Fig. 6: Research framework.

The yellow arrow indicates the theoretical flow direction of urban domestic wastewater, while the green and orange dashed arrows represent the directions of infiltration and exfiltration, i.e., total nitrogen (TN) loads entering the network from the outside and loads discharged from the network into the external environment due to exfiltrated or overflowed sewage, respectively. Red dashed arrow indicates TN loads that are not collected into the sewage network and are discharged directly into the environment. The uncollected TN loads are accounted by the Chinese Second National Pollution Source Census (CSNPSC), along with loads discharged from wastewater treatment plants after treatment, which is indicated by the purple arrows. Net leakage of TN is the difference between the exfiltration and infiltration loads, which is indicated by the red arrow. The uncollected load and the net leakage load together constitute the gross leakage. The unit of each TN concentration C is mg/L, the unit of volume V is L capita⁻¹ day⁻¹, and the unit of daily per capita TN load PL is g capita⁻¹ day⁻¹.