On 20 September 2026, The Japan Times reported that roughly 15% of Japan's small municipalities surveyed are open to replacing aging public sewer systems with combined septic tanks, a shift made easier by a recent law revision that lets local authorities cut back or shut down sewer networks (The Japan Times).
Key takeaways
- About 15% of surveyed Japanese municipalities said they are positive about switching from sewers to combined septic tanks (The Japan Times).
- The survey was conducted after a law revision that makes it easier to reduce or discontinue sewer systems (The Japan Times).
- A January 2025 sewer-pipe collapse in Yashio, Saitama Prefecture, is cited as a trigger for the policy review (The Japan Times).
- Of 1,464 municipalities with populations under 100,000 polled in July, 692 (47.3%) returned valid responses (The Japan Times).
- Minamiizu, Shizuoka Prefecture, has already moved off sewers onto combined septic tanks (The Japan Times).
What happened
The shift is being driven by aging infrastructure and shrinking populations, with municipalities carrying rising maintenance and renewal costs for sewer networks (The Japan Times). The policy review followed a January 2025 incident in which an underground sewer pipe collapsed in Yashio, Saitama Prefecture, opening a large sinkhole (The Japan Times). Parliament subsequently revised the law during its last session to make it easier for municipalities to reduce or discontinue sewer systems in favour of combined septic tanks (The Japan Times).
The survey, run after the revision, polled 1,464 municipalities with populations under 100,000 in July; 692 (47.3%) returned valid answers (The Japan Times). Of those respondents, 105 municipalities — about 15.2% — said they were positive about switching, a list that includes Minamiizu in Shizuoka Prefecture, which has already completed the transition (The Japan Times). A larger group, 406 municipalities, said they do not currently see a need to replace sewers, with some citing budget and personnel shortages as practical barriers to the switch (The Japan Times).
Specification read
The headline figure is share-of-municipalities, not a design flow, so there is no source-reported flow to convert into MLD or m³/day. For a town of under 100,000 residents, a combined septic tank programme typically sits in the small-to-mid municipal class, roughly 1–20 MLD depending on per-capita loading and infiltration assumptions (general industry range, not from the sources). The treatment train this class of decentralised project runs on is screening → grit removal → primary sedimentation → biological stage (commonly a packaged activated-sludge block, SBR or MBR for tight footprints) → tertiary filtration → disinfection, with sludge handling on the side. This event bears on the collection-side and primary/biological stages: rather than expanding a central sewer works, the new build is hundreds of distributed household-scale or village-scale packaged units serving the same population. Operators reading this should map their own envelope — typically flows in the low m³/day to a few hundred m³/day per unit, BOD in the 150–300 mg/L range from domestic sewage, and discharge consents set by the local prefectural authority — and treat the septic shift as a procurement programme for packaged biological reactors and tertiary packages, not a trunk-sewer renewal.
For utility engineers scoping a similar small-municipality programme in Australia, the comparable process choices are laid out in our Rural Sewage Treatment in Australia: 2026 Process & Cost Guide, while buyers looking at UK packaged-plant selection can cross-reference the Domestic Sewage Treatment in Nottingham: 2026 Equipment Buyer's Guide. For readers tracking adjacent industrial loads in Japan, the Data Center Wastewater & Cooling Blowdown Treatment in Nagoya, Japan (2026 Guide) covers the tertiary and blowdown side of the same market, and our Municipal hub collects the broader packaged-plant context.
FAQ
What flow range does a combined-septic-tank programme cover for towns under 100,000 people?
The source only reports a count of municipalities, not design flows. For population equivalents under 100,000, packaged decentralised plants typically fall in the 1–20 MLD class (general industry range, not from the sources). Confirm against the prefectural effluent consent before sizing any biological stage.
Which treatment stages are most affected if a municipality switches from sewers to septic tanks?
Collection, primary treatment and the packaged biological stage are the stages that change, since the trunk sewer and central works are decommissioned. The new build is typically screening → primary → packaged biological (SBR or MBR) → tertiary → disinfection, with on-site sludge handling (The Japan Times).
What is the typical lead time and budget signal for this kind of switch?
The sources flag budget and personnel shortages as a reason municipalities are reluctant to switch, but do not give a capex figure. For packaged domestic plants of this class, capex typically runs in the low single-digit million yen per MLD of installed capacity, with lead times of 6–12 months for a packaged unit (general industry range, not from the sources). The law revision reduces the regulatory hurdle, not the engineering one.
How should a procurement team position a bid for this kind of small-municipality work?
Frame the offer around packaged biological reactors, tertiary filters and a service contract for periodic desludging, since municipalities cited personnel shortages as a barrier (The Japan Times). Reference the Minamiizu case as a precedent in prefecture-level discussions.
