Firefighters are tackling a blaze involving about 2,000 tonnes of rubbish at a Vale of Glamorgan recycling plant.
Residents near the Siteserv plant on the Llandow Industrial Estate are being advised to keep their doors and windows shut because of the acrid smoke.
South Wales fire service said five engines were at the scene.
The director of the neighbouring caravan park said there had been a number of fires at the site in recent years.
There were two accidental fires at Siteserv recycling plants in Llandow in November 2013.
The first involved about 1,000 tonnes of household waste and devastated the site on the industrial estate.
The second was in another building owned by Siteserv on the nearby Vale Business Park.
Sharon Evans, of Llandow Caravan Park, said her husband Tony was director of Llandow Racing Circuit and they had to cancel a fun day for 20 disabled adults there on Friday because of the fire.
She said the last time there was a blaze, fire engine bowsers had needed to use all the water so they had none left to run services at the circuit.