Study On Variations And Influence Factors Of Nitrous Oxide In A Sewage Sludge Treatment System

Nitrous Oxide?N₂O?is a potent greenhouse gas and the amounts of anthropogenic N₂O emissions have increased in the atmosphere.It is expected to be one of the major ozone-depleting substances in the future.Inbiologicalwastewatertreatmentsystems,nitrficationand denitrification process have been identified as important sources of N₂O emission.Although N₂O emission and influencing factors in biological wastewater treatment processes have been widely studied,emission from sludge treatment systems is still less understood.At present,it is more significant to study the variations and influencing factors of N₂O in sludge treatment system.In this study,experiments were carried out at Takasu wastewater treatment plant?WWTP?and Kochi university in Japan.The wastewater treatment system of Takasu WWTP includes three kinds of anoxic/aerobic?A/O?activated sludge processes.N₂O variations in a sludge mixing tank,which received three kinds of sludge?A:primary sedimentation sludge,B:excess sludge,C:sludge from other WWTP?,were observed at Takasu WWTP.In addition,lab-scale experiments replicating the mixing sludge tank were conducted to examine the mechanisms of N₂O emission.Firstly,N₂O variations in three kinds of sludge?primary sedimentation sludge,excess sludge,sludge from other WWTP?and mixed sludge in the sludge mixing tank were investigated at Takasu WWTP.The results showed that dissolved?D-?N₂O concentration dramatically increased after mixing of the three kinds of sludge and the highest N₂O concentration reached 1300?g N/L.In addition,DO increase inhibited denitrification and N₂O accumulation under the anoxic condition in the mixing sludge tank.The investigation demonstrated that mixing sludge tank was the main source of N₂O in the sludge treatment system,which was reported as one of the important N₂O emission sources in Takasu WWTP.Secondly,lab-scale experiments replicating the mixing tank were conducted to examine the mechanisms of N₂O emission.Dissolved D-N₂O concentration dramatically increased after mixing of primary sedimentation sludge?A?and sludge from other WWTP?C?,and these two kinds of sludge were considered as the main contributors to the increasing of D-N₂O in the mixing sludge.The results were in agreement with the field observation in the sludge treatment system.Thirdly,primary sedimentation sludge?A?and sludge from other WWTP?C?were selected for further experiments to clarify the variation of N₂O.The 0.5 mg N/L of nitrate and different concentrations of hydrogen sulfide?H₂S??0,0.1,0.2,1 mg S/L?were added to the lab-scale reactor at different p H?p H=5.0?7.0?to study the variations of D-N₂O.The results showed that D-N₂O accumulated to a higher concentration at additional 0.5 mg N/L of nitrate.G-N₂O also showed an upward tendency.With the increasing of H₂S concentrations,N₂O production rate became slowly and N₂O concentration decreased.In addition,the amounts of net N₂O remained stabled even if at original H₂S concentration at p H=5.0.However,there existed lower D-N₂O and G-N₂Oconcentrationatp H=7.0.AfteraddingH₂S,D-N₂O concentration became lower at p H=7.0.In addition,the amounts of net N₂O existed negative value at p H=7.0.Finally,the 0.5 mg N/L of nitrate and different concentrations of hydrogen sulfide?H₂S??0,0.1,0.2,1 mg S/L?were added to the lab-scale reactor after mixing of A and C sludge under different p H?p H=5.0,7.0?to clarify the effect of nitrate and H₂S on D-N₂O variations and the mechanisms.At p H=5.0,nitrate was dramatically reduced to D-N₂O,and H₂S was found to have inhibitory effect on N₂O reduction to N₂ even at the original H₂S concentration of 0.26 mg S/L.Furthermore,retardation of the D-N₂O peaks due to the inhibition of nitrite reduction was observed at additional H₂S concentration of 1 mg S/L at p H=5.0.Although,nitrate reduction was inhibited,N₂O reduced to N₂ due to the high activity of N₂O reductase at p H=7.0.These results suggested that p H and H₂S jointly play a significant role in N₂O emission from sludge treatment system because of the inhibition of NO₃^--N and N₂O reduction by H₂S and p H.