Changqiao Water Plant in Shanghai's Xuhui District has been releasing colored waste water into a river near Chunshen Harbor, linking the Dingpu and Huangpu rivers. [Photo/Xinmin.cn]

"The blue and green is from chlorine disinfection used in tap water and does not pollute the river," the official said.

He said the yellow comes from sand while the dark color is due to activated carbon used in purification. These do not harm water quality, said the official.

Meanwhile, more than half of Shanghai's rivers and lakes are heavily polluted, their beds seriously contaminated, the city's first water census has found.

Some 53 percent of local surface water fails even to reach Grade V — the worst of five water quality levels, the Shanghai Water Authority said yesterday.

Only 3 percent of rivers and lakes achieved better than Grade III, a level that indicates the water can be used for residents and aquaculture.

Some 23 percent were Grade IV, meaning the water can be used for industry and scenic purposes — such as fountains.

And 20 percent make Grade V, water that can only be used for irrigation.