Northamptonshire fires: Waste centre and factory ablaze
Firefighters could remain at the scene of a blaze involving about 200 tonnes of household waste for up to 48 hours.
The fire started at the waste transfer station in Rushton, Northamptonshire at about 22:30 BST on Tuesday.
Northamptonshire Fire and Rescue Service (NFRS) said it would take at least another 24 hours to put out, due to the fire's "deep seated nature".
It was also called to a blaze at an industrial unit in Studland Road, Northampton just before midnight.
Two crews remained at the scene of the single-storey building fire in Northampton on Wednesday afternoon, with the cause "under investigation".
The fire service said plumes from both fires "have decreased substantially" but was advising residents to keep doors and windows shut if their property was affected by smoke.
There are not believed to have been any casualties at either of the fires.
A statement from Mick George Ltd, whose head office is in Huntingdon in Cambridgeshire, said there were staff permanently on site.
"The cause of the fire has not yet been identified, however it is most likely a consequence of the current record-breaking heatwave," a spokesman said.
"The management team at Rushton Transfer Station acted quickly on discovering the fire and contacted NFRS, who were very quickly on the scene and brought the fire under control.
"There were no injuries, no risk to public health and the Environment Agency has been informed."