"Thai police in the globalised world must have one hand holding pistols and arresting crooks and the other hand making biodiesel," Lieutenant-Colonel Tepvisit Potigengrit, head of a biofuel project at a Bangkok police station, told Reuters.

The campaign began in May at three police stations in Bangkok. By the end of this year, police plan to have 80 of their 1,500 stations nationwide run their pickup trucks on biodiesel.

The cost of making biodiesel from used edible oils is 7 baht (US$0.20) a litre. Conventional diesel costs 28.64 baht.

If all 3,500 police patrol trucks were to run on biodiesel, the force would save 200 million baht (US$6 million) a year, a police spokesman said.

It would also be a solution for up to 4.7 million litres of used cooking oil dumped into Bangkok sewers each year, Tepvisit said.

Each police station taking part in the project is required to make room for a boiler capable of making 150 litres of biodiesel a day, Tepvisit said.

The main problem is commercial refiners willing to pay street vendors and households up to 20 baht (US$0.60) a litre of the once unwanted waste, which means Tepvisit's station can only get enough to make biodiesel three times a week.

One answer, he hopes, is an offer to return half the used cooking oil to contributors as biodiesel.

Another is an appeal to self-interest.