Tadashi Yamamura, president of the Japan External Trade Organization, asked for postponing the assessment until September 17 in letters he sent to Prime Minister Nguyen Xuan Phuc, the Ministry of Natural Resources and Environment and the Hanoi People’s Committee.

Since May city authorities have had the Japanese team over to clean a section of the To Lich River using nanotechnology. The results were positive, with the river’s usual stench and sludge at the bottom reducing within just a week.

But last Tuesday a floodgate in Hanoi’s West Lake was opened to maintain the lake’s level and to flush the To Lich.

Yamamura said this "wiped out" all the microorganisms used by the Japanese to clean the river.

"We virtually have to start over, and we need a minimum of over a month to reactivate the microorganisms. Only then can we get an accurate, objective sample [from the To Lich River]."

The Japanese technology, nano-bioreactor, does two things: infuse oxygen into the river to activate beneficial microorganisms, and create an environment for these microorganisms to live in.

The To Lich River used to be a branch of the Red River but was delinked by the French by filling a section as part of a city plan in 1889.

Over 200 sewage outlets empty 150,000 cubic meters of untreated household wastewater every day into it, according to the city Department of Natural Resources and Environment. Wastewater from factories also contributes to the river’s pollution.

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

Hanoi has unsuccessfully tried to clean it many times in the last decade.