Hong Kong authorities will replace old pipes lined with bitumen in the long run and are planning to increase cleansing efforts, an official has said, after test results confirmed the presence of the banned substance in the water supply at two public housing estates.

But the government’s late response failed to ease residents’ worries, who said the black particles were melting in the water and becoming sticky, questioning if filters could do the job.

Bitumen, which has been found in the freshwater supply at Queens Hill Estate and Shan Lai Court in Fanling, is a substance produced through the distillation of crude oil and known for its waterproofing and adhesive properties. It has been banned from use in water pipes since 2005.

“It is obvious that the black particles have decreased after the Housing Department added the filters,” director of Water Supplies Roger Wong Yan-lok told a radio programme on Friday.

Asked if the government would replace the ageing pipes, Wong said it was part of the department’s long-term plan.

“We will replace the pipes in a risk-based approach in the long term,” he said, adding that 700km long water pipes in the city had bitumen coating.