Nagae Water Purification Plant

Nagae Water Purification Plant

The Nagae Water Purification Plant (長江浄水場, Nagae Jōsuijō) in Nagae, Onomichi City, Hiroshima Prefecture, was completed in 1925; its receiving well, slow sand filter basin and distribution reservoir were registered as Registered Tangible Cultural Properties (structures) in 2004, and its venturi meter house in 2011 (Agency for Cultural Affairs). All four are owned by Onomichi City.

According to the Onomichi City Water and Sewerage Bureau, a town whose centre stood largely on reclaimed land, with salty groundwater and no large river, had long wanted piped water. In 1921 the city council approved a scheme to dam a reservoir at Hisayamada and build a treatment plant in Nagae. Approval from the Home Ministry came in 1922 and work began the following year; the plant was completed on April 9, 1925, and supply began on the 18th, for 37,000 people and up to 4,500 cubic metres a day.

Most of the cost was met by a gift from Yamaguchi Gendo, a businessman born in Onomichi. The bureau's chronology gives the donation as 1,035,000 yen, 74% of the total cost. The engineer Mizuno Hironoshin planned and designed the plant under the guidance of Sano Tojiro (Agency for Cultural Affairs).

Layout and the Path of the Water

The Nagae Water Purification Plant sits on ground made by cutting about 12 metres off the crest of the Makigatoge pass (槇ヶ峠) (Agency for Cultural Affairs). Raw water comes from the Hisayamada Reservoir by gravity alone, using the difference in height, with no pumping power at all (Onomichi City Water and Sewerage Bureau).

The water enters the receiving well, is filtered through sand and its microorganisms in the slow sand filter basin, and then passes to the distribution reservoir. The four filter basins are arranged in a fan around the reservoir, which is a circle sharing their centre (Agency for Cultural Affairs; Onomichi City Water and Sewerage Bureau). The venturi meter house is a small building at the south end of the plant (Agency for Cultural Affairs). Each structure is described on its own page.

The dam of the Hisayamada Reservoir in Hisayamada-cho, the source of the water, was also registered in 2004 (Agency for Cultural Affairs). It lies about 3 kilometres west-northwest of the plant in a straight line.

The Plant Today

Of the three plants that supply Onomichi, the Nagae Water Purification Plant is the one that treats water from the city's own source, the Hisayamada Reservoir; the other two belong to Hiroshima Prefecture (Onomichi City Water and Sewerage Bureau). The bureau notes that the original equipment is still at work and still uses slow sand filtration.

The bureau's chronology also records that rapid filtration equipment for the plant was begun in 1951 as the fourth expansion project. Onomichi's waterworks marked its centenary on April 18, 2025.

Visiting

The Onomichi Waterworks Memorial Hall opened on the plant grounds on May 25, 2025 (Onomichi City Water and Sewerage Bureau). According to the water bureau it is open from 9:00 a.m. to 4:00 p.m., closed on Wednesdays (except public holidays, from April 2026) and from December 29 to January 3, and admission is free. Inside, a cross-section model of a slow sand filter basin and a relief model of the area explain how Onomichi's water system, including the Nagae plant, works.

The plant itself is a working water facility, and no official information on public access to the filter basins and other structures was found. The bureau says it accepts requests to tour its facilities and describes staff guiding a primary school class around the plant; whether individual visitors can join is best checked with the bureau's General Affairs Division.

The address is 3-6-52 Nagae, Onomichi, the same site as the water bureau's offices. The plant stands on the hills north of central Onomichi, about 1.5 kilometres north of JR Onomichi Station in a straight line.