The opening of the floodgate, near Trich Sai Street, depends on the water level in the lake, a spokesperson for the Hanoi Sewage and Water Drainage Company said.

"As per regulations, the West Lake’s water level must be maintained at 5.7 meters, but the current level is almost six meters."

An ongoing trial to improve the To Lich River’s water quality using technologies from Japan and Germany would continue, he added.

After nearly two months, the water in the river has changed color from black to green in some areas and less foul-smelling than usual. The sludge at the bottom has also been decomposed.

"It’s been a long time since I last saw green water in the To Lich," Nguyen Thi Xuan, a local resident, said.

"If the floodgate can be opened once a week to clean the river, it would be great."

The final results from the foreign intervention are expected to be announced at the end of this month.

This is not the first time water from the West Lake, Hanoi's largest freshwater body, has been let into the To Lich to clean it. The company did it once last year and the results were encouraging.

The To Lich 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 contribute to the river’s pollution.