In recent years, due to the effects of climate change, torrential downpours from guerrilla rainstorms and linear precipitation zones have occurred frequently across the country. According to meteorological observation statistics, the annual frequency of short-duration heavy rainfall exceeding 50mm per hour has increased to approximately 340 times (10-year average), about 1.5 times higher than in the initial statistical period of 1976-1985 (approx. 226 times) (*1). Under these circumstances, responding to inflow volumes exceeding design assumptions during rainy periods has become a major challenge for wastewater treatment plants nationwide. In particular, combined sewer systems, which collect rainwater and sewage in the same pipes, are used in approximately 190 cities nationwide and have a structural problem where a portion of the mixed rainwater and sewage is discharged untreated into rivers and seas during rainfall (*2). Even in separate sewer systems, increased treatment plant load due to rainwater infiltration (I/I) is pointed out, concentrating the burden of rainy day operations on the field staff responsible for plant operation and management.

Against this backdrop, Seisui Kogyo Co., Ltd. (Head Office: Chiba City, Wakaba Ward; President: Kenichi Imoto; hereinafter, Seisui Kogyo, https://seisui-kk.com), an expert in water and sludge treatment, conducted a survey on the operational status of wastewater treatment plants among 106 individuals (municipal employees) involved in the operation of wastewater treatment plants nationwide, in the context of increasingly severe localized heavy rainfall. The results are as follows:

*1 | Japan Meteorological Agency | 'Changes in Extreme Weather Events such as Heavy Rain and Extremely Hot Days' | https://www.data.jma.go.jp/cpdinfo/extreme/extreme_p.html

*2 | Ministry of Land, Infrastructure, Transport and Tourism | 'Regarding the Future Policy for Combined Sewer Systems (Proposal)' (June 2023, Committee for Comprehensive Evaluation and Future Policy of Urgent Improvement Projects for Combined Sewer Systems) | https://www.mlit.go.jp/mizukokudo/sewerage/content/001617353.pdf

01 | Approximately 90% of municipal employees in charge of sewage-related departments experience malfunctions, defects, or reduced processing capacity in grit removal equipment.

02 | 58.0% of equipment has been in operation for '10 years or more,' with 24.0% being '20 years or more.'

03 | 87.0% face or expect to face the constraint of 'cannot stop operations' during equipment replacement.

Download the full survey here.

Survey Overview

Survey Name: Survey on the Operational Status of Wastewater Treatment Plants Under Intensifying Localized Heavy Rainfall

Survey Method: Internet survey conducted as part of the research marketing service 'Risapy®︎' provided by IDEATECH.

Survey Period: June 15, 2026 - June 17, 2026

Valid Responses: 106 individuals (municipal employees) involved in the operation of wastewater treatment plants nationwide.

*Proportions may not sum to exactly 100% due to rounding of the second decimal place.

≪Usage Conditions≫

1. Please clearly state 'Seisui Kogyo Co., Ltd.' as the source of information.

2. When using on a website, please include the following link as the source.

URL: https://seisui-kk.com/

Approximately 90% of Municipal Employees Feel Increased Load on Wastewater Treatment Plants Due to Frequent Heavy Rains

When asked, "Q1. Do you feel that the frequency of guerrilla rainstorms and linear precipitation zones has increased due to recent climate change, leading to a greater burden on wastewater treatment plants?" (n=106), 54.7% responded 'Strongly agree' and 34.0% responded 'Somewhat agree.'

Strongly agree: 54.7%

Somewhat agree: 34.0%

Somewhat disagree: 7.5%

Strongly disagree: 1.9%

Don't know/Cannot answer: 1.9%

Against the Background of Frequent Heavy Rains in Recent Years, About Half Have Experienced Inflow Exceeding Design Assumptions '5+ Times a Year'

When asked, "Q2. In the wastewater treatment plants you operate or manage, approximately how many times per year in recent years (about the past 3 years) has the inflow volume exceeded design assumptions due to guerrilla rainstorms or torrential downpours?" (n=106), 29.2% responded 'About 5 to 9 times a year' and 22.6% responded 'About 3 to 4 times a year.'

10+ times a year: 16.0%

About 5 to 9 times a year: 29.2%

About 3 to 4 times a year: 22.6%

About 1 to 2 times a year: 13.2%

Less than once a year: 8.5%

Has not occurred in the past 3 years: 5.7%

Don't know/Cannot answer: 4.7%

Concerns Felt as 'Not Sufficiently Shared with Citizens/Superiors,' with 'Increased Odor/Noise' and 'Ambiguity in Overflow Notification Rules' Both at 41.5% as Most Frequent

When asked, "Q3. In the rainy day response at the wastewater treatment plants you operate or manage, what are the concerns that you feel are not sufficiently shared with citizens or administrative superiors? (Multiple answers allowed)" (n=106), 41.5% responded 'Increased odors and noise generated at the treatment plant,' 41.5% responded 'Ambiguity in notification standards and information disclosure rules in case of overflow,' and 35.8% responded 'The reality that untreated or partially treated water is discharged into rivers and seas during rainy periods.'

