Sue Water Purification Plant No. 2 Slow Sand Filter

Across the Walkway

The Sue Water Purification Plant No. 2 Slow Sand Filter (末浄水場二号緩速ろ過池, Sue Jōsuijō Nigō Kansoku Rokachi) is a reinforced concrete filter bed built in 1931 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 lies directly north of the No. 1 filter, across the service walkway. It covers 1,375 square metres and has its own clear-water well to collect the filtered water.

The Sue Water Purification Plant No. 2 Slow Sand Filter is the easternmost of the north row, facing No. 1 across the walkway. Slow sand filtration passes water gently down through a bed of sand. A living film forms on the surface of the sand and does much of the cleaning, so the method relies little on chemicals but needs a great deal of space. At Sue the answer was six filters, three on each side of the walkway that carries the supply axis west.

The Kanazawa City Public Enterprise Bureau dates the completion of all six slow sand filters to June 1931 and gives each clear-water well an area of 14 square metres. The original filters still carry the slow-filtration line today.

Inside the Filter Bed

The filter was registered for its contribution to the historic landscape. In its entry for the Sue Water Purification Plant No. 2 Slow Sand Filter, the Agency for Cultural Affairs describes the part normally hidden under water: the bed itself.

From the bottom up it is built of brick, then perforated reinforced concrete slabs, then sand. Filtered water drops through the holes in the slabs into the space held up by the brick. Down the centre runs a collecting channel about 0.9 metres wide, falling toward the clear-water well at a gradient of 1 in 118, so that the water gathers by gravity.

To allow for expansion the filter was divided into 25 sections, and the joints were treated in the same way as those of the sedimentation basins, where the Agency records asphalt and lead sheet.

A fall of 1 in 118 is less than 1 centimetre over 1 metre. Holding a slope that shallow and that even across the floor of so large a filter meant controlling levels from the formwork up, a small figure that says a good deal about the care taken in early Showa civil works.

Seeing It

The Sue Water Purification Plant No. 2 Slow Sand Filter can be seen up close only on the plant's pre-booked group tours, which the bureau says cover the sedimentation basins and filters and last about 40 minutes. Tours are for groups such as local governments and community associations, and applications are due two weeks ahead.

The filter field lies well west of the front garden and cannot be approached from the area open without booking. From the walkway, look for the line of the central channel running toward the well.

Nothing is published on photography, so ask the guide. Directions and garden hours are on the Sue Water Purification Plant page.