Hong Kong authorities have launched an investigation after black particles, suspected to be bitumen, were reported in the drinking water of a private residential estate in Quarry Bay, the fifth such case this year.

The Water Supplies Department said on Sunday that it had followed up on complaints about polluted drinking water at Taikoo Shing residential development in Quarry Bay, and collected water samples for laboratory testing.

“Results are pending. Based on past experience, the black particles are believed to be bitumen protective coating in the water pipes,” a department spokesman said. Bitumen is a black, sticky substance obtained from crude oil.

The estate’s management office had received reports of black particles appearing in tap water at several residential blocks, mainly from Phases 4 and 5, on Saturday night, and alerted the department.

The department said it had carried out a temporary rerouting of the water supply near Taikoo Shing on the same night to facilitate pipe maintenance works, with changes in water pressure possibly stirring up sediments in the pipes.

Investigations also revealed that damage to one of the filter facilities at the estate’s public water inlet, which may have allowed a small quantity of sediment to enter the estate’s water pipes, caused particles to appear in some households’ drinking water.

The department said it routinely inspected the inlet filters, with the most recent regular check on November 3 showing no abnormalities. The damaged filter was repaired on Saturday night, and the department had strengthened pipe flushing in the affected system.