Increased odors and noise generated at the treatment plant: 41.5%

Ambiguity in notification standards and information disclosure rules in case of overflow: 41.5%

The reality that untreated or partially treated water is discharged into rivers and seas during rainy periods: 35.8%

Water quality impact on downstream recreational areas (river play, fishing, etc.): 27.4%

Normal operational capacity being strained by rainwater infiltration (I/I): 22.6%

Capacity of grit chambers and primary sedimentation tanks not keeping up with modern rainfall volumes: 22.6%

Staff being forced into long working hours during emergency response: 14.2%

Other: 0.0%

None in particular: 7.5%

Don't know/Cannot answer: 1.9%

Rainy Day Operation Rules: Over Half (54.7%) Operate 'Personally,' with Only 22.6% Having Clearly Established Rules

When asked, "Q4. In the wastewater treatment plants you operate or manage, are there clear operational rules (treatment priority, water quality management standards, emergency response procedures, etc.) for rainy day treatment?" (n=106), 22.6% responded 'Clear rules are established,' and 54.7% responded 'Some rules exist, but they are operated personally.'

Clear rules are established: 22.6%

Some rules exist, but they are operated personally: 54.7%

No rules, decisions made on a case-by-case basis: 17.0%

Other: 0.0%

Don't know/Cannot answer: 5.7%

Challenges in Rainy Day Response: 1st Place 'Insufficient Capacity of Grit and Primary Sedimentation Tanks,' 2nd Place 'Delayed Development of Rainwater Retention Basins/Storage Facilities'

When asked, "Q5. Please tell us the challenges you feel in the rainy day response at the wastewater treatment plants you operate or manage. (Up to 3 answers allowed)" (n=106), 41.5% responded 'Insufficient capacity of grit and primary sedimentation tanks,' 36.8% responded 'Delayed development of rainwater retention basins and storage facilities,' and 30.2% responded 'Processing capacity of existing treatment facilities not keeping up with rainy day inflow volumes.'

Insufficient capacity of grit and primary sedimentation tanks: 41.5%

Delayed development of rainwater retention basins and storage facilities: 36.8%

Processing capacity of existing treatment facilities not keeping up with rainy day inflow volumes: 30.2%

Concerns about deteriorating water quality and impact on rivers/seas during rainy periods: 23.6%

Increased normal-time treatment load due to rainwater infiltration (I/I): 21.7%

Shortage of staff with specialized knowledge for rainy day response: 18.9%

Ambiguity in rules for emergency notification and information disclosure: 15.1%

Insufficient budget secured for response: 6.6%

Other: 0.0%

None in particular: 4.7%

Don't know/Cannot answer: 2.8%

Final Decision on Discharge/Treatment Switching During Rainy Periods Relies on 'Site Manager' for About 40%

When asked, "Q6. In the wastewater treatment plants you operate or manage, when inflow exceeding assumptions or water quality deterioration occurs during rainy periods, who primarily makes the final decision regarding discharge permissibility and switching treatment methods?" (n=106), 35.8% responded 'Decision by site manager (facility chief, etc.)' and 27.4% responded 'Mechanically handled according to manuals/standards.'

Decision by site manager (facility chief, etc.): 35.8%

Mechanically handled according to manuals/standards: 27.4%

Decision made in consultation with head office/contracting party: 15.1%

Left to the judgment of the on-duty staff/individual personnel on site: 11.3%

Varies depending on the situation each time: 5.7%

Other: 0.0%

Don't know/Cannot answer: 4.7%

77.3% Say They 'Can Respond' with Current Systems, While 20.7% Perceive They 'Cannot Respond'

When asked, "Q7. In the wastewater treatment plants you operate or manage, can rainy day response be sufficiently handled with the current operational system (existing facilities, staff, budget)?" (n=106), 16.0% responded 'Sufficiently able to respond' and 61.3% responded 'Somewhat able to respond.'

Sufficiently able to respond: 16.0%

Somewhat able to respond: 61.3%

Not very able to respond: 19.8%

Completely unable to respond: 0.9%

Don't know/Cannot answer: 1.9%

Even Among Those Who Can Respond Currently, Over 60% Predict 'Response Will Become Difficult Within 5 Years'

When asked, "Q8. For those who answered 'Sufficiently able to respond' or 'Somewhat able to respond' in Q7. Considering the increasing frequency of heavy rains due to climate change, by what time do you think the current operational system (existing facilities, staff, budget) will become difficult to handle rainy day response at wastewater treatment plants?" (n=82), 41.5% responded 'Will become difficult within 5 years' and 24.4% responded 'Will become difficult within 10 years.'

Will become difficult within 3 years: 19.5%

Will become difficult within 5 years: 41.5%

Will become difficult within 10 years: 24.4%

Will become difficult more than 10 years from now: 3.7%

Will not become difficult: 3.7%

Don't know/Cannot answer: 7.3%

Effective Future Rainy Day Measures: 'New Construction of Rainwater Retention Basins/Storage Pipes' (52.1%) and 'Introduction of Dedicated Rainy Day Treatment Processes' (45.2%) Rank High

When asked, "Q9. For those who answered 'Will become difficult within 3 years,' 'Will become difficult within 5 years,' 'Will become difficult within 10 years,' or 'Will become difficult more than 10 years from now' in Q8. Please tell us what measures you think are effective for future rainy day response. (Multiple answers allowed)" (n=73), 52.1% responded 'New construction of rainwater retention basins/storage pipes' and 45.2% responded 'Introduction of dedicated rainy day treatment processes.'

