Authorities have uncovered two large workshops that were turning discarded kitchen waste into cooking oil - a practice that is illegal and carries potential health risks.

One of the sites, in Dongguan of Guangdong province, had already been punished in February this year for illegally producing cooking oil.

It had re-registered as a bio-energy technology company and was selling more than 10 tons of such oil a day, China Central Television reported.

A second site, in Shenzhen, was found with more than 32 tons of illegal oil.

Wu Shengbin, an official with the market supervision and management bureau of Shenzhen, said the oil is OK for industrial use, but not for cooking.

Kitchen waste contains large amounts of sewage, garbage and detergent, said Huang Fenghong, director of the Processing Center of the Oil Crops Research Institute under the Chinese Academy of Agricultural Sciences.

Refining at illegal workshops does not sufficiently reduce the bacteria and hazardous substances. Long-term intake of the oil may lead to cancer and other diseases.

The oil, which appears the same as normal cooking oil after purification, has been used by canteens at some construction sites, factories and cheap restaurants in Dongguan.

A worker at a construction site canteen said the slop oil costs only 4 yuan (59 cents) per kg compared with 20 yuan per kg for peanut oil. His canteen uses more than 30 kg of oil a day, which means a savings of nearly 500 yuan.

Cooking oil as cheap as 2 yuan per kg and labeled as palm oil was found at a farmers' market in Dongguan.

The problem is difficult to eradicate because the workshops are scattered and the sector is supervised by different government agencies without an effective coordination, Huang said.

The lack of a standard for cooking oil also makes it hard to single out slop oil from normal cooking oil, said a worker with the industry and commerce administration of Dongguan.

In Guangzhou, city authorities are formulating the regulation of kitchen waste and plan to build a slop processing center. In Shenzhen, the market supervision and management bureau was established in September. It combines the roles of three previous departments and aims to improve food safety and market supervision.

Guangdong is not alone in the fight against slop oil. About 17 tons of such oil was found every day during a campaign this January in Changsha, Hunan province. Ten tons were discovered at a site in Chongqing municipality in November last year.

China produces more than 60 million tons of kitchen waste every year, according to statistics from the department of environmental science and engineering of Tsinghua University.

About 14.1 million tons of edible oil was sold in China in 2006 and up to 1.5 million tons of slop oil was used for cooking every year, reported Science and Technology Daily early this year.