Emission of Nitrous Oxide in Nitrification and Denitrification Process of Municipal Wastewater.

Production of nitrous oxide was examined in nitrogen removal process treating domestic wastewater from combined sewerage system. Laboratory scale experimental units were set up to simulate circulated process consisting of denitrifying reactor and nitrifying reactor, and intermittent process employing time sequencing of anoxic and aerobic condition. Nitrogen removal was satisfactory and nitrogen conversion from influent total nitrogen to nitrous oxide gas was no more than 0.1% at solid retention time of 10 or 20 days. Continuous monitoring of nitrous oxide revealed that its emission has significant variation. Nitrous oxide emission from nitrification step was greater than that from denitrification in usual situation. However, there were a few days lasting events of abrupt emission of nitrous oxide from denitrification. Such events of high emission contributed to overall nitrous oxide emission to some extent. There was almost no difference in nitrous oxide production between the circulated process and the intermittent process. Hypothetical comparison of greenhouse effect between reduction of nitrous oxide and increased aeration and addition of methanol to reduce nitrous oxide emission indicates that such control methods are justified from the viewpoint of net greenhouse effect when nitrous oxide conversion from wastewater nitrogen is in the order of one percent.