New construction of rainwater retention basins/storage pipes: 52.1%

Introduction of dedicated rainy day treatment processes: 45.2%

Enhancement of processing capacity of permanent treatment facilities: 34.2%

Renovation of sewer pipes (rainwater infiltration countermeasures, etc.): 34.2%

Temporary increase in processing capacity with temporary treatment facilities: 24.7%

Rainwater runoff control across the entire watershed (green infrastructure, etc.): 24.7%

Inflow prediction and operational optimization using AI/IoT: 16.4%

Mutual utilization of processing capacity through wide-area cooperation: 8.2%

Other: 0.0%

None in particular: 0.0%

Don't know/Cannot answer: 0.0%

Over 80% Wish to Utilize Specialized Temporary Water Treatment Service Providers

When asked, "Q10. Would you like to utilize the support of specialized temporary water treatment service providers who possess equipment and expertise for enhancing rainy day processing capacity and emergency response?" (n=106), 38.7% responded 'Strongly agree' and 43.4% responded 'Somewhat agree.'

Strongly agree: 38.7%

Somewhat agree: 43.4%

Somewhat disagree: 10.4%

Strongly disagree: 3.8%

Don't know/Cannot answer: 3.8%

Summary

This survey was conducted among 106 individuals (municipal employees) involved in the operation of wastewater treatment plants nationwide to investigate the operational status of these plants under increasingly severe localized heavy rainfall. The results revealed that while 88.7% feel an increased load due to frequent heavy rains, 71.7% of rainy day operation rules remain personalized or unprepared. Specifically, the frequency of inflow exceeding design assumptions was '5+ times a year' for 45.2%, and including '3+ times a year,' it reached 67.8%. Regarding rainy day operation rules, 54.7% were operated personally, and 17.0% had no rules and were decided on a case-by-case basis, with only 22.6% having clearly established rules. Furthermore, the final decision on discharge permissibility and treatment method switching was left to the site in 47.1% of cases, combining 'site manager' and 'on-duty/individual staff.' While 77.3% stated they 'can respond' with the current system, 61.0% of those predicted that 'response will become difficult within 5 years.' Finally, regarding effective measures, 'new construction of rainwater retention basins/storage pipes' (52.1%) and 'introduction of dedicated rainy day treatment processes' (45.2%) ranked highest, and 82.1% were positive about utilizing specialized temporary water treatment service providers.

This survey highlights structural challenges in wastewater treatment plant rainy day response across facility capacity, operational systems, and information sharing. Even as inflow exceeding assumptions becomes increasingly common, operations tend to rely on site-level judgment, and the limits of current responses are being recognized in the field. Beyond hardware improvements, the standardization of operational rules and how to integrate external specialized resources will likely be key discussion points going forward.

Download the full survey here.

Seisui Kogyo Can Handle Inflow Exceeding Assumptions and Insufficient Processing Capacity During Rainy Periods with Temporary Water Treatment

With an extensive track record of 2,650 projects nationwide. We solve the problem of increased inflow during rainy periods with temporary water treatment plants using rental water treatment equipment.

Based on our extensive experience of 2,650 projects cultivated at wastewater treatment plants, various factories, civil engineering sites, and disaster recovery sites, Seisui Kogyo proposes temporary water treatment plants tailored to the inflow volume, water quality, treatment period, and installation conditions of each site. By combining water treatment equipment used in permanent facilities, we can respond to various needs such as increased inflow during rainy periods, insufficient processing capacity, alternative treatment during equipment downtime, and temporary water treatment during recovery work.

Are you facing any of these issues with rainy day wastewater treatment plant operations or responding to inflow exceeding assumptions?

Want to handle inflow exceeding assumptions during heavy rains

Existing equipment processing capacity is insufficient during rainy periods

Want to utilize external contractors during inflow increases or equipment troubles

Seisui Kogyo provides treatment according to site conditions for increased inflow and insufficient processing capacity during rainy periods at wastewater treatment plants.

Click here for examples of temporary water treatment.

Company Profile

Company Name: Seisui Kogyo Co., Ltd.

Established: April 1974

President: Kenichi Imoto

Location: Chiba Research Park, 424-18 Kaminizumi-cho, Wakaba-ku, Chiba City, Chiba Prefecture

Business Activities:

Planning for wastewater and sludge treatment

Rental of wastewater and sludge treatment plants

Rental of decanter-type centrifuges

Rental of various water treatment equipment

[NETIS] Civil Engineering Muddy Water Recycling System (Disaster Response)

[NETIS] Contaminated Soil Classification System (Disaster Response)

IHI Corporation Business Partner

URL: https://seisui-kk.com

FACT BOX

  • Source: PR TIMES
  • Category: Survey報告
  • Organizations: IDEATECH