Seoul's decade-long effort to suppress street-level sewer odors is faltering, with annual complaint counts rebounding to 2,302 in 2024 after falling to 1,660 in 2020, according to a 2025 report by the Seoul Institute cited on 11 August 2026 by regional outlet Asia News Network (Asia News Network). The figure is now higher than the 3,095 logged in 2015, the year the city launched its odor-reduction programme.
Key takeaways
- Seoul sewer-odor complaints dropped from 3,095 in 2015 to 1,660 in 2020, then rose again to 2,302 in 2024 (Asia News Network).
- Only 20.4% of Seoul's sewer network had been converted from combined to separate stormwater/wastewater systems as of 2025 (Asia News Network).
- Sewer-related smells account for roughly half of the city's civil odor complaints, per a 2025 Seoul Institute report (Asia News Network).
- Seoul has considered separating its combined sewer system since the early 1990s, but replacement has been slow and expensive (Asia News Network).
What happened
Theodor problem comes from hydrogen sulfide produced when organic matter in sewage breaks down in oxygen-deprived pipes, the outlet reported, with sharp smells reaching pedestrians through storm drains and maintenance holes during hot, dry spells (Asia News Network). Many Seoul buildings still rely on septic tanks as an interim step before discharge, and dissolved organics in that flow continue to feed the reaction (Asia News Network).
Underlying the rebound is a structural constraint. Seoul's combined sewer carries building wastewater and street stormwater through the same underground pipe, and the city has been trying to split the two networks since the early 1990s (Asia News Network). As of 2025, only 20.4% of the network had been converted, and progress has been held back by the cost and difficulty of replacing buried infrastructure (Asia News Network).
City officials are now weighing operational fixes rather than full separation. A Seoul city official said the administration is "considering measures such as changing sewer structures and dredging accumulated sediment," and pointed to high declines in wastewater flow and sediment buildup inside pipes as drivers of odor (Asia News Network). Park Seok-soon, an environmental engineering professor emeritus at Ewha Womans University, argued for artificially circulating water through the network: "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 (Asia News Network).
The wider region is also looking at urban waterway restoration. A separate report on 8 August 2026 described how the Tamil Nadu government cited Seoul's Cheonggyecheon stream restoration, completed in October 2025 at a cost of around $281 million over 5.8 km, as one of two possible models for reviving Chennai's Cooum river (The Federal).
Specification read
For an operator, the reported 20.4% conversion figure is the key procurement signal: roughly four-fifths of Seoul's sewer catchment still runs as a combined system, so any new build or upgrade here is a combined-sewer odor-control project, not a greenfield separate-system job. The complaint trajectory, down from 3,095 to 1,660 then back up to 2,302 between 2015 and 2024, frames a buyer question in the same units: how many m³/day of dry-weather combined flow are sitting in the network waiting to turn septic. No daily flow, BOD or COD value is given in the source, so a sizing class for this category of work falls in a general industry range of around 50–500 m³/day per interceptor or dosing station for odor treatment at trunk-sewer choke points (general industry range, not from the sources).
The treatment train this class of project requires runs screening → grit removal → primary clarification → biological treatment (typically an anoxic-aerobic configuration such as A2O or an MBBR, or an SBR for smaller dry-weather flows) → tertiary clarification or Integrated Sewage Treatment skids → disinfection, with chemical or biological scrubbing for H2S at the sewer headspace and dosing points for nitrate or iron salt addition to suppress sulfide generation in the collection system itself. The event reported here bears on the headspace and collection-stage end of that train, not the activated-sludge end, which is why Air Dust Treatment scrubbing equipment and in-pipe dosing units are the relevant procurement categories, served by the Municipal market. If your plant handles combined-sewer flow in the order of 50–500 m³/day during dry weather, sees periodic H2S spikes above 5 ppm at the head of works, and is being asked by the regulator to act on complaints rather than just monitor them, the Seoul trajectory is the cautionary template: complaint counts will not fall on a one-time device install, and the procurement decision should be framed as a multi-year odor-management programme, not a single capital purchase.
FAQ
What is the main cause of sewer odor in Seoul?
Hydrogen sulfide produced when organic matter in sewage decomposes in oxygen-deprived combined-sewer pipes, released through storm drains and maintenance holes during hot, dry weather, according to Asia News Network reporting on 11 August 2026 (Asia News Network).
How much of Seoul's sewer network has been converted to a separate system?
As of 2025, 20.4% of Seoul's sewer network had been converted from combined to separate stormwater and wastewater systems, with the rest still on combined operation (Asia News Network).
What does a buyer actually need to specify for this class of project?
A combined-sewer odor-control scope typically calls for sulfide dosing or injection at trunk-sewer choke points, plus scrubber or biofilter units on the headspace end, sized to peak dry-weather H2S load; specific H2S peak figures were not given in the source, so any quoted peak load should be requested from the supplier as site-measured data (general industry range, not from the sources).
How long does a complaint-reduction programme like Seoul's take to show results?
Seoul's experience suggests at least a five-year window between intervention and sustained complaint decline: complaints fell from 3,095 in 2015 to 1,660 in 2020 after odor-reduction devices were installed in large septic tanks, before rebounding to 2,302 in 2024 (Asia News Network).
