The blaze, which may have been caused by fireworks, happened at United Site Services.
According to United's website, it is the largest provider of portable toilets and restroom trailers in the U.S. with 52 locations across the country.
Bexar County spokeswoman Monica Ramos said the toilet fire is still under investigation.
The Converse and Kirby Fire Departments worked together to flush out the flames.
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