Hong Kong authorities have vowed to work through the night and into the morning to clean all water tanks at a private residential estate in Quarry Bay that were found to contain bitumen, the city’s fifth such case this year.

The Water Supplies Department said on Sunday night that its laboratory tests had confirmed that the black particles found in the polluted drinking water of 14 blocks of the Taikoo Shing residential development in Quarry Bay were bitumen.

Bitumen is a black, sticky substance obtained from crude oil.

“In order to minimise the inconvenience brought to the residents, the department, alongside the estate’s management office, will start cleansing the underground water tanks of the affected blocks from 12.30am and expect to have it completed in the early morning,” it said.

The department added that it would also clean the rooftop water tanks during the off-peak usage time on Monday morning.

It said it helped to wash the water meters of 100 flats by 9pm on Sunday, adding that the work was ongoing.