Industry-Gakuin Technical Association, in response to the collapse accident in Yashio City, Saitama Prefecture Analysis of Various Deterioration Factors of Combined Sewerage System

Japan Association of Industrial and Academic Technology (Location: Chiyoda-ku, Tokyo; President: Tetsuo Suda) reconfirmed the published mechanism of deterioration caused by rainwater inflow and examined concrete measures that can be taken with the cooperation of local residents to extend the life of combined sewer systems, in response to the collapse that occurred on January 28, 2025 on a prefectural road in Yashio City, Saitama Prefecture. This release describes the results of the study. In this release, we will share the results of the study and introduce our proposals for life extension measures and related products and their maintenance management.

Image of the inside of a combined sewerage system.

Although an investigation into the cause of the recent collapse is still underway, the Association has confirmed that the section in question is a combined sewer. Shocated sewers generally have a service life of 50 years or more, but this accident occurred eight years earlier than that, which has come as a great shock.

In Tokyo and other historically developed cities, combined sewers are often used, and it is said that the inflow of rainwater accelerates the deterioration of the pipelines and shortens their service life by approximately 20% compared to separate sewers. It is thought that similar accidents could already occur at any time in these areas.

The following three factors are the main causes of deterioration due to rainwater inflow. 1.

1. internal pressure loading during heavy rainfall (affecting about 40-50%)

(2) Pipe wear due to wet blasting (affecting approximately 30-40%)

Chemical corrosion and blockage (approx. 10-20% impact)

The same is assumed for chemical degradation due to hydrogen sulfide, earthquakes, and external factors such as ground subsidence.

In addition, factors that shorten the response time were analyzed and categorized as shown in the response time comparison table below.

Comparative Table of Years of Response

In areas where combined sewer systems are used, it is thought that the deterioration of piping will increase with the frequency of heavy rainfall. As a result, the risk of people being caught in sudden road cave-ins caused by the frequency of recent heavy rains has increased.

Physical deterioration of piping can be controlled to a certain extent with appropriate maintenance. For example, deterioration due to internal pressure loading during heavy rainfall is generally assumed to be 10% by design. This problem is often considered to be the result of extreme weather conditions, but on the other hand, the reduced infiltration rate of soil due to urbanization plays a major role, and the immediate flow of rainwater into drainage channels is a contributing factor to the increased load.

The Rainwater Harvesting Promotion Law enacted in 2015 also aimed to reduce internal pressure loads through decentralized rainwater harvesting and time-differentiated drainage, but due to a lack of enforcement, inadequate subsidy systems, and further problems such as trash and chemicals in the water collection infrastructure, it has not become widespread enough. In particular, with the current catchment system, solids flow into the sewers along with rainwater, causing wet blast phenomena in the pipes and contributing to further deterioration.

Furthermore, proper maintenance of stormwater drainage channels creates a favorable environment for mosquito breeding in stormwater catch basins, contributing to the increase in mosquito populations in urban areas. Although chemicals are applied to control mosquito breeding, they tend to runoff into the sewers during rainfall, which is one of the factors accelerating the chemical degradation of sewer pipes and water quality deterioration.

Based on the above discussion, the main causes that accelerate the deterioration of combined sewers are the inflow of substances other than rainwater into the on-site rainwater collection and drainage system and the frequent occurrence of heavy rainfall. Therefore, we concluded that to extend the service life of combined sewer systems, both site owners and users should take measures to prevent the inflow of non-rainwater, and that temporary water storage based on the site area can achieve a service life similar to that of a separate sewer system.

■Current water collection method and maintenance management that allows rainwater and solids to flow into the system at the same time

Photographs of the current situation

The above photo shows the current situation. However, why is the irrational water collection method that allows rainwater and solids to flow into the grating at the same time still being used today? Furthermore, why do they continue to dispose of the garbage left on the grating and the incoming solids separately?

The following issues exist in the background

1. technical factors

There has never been a product that effectively traps sediment and gravel while maintaining drainage capacity.

