What Is Causing Seoul's Sudden Sewer Smells

Seoul residents and visitors keep encountering a sudden, sharp smell along sidewalks, often gone within seconds but persistent enough to drive repeated complaints. The odor problem is not new, but it is getting worse again. According to a 2025 report from the Seoul Institute, sewer-related smells now make up about half of all civil complaints about bad odors in the city.

The smell is caused mainly by hydrogen sulfide, a gas produced when organic material in sewage decomposes in conditions without oxygen. Seoul's sewer network, built when the city was smaller and now stretched by rapid urban growth, still relies heavily on septic tanks and combined pipes that carry both wastewater and stormwater. That system leaves dissolved organic matter in the flow and allows the gas to accumulate, especially during hot, dry periods.

Complaints fell from 3,095 in 2015 to 1,660 in 2020 after the city installed odor-reduction devices and other measures. But they climbed back to 2,302 in 2024, raising doubts about whether the city's decade-long effort is enough. Officials say they are considering dredging sediment and changing sewer structures, while researchers argue the city needs better prediction tools and more proactive management.

Why Seoul's Combined Sewers Keep the Smell Coming Back

Hydrogen sulfide turns routine sewage into a street-level nuisance

The rotten-egg smell is not the sewage itself but hydrogen sulfide. It forms when organic matter breaks down in oxygen-starved pipes, a process that slows during heavy rain but intensifies during hot, dry spells. When water flow drops and sediment builds up, the gas can escape through the most accessible openings: storm drains and maintenance holes along sidewalks.

Seoul's combined pipe system makes the smell a structural issue

Many buildings use septic tanks before discharging into underground sewers, removing solids but leaving dissolved organic matter in the wastewater. Because Seoul still carries wastewater and stormwater through the same pipes, the system is harder to flush and manage. Although the city has discussed separating the two flows since the early 1990s, only 20.4 percent of the network had been converted as of 2025, and replacing underground pipes remains slow and expensive.

A complaint-driven approach limits the city's options

Seoul Institute researcher Ki Dong-won said the current system reacts to complaints rather than preventing odors. There are no established models for how sewer odors spread above ground, and odor maps are not yet connected to performance indicators for reduction facilities. Seoul's legal framework also mainly requires managing odors that exceed allowable limits, rather than requiring local governments to prevent them proactively. Ewha Womans University professor Park Seok-soon suggested a more manageable fix: artificially circulate water through pipes to stop sewage from stagnating. He noted that many Japanese cities also use combined sewers, but they manage them more strictly.

What Seoul Residents Should Expect From the City's Next Steps

As Seoul works through the problem, residents should expect a case-by-case response rather than a quick, citywide fix.

  • Odors are most likely during hot, dry periods, when hydrogen sulfide builds up and escapes through storm drains and maintenance holes.
  • Only 20.4 percent of Seoul's sewer network had been separated into distinct wastewater and stormwater pipes as of 2025, so structural conversion will not solve the problem soon.
  • City officials are considering dredging sediment and altering sewer structures, but the legal framework is complaint-driven and does not yet require proactive prevention.