Sewer inspection: looking inside pipes that never stop flowing

The most dangerous form of sewer deterioration is hydrogen-sulfide corrosion. Where sewage stagnates or is churned at drops, hydrogen sulfide is released, turns into sulfuric acid in the air space at the crown of the pipe, and dissolves the concrete from the inside. Pipes also suffer joint displacement and infiltration, cracking and breakage, root intrusion, and sagging (settlement).

Once a pipe is broken, the surrounding soil is drawn into it along with groundwater. A void forms underground, and one day the road above gives way. Road collapses caused by sewer pipes happen in large numbers across Japan every year. Most are small holes, but the larger the pipe, the more soil it can swallow at once.

Japan has roughly 490,000 km of sewer pipes. Much of it was built in a rush from the high-growth era onwards, so the length past the commonly assumed 50-year service life is about to rise sharply. Since the 2015 amendment of the Sewerage Act, sections at high risk of corrosion (such as large drops and inverted-siphon chambers exposed to hydrogen sulphide) must be inspected at least once every five years.

The problem is worst in large trunk sewers. They gather wastewater from whole basins, so the flow cannot be stopped and the water is deep. Sending a person in means gas, current and darkness all at once. The most important pipes are the hardest ones to see — that is the reality of sewers.

After the collapse, the pipe was in no state for people to enter: hydrogen sulfide, a flow that could not be stopped, and collapsed soil. Small drones were therefore sent into the pipe through manholes to film the inside, and were reported to have been used to check conditions within the pipe (e.g. small Japanese-made drones designed for confined spaces). The national government then asked local authorities to carry out priority inspections of large-diameter sewers nationwide, including with drones.

In sewers the drone is the first pair of eyes, sent in instead of a person. It is most useful in large-diameter pipes where the water is too deep for a crawler camera yet conditions are too dangerous for people.

The basic pattern is to enter at one manhole, fly straight down the centre of the pipe, and recover at the next manhole — or fly back. Holding a steady distance from the wall with fixed lighting lets you stitch the images into a developed (unrolled) view of the pipe wall or a 3D model afterwards.

A sewer is one of the harshest places a drone can fly: no GNSS at all, total darkness, tight space, flowing water below and hydrogen sulfide or methane in the air. Radio travels in straight lines, so links tend to drop beyond bends and drops.

• GNSS-denied: position comes from cameras, LiDAR and inertial sensors. But a pipe looks the same for long stretches, so distance travelled drifts easily. Re-anchor at each manhole. GNSS-denied
• Confined space: assume contact with the wall and choose a guarded airframe. Check pipe diameter against airframe size, and how prop wash recirculates. Confined spaces
• Darkness: lighting power and direction decide image quality. Fix exposure and expect spray and mist to scatter light. Darkness
• Water & humidity: downward range sensors are easily fooled by water. Humidity fogs lenses and spray soaks the airframe. Water, spray & humidity
• Oxygen deficiency & toxic gas: the drone may not mind, but people working at the manhole do. Measure oxygen and H₂S and ventilate before work. Oxygen deficiency & toxic gas
• Explosive atmospheres: methane and other gases can accumulate. Ordinary drones are not explosion-proof. Agree a gas threshold above which you do not fly, before you arrive. Explosive atmospheres

Raw footage is just video of a dark pipe. It becomes an inspection only when you record where (how many metres from which manhole), what (corrosion, cracks, infiltration) and how bad. Sewer inspection records defect types and severities in fixed categories and assigns each pipe an urgency rating that sets the order of repairs.

In Japan, sewers are managed by municipalities (and basin-wide trunk systems by prefectures). The 2015 amendment of the Sewerage Act set maintenance and repair standards: sections at high risk of corrosion must be inspected at least once every five years and the results recorded. Methods and ratings follow national guidelines and the Japan Sewage Works Association’s manuals. After the Yashio collapse, the prefecture set up an independent cause-investigation committee, and in March 2025 the national government requested special priority surveys of large pipes nationwide while reviewing how inspections are done.

On the flying side, Japan’s Civil Aeronautics Act does not cover flight in enclosed indoor spaces; the official interpretation treats tunnels and underground passages whose inside is clearly separated from the outside as indoor, so flying only inside a pipe usually needs no aviation permit or approval. Take-offs or test flights on the road outside the manhole are another matter. Work on the road needs road-use permission, and anyone approaching a manhole must follow the Ordinance on Prevention of Anoxia for gas measurement and ventilation. Abroad, coding systems for in-pipe defects are widely used (e.g. NASSCO PACP in the US, EN 13508-2 in Europe). Compare countries in world regulations.

• Under the water: corrosion or damage below the waterline is invisible to a flying drone. The invert needs lower flows or other equipment.
• Voids outside the pipe: a drone sees only the inner surface. Voids outside the pipe must be found from the road with ground-penetrating radar and similar methods.
• Wall thickness and strength: images show surface roughness, not the remaining thickness or strength of the concrete. LiDAR measures shape; real strength needs testing.
• Long reaches and sudden surges: battery and radio limit how far one flight can go. If rain raises the water fast, the aircraft may not be recoverable.
• Rating and accountability: interpreting defects and setting repair priorities belongs to owners and engineers.

What follows is the editors’ outlook. The manholes of major trunk sewers have drone hatches and charging cradles. On dry nights, drones fly fixed sections from manhole to manhole, imaging the pipe wall at the same height and lighting as last time. By morning, only the sections where corrosion has advanced glow red on the map.

In-pipe imagery, surveys of voids beneath the road and flow and gas sensors come together on one map, so that “the ground under this junction is at risk” is known before an accident. People enter pipes only to repair them or when confirmation is truly needed. Sewer inspection has shifted from a job that risks lives to a job of watching over the ground the city stands on.