<p>The sewage industry is a significant source of urban greenhouse gas emissions, and the methane produced during its treatment has a significant warming effect. Accurate measurement of methane emissions from domestic wastewater is essential for sustainable urban development. To this end, this study proposes the concept of the “First” biochemical digestion reaction of domestic sewage and calculates its carbon emissions based on 2334 gas sensor data (2019–2021) and Fick’s law. Subsequently, the spatial-temporal distribution of the unorganised carbon emissions from septic tanks in Chongqing were analysed by combining wavelet analysis and the inverse distance weighted method. The results found that the annual carbon emissions (in terms of CO₂ equivalent) from domestic sewage in the main urban area were 13.30 × 10⁷ kgCO₂-eq in 2019, rose to 17.09 × 10⁷ kgCO₂-eq in 2020, and declined to 6.542 × 10⁷ kgCO₂-eq in 2021. The spatial distribution differences widened year by year, and inter-monthly changes showed a summer peak (rising from February to July and falling for 5 consecutive months from August). This study provides data support for carbon emission reduction from urban sewage in western cities and suggests enhanced seasonal emission regulation for sustainable development.</p>