海水淡化能解决水资源短缺问题?

Most of China's economic zones are located along its eastern and southern coastlines. About 70 percent of the country's GDP comes from these areas which comprise one-fourth of the country's territory. As water shortages along the coastlines and beyond become more severe and threaten both industry and residential areas, many are looking at seawater desalination as a significant supplementary resource.

There are two main technologies used in seawater desalination-thermo and membrane desalination. To put it more simply, thermo desalination vaporizes seawater, while membrane desalination filters salt from seawater through a special membrane.

Tianjin is a large coastal city that has developed a seawater desalination industry. Its desalination plants can process about 225 thousand tons of seawater a day using both desalination methods. The water is mainly used by manufacturing enterprises along the coast.

Zhang Kai is deputy director of the Tianjin Water Environment Research Institute. He says although desalinated seawater is potable, moving it though the city's tap water pipes to homes and offices has limits.

"Because many of the ions are removed in the desalination process, the ion balance is broken. The desalinated water may erode pipes as it is transported. So when it comes out from normal water pipe, it's yellow, because it contains a lot of rust. The water quality is different from that of tap water."

Beijing is one metropolis with a severe water shortage. The city is about 100 kilometers from to Tianjin-fairly close to the seawater desalination plants. But transporting desalinated water from the Tianjin plants to China's capital would not be cost effective. And as Beijing sits at a higher altitude than Tianjin, there would be additional problems in transporting desalinated water.

Even in Tianjin, the adoption of desalinated water on a large-scale basis faces some obstacles. One of them is from the market. Zhang Kai says:

"For instance, the current price of surface water in Tianjin is more than 1 yuan per ton. Desalinated seawater costs three to five yuan per ton, and the equipment depreciation is not even counted. If it's counted, the price may rise to four to eight yuan per ton. That's because the technology and infrastructure cost more money than tap water."

Zhang says another problem with using desalinated water is environmental damage.

"Seawater desalination has some effect on neighboring environments. The wastewater from desalination has a high-salt density. Discharging it has some effect on the ecosystem. Unlike the situation in Singapore and Australia, China's Bohai Bay area is an inner sea. There is no strong current which can effectively take the highly salty water back to the Pacific."

Despite these limitations, desalinated seawater will play a greater role in the near future in helping to alleviate the country's water shortages.

Zhang Kai says Tianjin has plans to better use its abundant regional seawater resources. By the year 2015, the city expects it desalination plants to process 460-thousand tons of seawater a day.

For CRI, I am Li Dong.