2. recognition issues

Stopping sediment on the grating has led to a misunderstanding that the grating is clogged due to its appearance, leading to complaints from maintenance managers, and correct awareness has not been adequately shared.

3. continuation of customary practices

There was a sense that continuing with the conventional method would not cause waves and would be less likely to meet organizational resistance.

4. problem of vested interests

Changes to the current maintenance and management system may have harmed the interests of some stakeholders, and thus positive improvements were not promoted.

If this were a one-of-a-kind antique in the world, it would be treated with care, even if it is an artifact. However, while Japan has abundant water resources, it also suffers a lot of damage from rain, so few people are aware of the problem of solids entering storm drains and the resulting shortened response time of combined sewer systems. Although this fact was known among experts, the general public was largely unaware of it.

However, through this accident, we have been reminded of how important it is to extend the life of wastewater infrastructure that extends underground. In order to properly maintain and manage combined sewers, it is essential for property owners and users to be aware of the importance of "not letting the factors that scour the inside of the pipes flow. It is hoped that increased awareness of combined sewers will promote a movement to prevent solids from entering storm drains on the property.

The following table of remedial measures and barriers shows the factors and percentages that are shortening the useful life of combined sewers. Please refer to the table for reference.

Improvement Measures and Impairments Table

An important point to be read from the above table is that two of the three main factors are inflows from catch basins. As a countermeasure to this, we can see that 100% improvement is theoretically possible by making the catchment outlets of the on-site drainage channels extremely fine-segmented and by properly treating the sediments on the grating (*unless budget constraints are considered). In other words, this is a challenge caused by our lifestyle and a problem that can be adequately solved by taking appropriate measures.

This improvement is expected to eliminate 50% of the conventional physical deterioration factors in a simple calculation, resulting in a roughly 5 to 10 year extension of the service life of the sewerage system. In addition, mosquito breeding, another issue for onsite drains, will be controlled, eliminating the need for the use of IGR agents (insect growth regulators) that were previously required. Costs for manpower and chemicals will be reduced.

This measure also promotes rainwater utilization because it eliminates the influx of chemicals and the burden of garbage clogging and waste disposal in the catchment infrastructure, which have been issues that have hindered the widespread use of rainwater. As a result, it also serves as a measure to compensate for the increased storage facilities for rainwater and reduced soil infiltration, thereby reducing the internal pressure load during heavy rainfall.

Thus, proper control and treatment of solids during water collection in on-site storm drains can lead to a cascade of related problems, ultimately leading to the development of a sustainable urban environment.

The following picture shows the situation after the introduction of the ultra-fine sorting and collection system and "Sand Stop G" as improvement measures shown in the table, which will improve rational maintenance management.

Photo of the situation after ultrafine sorting and collecting

The Sewerage Bureau has been calling for the prevention of the inflow of solid waste, and the introduction of this product will reduce the burden on the sewerage system in a manner that is consistent with this policy.

■Introduction of Sand Stop G and Separate Water Collection Mat (Sand Stop Type)

We are now proposing the Sand Stop G and a water collection mat for existing grating (sand stop type) as new products that effectively capture sand, gravel, fallen leaves, branches, and debris that accumulate on grating and realize ultra-fine sorting. The introduction of these products will solve issues (1) through (7) in the list of issues below in one fell swoop, significantly reducing costs and personnel burdens associated with sewer maintenance and management. As a result, the system will extend the service life of infrastructure and reduce manpower, contributing to the development of a sustainable urban environment.

List of Issues

Sand Stop G is a product that combines excellent water collection capacity and solids catching capability by integrating two types of mats in the grating manufacturing process. By introducing this product, the design slope and drainage cross-section of storm drainage channels can be secured while improving the efficiency of maintenance and management. In addition, the sorted water collection mat (sand stop type) is a component that can be retrofitted to existing grating to provide the same performance as Sandstop G. With simple on-site processing, it reduces the inflow of sand, gravel, fallen leaves, and debris, thereby reducing the burden on stormwater drainage infrastructure.

