Construction on a 21 kilometer sewer system to collect wastewater from To Lich River commenced Monday, to include a 11 kilometer subterranean section linking Cau Giay and Thanh Tri Districts.

The project, based on a plan Hanoi put forward for opinions in February, is expected to complete in four years.

"In order to save on site clearance and reduce the impact on traffic, pipe jacking will be employed. This marks the first time this technique is applied for construction projects in Hanoi," management board director Nguyen Van Hung said.

Pipe jacking involves installing pipelines underneath roads, buildings and more, with minimal disruption to the surface and reduced downtime on its surrounding environment. Apart from manholes at both ends of the pipeline where the jacking machine is lowered and later removed, most of the action takes place beneath the surface.

The sewer will run 6-19 meters under the river bed.

The capital has chosen a Japanese company, whose name is yet to be revealed, to build the interceptor, a component that helps control water flow.

The entire 21 kilometer sewer system is part of a project to treat wastewater at Yen Xa plant in Thanh Tri District.

Costing VND16 trillion ($687.2 million), the plant, under construction since 2016, will have a facility to treat 270,000 cubic meters of wastewater a day and a system to collect water from not only To Lich but also the Lu River and Ha Dong new urban area, via a sewer system running 52 kilometers.

Hanoi chairman Nguyen Duc Chung said building an underground sewer to rehabilitate To Lich would save time and costs, and avoid affecting traffic.

"Once the Yen Xa plant is completed and the sewer system is connected all wastewater discharged into To Lich would receive 100 percent treatment to revive the river," Chung said.

He has assigned the municipal construction department to upgrade a 16 kilometer section along both banks of the river by growing trees and creating spaces for pedestrian streets while the transport department was tasked with building three walking bridges.

To Lich used to be a branch of the Red River, but was delinked by the French in 1889 as part of an urbanization plan.

Over 200 sewage outlets empty 150,000 cubic meters of untreated household wastewater into it per day, according to the Department of Natural Resources and Environment. Wastewater from factories adds to the pollution.

The river runs 14 kilometers (8.7 miles) through the downtown districts of Thanh Xuan, Hoang Mai and Thanh Tri and has become infamous for its stench and black water.

Japan-Vietnam Environmental Improvement Company tried last year to clean part of the river by installing nanotechnology equipment along a 300 meter section.

The equipment was provided by Japan's Trade-Environment Promotion Organization. Nano-bioreactor technology works by infusing oxygen into the river to activate beneficial microorganisms and create an environment for them to thrive.

The Japanese team ended its pilot project in November without indications of success, later confirmed by Hanoi authorities.