(CNN) -- China's third largest freshwater body of water, Lake Tai, is at once placid and majestic, until it reveals its dirty secret.

Lake Tai, or Tai Hu in Chinese, was once considered an ecological treasure in the heart of eastern China's land of fish and rice cultivation in Jiangsu province.

Today, the lake is lined with green algae, dead fish and industrial waste, unmistakable consequences of more than 20 years of heavy factory production along its perimeter. It is a story of environmental disintegration that Wu Lihong, an environmental activist recently freed from prison, has been trying to tell for decades.

Wu's crusade started in 1989 when he began documenting the lake's demise. The former factory salesman snapped hundreds of photographs of factories and wastewater pipes along the shores, took samples of the water, filed petitions, and warned government officials the lake was doomed.

As China's economy surged forward, local governments encouraged the industrialization of Lake Tai's surroundings, giving incentives to build dams and small chemical plants.

The building boom caused Lake Tai to degenerate from national treasure to national disaster.

"You have all the pollution problems you could imagine surrounding one lake," said water resources expert Deborah Seligson, of the World Resources Institute. "You have agricultural runoff of fertilizers, pesticides of all kinds, organic pollutants ... and you have industrial pollution from factories."

Wu first embarked upon his environmental campaign after realizing the air in his hometown smelled of industrial byproducts, creating a putrid smell he couldn't stand. He began taking photographs of untreated wastewater discharged from factories and sending them to local environmental organizations. His efforts expanded in the early years of this century, including assembling petitions and calling up local party officials.

Over the years, government representatives praised Wu's efforts. In 2005, the National People's Congress even presented him with an "Environmental Warrior" award for his work.

Wu used his new platform to call for further reforms and draw more attention to the deteriorating lake. He began to dial up higher-ranking Communist Party officials in Beijing and his provincial capital of Nanjing to inform them of his findings, skipping over local government officials.

"When the government gave me the award, they expected me to be on their side, but I had to be honest," Wu said.

Wu said his surge forward caused him to become the subject of intense government and regulatory scrutiny.

"The police would just sit in front of my home and watch me," he said.

Other government officials and factory bosses tried to get him to accept illegal gifts and bribes, Wu said.

Finally in 2007, the local police arrested him, charging him with blackmail, fraud and extortion. Wu was convicted and imprisoned for three years.

As he headed for prison, Wu's wife Xu Jiehua had her doubts about her husband's work.

"At first I thought he should just give up," Xu said. "But then I realized that he was doing the right thing and should keep going."

Later that year, Wu Lihong's worst fears materialized. Lake Tai succumbed to a massive algae bloom, turning the water a fluorescent green from a scum that covered the lake, thriving on pollutants. It left more than 2.3 million people on its northern shores without drinking water.

In the aftermath, the Chinese government forced thousands of nearby factories to close, added water treatment facilities and pledged $14 billion to clean up the lake by 2020.

During this time, Wu remained in prison, bearing silent witness to the peril he predicted. He was released in April 2010.

In response to whether the blackmail, extortion and fraud charges against Wu were unjust, a local government official disagreed.

"We welcome people to be enthusiastic about protecting the environment ... but if you violate the law, it's the same punishment for everyone," said Liu Yamin, director of the Wuxi City Environmental Protection Bureau.

Liu said the local government was attempting various cleanup efforts of the lake and had a longer term plan to resolve the problem, but noted they had only achieved basic improvements and that the pollution remains a huge challenge around Lake Tai. Many large plants are still operating, though all factories directly along the water's edge have been shut down.

Fishermen say the water seems cleaner though their catch is dwindling, while fishing boat operators have reported an increasing presence of green algae in June and July.

Zhou Xunluo, a resident of 25 years in the area, recently noted the algae bloom on a river leading from the lake.

"Look how bad the algae is!" said Zhou. "Without Wu Lihong and his hard work, this would be much worse."

Wu has quickly jumped back into work and said he plans to continue his efforts "at any cost."

"The environment belongs to the people, and we need to protect it," he said. "I just want the water to be clean, for the next generation."