Imbe Water Purification Plant No. 5 Filtration Pond

A Filter Bed Recorded to the Centimetre

Imbe Water Purification Plant No. 5 Filtration Pond (忌部浄水場五号濾過池) is a concrete slow sand filter bed dating from 1919 at the Imbe water treatment plant in Higashi-Imbe-cho, Matsue City, Shimane Prefecture, and registered as a Registered Tangible Cultural Property of Japan on 18 April 2008. It sits east of bed No. 4 and matches Nos. 3 and 4 in size and form, one of the four founding beds, with an area of 695 square metres.

The Agency for Cultural Affairs records its shape in unusual detail. The surfaces are finished in mortar. The bed is 2.7 metres deep. The walls rise at a batter of 1 bu 7 rin, and the floor falls toward a central groove at 1 in 150. Beneath a flat sheet of water lies a carefully calculated cross-section.

1919: The Plant Is Finished

The date of the Imbe Water Purification Plant No. 5 Filtration Pond, 1919, is the year the Imbe plant was completed. According to the Matsue City Water and Sewerage Bureau's chronology, the Senbon dam was finished in March 1918, water reached the city on 1 June 1918, and the plant was completed in March 1919.

Of the four founding beds, Nos. 3 and 4 are dated 1918 and Nos. 5 and 6 are dated 1919. Work carried on after supply began, and the sequence of dates shows the row being completed from west to east.

The registration criterion is contribution to the historic landscape of the country. Of the six beds, No. 5 is the only one whose depth appears in the Agency's description, which makes its cross-section the easiest to picture.

Reading the Slopes

Try sketching the section of the Imbe Water Purification Plant No. 5 Filtration Pond from its numbers. A batter of 1 bu 7 rin means the wall leans 0.17 horizontally for every 1 of height. Over a depth of 2.7 metres, the top of the wall sits about 46 centimetres out from its foot. That is why the walls, seen from the rim, open slightly outward rather than dropping straight down.

Bed No. 1, added in 1929, is recorded with a batter of 1 bu, so No. 5's walls lean more. The founding beds and the later additions differ slightly in the angle of their walls.

The floor gradient of 1 in 150 works out, over the roughly 11 metres from the side of a 22-metre bed to its centre, to a fall of about 7 centimetres. You would never see it, but it is enough to steer filtered water to the groove.

Visiting

The Imbe Water Purification Plant No. 5 Filtration Pond is 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 in advance.

If you visit, look down from the rim at the angle of the walls; how much of them shows depends on the water level. Follow staff instructions on photography and on how close you may go.