Yonai Water Purification Plant Slow Filtration Sedimentation Basin

The Yonai Water Purification Plant Slow Filtration Sedimentation Basin

The Yonai Water Purification Plant Slow Filtration Sedimentation Basin (米内浄水場緩速沈殿池, Yonai Jōsuijō Kansoku Chindenchi) is a reinforced concrete settling basin built in 1934 at the Yonai Water Purification Plant in Kamiyonai, Morioka City, Iwate Prefecture, and listed as a Registered Tangible Cultural Property on 23 August 1999. Registered as No. 03-0013, it consists of two basins, No. 1 and No. 2, with a combined area of 1,458 square meters.

A sedimentation basin holds water still long enough for silt and fine sand to sink. Slow sand filters clog quickly if the water reaching them is muddy, so this stage protects the filter beds downstream. Morioka City also writes the name as the slow filtration system sedimentation basin (緩速系沈殿池).

Why the Yonai Water Purification Plant Slow Filtration Sedimentation Basin was listed

The Agency for Cultural Affairs notes that the Yonai Water Purification Plant Slow Filtration Sedimentation Basin was built at the same time as the receiving well, on a raised terrace on the east side of the grounds. It adjoins the well on the north and takes the metered water from it. The slow sand filter beds spread out to the south, and settled water moves on to them.

Morioka's first waterworks, which began supplying the city in 1934, treated water in three steps: receiving well, sedimentation, filtration. This basin is the second step. It was listed for contributing to the historic landscape of the country as part of the group that still shows that original sequence.

Inside the Yonai Water Purification Plant Slow Filtration Sedimentation Basin

The Yonai Water Purification Plant Slow Filtration Sedimentation Basin measures 54 by 27 meters with an effective depth of 3 meters. A central partition wall divides it into two pools, each a 27-meter square. From the northern edge the two squares sit side by side and the plan is easy to read.

The side walls are reinforced concrete, but their inner faces are lined with brick and the tops are finished with coping stones. The same brick lining appears in the filter beds to the south. Concrete structure, brick facing and stone coping together show how carefully even a working basin was finished in the early Showa period.

Splitting the basin in two probably let staff drain one side and clear the settled sludge while the other kept working, a common arrangement in plants of this type.

Seeing the Yonai Water Purification Plant Slow Filtration Sedimentation Basin

The Yonai Water Purification Plant Slow Filtration Sedimentation Basin lies inside a working plant and the grounds are normally closed. Visitors can walk in during the spring cherry blossom opening or on the waterworks bureau's tour days.

The basin sits on the higher ground on the east side. The timber Waterworks Memorial Hall was placed to overlook both the settling basin and the filters, so standing in front of the hall and looking east is a good way to see how the Yonai Water Purification Plant Slow Filtration Sedimentation Basin relates to the filter beds below it. Staff on the day decide how close you may go and whether photos are allowed.