A Buried Holding Tank for Clean Water
The Sanjo City Waterworks Bureau Osaki Water Purification Plant Regulating Reservoir (三条市水道局大崎浄水場調整池) is a 1933 reinforced concrete tank at the Osaki waterworks in Nakashin, Sanjo City, Niigata Prefecture, that holds filtered water for a time before it moves on, and it was listed as a Registered Tangible Cultural Property on October 28, 2011. It lies south of the transmission pump house.
It measures 9.1 meters east to west and 14.5 meters north to south, 3.6 meters deep, with an area of 132 square meters. It is sealed with a concrete roof and a covering of earth, and six ventilation shafts rise through the mound. The Niigata branch of the Japan Society of Civil Engineers notes that the mound is planted as well.
For something called a reservoir, it shows no water at all. Above ground you see only a low grassy mound with vents poking out of it.
Balancing Supply Before Distribution
The Sanjo City Waterworks Bureau Osaki Water Purification Plant Regulating Reservoir stores treated water until it is needed and evens out how much is sent on. The Agency for Cultural Affairs places it, with the distribution reservoir, in the final stage of the plant's process.
The flow coming out of the filters never matches the flow the town wants at every hour. This tank absorbs the difference. The transmission pump house next door to the north draws from it and pushes water up to the distribution reservoir on the hill, so the two structures work as a pair in both process and layout.
The roof and earth cover presumably protect the clean water from debris, sunlight and swings in temperature. Buried like this, the tank looks less like a facility than a small knoll.
What to Look For
The six vents are the clue. Their regular spacing on the mound traces the outline of the tank hidden underneath. The distribution reservoir up the hill uses the same system on a larger scale, with 36 vents.
The city's reference photograph of the Sanjo City Waterworks Bureau Osaki Water Purification Plant Regulating Reservoir is taken from the south, with the vent-studded mound in front and the transmission pump house behind. That view shows best how the two relate.
Visiting
The tank lies inside the working plant, which is normally closed. During the spring opening, look for the mound with its vents just south of the transmission pump house. It holds drinking water, so do not climb the mound or touch the vents.
Directions and opening conditions are collected on the Sanjo City Waterworks Bureau Osaki Water Purification Plant page.
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Q&A
- What is the regulating reservoir at the Osaki Water Purification Plant for?
- It holds filtered water for a time and evens out the amount sent on, forming the final stage of the process with the distribution reservoir.
- When was the Osaki plant regulating reservoir built?
- It was built in 1933 and listed as a Registered Tangible Cultural Property on October 28, 2011.
- How big is the regulating reservoir of the Osaki Water Purification Plant?
- It measures 9.1 meters by 14.5 meters and is 3.6 meters deep, with an area of 132 square meters.
- Can you see the water in the Osaki regulating reservoir?
- No. It is sealed with a concrete roof and an earth mound; only six ventilation shafts show above ground.
- Can you visit the regulating reservoir at the Osaki plant?
- The grounds are normally closed, but it can be seen from outside, south of the transmission pump house, when the city opens the grounds in spring.
Basic Information
| Name | Sanjo City Waterworks Bureau Osaki Water Purification Plant Regulating Reservoir |
|---|---|
| Location | 4420-3 Nakashin, Sanjo City, Niigata Prefecture |
| Designation | Registered Tangible Cultural Property (Building) |
| Year Designated | Registered 2011/10/28 (Heisei 23) |
| Quantity | 1 structure |
| Dimensions | Reinforced concrete, 9.1 meters east to west by 14.5 meters north to south, 3.6 meters deep, area 132 square meters |
| Owner | Sanjo City Waterworks |
| Public Access | Normally closed as a working waterworks; the grounds may open to walkers in cherry blossom season |
References
- Cultural Heritage Database: Sanjo City Waterworks Bureau Osaki Water Purification Plant Regulating Reservoir (Agency for Cultural Affairs, in Japanese)
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00008841
- Registered Tangible Cultural Property: Sanjo City Waterworks Bureau Osaki Water Purification Plant (Sanjo City, in Japanese)
- https://www.city.sanjo.niigata.jp/soshiki/shimimbu/shogaigakushuka/bunkazai/bunkazai/1740.html
- Sanjo City Waterworks Bureau Osaki Water Purification Plant (Japan Society of Civil Engineers, Niigata branch, in Japanese)
- https://jsce-niigata.com/introduction/article/049/art049.html
Last verified: 2026.09.25
