Blunt refusals, threats and beatings, that's the fate authorized collectors of restaurant waste in Zhengzhou, Henan province, suffer often as unscrupulous gutter oil dealers get the upper hand in the battle for the leftovers from the city's eateries.

Xi Kezhong, general manager of United Elites Bio-energy Co, said his employees face a daily struggle to collect waste from restaurants, despite the fact that his company is the only authorized collector in the city.

The company collects restaurant waste and turns it into diesel fuel, while waste going to unauthorized dealers may be recycled as cooking oil, which is believed to be hazardous to health.

The company employs more than 80 workers to collect leftovers from the city's 12,000 restaurants, but most said they were often turned away.

Liu Shichao, 47, head of the company's oil recycling department, said Zhengzhou's restaurants produce more than 10 tons of oil every day, but the company could only recycle around 2 tons, with the rest being snapped up by unauthorized dealers.

"The reason our workers are refused is that we do not pay the restaurant owners for the leftovers, because we seldom make any profits from producing diesel fuel from the leftover oil," Liu said.

"What we offer the restaurant owners is to clean their kitchens, but this is not so attractive compared with the money paid by unauthorized dealers."

A restaurant owner in the city said on condition of anonymity that an unauthorized collector paid him 600 yuan ($95) a year to buy the waste from his restaurant.

He said he didn't know whether the leftovers would be used to produce gutter oil.

"I did not ask the dealer about the use of the leftovers because I don't care," he said. "I just care about who gives me money."

Selling recycled cooking oil has become a profitable business, prompting some unauthorized dealers to resort to violence.

Wang Fengguo, 48, a worker at United Elites, said that he was attacked by unidentified men when he was processing leftovers near a restaurant on Feb 13.

"A man told me that the leftovers had been contracted to him," Wang said.

"Several minutes later, three other men came to beat me."

Xi, United Elites' general manager, said that the government should strictly supervise restaurants in order to prevent leftover oil being sold to unauthorized dealers.

"The government should make more restaurant managers aware of the policies on recycling used oil," Xi said.

Gutter oil has become a major food safety concern in China, and the government launched a nationwide food safety campaign in August to crack down on the practice.

"When the courts rule in gutter oil cases, for those deserving death, death penalties must be resolutely given," said a notice issued in February by the Supreme People's Court, the Supreme People's Procuratorate and the Ministry of Public Security.

Police had arrested 100 gutter oil manufacturers nationwide since August, and around 800 suspects in 135 cases, in a special campaign launched last year, according to a report by Xinhua News Agency in February.