A 1929 Filter Still at Work
At the center of the Kotakano purification plant, in the eastern part of Toyohashi, lie five long open basins of reinforced concrete. Together they cover about 5,236 square meters. They are slow sand filter basins, and what sets them apart from most cultural properties is simple: they are still running. Nearly a century after they were built, river water still passes through them on its way to the city's taps.
The basins date from 1929 (Shōwa 4), when the first four were completed as part of Toyohashi's new municipal waterworks. A fifth basin was added in 1931. The whole arrangement measures roughly 130.9 meters long and 40 meters wide, with an effective storage capacity of about 5,320 cubic meters. The five tanks run north to south, each fed by an inlet on its west side and drained through a regulating well on the east.
How Slow Sand Filtration Works
Slow sand filtration is a gentle, biological method. Water moves downward through a thick bed of fine sand at a slow pace, and a living film of microorganisms that forms on the sand surface does much of the cleaning. The technique came out of nineteenth-century Europe and was once common, but it needs space and patience, and most modern plants have replaced it with faster chemical processes. A working set of slow sand filters from the 1920s is now a rare survival.
Toyohashi could rely on the method because its source water was already clean. The supply was drawn from the subsurface flow of the Toyokawa River, naturally filtered through gravel before it ever reached the plant. The system was planned under Nishiōeda Satoru, an engineer trained in Tokyo's waterworks, and it began serving the whole city after the opening ceremony of March 1930.
Details Worth Noticing
For a piece of working infrastructure, the basins were finished with real care. Granite was laid along the rims of the tanks, and the inlets were shaped as horseshoe arches rather than plain openings. These are the small marks of an era when civil engineers treated even a filter bed as something to build properly.
The basins were registered as a Tangible Cultural Property on 2 November 2018, recognized for their contribution to the historic landscape of the country. Because the plant is an active utility, the filters are not casually open to visitors, but they remain the quiet heart of a system that has supplied Toyohashi without a break since 1930.
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Q&A
- Are the filter basins really still in use?
- Yes. They form part of the active Kotakano purification plant and continue to treat water for the city, which makes them an unusual example of a working cultural property.
- How many basins are there, and when were they built?
- There are five. The first four were completed in 1929 (Shōwa 4) and the fifth was added in 1931. The full set spans about 130.9 by 40 meters.
- What is slow sand filtration?
- It is a method in which water passes slowly through fine sand, where a natural film of microorganisms removes impurities. It is effective but needs space, and few large plants still use it today.
- Can the basins be visited?
- The Kotakano plant is an operating facility, so the basins are not generally open to the public. Anyone interested in the site should contact the Toyohashi City Waterworks and Sewerage Bureau about events or tours.
- Why is the source water suited to this method?
- The water is drawn from the subsurface flow of the Toyokawa River, which has already been filtered through gravel and is clean to begin with. That made gentle slow sand filtration a practical choice.
Basic Information
| Name | Kotakano Purification Plant Slow Sand Filter Basins, Toyohashi Municipal Waterworks (Odakano Jōsuijō Kansoku Rokachi) |
|---|---|
| Location | 2-9-3 Higashi-Odakano, Toyohashi, Aichi |
| Designation | Registered Tangible Cultural Property (civil structure) |
| Registered | 2 November 2018 (registration no. 23-0527) |
| Built | 1929 (Shōwa 4); fifth basin added 1931 (Shōwa 6) |
| Count and structure | 1 unit; reinforced concrete, area 5,236 sq m, five open basins |
| Owner | Toyohashi City Waterworks and Sewerage Bureau |
| Access | Active treatment facility; not generally open to the public |
References
- Agency for Cultural Affairs, National Cultural Properties Database: Kotakano Slow Sand Filter Basins
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00012556
- Cultural Heritage Online: Kotakano Purification Plant Slow Sand Filter Basins
- https://bunka.nii.ac.jp/heritages/detail/404131
- Toyohashi City Waterworks and Sewerage Bureau: History of the Water Supply
- https://www.city.toyohashi.lg.jp/3984.htm
Last verified: 2026.06.25