That will be in 2029 when the Sha Tin sewage treatment works are relocated to the caverns, the Drainage Services Department said yesterday.
And in doing so the department introduced a "caverns detector" robot it developed for use during construction.
The existing facility, in Ma Liu Shui near the Shing Mun River, spans 28 hectares and has a treatment capacity of 340,000 cubic meters per day.
Yet following the relocation to caverns at Nui Po Shan, the treatment capacity will stay the same despite occupying only 14 hectares of space.
The relocation, which will cost between HK$40 billion and HK$50 billion, means the existing site can be used for less odious purposes.
And advanced sewage and sludge treatment technologies will be introduced in the caverns, director of drainage services Alice Pang Nga-nei said yesterday.
Pang said construction of the main treatment unit will commence in the caverns within months.
At the other end of the relocation program, demolition work of the existing facility should be finished in 2031.
She said the first stage of the caverns project started in February 2019, including building a primary tunnel that links them. And construction has gone smoothly.
"The tunnel spans a length of 4.8 kilometers, with a height of 20 meters and a width of 25 meters," Pang said. "This is equivalent to three traffic lanes.
"So far we have completed the blasting and excavation of the 350-meter upper deck of the tunnel."
Pang also said the smooth operation of the first stage had to do with the work of the robot developed by her department.
"To reduce risks the department spent HK$200,000 developing the robot named Caverns Detector, which replaces workers in inspecting the site after completing a blasting procedure," Pang said.
"The robot then carries out 3D scanning of the environment and sends screenshots to the staff using 5G technology. Geotechnical engineers then judge air quality and structural safety."
Pang noted that the caverns holding the new treatment works should reduce odors as they are enclosed. The Sha Tin sewage treatment works has been in operation since 1982, she noted, and "as the facility is open the odors travel in the wind and affects residents."
Planners will consider what use the Ma Liu Shui site can be put to when the entire relocation project nears completion.
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