Reducing methane emissions from gravity sewer pipelines by ultrasonication

Methane (CH₄), a potent greenhouse gas (GHG), is a major concern in sewer pipelines, and its emissions can increase with food waste (FW) introduction via disposer systems. To mitigate these emissions, several chemical approaches have been attempted, yet they face environmental and economic challenges. In this study, ultrasonication was applied inside sewer pipelines at different intervals (30 min once every 4, 5, and 6 weeks) to evaluate its long-term effect on CH₄ emission reduction. Two lab-scale gravity sewer pipelines (7 m long, 80 mm diameter) were constructed and fed with: only domestic wastewater (R1) and domestic wastewater plus FW (R2). FW addition increased CH₄ emissions from 52 to 110 mL CH₄/L_sewage, due to higher organics entering the sewer pipelines (500→1000 mg COD/L). Following ultrasonication, CH₄ emissions decreased by 40–56 % and 34–39 % in R1 and R2, respectively, attributed to biofilm detachment, as confirmed by changes in extracellular polymeric substances concentration and shear stress test. The average daily GHG reduction by ultrasonication was 10.5, 9.2, and 7.7 g CO₂ eq./d for R1, and 15.8, 14.6, and 13.9 g CO₂ eq./d for R2, at 4-, 5-, and 6-week intervals, respectively, clearly indicating a revival in CH₄ emissions. Considering the energy use, net GHG reduction effectiveness significantly dropped with longer intervals in R1 (38→28 %), while remaining relatively stable (28–30 %) in R2. These results emphasize the importance of shortening ultrasonication intervals, particularly when only domestic wastewater is conveyed in the sewer pipelines.