An animal rendering and waste oil company is raising a stink over the city of South St. Paul's foul odor ordinance, charging that it's unconstitutionally vague.

Sanimax USA, a Green Bay, Wis., corporation, filed a federal lawsuit in St. Paul on Feb. 21 that says the city's methods of identifying the source of bad odors violates the company's constitutional right of due process. City officials did not respond to requests for comment.

South St. Paul's ordinance outlaws the emission of odors that are "offensive or obnoxious." It also allows the city to designate businesses as significant odor generators, subjecting them to greater regulation and potential fines.

The city notified Sanimax in December that it made the list of malodorous malefactors.

Sanimax contends that the two methods used by the city to identify offenders — a study it commissioned and a public complaint system — are unconstitutional.

The company's lawsuit notes that the ordinance fails to define the terms "offensive" or "obnoxious" and that the technology used to measure them — an olfactometer called Nasal Ranger — is inadequate.

"The technology is dependent on the odor sensitivities of the user," the company's complaint says. "In addition, [it] is unable to determine the source of an odor — whether it comes from Sanimax, or whether it comes from one of the other odor generating facilities in the city."

In addition, the source of "seven verifiable odor complaints" in a six-month period are designated as significant odor generators. The Sanimax suit says the definition of a "verifiable odor complaint" is unclear, however.

"Sanimax was not provided information about who made the complaints, when the complaints were made or any further information to allow it to internally investigate the merit of the odor complaint," the suit says.

In 2013, Sanimax and three other odor-generating businesses commissioned a study to identify stench producers in South St. Paul. Sanimax was identified as a potential source in the study, which was provided to the City Council. The city commissioned its own supplemental study that also identified Sanimax as a significant odor generator.

Bad odors are nothing new to South St. Paul, which was home to stockyards, the last of which closed in 2008. Related industries remain, including a composting site, meat processors and leather makers.

Sanimax collects animal hides and skins, animal and meat by-products and discarded oils and grease, for various processes.

Gabriel Sanchez is a University of Minnesota student on assignment for the Star Tribune.