SEOUL: Occasionally, a sharp odour hits pedestrians as they walk along Seoul’s sidewalks or wait at bus stops in South Korea.
Although the smell often disappears within seconds, similar complaints have been repeatedly raised by residents and visitors, and posts describing sudden bursts of sewer-like odours can easily be found in both Korean and international online communities.
Much of the smell appears to come from the sewer system.
According to a 2025 report by the Seoul Institute, sewer-related smells account for about half of the city’s civil complaints regarding bad odours.
While complaints declined after Seoul launched sewer odour reduction measures in 2015, they have since rebounded.
The city has made efforts to tackle the issue for over a decade, but questions remain over whether its mitigation measures are enough to eliminate the persistent smell.
What causes Seoul’s sewer odour, and why has it been so difficult to address?
Organic matter rotting beneath the streets
The main source of the smell is hydrogen sulfide, a gas known for its rotten egg-like odour. It forms when organic matter in sewage decomposes in oxygen-deprived environments.
Seoul’s sewer system has struggled to keep pace with the city’s rapid urban growth, and many buildings have installed septic tanks as an interim measure before sewage is discharged into underground sewer pipes.
While these tanks remove some solid waste, dissolved organic matter remains in wastewater flowing into underground sewers, where it can decompose and produce hydrogen sulfide.
During heavy rainfall, increased water flow helps flush sewage through the system and reduces the build-up of odour-causing gases.
But during hot, dry periods with little rainfall, hydrogen sulfide can accumulate in sewers and escape through openings such as storm drains and maintenance holes.
“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,” a Seoul city official said.
“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.”
Another city official said Seoul’s combined sewer system, where wastewater from buildings and stormwater from streets flow through the same underground pipes, can also contribute to odour problems.
Since the early 1990s, Seoul has considered converting the system into one in which wastewater and stormwater are carried through separate pipes.
However, progress has been slow due to the difficulty and high cost of replacing underground sewer networks.
As at 2025, only 20.4 per cent of Seoul’s total sewer network had been converted to a separate system.
Why the problem persists
Seoul has made sewer odour reduction a priority in its environmental policies, with a dedicated team overseeing odour mitigation projects.
The current legal framework mainly focuses on managing odours that exceed allowable limits, rather than requiring local governments to proactively prevent sewer odours.
Experts say the city needs more fundamental and long-term solutions, arguing that a major city like Seoul should take a more proactive approach to tackling persistent odour problems.
The number of sewer odour complaints in Seoul fell from 3,095 in 2015 to 1,660 in 2020, partly due to the city’s efforts, including the installation of odour reduction devices at large septic tanks. However, complaints rose again to 2,302 in 2024.
Ki Dong-won, a researcher at the Seoul Institute, said a complaint-driven approach has made it difficult to establish long-term and standardised solutions.
“Korea has yet to develop models that can predict how sewer odours spread above ground or analyse their diffusion paths, making it difficult to identify areas vulnerable to odour complaints,” Ki said.
“Efforts such as creating sewer odour maps are underway, but they are not yet linked to performance indicators or evaluation standards for odour reduction facilities.”
Park Seok-soon, an environmental engineering professor emeritus at Ewha Womans University, said that water should be artificially circulated through sewer pipes to prevent sewage from stagnating and decomposing, given the difficulty of replacing combined sewer systems.
“When water remains stagnant in storage facilities for a long time, water quality deteriorates. I believe introducing water from hydrants or other facilities could help circulate water through sewers,” Park said.
“Many major cities in Japan and other developed countries still use combined sewer systems. Ultimately, it comes down to how strictly they are managed.” - The Korea Herald/ANN
