Sue Water Purification Plant No. 2 Slow Filtration Sedimentation Basin

The Middle Basin

The Sue Water Purification Plant No. 2 Slow Filtration Sedimentation Basin (末浄水場二号緩速沈澱池, Sue Jōsuijō Nigō Kansoku Chindenchi) is a reinforced concrete settling basin built in 1930 at the Sue Water Purification Plant in Suemachi, Kanazawa City, Ishikawa Prefecture, and was registered as a national Registered Tangible Cultural Property on 28 August 2001. It adjoins the No. 1 basin on its north side, the middle of three in a row, and covers 2,848 square metres.

Its job is to hold water still ahead of slow sand filtration so that silt can settle. Raw water from the receiving well is divided evenly among the three basins from the east. This one sits between the other two and matches them in size and build.

The Kanazawa City Public Enterprise Bureau dates the basins to December 1930 and gives their depth as 5.5 to 5.85 metres. The original basin remains in use on the slow-filtration line.

Keeping the Water In

Like its neighbours, it was registered for its contribution to the historic landscape, as part of the band of water the three basins form.

For this basin the Agency for Cultural Affairs writes about watertightness. Reinforced concrete swells and shrinks with temperature, and a large continuous slab tends to crack. The Sue Water Purification Plant No. 2 Slow Filtration Sedimentation Basin was therefore divided into 12 sections, with joints to take up the movement.

Those expansion joints were made with asphalt and lead sheet: the pliable asphalt fills the gap, the lead blocks the path of the water. The whole inner surface was then rendered in mortar to give a smooth, dense skin.

Water does not only try to escape; it also pushes up from below. Clay pipe laid under the basin carries off groundwater, a measure the Agency notes is shared with the other basins.

Reading What Cannot Be Seen

When the basin is full, the twelve sections and the lead in the joints are mostly hidden. Along the rim and the upper walls, though, the layout of the sections can sometimes show as regular lines. On a tour, look for joints at even intervals along the top of the walls.

The Agency's entry for No. 1 describes the section and stair, this one the watertightness, and No. 3 the way water enters. Three identical basins, three different descriptions: read together they give a full picture of an early Showa settling basin.

Seeing It

The Sue Water Purification Plant No. 2 Slow Filtration Sedimentation Basin is shown only on the plant's pre-booked group tours, which include the sedimentation basins and filters and take about 40 minutes.

The front garden, open without booking, lies west of the basins, but published sources do not say how much of them is visible from there.

The bureau says nothing about photography, so check with the guide. Directions and garden hours are on the Sue Water Purification Plant page.