1. How Wastewater Travels Through Sewer Pipes to Treatment Plants
Water that flows from home toilets, kitchens, and bathtubs is called wastewater. It travels through sewer pipes to treatment plants, where it is cleaned before being released into rivers or the sea. In Tokyo, these plants are called "Water Reclamation Centers."[1]
In the 23 special wards, the sewer system serves about 9.49 million people and is divided into 10 treatment districts. As of the end of fiscal year 2024, the total length of connected sewer pipes was about 16,221 km (roughly 10,078 miles). This is about 40% of the Earth's circumference (about 40,000 km). There are 13 Water Reclamation Centers, processing 4.624 million cubic meters of wastewater daily.[1]
Nationwide, as of the end of fiscal year 2025, 101.41 million people (82.2% of the total population) are connected to the sewer system. Septic tanks account for 9.5%, and agricultural settlement drainage facilities account for 2.2%. The combined "wastewater treatment population coverage rate" is 94.0%. About 7.4 million people are in situations where they cannot use wastewater treatment facilities.[2]
2. How Tiny Microbes Clean Wastewater at Treatment Centers
At Water Reclamation Centers, wastewater passes through pool-like ponds in sequence. First, in a grit chamber, large trash is removed and sand settles. Next, in a primary settling tank, the water flows slowly so that easily settleable dirt sinks to the bottom.[3]
The most important step is the aeration tank. Air is pumped into a mixture called activated sludge, which is full of microbes, and it is stirred for about 6 to 8 hours. The microbes break down the dirt in the water. Fine particles stick to the microbes, forming clumps that are easy to settle.[3] While people sometimes say microbes "eat" the dirt, this is just a simple way to explain their ability to break it down.
The water is then settled in a secondary settling tank for 3 to 4 hours, separating the treated water from the sludge. The treated water is disinfected with chlorine to kill bacteria like E. coli before being released into rivers or the sea.[3]
In Japan, most wastewater treatment uses microbes, and many plants use the activated sludge method.[4] Adding the time in the aeration tank and the secondary settling tank gives a total of 9 to 12 hours. This does not include time in the primary settling tank or time flowing through pipes. Treatment order and times vary by plant, so these are rough estimates.
3. How Japan's Sewer System Began After a Cholera Epidemic
The history of Japanese sewers can be traced through a timeline by the Japan Sewage Works Association. The timeline lists a cholera epidemic in Japan in 1879 that caused many deaths, alongside the construction of a modern sewer system in Kanda, Tokyo, in 1884. The old Sewage Act was established in 1900.[5]
In 1922, Japan’s first wastewater treatment plant (the Mikawashima Sewage Disposal Plant) was built in Mikawashima, Tokyo.[5] The building of its pump station is designated as an Important Cultural Property of Japan as a representative remnant of the country's first modern wastewater treatment plant.[6]
The activated sludge method, which uses microbes, was used at a UK sewage treatment plant in 1914. In Japan, wastewater treatment using the activated sludge method began 16 years later, in 1930, in Nagoya.[5] This was 8 years after the Mikawashima plant opened in 1922.
From the 1950s to the 1970s, as industry boomed, pollution occurred, and rivers and seas across the country became dirty. The current Sewage Act was established in 1958, and the Water Pollution Control Act was established in 1970.[5]
4. How Sewers Handle Both Wastewater and Rainwater on Rainy Days
There are two ways to build sewer pipes: the separate system, which uses different pipes for wastewater and rainwater, and the combined system, which uses the same pipes for both.[7] Most of Tokyo’s 23 special wards use the combined system.[3]
According to the Ministry of Land, Infrastructure, Transport and Tourism, large cities like Tokyo are located downstream of rivers. Because preventing urban flooding and improving the urban environment were urgent tasks, the combined system was used. In 1970, the Water Pollution Control Act was established,[5] and the Sewage Act was revised. Protecting water quality in rivers and seas became part of the sewer system's role, so subsequent sewers began to use the separate system.[7] This connects to the pollution era mentioned in the previous section.
Combined sewers collect dirt from roads and ground along with rain on rainy days. Up to a certain amount, the Water Reclamation Center treats this water. However, if the rain is heavy, the treatment plant cannot process all the collected wastewater. Therefore, the particularly dirty wastewater from the beginning of the rain is stored in large ponds called rainy-day storage basins and treated after the rain stops. If the storage basins fill up, the water undergoes simple treatment (settling and disinfection only) before being released into rivers or the sea.[3]
Regarding rainwater, sewers also have the role of draining rain that falls on the city (internal water) into rivers. Urban flooding includes "internal flooding" (when city rain cannot drain into rivers) and "external flooding" (when rivers overflow).[9] The Tokyo Metropolitan Government Bureau of Sewerage has developed pipes and other facilities to handle rain of 50 mm (about 2 inches) per hour, and is progressing toward handling 75 mm (about 3 inches) per hour in areas with significant damage.[8] The goal is to handle 75 mm across all wards.[1]
These figures are for Tokyo’s 23 special wards; development methods differ by municipality.[8] During the Typhoon of 2019 (Reiwa 1 East Japan Typhoon), internal flooding damaged about 30,000 houses nationwide, and 16 wastewater treatment plants across the country were flooded, temporarily stopping their treatment functions.[9]
5. What the Leftover Sludge From Treatment Is Used For
About 80% of the sludge produced during treatment is organic matter (biomass). The percentage used for energy, such as biogas power generation or solid fuel (the sludge energy utilization rate), was about 31% in fiscal year 2024.[10]
If the energy in the organic matter of the sludge is converted to heat, it is about 12 billion kWh, which is about 1.6 times the electricity used by wastewater treatment plants in one year.[10] Only a portion of this energy is actually utilized.
Another resource is phosphorus. Phosphorus needed for chemical fertilizers is completely dependent on imports. Of Japan’s annual phosphorus demand (about 300,000 tons), about 20%, or roughly 50,000 tons, is contained in sewage sludge.[10]
In the May 2015 revision of the Sewage Act, sewer administrators were required to strive to reuse sludge as fuel or fertilizer.[10] This means that what remains of used water can become raw material for electricity or fertilizer. The pipes carrying this water are under the roads. One of the entrances connected to the pipes is the manhole cover on the roadside.
6. How to Explore the Sewer System Near You
You can explore the sewer system near you. Do this with an adult.[12]
Look at manhole covers. Check the patterns and text. In Tokyo’s 23 special wards, the center of the cover has four text caps. Yellow indicates "combined pipe/wastewater pipe," blue indicates "rainwater pipe," and the fourth one shows the year the sewer pipe was installed.[11] What is written on the cover in your town?
Research your town’s sewer system. Check the municipal website to see if the sewer is a combined or separate system, or if the area uses septic tanks. If it is a combined system, there may be information about measures taken on rainy days.
Look for tours of Water Reclamation Centers. The Tokyo Metropolitan Government Bureau of Sewerage offers facility tours.[12]
Safety Note: Do not step on manhole covers or try to open them. Do not stand in places where cars pass. Do not go outside to check conditions during heavy rain.