EVERETT — Cedar Grove Composting has been given until the end of this month to track down the sources of offensive odors coming from its Smith Island site, and tell how it plans to deal with them.

The Puget Sound Clean Air Agency has given the business until Sept. 30 to have a plan in place, said Jim Nolan, the Seattle-based agency’s compliance officer.

After that, the agency and the business will negotiate a timeline for action, he said.

The agency has the power to fine the company.

“We’re preserving all our options,” Nolan said.

Complaints about a foul stench coming from Smith Island skyrocketed in June. People who live in Marysville and north Everett complained of an odor that went from occasional and bearable to persistent and overpowering.

Three times in June and early July, investigators from the Clean Air Agency tracked the stink to Cedar Grove Composting. The smell was traced to the operation’s grinder, where yard debris and food waste from around Snohomish County is ground into compostable material. The machine is outdoors.

The company hit its peak composting volume for the year in June, when it ground up 900 tons of material a day, compared with about 750 to 800 tons a day before, company vice president Jerry Bartlett said.

In the past month, complaints are “way down,” Nolan said.

Some say the air at their homes smells better than it did earlier this summer; others say it doesn’t.

“I have not noticed anything in quite some time,” said Michelle Matthews, who lives near Marysville-Pilchuck High School, several miles north of downtown Marysville.

Wendy McKenna, who lives northeast of downtown, said the smell is still there.

“It’s always here,” she said. McKenna said she and some of her neighbors have developed sinus problems that they believe are caused by the stink.

After the first round of complaints, Cedar Grove began adding odor control liquid to the grinder. It cleaned out its leachate tank, which collects pungent drainage from the lawn clippings and other organic material after it is trucked in. Incoming loads are inspected more closely for foul-smelling material and the companies that send waste to Cedar Grove have been notified that especially foul-smelling loads will be sent back, company officials said.

Now, the company is investigating seven different possible sources, according to a letter from Cedar Grove’s attorney to the Clean Air Agency. These include the grinder, the leachate collection system, and the addition of leachate back into the material to keep it moist enough for the grinder.

The company hasn’t ruled out using a different type of grinder or enclosing the grinder in a building, Bartlett said. If the company has to build a new building or spend a significant amount of money to address the problem, the air agency will likely increase the time Cedar Grove is given to get it done, as opposed to whatever quick fixes might be available, Nolan said.

The company also is looking into having haulers reject loads from local cities and garbage haulers that have already become “anaerobic” — meaning they’ve begun to compost and are already ripe.

Individuals can help, too, by not allowing their organic waste to sit before putting it out for collection, Bartlett said.

“It’s educating the public that if they mow their lawn in May and keep it until June, that’s a problem.”

Reporter Bill Sheets: 425-339-3439 or sheets@heraldnet.com.