Seoul's persistent sewer smell baffles city officials

A persistent, sharp, sewer-like odor occasionally assaults pedestrians in Seoul, a recurring issue that has plagued residents and visitors for years. Complaints about these sudden bursts of smell are easily found in online communities, highlighting the city's ongoing struggle with its olfactory environment.

According to a 2025 report by the Seoul Institute, these sewer-related smells are responsible for roughly half of all civil complaints concerning bad odors. While initial measures launched in 2015 saw a decline in complaints, they have since rebounded, indicating the limitations of past mitigation efforts. The city has grappled with this problem for over a decade, yet questions linger about the sufficiency of its strategies to fully eliminate the pervasive smell.

The main culprit behind the stench is hydrogen sulfide, a gas with a distinct rotten egg odor. It forms when organic matter in sewage breaks down in environments lacking oxygen. Seoul's sewer system, strained by rapid urban expansion, often sees buildings installing septic tanks as a temporary solution before wastewater enters underground pipes. Although these tanks filter some solids, dissolved organic matter remains, decomposing and generating hydrogen sulfide.

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Odours can be caused by diverse and complex factors, so they must be addressed based on expert diagnoses and on a case-by-case basis.

While heavy rainfall can help flush the system and reduce odor buildup, hot, dry periods allow hydrogen sulfide to accumulate and escape through openings like storm drains. A Seoul city official acknowledged the diverse and complex factors contributing to odors, emphasizing the need for expert diagnoses and case-by-case solutions. "High drops in wastewater flow and sediment build-up inside sewer pipes can contribute to odours, so the city is considering measures such as changing sewer structures and dredging accumulated sediment," the official stated.

Seoul's combined sewer system, which carries both wastewater and stormwater in the same pipes, further complicates the issue. Although the city has considered separating these systems since the early 1990s, progress has been slow and costly. As of 2025, only 20.4 percent of the network has been converted, leaving the majority susceptible to odor issues. The city has a dedicated team for odor mitigation, but the fundamental challenges of infrastructure and the complex nature of the problem mean the smell continues to be a part of Seoul's urban experience.

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High drops in wastewater flow and sediment build-up inside sewer pipes can contribute to odours, so the city is considering measures such as changing sewer structures and dredging accumulated sediment.