As the residents of Harbin gingerly switched on their water taps yesterday, environmental experts warned that it could take a number of years to get rid of the toxic chemicals that spilled into the city's main river.

Harbin, in the north-east of China, suffered a five-day shut-down of its water supply last week because of a chemicals spill in the Songhua river caused by an explosion two weeks ago at a plant 200km up-river.