Imbe Water Purification Plant No. 4 Filtration Pond

Sand, Gravel and Brick

Imbe Water Purification Plant No. 4 Filtration Pond (忌部浄水場四号濾過池) is a concrete slow sand filter bed dating from 1918 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 lies east of bed No. 3 and was built to the same size and pattern, one of the four founding beds. Its area is 695 square metres.

The Agency for Cultural Affairs is unusually specific about what happens inside. The floor is laid with layers of fine sand, gravel and brick, and raw water is purified as it passes down through them. The filtered water gathers in a groove cut north to south along the centre of the floor and runs at a gradient of 1 in 100 to the regulating well on the north side.

How Slow Sand Filtration Works

The Imbe Water Purification Plant No. 4 Filtration Pond uses slow sand filtration. Water seeps through a bed of sand, and a living film of microorganisms that forms on the surface does much of the cleaning, along with the straining action of the fine grains. The method developed in Britain in the nineteenth century and was widely used in Japan's early modern waterworks.

Founding records for Imbe give a filtering rate of 10 shaku, about 3.03 metres, a day, roughly three cubic metres of water through each square metre of sand daily. That slowness gives the method its name, and it is why a plant needs several large beds side by side.

Slow sand beds do not cope well with muddy water. After a typhoon in 1934, soil from landslides upstream washed into the system and the plant recorded extremely high turbidity. The third expansion, begun in 1935, added sedimentation basins to settle out silt before it reached the beds. The plant still combines settling with slow sand filtration today. The registration criterion is contribution to the historic landscape of the country.

Picturing What Lies Beneath

From the rim you see only a flat sheet of water, the granite coping stones and the well housing to the north. The layers of sand, gravel and brick, and the groove in the floor, are all out of sight.

Keep that central groove in mind as you look toward the north well, and the bed reads as a single machine drawing water in one direction. On a bed 31 metres long, a gradient of 1 in 100 works out to a fall of about 30 centimetres from end to end.

The collecting well, which gathers the filtered water and sends it on, stands just north of this bed. From the north rim you can see how water passes from the regulating well to the collecting well.

Visiting

The Imbe Water Purification Plant No. 4 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.

On a visit, go round to the north side to see the regulating well and the collecting well together. Follow staff instructions on photography and on how close you may go.