The appliance of science cleans Lake Taihu
For generations, poets used the term bishui, or "emerald water", to describe Lake Taihu, China's third-largest freshwater body, famed for its scenery and clear water that reflected the surrounding green hills.
The lake, which straddles the provinces of Jiangsu and Zhejiang in East China, is a cradle of fisheries and tourism. It is also the water source for industries and cities, from Wuxi in Jiangsu to Shanghai, nurturing more than 44 million people.
As nearby cities flourished during the past three decades, industrial effluent and waste from human activities ravaged the lake. A buildup of nutrients, often caused by fertilizers leaking into the water, resulted in eutrophication, a process that occurs when blue-green algae blooms and strips the oxygen from the water, causing it to stagnate and become putrid, tea-green sludge.
"It's a terrible eyesore," said Shen Ji, director of the Nanjing Institute of Geography and Limnology at the Chinese Academy of Sciences. According to Ji, China has about 920 freshwater lakes, most of them located along the middle and lower stretches of the Yangtze River, and more than 85 percent have been affected by algae pollution.
Now, the academy's scientists are trying to turn the tide via new technology: from bionic platforms that "eat" algae, to a giant razor that "shaves" the sediment, and a powder that turns algae into stone.
Although still in their infancies, these technologies, coupled with improved monitoring and prediction systems, have raised the amount of algae collected at the annual clean up from 0.8 million tons in 2008 to 1.6 million tons last year, according to Qin Boqiang, director of the academy's Taihu Laboratory for Lake Ecosystem Research.
"It will take decades for Taihu to fully recover," Qin said, adding that China isn't the only country facing the problem. "From Lake Okeechobee in the United States to the Nakdong River in the Republic of Korea, algae pollution has become a global ecological hazard. But these new technologies are helping us to make great progress, and we hope our experience at Taihu will help other countries to tackle the issue."
Inspired by nature
Silver carp has long been famous as a classic Chinese dish, but the fish is now also the inspiration for pollution control. Over the course of its life, one silver carp can consume 50 kilograms of algae, but dumping a large number of them into the lake would be expensive and could disturb the delicate ecological balance.
In response, Li Wenchao, an environmental engineer at the academy, devised a way to emulate the fish's "feast-and filter" mechanism and built a bionic platform that removes algae in an efficient, environmentally friendly way.
The 12-meter-long, 11-meter-wide floating platform loaded with filters is modeled on the gills of the silver carp. It travels across the water at 5 km an hour, gulping the algae in its path and leaving a trail of filtered water in its wake.
"It is like a vacuum cleaner for algae," Qin said. "The platform eats the algae, filters and shreds it through 200 layers of 'teeth', and then concentrates it into a thick pulp which is stored on deck. The clean water is then pumped back into the lake.
Traditionally, floating algae is cleared with large hand-held nets, which is slow, costly and ineffective. The platform can clear algae invisible to the naked eye, and can clean 1 metric ton of water for less than 0.05 yuan. It can be used all day without damaging the environment.
The prototype was first used in 2011, but by last year, newer, more-compact models had emerged. About 20 lakes and reservoirs now use the platform, ranging from Lake Xingyun in Yunnan province in the south of the country to the Yuqiao reservoir in Tianjin in the north.
The downside is that the platform costs about 2 million yuan ($290,000) and it is only effective in medium or large lakes, meaning its use is limited to waters that have economic significance, such as tourist attractions and reservoirs, according to Qin. "Even so, the bionic platform is an effective method of emergency algae control," he said.
While the platform is highly effective at surface level, it can't address the bigger problem－stopping the algae from blooming in the first place. To do that, the scientists have to tackle the lake-bed sediment in which the plant thrives.
Researchers have discovered that algae outbreaks have four stages: dormant in sediment; resuscitation; mass reproduction; and surface outbreaks. "If the algae is removed from the sediment, it is gone for good," said Yang Guishan, an ecologist at the academy.
The two main fuels for algae outbreaks－nitrogen and phosphorous－are also deposited in the sediment, so it is important to clean the soil when the algae is dormant, he said.
Dredging and replacing the lake bed have been tried, but they are expensive because of the high cost of labor and transportation of clean soil.
In December, the academy's scientists successfully tested a new dredging platform that shaves off the sediment's top layer, cleans it and pumps it back into the water.
The dredger is 21 meters in length and has a 6-meter-long arm attached to one end. The end of the arm has two rotating paddle blades and a number of water pumps.
The pumps disturb the water near the top layer of sediment－where most organic waste and algae are deposited－while the paddle blades collect the mixture and another pipe sucks it onto the platform. The mud is then filtered before being pumped back into the water. The manufacturer claims the process can remove more than 80 percent of the algae and organic waste in the sediment, meaning there is no need to transport fresh soil.
The platform can clean 1 hectare of soil an hour, and can hold about 50 metric tons of algae and waste.
Again, there's a downside. The dredger is a prototype, and it will be years before it can be used widely. Moreover, its size and the arm's limited reach mean it only works in large, shallow lakes, and it can only be used to clean sediment that does not have a high level of biodiversity in its top layer, according to Qin.
"But circulatory cleaning, soil filtering and other technologies on the platform are already showing potential and they could be implemented earlier," he said.
Turned to stone
When they came to consider famous tourist lakes, the scientists decided to employ a more sensitive approach, because giant platforms carrying large amounts of toxic algae could ruin the scenery and damage the local tourism industry.
Last year, scientists developed a powder that glues algae into large clusters. As a cluster sinks, it acts on the phosphorus in the water, transforming it into insoluble calcium phosphate which traps both the algae and its nutrients. Just 0.3 kilogram of the powder can clean 1 square meter of water and remove 95 percent of the phosphates.
"It's like killing two birds with one stone," said Zheng Maosong, deputy director of the academy's Xuyi Center of Attapulgite Applied Technology Research Development and Industrialization, which helped to create the powder.
The key ingredient is purified attapulgite, a natural nanomaterial whose unique rod-like structure gives it strong adhesive qualities.
The mines in Xuyi, a city in Jiangsu, contain an estimated 889 million tons of attapulgite, accounting for 48 percent of the global reserve, said Wang Aiqin, a researcher at the center.
The powder is a relatively new invention, so its effect on the ecosystem will have to be studied before it can be used on a large body of water, such as Lake Taihu, Zheng said.
"Taihu is frequently buffeted by strong winds that stir up the sediment, and when that's coupled with the effects of global warming, the stones created by the mixture may break and release the pollutants back into the clean water," he said.
At the moment, the process is only suitable for use in small lakes or sewage works that have more-stable water dynamics, according to Zheng, who said he sees a strong future for the powder once it has been fully tested.
Contact the writer at Zhangzhihao@chinadaily.com.cn