Five-second answer. A sewer system is not just a pipe that carries dirty water away. Nagoya has about 7,900 km of sewer lines, and the system also requires treatment plants, microbial treatment, sludge handling, cleaning, camera inspection, stormwater management and continuous renewal of old pipes.[1][2][3]
There is a second surprise. After seeing that amount of buried infrastructure, it is tempting to think Japan’s debt must be mostly concrete hidden underground. The Ministry of Finance says the ordinary government bond balance increased by about ¥975 trillion from the end of FY1990 to the end of FY2026. But within the roughly ¥785 trillion of spending-side increase factors shown by the ministry, social-security spending accounts for about ¥524 trillion, while public works account for about ¥72 trillion.[4]
So this article follows an unreasonable route:
wastewater → microbes → final settling → sludge cake → giant pipes → maintenance → underground cavities → sinkholes → the Horikawa → government bonds → intergenerational fairness.
We entered a sewer museum and somehow exited through public finance.
The 30-second map
- Wastewater is separated into cleaner water and sludge using settling and microbes.
- Some activated sludge from the final settling tank is returned to the reaction tank; only the excess goes to sludge treatment.
- Sludge is thickened, dewatered and treated, and some outputs are recovered as materials or fuel.
- Nagoya has about 7,900 km of sewer pipes, around 20% of which are more than 50 years old.
- Pipe damage can allow soil to escape into the pipe, creating cavities that may lead to road collapse.
- Modern Horikawa odors are not century-old feces; sediment, oxygen, flow and wet-weather discharges matter now.
- In the debt data, social security is a much larger spending-side factor than public works.
1. Is a sewer just a drainpipe? — No, it is a second urban system under the first one
Nagoya has about 7,900 km of sewer pipes. The city compares that distance with traveling from Nagoya to Seattle or Stockholm.[1]
That does not mean 7,900 km of human-sized tunnels. Some pipes are small local lines; others are large trunk sewers that collect many flows.
Still, a full-scale large pipe changes the mental picture.
This is less plumbing and more a second map under the city.
That also explains why movies love manholes. A character can disappear from the street into a network that really does exist below it.
Reality is less cooperative. Sewers can contain oxygen-deficient air, hydrogen sulfide, sudden flows, drops and other hazards. Hollywood protagonists routinely enter a confined-space worksite without a safety briefing.
Great for screenwriters. Terrible for safety officers.
2. Did Nagoya start sewer service in 1912? — Yes, but early sewers mostly collected wastewater and sent it to rivers
Nagoya began sewer service on 15 November 1912. In the early period there were no treatment plants, so collected wastewater was discharged into the Horikawa and Shin-Horikawa.[5]
From a modern perspective:
“Isn’t that just express delivery for dirty water?”
For the time, however, quickly removing wastewater and stormwater from dense urban areas was a major sanitary improvement. The new problem was that the receiving rivers inherited the pollution.
There is also an important correction to the poop joke. Human night soil was not simply poured into the sewer from the first day in 1912. It was handled through other systems, including collection and agricultural use. Nagoya began introducing it to the sewer in 1926 after passage through facilities equipped with septic treatment.[6]
So the idea of 100-year-aged poop quietly fermenting in the Horikawa is funny, but historically wrong.
The century-aged vintage has been cancelled.
3. Why did Nagoya need treatment plants by 1930? — Pollution and odor had moved the problem downstream
Collecting wastewater removes it from a street. It does not remove the pollution from the water.
As the Horikawa and Shin-Horikawa became polluted and foul-smelling, Nagoya tested the activated-sludge process and in October 1930 opened treatment at Horidome and Atsuta. The city describes this as Japan’s first practical sewage treatment using activated sludge.[5]
The idea is simple before the technical name appears: let microbes take up organic pollution, then settle the microbes out of the water. That process is called activated sludge treatment.
Nagoya built experimental facilities because there was no domestic operating precedent, then pushed the method into real service.
For 1930, the sewer department was shipping fairly aggressive new technology.
4. How does wastewater become cleaner? — Settle it, let microbes eat the pollution, then settle it again
A simplified treatment train looks like this.[2]
- Grit chamber: remove sand and large debris.
- Primary settling tank: slowly settle finer solids.
- Reaction tank: add air and activated sludge full of microorganisms so they take up organic pollution.
- Final settling tank: settle the microbial flocs and separate them from clarified water.
- Disinfection: disinfect the clearer water before returning it to a river.
The aggressively simplified museum version is:
“Microbes eat your poop.”
Directionally, yes. More precisely, they consume dissolved and fine organic matter in wastewater rather than holding a cartoon buffet around intact feces.