Major advantages of introduction

Prevents deterioration of combined sewers: Prevents inflow of solids and extends the service life of facilities.

Labor-saving maintenance and management: Reduces cleaning burden and management costs.

Reduction of environmental impact: Reduction of chemical use and promotion of rainwater utilization by ensuring soil infiltration.

Reduced mosquito breeding: Improved sanitation through proper management of rainwater catch basins

Sand Stop G cross-sectional view

Brochure link: https://sangaku.org/sand%20stopPamphlet20250211n.pdf

Recommended Installation Locations

SandStop G and the sorting water collection mat (sand stopper type) are best suited for the following locations where leaves and sand tend to accumulate and people tend to gather, which are also the locations that place the greatest burden on combined sewer systems.

Parks and cemeteries: preventing the inflow of fallen leaves and sediment and helping to maintain the environment

Schools, public facilities, housing complexes, residential areas: Ensure a safe and sanitary drainage environment

Nursing care facilities, commercial facilities, outdoor parking lots: Improves drainage function during rainy weather and reduces management burden

Commuting and school routes, tourist attractions, shopping areas: Improve drainage in areas with heavy pedestrian traffic

■Demonstration data on water catchment performance

Click on the links below to view the water collection capacity of Sand Stop G and the water collection performance restoration verification using an air blower (for fallen leaves and sand).

［Water collection performance verification]

https://youtu.be/4KCzEgEumV8?si=L64f-l7EKc-ZE661

Water collection performance verification

［Water collection performance restoration verification by air blower (leaves)

https://youtu.be/XiT3cmmLptY?si=EI5gnJxJLCZ9swZb

Fallen leaves

［Water collection performance restoration verification using an air blower (sand version)]

https://www.youtube.com/watch?v=MDiIDpPXzkE

Sand

■ Request for your support

The Association's mission is to raise issues with public agencies and propose solutions to local governments.

To promote this activity more strongly, we need your support.

<Supporting Member Wanted (1,000 yen per unit per year including tax)

If you would like to support our activities for any of the following reasons, please register as a member using the two-dimensional code.

You live in a combined sewerage system area.

Family members, relatives, or acquaintances live in the area.

You have a place of work or are on your way to work or school.

Have an opportunity to use the sewerage system for sightseeing, shopping, etc.

I want to contribute to environmental preservation.

Support link: https://my.formman.com/t/jQdw/

Support Link

In addition, inquiries from those who wish to promote this product commercially are accepted through the following e-mail address.

Mail: sangaku.aiuto@gmail.com

"Why don't we all work together to reduce the physical deterioration factors of combined sewers and extend their lives?"

By preventing the inflow of solids into the sewers, the introduction of this product will reduce the physical deterioration of the facilities and extend the corresponding service life. Furthermore, it is expected to have multifaceted effects such as reducing the burden of maintenance and management, optimizing storm water drainage, and inhibiting mosquito breeding.

We have built the infrastructure that forms the foundation of our society with an eye toward the happiness of our families and our future. However, before passing it on to the next generation, I believe that awareness of the need for proper maintenance and management on a daily basis has not yet fully taken root.

Why didn't we routinely take measures to prolong the life of our buildings, even though we knew they would deteriorate?"

To avoid such questions in the future, now is the time for each and every one of us to change our awareness, extend the life of our sewage infrastructure, and work toward the realization of a sustainable urban environment.

Through the introduction of "Sand Stop G" and "Separate Water Collection Mats," we hope to build a better future for all of us.

Company Profile

Company name: Japan Society of Industrial and Academic Technology

Representative: Tetsuo Suda, President

Address: Resona Kudan Bldg. 5F, 1-5-6 Kudan-Minami, Chiyoda-ku, Tokyo 102-0074, Japan

　　　　 Resona Kudan Bldg. 5F KS Floor

Establishment: September 2014

URL ： https://sangaku.org/

For inquiries regarding this matter, please contact

Secretariat of Protecting Combined Sewerage Systems with Sand Stop G

Mail: sangaku.aiuto@gmail.com