They have installed nanotechnology equipment in a 300-meter section of the severely polluted river, and it is expected to start reducing the stench within three days and decompose waste and mud at the bottom in two months.

The nano-bioreactor technology, provided by Japan, has also been used in the West Lake, the largest of the city’s lakes.

Experts and workers installed water treatment devices in the bed of the To Lich in the section between the Buoi-Hoang Quoc Viet intersection and Cau Giay District. Five such devices were seen on the first day. The Japanese team said they will add more devices later, but has not revealed how many would be used.

According to one of the experts, the devices create and diffuse nano-gas streams into the surrounding water to stimulate microorganisms, which would release oxygen, decompose the mud in the riverbed and make the water cleaner.

The project's cost, defrayed by the Japanese, has yet to be announced.

Speaking at the project launch, Dr Tadashi Yamamura, chairman of Japan's Trade-Environment Promotion Organization, which provided the equipment, said the technology has already proved successful in many rivers in Japan, the U.S. and China.

"Japan's bio-nanotechnology fully decomposes the layer of mud at the bottom without the need for mechanical dredging, and would radically tackle the source of pollution that's been producing the To Lich River's stench."

Nghiem Vu Khai, vice chairman of the Vietnam Union of Science and Technology Associations, said the nano-bioreactor technology has already been used successfully in a lake in the northern port city of Hai Phong, two hours east of Hanoi.

Its use in cleaning a section of the To Lich River would be the first in a chain of activities to clean rivers and lakes in Hanoi, and potentially in other places as well.

But he stressed that nano-bioreactor technology is not something that would keep the environment clean forever, and there needs to be a general solution for treating garbage and wastewater at source.

The efforts to clean the To Lich River and the West Lake began after a Japanese delegation headed by Yamamura met Prime Minister Nguyen Xuan Phuc last month and offered to provide their bionanotechnology equipment for free.

Phuc hailed the proposal as "a good idea, very practical for Vietnam."

He hoped the new technology would be successful in treating wastewater and polluted water sources in Hanoi as well as elsewhere in the country.

The To Lich used to be a branch of the Red River and connected to the West Lake. It was however disconnected from both after the French filled a section of the river as part of a replanning of Hanoi in 1889.

It runs around 14 kilometers (8.7 miles) through the downtown districts of Thanh Xuan, Hoang Mai and Thanh Tri.

Over 200 sewage outlets empty into the river 150,000 cubic meters of untreated household wastewater every day, according to the city Department of Natural Resources and Environment.

Its downstream is also polluted by industrial wastewater from factories.