The final settling tank is easy to underestimate. The sludge that settles there is not all thrown away. Some is returned to the reaction tank so the microbial population can work again; only excess sludge is removed for sludge treatment.[2]
Corporate translation:
productive microbes go back to the team; surplus headcount gets transferred.
Even microbes have internal mobility.
5. What is dewatered sludge cake? — Sludge with much of the water squeezed out, not dessert
Sludge contains a lot of water. Nagoya thickens it, adds conditioning chemicals and mechanically removes water before further treatment such as incineration.[7]
The solid mass after dewatering is commonly called dewatered sludge cake. The word cake is an engineering term for the solid layer left after filtration or dewatering.
In other words:
only the name has wandered in from the bakery industry.
As a resource, however, the material has a respectable career. Nagoya historically sold dried sludge as fertilizer. Today, sludge incineration ash is used for cement and other materials, while solidified fuel can substitute for coal. City data for the end of FY2021 reported 6,534 tonnes of sludge ash with a 99.8% beneficial-use rate.[8]
This does not mean human waste goes straight back onto a dinner plate. It means treated sewage-derived materials are routed into other resource cycles.
Terrible food branding, surprisingly good job placement.
6. Is Nagoya really hollow underneath? — Not one giant cavern, but the 7,900 km artificial network is real
Hearing “7,900 km of sewer” makes the city sound like an ant colony.
Pipe diameters vary, so Nagoya is not floating above 7,900 km of human-sized empty tunnels.
But the amount of buried infrastructure is still enormous.
About 20% of Nagoya’s 7,900 km network is more than 50 years old, and the city renews older sections based on condition.[1]
Across Japan, sewer pipelines totaled about 500,000 km at the end of FY2024. Roughly 40,000 km had exceeded the standard 50-year service life. The transport ministry projects about 110,000 km in ten years and 230,000 km — roughly 45% of the network — in twenty years.[9]
The nasty feature of underground infrastructure is visibility.
You can see rust on a bridge and cracks in pavement.
A sewer can be deteriorating while the road above still looks perfectly normal.
7. What does sewer cleaning actually do? — High-pressure washing, vacuuming and camera inspection keep the city’s HP bar alive
Sediment and rubbish naturally accumulate in sewer pipes. If left there, they can obstruct flow, create odor and contribute to overflow.[3]
Nagoya uses high-pressure cleaning trucks and sludge-vacuum vehicles, and it inspects pipes with self-propelled TV cameras. Serious damage can trigger emergency repair or renewal.[3]
The workflow sounds easy:
blast it loose → suck it out → send in a camera → repair what is broken.
The target network is 7,900 km.
Suddenly, not easy.
When sewer cleaning or maintenance is a job done by someone in your wider circle, a roadside service truck stops looking like “some machine sucking gross stuff” and starts looking like a device keeping the city’s HP bar from reaching zero.
Cleaning, structural inspection and renewal are not necessarily one person’s job. The accurate unit here is sewer-network maintenance as a whole.
8. Why can a road collapse over a sewer? — Soil can escape through damage and a cavity can grow underground
There are several possible sewer-related collapse scenarios. One representative sequence described by Japan’s transport ministry starts with corrosion or damage, allows soil to flow into the sewer through an opening, forms a small cavity, and lets that cavity grow until the pavement loses support.[10]
From street level:
Yesterday: normal road.
Below: a little soil disappears.
Today: still a normal road.
Below: the cavity grows.
Later: the surface can no longer support the load.
So a sinkhole may look sudden only because the underground part happened first.
On 21 April 2026, the transport ministry published results from a national special priority inspection covering 5,332 km. As of the end of February, 748 km required measures and 96 underground cavities had been identified. The ministry said all 96 cavities had been addressed by the time of publication.[11]
That is not evidence that every road is about to collapse. It is evidence from a targeted inspection program that found defects and cavities and then acted on them.
9. Why do movies use manholes for escapes? — They are a ridiculously convenient underground alternate route
After seeing a giant sewer, the movie trope becomes easier to understand.
For a writer, the benefits are absurd:
- break line of sight,
- bypass surface roadblocks,
- get a dark maze for free,
- emerge somewhere else,
- and somehow bring the villain into the same tunnel.
Urban infrastructure is doing unpaid scriptwriting.
Real sewers are hazardous workplaces, not public escape corridors. The fun part is not that movies invented an underground city from nothing; it is that they take a real urban network and make it outrageously convenient.
10. Why can the Horikawa still smell? — Not century-old poop, but present-day sediment, oxygen, flow and wet-weather discharge
Once you learn that early sewers discharged to the Horikawa, it is hard not to suspect the current smell is historical sewage refusing to leave.
It is not.
