The director of the Water Supplies Department said on Friday that black particles, since identified in laboratory tests as bitumen, found in tap water at two Fanling housing estates probably came from a steel pipe installed around 1987.

The government said a day ago that it believes that black particles found in tap water in Queen's Hill Estate are mostly residual bits of bitumen and resin.

Speaking on an RTHK radio programme, Roger Wong said the upstream pipe was located 2.6 kilometres from Queen's Hill Estate.

"That steel pipe is about 400 metres long," he said.

"Old pipes like this one were coated with bitumen, and the coating might have peeled off.

"That's why we believe the bitumen came from flaking of the coating in this old steel pipe."

The practice of using bitumen as a coating for pipes to prevent rust has been banned for years.

As for the bits of resin, Wong said, they came from protective coating that peeled off from valves.

Officials flushed pipes in Queen's Hill Estate and Shan Lai Court in 2022, and Wong said the flushing has to be carried out on a regular basis and based on certain procedures.

His department will review the maintenance and flushing records with estate management companies and the Housing Department as part of the next steps forward to determine the origin of the particles and eliminate them at the source.

Wong also said it is unlikely that impurities will be able to enter the water supply system now given that more filters capable of stopping particles measuring 0.1 millimetres in diameter from getting through have been installed at the two estates.

But he suggested residents, particularly those who have not used their taps for quite some time, leave their taps running until no black particles are seen.

Meanwhile, Chan Hon-fai, chairman of the Drinking Water Safety Advisory Committee, told RTHK's Hong Kong Today that he expects the situation to improve over the next few days.

"As the area's population grows, the water flowing into the pipes increases, and this will increase the velocity," he said.

"My view is that this increasing flow must have dislodged the particles.

"I think they have to do a more thorough cleansing of the pipes because now that they have strainers at the junction between outside government pumping pipelines and the internal pumping system, there will be no more particles coming from the internal pumping system.

"These particles can be removed by total flushing."

Chan said while the water may be safe to drink, he does not recommend it, and residents should filter out particles in the water first before they do or use it for other purposes like flushing.