N2 O concentration was detected with large fluctuation in mixing sludge tank.These results demonstrated that N2 O was produced after mixing of these sludge and the mixing sludge was the main N2 O emission source of the sludge treatment system.