During postwar urban growth, the river’s water quality became extremely poor. In 1966, its BOD reached 54.8 mg/L. BOD means biochemical oxygen demand; in plain language, it is a common indicator of how much oxygen-consuming organic pollution is present. Hydrogen sulfide and other odors rose from bottom sediment, and the river was even called “dead.”[12]
Sewer development, sediment removal, surface cleaning and added water flow later improved conditions greatly.[12]
Yet flow, bottom sediment and low-oxygen conditions can still matter.
Older parts of Nagoya also use combined sewers, meaning sewage and rainwater share the same pipe. During heavy rain, flows beyond treatment capacity can result in stormwater containing some sewage being discharged directly to rivers. Nagoya says it met the national wet-weather effluent standard by the end of FY2023, while additional measures continue for the Horikawa and Shin-Horikawa.[13]
So:
old poop: mostly history.
sediment, oxygen, flow and wet-weather hydraulics: still current.
The final boss was not a preserved 1912 specimen. It was the modern city continuously producing and moving water.
11. Why does a virtual sewer-exploration game work? — Because it turns forbidden infrastructure into a dungeon
The Metawater Sewerage Science Museum Nagoya was extensively renewed in 2020. Its five zones cover daily life and sewers, treatment, sewer maintenance, disaster preparedness, and Nagoya’s sewer history and future. A July 2026 city notice specifically promotes a game that lets visitors virtually explore inside a sewer.[14]
During the visit behind this article, people were clustering around interactive exhibits. That is a field observation, not an official popularity ranking.
Still, the pitch is objectively funny:
“Come explore the sewer!”
You people know this is a sewer, right?
But remove the word dirty for a second and the design makes perfect sense: normally inaccessible, underground, huge, dark, moving, slightly scary, secret-base energy.
A sewer minus the disgust factor is basically a dungeon.
Public outreach cooked with difficult ingredients and somehow won.
12. Did sewer construction cause Japan’s debt? — In the spending-side Pareto, social security is about two-thirds; public works about 9%
This is where the field trip reaches public finance.
After 7,900 km of pipes, treatment plants, pumps, cleaning, cameras, renewal and sludge handling, the joke writes itself:
“Did Japan bury its national debt under the streets?”
The numbers say no.
The Ministry of Finance says the ordinary government bond balance increased by about ¥975 trillion between the end of FY1990 and the end of FY2026.[4]
Ordinary government bonds are a defined category that includes instruments such as construction bonds, special deficit-financing bonds and reconstruction bonds, whose interest and redemption are mainly financed by tax revenues and related resources. The number is not a universal total of every form of government liability.[15]
For the roughly ¥785 trillion of spending-side increase factors shown directly by the ministry, the Pareto-style ranking is:[4]
| Rank | Spending-side factor | Contribution | Share of ¥785T | Cumulative |
|---|---|---|---|---|
| 1 | Social-security spending | ~¥524T | 66.8% | 66.8% |
| 2 | Other spending excluding debt redemption | ~¥95T* | 12.1% | 78.9% |
| 3 | Local allocation tax grants, etc. | ¥94T | 12.0% | 90.8% |
| 4 | Public works | ~¥72T | 9.2% | 100.0% |
*The ~¥95T figure is a rounded residual: ¥785T minus ¥524T, ¥94T and ¥72T.
This corrects an important accounting mistake. The table is not a 100% decomposition of the full ¥975T increase. The ministry separately identifies revenue-side tax shortfalls caused by past economic weakness and tax cuts as major factors.[4]
And public works is a broad category containing much more than sewerage.
The 7,900 km underground suspect just acquired a pretty strong alibi.
13. Is Japan’s FY2026 budget “healthy”? — It is not planless, but the accumulated debt is still heavy
Japan’s FY2026 general-account budget totals about ¥122.3T. Ministry of Finance figures show ¥39.1T for social security, ¥20.9T for local transfers and ¥31.3T for debt service. Tax revenue is projected at ¥83.7T, new bond issuance at ¥29.6T, and the bond-dependency ratio at 24.2%.[16]
The general-account primary balance is projected at about ¥1.3T in surplus.[16]
The primary balance asks, roughly, whether current policy spending can be covered by current revenues before the interest and principal costs of past debt are counted.
So a ¥1.3T primary surplus does not mean the entire national budget is in the black. The same ministry projection puts the FY2026 general-account fiscal balance at minus ¥11.7T, debt service at ¥31.3T and interest payments at ¥13.0T.[16]
Government: “We are attempting to balance this.”
Citizen: “More disciplined than expected. Good luck — not my problem.”
Tax system: “You are part of the funding model.”
Citizen: “Annoying.”
