GUANGZHOU: 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.