The water quality of China's third-largest freshwater lake has been greatly improved since a massive algae bloom broke out in 2007, but pollution management remains a major problem, according to environmental protection authorities.

Zhang Limin, deputy director of the General Office of Taihu Lake Water Pollution Prevention and Control in Jiangsu province, said that compared with 2013, the number of blue-green algae blooms and the algae density in the lake both decreased last year.

According to the general office, all nine water sources and the waterworks it investigated last year provided water with quality that met or exceeded that required by the national standard. A total of 700 million tons of water was supplied to Jiangsu and neighboring provinces during the year.

In 2007, a massive outbreak of blue-green algae, which mostly resulted from the excessive runoff from agricultural fertilizers and household cleaning chemicals, threatened tap water supplies to 2 million residents in lakeside Wuxi.

Pollution at Taihu Lake has attracted a lot of attention in China since then because of the lake's location, its size and the 40 million people who live within its watershed. The lake covers 2,428 square kilometers and is located in the densely populated Yangtze River Delta.

The general office also investigated 223 businesses around the lake last year, among which 59 were closed for their failure to treat wastewater properly.

Since 2007, the Jiangsu government has allocated 2 billion yuan ($318 million) every year to manage water pollution in the lake. Wuxi also partners with other lakeside cities, including Suzhou and Changzhou, to carry out pollution control methods.

Yixing, a county-level city under the jurisdiction of Wuxi, has invested more than 5 billion yuan to complete a sewage collection and disposal system and a garbage treatment system that serves the whole city. It has also closed about 600 chemical factories and businesses and more than 400 glazed tile enterprises.

Zhu Tiejun, director of the provincial General Office of Taihu Lake Water Pollution Prevention and Control, said that due to the large number of businesses and the large population around the lake, the industrial wastewater and sewage discharged into the lake is still a major problem.

"It's common that the water treated by the factories cannot meet the national standard when it's discharged into the lake," Zhu said.

The many livestock and poultry farms around the lake have also become a significant source of pollution.

According to Zhang, the sewage from the livestock farms equals that of 6.4 million pigs. One pig produces seven times more sewage than a human being, so the sewage discharged into the lake equals that generated by more than 40 million people.

However, the local governments cannot close the farms because they say they cannot afford to pay the farmers for doing so.

"To close the farms, local governments must compensate the farmers," said Wang Yunxian, director of the Changzhou branch of the general office. "It's their source of income and how they support their families. Local governments cannot afford compensation to the farmers."

According to the Yixing government, it paid 100 million yuan last year to close the farms around just one river that flows into the lake.

Zhu added that some local governments are reluctant to close businesses or farms because of the impact it would have on economic development in the area.

cangwei@chinadaily.com.cn