The driver told the police he had made 12 similar trips since last month, carrying waste from a leather manufacturer to the feed miller Le Phu Production and Commerce.
Such tanning waste is considered a dangerous health hazard. During the hide-tanning process, the cattle skin is soaked in acid and other solutions that contain heavy metals such as mercury and chromium that can cause cancer.
Police said they found around seven tonnes of tanning waste unprocessed at Le Phu Company and nearly 15 tonnes of processed materials, not including material being aired.
Le Van Tam, director of Le Phu Company, said his company has made animal feed from tanning waste since last year. The waste is steamed at 140°C, then aired, dried and mixed with several substances to become animal feed, Tam said.
Police have taken samples of tanning waste from Le Phu Company for testing and have orderd the company to halt all sales pending further notice. Le Phu was not authorized to treat tanning waste.