Strange odor and vibration of the sewage system…The hell of “7 years of recovery” from the Yashio sinking in Saitama! The nightmare that strikes the residents and the shudder of the “remaining 4km danger zone

Seven Years Until Restoration…Hopelessness Strikes Residents

It was January 28, 2011, when a sewer pipe broke and a road caved in in Yashio City, Saitama Prefecture. More than a year has passed since then. A 40-meter-diameter hole still remains at the site of the cave-in, and nearby residents are still suffering from the foul odor and vibration caused by the construction work. The prefectural government plans to close the hole in April and reopen the road to traffic with two temporary lanes, but it will take five to seven years for the construction to be completed. Why is it taking so long?

When the sinking accident occurred, we re-inspected the sewer pipes and found that there were areas of corrosion as far as 4 km before and after where the sinking occurred. That’s why we had to build another route, but we haven’t decided where to make the new route.

says Hiroaki Morita, a professor at Nihon University’s College of Industrial Engineering and chairman of the committee established by the prefectural government to study restoration methods.

According to Professor Morita, once the route is determined, one year is spent on geological surveys and one year on design, followed by the actual digging and burying of the pipes, but only 5 m is excavated per day, and 4 km requires 800 days, or a little over two years. The total time required for the project is a little more than four years, but why seven years?

If construction starts and proceeds smoothly, it will take a little over four years, but before construction can begin, we have to go through urban planning procedures and obtain the approval of residents. The question is how long these things will take.

Sewer pipes will be built under prefectural or city roads. Although they do not pass under residents’ homes, accidents such as this one make people understandably anxious about having sewer pipes run near their homes.

Gas” that melts thick concrete

The reason the road caved in the first place was due to corrosion of the concrete caused by hydrogen sulfide, according to an interim report by a prefectural third-party committee. The sewer pipe that caved in was approximately 5 m in diameter, and the concrete was 50 cm thick. If even such thick concrete can corrode, there must be many dangerous sewer pipes all over Japan.

Hydrogen sulfide rarely forms in sewer pipes. The Ministry of Land, Infrastructure, Transport and Tourism reports that only about 3,500 km of the 500,000 km of sewer pipes are corroded.

According to Professor Morita, there are two conditions for hydrogen sulfide to be generated.

The original component of hydrogen sulfide is sulfur. When it becomes free of oxygen, it changes into hydrogen sulfide. Hydrogen sulfide is then generated when the sewage containing hydrogen sulfide is stirred.

There are microorganisms in sewage. When those microbes eat the organic matter in the sewage, they consume oxygen. If there is no slope in the sewer pipe and the sewage flows slowly, the microorganisms consume oxygen at a faster rate than the rate at which oxygen is dissolved from the air in the sewer pipe, resulting in an anoxic condition.

Furthermore, if there is a step in the sewer pipe that allows the sewage to flow down from the top, this will stir up the sewage and produce hydrogen sulfide. It was this combination of unlucky conditions that led to the cave-in accident.

The new pipe will be wrapped with plastic on the inside, so there should be no more worry about hydrogen sulfide corrosion.”

Fear of 4km of “danger zone” still remains

A new sewer pipe has been installed in the area damaged by the cave-in, the road will cover it, and the stench that had plagued the residents will be gone. The stench that had been bothering the residents was gone. But it was not to be.

The sewer line is now covered to prevent the water from flowing into the damaged area, and a detour route has been created to allow the sewage to flow through the damaged area. But we have to remove the cover and connect the new pipe to the old one, but the amount of water is so great that we are not sure if we can make a good connection.

That is not the only problem.

If it takes time to obtain the residents’ approval, the four-kilometer-long sewage pipe, which has been affected by corrosion, could be in a dangerous state.

Reinforcing the inside is nearly impossible because of the hydrogen sulfide that has been produced.” The outside could be hardened with ground improvement materials to prevent it from caving in, but this would be enormously expensive. However, we believe that measures need to be taken for what concerns human lives.”

Even though the stench has disappeared, there is still a period of time when one cannot let one’s guard down.

Hiroaki Morita is a professor of civil engineering at Nihon University’s College of Industrial Engineering. He studied eutrophication in Kasumigaura at the Water Quality Laboratory of the Public Works Research Institute of the Ministry of Construction. Subsequently worked on themes such as hydrogen sulfide corrosion, effective use of biomass, and disposals, as well as the realization of a recycling-oriented society and international deployment of civil engineering technology as research themes.’ He has been in his current position since 2003. His research interests include sewerage, propulsion methods, and organic waste treatment. He also serves as president of the Japan Association of Non-excavation Engineering.