Abstract
Reactive nitrogen, it is presumed that, has been over the planetary boundaries, thus nitrogen flows (NF) with human activities should be estimated appropriately for the sustainable management. Numerous studies, especially in Europe, have already quantitatively analyzed NF and the sectoral flows mainly for agriculture have been identified, while results on wastewater treatment sector remain limited. In addition, though Japan is one of the few Asian countries that has accumulated scientific knowledge on national NF by some researchers, NF of the only sewage treatment plant (STP) have been estimated for a domestic wastewater. However, Japan has been operating various domestic wastewater treatment systems (DWTS), as well as STP, in such a way as to match regional characteristics, but there is no reliable analysis on NF by DWTS including STP yet even though treatment capacities may be different. Therefore, this study evaluates national nitrogen inflow by DWTS between 2005 and 2019 by compiling the nitrogen inflow estimates for 1, 731 municipalities in Japan. The municipal nitrogen inflow is calculated by multiplying number of users by DWTS, namely sewage treatment, onsite treatment (called as “Johkaso”), community plant, the others and pit toilet, by per capita nitrogen intake as food, for a municipality, using statistical data. As the results, total nitrogen inflow in Japan was almost flat (504–540 GgN/year), in the target years, and NF into STP were constantly dominant (333–411 GgN/year). Furthermore, in the past 15 years NF into pit toilet and the others decreased, while NF into STP and Johkaso increased, instead. It is found that the NF into STP is 20–30% lower than that of the past study due to differences of system boundaries. Meanwhile, NF into STP using plant-based data differ by region, which are presumably caused by differences of practical usage of and other inflows into STP.
