Hydraulic retention time as a key driver of methane emissions in gravity sewer pipelines

초록

Methane (CH4) emissions from sewer pipelines are an underexplored contributor to greenhouse gases in the wastewater sector, with food waste (FW) discharge intensifying these emissions. This study investigated the effect of hydraulic retention time (HRT) on CH4 emissions in two lab-scale gravity sewers: R1 receiving domestic wastewater and R2 co-fed with FW. At long HRT (48 h), emissions reached 60-70 mL CH4/Lsewage in R1 and 130-140 mL CH4/Lsewage in R2. Stepwise HRT reduction (48-4 h) consistently decreased emissions, with major declines at 24-12 h in R2 (from 100 to 70 mL CH4/Lsewage) and 12-8 h in R1 (from 48 to 27 mL CH4/Lsewage). Overall, shortening HRT from 48 h to 4 h reduced emissions by 70-75% in both pipelines. Hydraulic analysis showed that decreasing HRT increased wall shear stress from 1.6 to 2.4 Pa, indicating increasingly turbulent conditions. In R1, where organic acids were negligible, and pH remained above 7.5, CH4 suppression was attributed to biofilm destabilization, supported by a 27% loss of extracellular polymeric substances between 12 and 8 h HRT. In contrast, in R2, emission reduction coincided with organic overloading, organic acid accumulation (up to 170 mg COD/L), and pH depression to 6.7-6.8.

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