The accurate conclusion is not “everything is healthy.” It is: the government is not borrowing without a framework, but the existing debt stock still creates a very large financing burden.
14. “Make the generation that created the bill settle it” — That is an intergenerational-equity question, but age alone is too crude
A natural working-age reaction is:
“If past generations received the benefit, do not send the entire bill to the people behind them.”
That is an intergenerational-equity question.
But a policy that simply charges everyone more because they are old is too crude. People of the same age have very different income, financial assets, property, health and ability to pay. Younger people also continue to use roads, sewers and other assets built decades earlier.
A serious design therefore asks:
- who receives which benefits and public services,
- who has what income and wealth,
- how taxes, social-insurance contributions and benefits are distributed,
- and which costs are being shifted to future taxpayers through debt.
The core question is not merely how old are you? It is how should benefits and burdens be shared within and across generations without turning future taxpayers into the default payer?
The emotional starting point can be “make that generation settle it.” The policy endpoint has to be more precise.
15. Conclusion — Follow the path of poop long enough and you can see the city’s maintenance bill and the nation’s invoice
A sewer museum starts with dirty water and refuses to stay there.
Wastewater.
Microbes.
Final settling tanks.
A catastrophically named sludge cake.
Huge pipes.
Cleaning and cameras.
Pipe damage, underground cavities and sinkholes.
The Horikawa’s pollution history.
Then the cost of maintaining all of it.
Then government debt.
Then the question of who pays.
You begin by following where excrement goes and end by following where public liabilities go.
The least glamorous conclusion may be the most important:
building a city is hard; keeping what you built working for a century is harder.
Sewers are easy to forget because they are underground. Stop cleaning, inspecting and renewing them, and failures can grow where nobody can see them.
The comparison with public finance is only a metaphor, but one thing is genuinely similar: invisible problems do not become harmless because they are invisible.
This sewer museum has an absurdly long range for a social-studies field trip.
Sources
- 名古屋市上下水道局 — 道路での下水道工事についてご協力をお願いします／管路約7,900km・約2割が50年超 water.city.nagoya.jp
- 名古屋市上下水道局 — 水の旅 ～下水処理のしくみ～／沈砂池・最初沈でん池・反応タンク・最終沈でん池・返送汚泥 water.city.nagoya.jp
- 名古屋市上下水道局 — これからも快適に下水道をご利用いただくために／高圧洗浄車・吸泥車・自走式TVカメラ water.city.nagoya.jp
- 財務省 — 日本の財政関係資料 令和8年4月「普通国債残高の増加要因」／1990年度末→2026年度末約975兆円、歳出増加要因約785兆円、社会保障約524兆円、地方交付税94兆円、公共事業約72兆円 mof.go.jp
- 名古屋市上下水道局 — 下水道の歴史／1912年供用開始・1930年堀留／熱田で日本初の活性汚泥法 water.city.nagoya.jp
- 名古屋市上下水道局 — し尿処理の解決策を求めて～下水道創設期の悪戦苦闘～／1926年から浄化槽を経て下水道へ投入 water.city.nagoya.jp
- 名古屋市上下水道局 — 水の旅 ～汚泥処理のしくみ～／濃縮・薬品混和・脱水・焼却 water.city.nagoya.jp
- 名古屋市上下水道局 — 下水道資源・資産の有効利用／汚泥焼却灰6,534t・有効利用率99.8%等 water.city.nagoya.jp
- 国土交通省 — 下水道の維持管理／2024年度末約50万km、50年超約4万km、10年後約11万km、20年後約23万km mlit.go.jp
- 国土交通省 — 道路陥没のシナリオ／土砂流出・空洞成長・路面支持力喪失 mlit.go.jp
- 国土交通省 — 下水道管路の全国特別重点調査の結果（2026-04-21）／対象5,332km、対策必要748km、空洞96か所 mlit.go.jp
- 名古屋市 — 堀川の浄化について／1966年BOD 54.8mg/L、硫化水素等の悪臭、浄化対策 city.nagoya.jp
- 名古屋市上下水道局 — 合流式下水道の現状とさらなる水質浄化／2023年度末に国基準達成、堀川・新堀川の追加対策 water.city.nagoya.jp
- 名古屋市上下水道局 — 名水レター7月号／メタウォーター下水道科学館なごや、下水道内バーチャル探検ゲーム water.city.nagoya.jp
- 財務省 — 国債とは／普通国債の種類と償還財源 mof.go.jp
- 財務省 — 令和8年度予算の後年度歳出・歳入への影響試算／一般会計122.3兆円、税収83.7兆円、新規国債29.6兆円、PB約+1.3兆円、財政収支▲11.7兆円、国債費31.3兆円、利払費13.0兆円 mof.go.jp
