Imbe Water Purification Plant No. 1 Filtration Pond

A Working Filter Bed from 1929

Imbe Water Purification Plant No. 1 Filtration Pond (忌部浄水場一号濾過池, Inbe Jōsuijō Ichigō Rokachi) is a concrete slow sand filter bed at Higashi-Imbe-chō in Matsue City, Shimane Prefecture, completed in 1929 and entered on Japan's national register of Tangible Cultural Properties on 18 April 2008. It is one of nine structures at the plant carrying that status, and unlike most registered buildings in Japan it has never been retired. Water still passes through it on its way to kitchen taps in Matsue.

The dimensions are on the record and worth carrying in your head before you arrive. The basin measures 22 metres east to west and 31 metres north to south, a plain rectangle covering 683 square metres. Its walls are given a slight batter, listed in the register as one bu. At the centre of the north wall sits a regulating well with a small roofed housing over it, the one part of the structure that rises above ground level and catches the eye.

Everything else lies flat. A filtration pond is, at heart, a shallow concrete box holding beds of fine sand, gravel and brick through which raw water sinks by gravity alone. There are no pumps in the process and no chemicals in the filtering itself. Bacteria colonise the top layer of sand and do much of the cleaning work. The method is called slow sand filtration, and Matsue has used it here since the plant opened.

Why the Register Includes It

The registration criterion applied to the Imbe Water Purification Plant No. 1 Filtration Pond is contribution to the historic landscape of the country. At the Imbe plant, nine structures along the water's route, from the receiving well to the collecting well, were registered on the same day in 2008. Within that set, No. 1 is the bed that shows how the 1929 expansion followed the founding design of 1918.

A Number That Does Not Mean What You Expect

Here is the detail that repays a second look at the plant map. No. 1 is not the oldest pond on site. Nos. 3 and 4 date from 1918 and Nos. 5 and 6 from 1919, all part of the original construction. No. 1 and No. 2 were built during the first expansion project, begun in 1927 and finished in 1929, when Matsue's population had outgrown what the founding facilities could supply. The Agency's own description notes that the new basin was built at roughly the same scale as the existing ponds, and the figures bear that out: 683 square metres against 692 to 695 square metres for the older four. The engineers of 1927 chose to match their predecessors rather than improve on them, which is why the whole row still reads as one composition.

What to Look For

The feature to seek out at the Imbe Water Purification Plant No. 1 Filtration Pond is the housing over the regulating well at the centre of the north side. Each of the plant's six beds has one, and no two are designed alike. No. 1 and its neighbour No. 2 are the two 1929 housings, so comparing them with those on the Taishō-era beds shows how much variety was given to huts with an identical job.

Granite coping stones run along the rim. The walls lean at a batter of 1 bu, 0.1 horizontally for every 1 of height, so from the edge they open slightly outward. Bed No. 5, from 1919, is recorded at 1 bu 7 rin, which means No. 1's walls stand a little more upright.

No. 1 is the westernmost bed in the row. The conduit from the former valve chamber and receiving well runs east past it, making it the bed closest to where raw water enters the plant.

Visiting

The Imbe Water Purification Plant No. 1 Filtration Pond lies inside an operating treatment plant and is not open to the public. Visits depend on whether the Matsue City Water and Sewerage Bureau is accepting them, so check before planning a trip.

If you get inside, stand at the western end of the row and look east along beds No. 2 to No. 6. You can see for yourself that the 1929 and 1918 beds line up at the same width. Follow staff instructions on photography and on how close you may go.