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 inward 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 this basin is the broadest of the three the system offers: contribution to the historic landscape of the country. That phrasing sounds vague until you see how the listing was assembled. On the same day in 2008, fourteen structures across Matsue's founding waterworks entered the register together, including the Senbon reservoir dam and its four-arch management bridge, nine components at the Imbe plant itself, and the three parts of the former Shōgiyama distributing reservoir some 3.5 kilometres north. The Agency for Cultural Affairs treated the water system as a single historic artefact rather than picking out a handful of photogenic pieces.

That system began with a survey. In July 1895 the British sanitary engineer W. K. Burton, employed by the Home Ministry, travelled through the villages south of Matsue looking for a water source and reported that Imbe offered the best option. Burton's student Nakajima Eiji, then a professor at Tokyo Imperial University, guided the design that followed. The Senbon dam went up as the first waterworks dam in the San'in region, and supply to the city began on 1 June 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 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.

Matsue has since carried out eleven rounds of expansion. The plant today draws on the Senbon and Ōtani dams and produces roughly a fifth of the city's daily supply.

What to Look For, and How to Get There

The regulating wells are the reason engineers and photographers make the trip. Each filtration pond has one, and their housings take semi-cylindrical or cylindrical forms with a run of engaged columns across the front, an ornamental treatment nobody would put on a piece of water infrastructure today. Granite coping stones cap the rims of the ponds. The result is a row of small classical pavilions sitting in the middle of a functioning industrial site, which is an odd and rather good thing to stand in front of.

Visiting takes planning. This is not a museum and there is no admission gate. The Matsue City Water and Sewerage Bureau has run guided tours of the Imbe plant for groups, taking around two hours, on weekdays by advance telephone booking with the plant itself. Arrangements are revised from time to time, so call the plant or the bureau before you build a day around it. Photography inside an operating waterworks depends on staff permission, and it is worth asking when you book rather than at the gate.

The plant sits in the Imbe district roughly five kilometres south of central Matsue, where the road out of the city leaves the flat ground along the Ōhashi River and begins to climb. A city bus stop named Suigenchi, meaning water source, is directly in front of the entrance. The Senbon dam lies about 500 metres to the southwest and needs no booking at all: its granite-faced wall, laid in the uncommon diagonal tanizumi pattern rather than the usual horizontal courses, is visible from the surrounding park, which local people know for its cherry blossom. The Japan Society of Civil Engineers named the dam a Civil Engineering Heritage in 2003.

Come on a clear weekday morning if you can arrange a tour, and give yourself time afterwards for the dam. Together they explain something a single structure cannot: that a city's water is a piece of designed landscape, laid out a century ago and still doing its job.