China's second biggest freshwater lake is slowly recovering from pollution despite experts saying it will take "dozens of years" to clean, the Guangzhou Daily reported Wednesday.

Blue-green algae, which normally appears on Taihu Lake in May or June, is now expected to form in August due to anti-pollution efforts, the report claims.

The lake, a major source of drinking water that straddles Zhejiang and Jiangsu provinces, has been seriously polluted by sewage, as well as industrial and agricultural waste, which has encouraged blue-green algae since the 1980s. The report said the blue-green algae choked the lake, affecting drinking water supplies for millions of local residents.

"I used to swim in the Taihu Lake when I was little and the water is very clear," said Wu Lihong, a local resident, "where some water turns milky or even dark after the industrial waste poured in," he added.

Taihu Lake is deemed as the heart of seven nearby cities, including Shanghai. It was heavily polluted when the regional economy started to take off in the 1980s.