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Environment

Tokyo Bay sewage raises Olympian stink at 2020 swimming site

Heavy rains overwhelm outdated pipes and send untreated waste into sea

The swimming leg of the Paratriathlon World Cup at Odaiba Marine Park was canceled due to high levels of E. coli bacteria.    © Kyodo

TOKYO -- Is Tokyo Bay's water safe? The cancellation of a triathlon event in August has raised concerns about pollution at one of the venues for the 2020 Summer Olympics, and thrown the spotlight on problems with the city's sewage treatment system.

High levels of E. coli bacteria detected at Odaiba Marine Park forced the swimming leg of the Aug. 17 Paratriathlon World Cup to be called off. The problem is that Tokyo's sewer system sometimes discharges inadequately treated, or even untreated waste, into Tokyo Bay when the city is hit by torrential rain.

"It smelled like a toilet," one athlete commented at a test event held days before the planned World Cup. 

Following multiple complaints of the piercing stench, the organizers of the event were forced to change the triathlon to a duathlon -- a running portion, followed by cycling, then more running -- because the water was too dirty to swim in. Testing revealed the presence of E. coli at more than twice the limit set by the International Triathlon Union.

Bacteria levels fell below the limit by the end of the day, allowing the triathlon mixed relay race to be held as planned on Aug. 18.

Officials said heavy rain in Tokyo from a typhoon had overwhelmed the capital's sewage treatment capacity, causing dirty water to flow into the venue.

Tokyo has two types of sewage treatment systems: combined and separate sewer systems. Eighty percent of central Tokyo's 23 wards use combined sewer systems, which collect rainwater runoff and household sewage in the same pipe. Separate sewer systems use different pipes for surface runoff and sewage.

While waste water in combined sewer systems are usually fully treated before discharge, on days of heavy rain, the sewage switches to quick, basic treatment with just chlorine. In times of a sudden downpour, the treatment system can be overwhelmed, and results in raw sewage flushing into rivers with no treatment. That river water flows into Tokyo Bay.

Just how much dirty water flows into the rivers? In fiscal 2017, a total of 170 million cubic meters of lightly treated wastewater was discharged from Tokyo's 11 combined sewer system facilities. The amount of untreated water released, on days of torrential rain, is not known.

Of the roughly 1,400 municipalities with sewage systems nationwide, 85% use the safer "separate" sewer systems. The fact that Tokyo has so many "combined" sewers stems from a cholera outbreak in the late Meiji period (1868-1912), when it hurriedly built the easier combined system to improve public hygiene.

On days of torrential rain, Tokyo's waste water treatment system can be overwhelmed, resulting in raw sewage flushing into rivers with no treatment.   © Kyodo

Replacing all combined sewer systems in Tokyo's 23 wards with "separate" sewers would cost north of $95 billion, according to an estimate by the metropolitan government, and would require household sewage systems to be retrofitted.

Instead, the metropolitan government has spent $90 million to $140 million annually to install temporary storage tanks to handle sewage during torrential rains. In areas along the Meguro River where such tanks have been installed, the number of discharges of untreated sewage has fallen 70%, a survey found. But the project will not be completed until fiscal 2023.

The Tokyo Olympic organizing committee considered moving the venue for next summer's open-water events to Yokohama, but the idea was shot down by the ITU. A committee official speculated that the ITU wants to hold races against the backdrop of the Rainbow Bridge, a telegenic suspension bridge that crosses the northern part of Tokyo Bay.

To prevent polluted water from flowing into Odaiba Marine Park, Tokyo 2020 organizers say they will install three layers of polyester screens around the venue before the games. Only one layer of screens was in place for the August qualifier.

In 2018, the three-layer method was tested and yielded satisfactory results. The screens were able to ensure adequate water quality, even after heavy rain. But the metropolitan government is not resting easy. "We don't know what will happen when the wind is strong," said one official. "Waves could spill over the screens."

Hiroaki Furumai, a professor at the University of Tokyo and an expert in urban environmental engineering, said local authorities have been slow to tackle the problem of rainfall runoff overwhelming the city's sewage treatment capacity because they have traditionally placed a higher priority on building sewer systems and preventing flooding.

If the Olympics draw attention to the problem and spur greater efforts to solve it, the mammoth international sporting event could leave a better water treatment system behind for Tokyo.

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