Chengdsdu City, capital of Sichuan Province, will build a wetland park at the 2010 World Expo that uses soil and vegetation to purify rainwater.

The 2,680-square-meter park in the Puxi section of the Expo site will be the city's Urban Best Practices Area showcase.

The 20 million yuan (US$2.92 million) park can purify about 30 cubic meters of water everyday and the water will be used to flush toilets on the Expo site.

An artist's rendition of the wetland park

The park will be shaped like a fish as Chengdu is known as the "land of fish and rice" due to its rich agricultural resources, said Liu Zong, deputy director of the Chengdu Water Authority.

A 200-cubic-meter underground pond will be built to collect rainwater for purification. If it doesn't rain, water from the Huangpu River will be pumped to the pond.

Another pump will then send the collected water to terraced fields. The pond water will start at the top of the 2.5-meter terrace and flow down through the 16 fields. The water will be purified as it flows through the fields.

An artist's rendition of the wetland park

The park will also include more than 60 species of plants and trees - some of which will be transplanted from Chengdu - including camphor and ginkgo trees and gardenia flowers.

The plants will cover an area of about 1,200 square meters and are chosen for their ability to absorb pollutants in water.

They will be planted in different areas to compose a natural purification system, said Lin Nong, chief engineer of the park.

Visitors will enter the park from the "mouth" of the fish. They can see how the water changes from dirty to clean, Lin said.

The park's "fish tail" will be a fun area. A "piano keyboard" will be built on the ground and when people step on a key, water will spray onto visitors.