Scientists in China have developed a way of making fertilizer from waste water discharged by potato processing plants, solving a pollution problem which has held back China's potato ambitions.

China sees potatoes as a new staple food to ensure food security, but protein-rich water discharged by starch processors, a major buyer of the spuds, has been blamed for polluting rivers and lakes.

"For years, there has been no technical solution to this problem, forcing environmental authorities to close more than 10,000 small plants, which has hurt the potato market and farmers," said Liu Gang, researcher with the Chinese Academy of Sciences, Lanzhou institute of chemical physics.

Liu's team has developed technology that can halve the chemical oxygen demand (COD) of the effluent by removing starch, fiber and protein. The processed water does not need to be dumped either, because of its high nitrogen, potassium and phosphorus content make it a perfect irrigation water.

LA four-year test has shown the water harmless to crops, and three starch companies are now using the technology to purify their discharge.

China is the world's largest potato producer, with 5.6 million hectares under the crops. Given its resistance to cold and drought, the tubers are more suitable for cultivation in China's arid west and northwest than wheat and rice.

The Ministry of Agriculture has plans to expand potato acreage to 10 million hectares to produce 50 million tonnes by 2020.

Chinese companies have developed buns, noodles and other products made from potato starch, products that are more familiar to the Chinese as staple food.