Environmental authorities in east China's Anhui Province are battling an intense blue algae outbreak in Chaohu Lake, the country's fifth largest fresh water lake, as they struggle to ensure clean water for local residents.

Blue algae grew by as much as 852 percent from the normal level on July 1st and has been hard to contain, said Zhu Yu, deputy director of environment monitoring center in Anhui.

The algae, which usually blooms in summer amid warm temperature, were found at Bakou and Chuanchang, two sources for drinking water at Chaohu Lake. Dozens of environment workers are collecting the blue algae from the lake.

"We are closely watching the water quality. Microcystin, a toxin which threatens drinking water, has not been detected. We have also stepped up purifying procedure to ensure water supply," Zhu said.

Emergency water supply is ready, Zhu said. About 50,000 tons of water can be supplied from other water sources in ten days.

He Zequn, deputy director of environment protection department, said Chaohu's blue algae blight has eased over the last few years, but this year it has apparently relapsed.

More than 220 million yuan (about 35 million US dollars) has been spent to contain blue algae this year. About 120,000 tons of blue algae have been collected.

"The relapse is a new warning to us. We have to keep up with efforts," he said.