A picture-perfect scene of flowing water and lush greenery and trees in Yunnan Province is a natural sewage treatment system that has applications around China.

This garden filter helps ensure that clean water is pouring into Lake Erhai, the second-largest fresh water lake in Yunnan Province.

The project is a pilot sewage disposal facility developed by Fudan University Professor Zheng Zheng, director of the Research Center for Basin Pollution Control.

Local government officials turned to Zheng to help address the pollution problem in one of the region's most beautiful lakes. Yunnan is famous for its unspoiled scenery, but waste from livestock farming and the booming tourism industry have spoiled the picture.

Some parts of Lake Erhai were so polluted that the water ranked at the bottom of national environmental water quality standards for surface water - levels 4 and 5 that human beings should not contact. That water was suited mainly for industrial, agricultural and landscape use - hardly the pristine water for which the lake was once famous.

The main culprit is a cattle ranch.

Though the Erhai pollution was not as serious as that in urban areas, the situation would get worse and spread throughout the lake if there was no intervention, says Professor Zheng.

A serious consequence would be a dangerously contaminated lake with blue-green algae blooms, one that could not support plant and animal life. It could pose a health hazard to humans.

Last year a government survey of China's environmental problems showed that water pollution levels in 2007 were more than twice the official estimate, largely because agricultural waste was ignored. Experts say the problem of water shortages and polluted water is severe today.

Purifying polluted water before it enters the environment is the best way to stop and prevent pollution, says Zheng. He and his team decided to use the power of nature, which he had been working on for years.

"Pollutants in most cases are resources that occur in the wrong places," says Zheng. "The problem is much easier to solve if we can find a way to put them in the right places."

The nutrient pollutants phosphorus and nitrogen contained in human and animal waste - while dangerous in the water - are actually necessary nutrition for many plants and creatures, according to Zheng. Water plants grow fast in water with high concentrations of phosphorus and nitrogen; and certain microbes also consume polluting elements quickly.

Instead of setting up a standard bricks-and-mortar facility, Zheng and his team built several small gardens near the cattle ranch. The major components are plants, earth, microbes, earthworms and processed layers filtering substances, mostly gravel.

Plants and shrubs with strong and deep roots were planted in earth 30-40cm deep, with earthworms directly beneath the plants. Beneath that is a layer of specially processed filters covered by a layer of various microbes.

When water flows down through the filters, microbes first consume the pollutants, such as phosphorus and nitrogen. The plants and earthworms above will also consume some of the pollution as nutrition.

The filtered water is then channeled into a pond containing water plants that again purify the water.

The gardens lie between the cattle ranch and the stream that feeds Lake Erhai. The system requires a considerable amount of space and a large catchment area; one to two cubic meters of earth are needed to filter one ton of water a day in this system.

In addition to the large area required, the system is not as fast as mechanical and chemical treatment plants used in urban areas. But it's very inexpensive and low-maintenance.

Rural and suburban areas would be the logical locations.

The garden pilot project aimed to filter 50 tons of waste water produced by the cattle ranch each day. Results are encouraging since the quality of the processed water has improved by two levels.

The garden-filtration program was also launched in villages in Guizhou and Jiangsu provinces where pollution is more serious. The team is now working on bigger filtration facilities, one for a cattle ranch in Yunnan that produces around 200 tons of sewage a day and another for a town in Changzhou City, Jiangsu Province.

Zheng is also considering a natural filtration system for further treatment of water processed by sewage disposal plants. Even the strictest standard for sewage discharge contains 10 times more pollutants than the two bottom levels of surface water, so the quality could be improved.

Urban green belts along river banks would be suitable sites if they are close to existing treatment facilities. And their green space and recreational function would not be affected.

So far there are no takers for the enhanced treatment of urban sewage. The idea of purify water with natural resources came to Zheng in 2000 when Chinese scientists realized the urgency of setting up sewage disposal systems in rural areas. While cities have sewage treatment systems, most rural areas still have none.

The problem of rural pollution is increasing as more factories move to rural areas, more fertilizers and pesticides are used in agriculture and livestock farming expands. These activities threaten water resources in rural areas and aggravate pollution in major nearby bodies of water.

Urban treatment

It's not economically feasible to build big treatment facilities in rural areas, but Zheng's natural treatment method could work. It is efficient and has low initial costs, low running costs and high stability. It requires little technology.

The "activated sludge" method developed by British scientists early last century is widely used in sewage treatment worldwide. Microbes in large ponds are used to treat sewage by consuming nutrient pollutants. But urban treatment plants produce huge amounts of sewage sludge, which also can become a pollution problem, if not treated properly.

The cost of treating the sewage sludge usually doubles the cost of sewage disposal. In many cases, the plants will just skip the treatment for sewage sludge.

In Shanghai, the sewage disposal system is comprised of 53 plants with a combined capacity of 6.9 million tons of sewage a day. Some sludge is treated to produce dry fertilizer; some isn't and is dumped. The effluent is discharged into the Huangpu River or Suzhou Creek.

Natural disposal would utilize the sludge, which would be consumed by microbes and absorbed by plants as nutrition. Also, the effluent could be further purified.

But natural sewage disposal methods, though they are eco-friendly, would not be feasible in big cities like Shanghai, according to Mao Weide, chief engineer of the New Water Treatment Technologies Center affiliated to the Ministry of Housing and Urban and Rural Development.

"There are millions of tons of sewage to be treated in Shanghai," says Mao, "and an extremely large amount of land would be needed to treat sewage in this way. But as everybody knows, land is as precious as gold in cities like Shanghai."

But he agrees with Zheng that in the right location, further natural purification of discharge could improve the water quality.

But discharged water doesn't meet standards for surface water.

It is like pouring ink in clean water, says Professor Zheng. "No matter how thin the ink is, the water will still darken as long as the ink is darker than the water.

"Pollution can only be stopped or relieved when the discharged water is at least as clean as the original surface water," he says.