MARYSVILLE — Complaints are surfacing once again about an offensive odor wafting through neighborhoods, and some again believe it is coming from Cedar Grove Composting in Everett.

Mike Davis of Marysville said the odor was especially bad last weekend and Monday. It smelled the same as the stink he’s endured the past four or five summers, he said. Cedar Grove began operations in 2004.

“It had that rotting corpse smell that is unbearable,” he said.

The Smith Island business processes food waste and yard debris from all of Snohomish County and has been the target of many complaints about odor in recent years.

Officials at the Puget Sound Clean Air Agency, which has jurisdiction over obnoxious odors, said they have received 12 complaints in the area since May 10.

Many of the instances were reported on evenings and weekends and the agency was unable to send out an inspector to determine the source of the stink, an official with the Clean Air Agency said.

At other times, the agency didn’t have an inspector available, said Mario Pedroza, managing supervising inspector for the Clean Air Agency. The agency has 12 inspectors to cover a four-county area, he said.

To cite a company for odor violations, the agency must send an inspector to trace the smell from the place of the call while the odor is still present.

Still, there’s a good possibility the smell is coming from the composting business on Smith Island, Pedroza said.

Other odor producers in the area, such as the Marysville wastewater plant, have characteristic smells that are easily distinguished from that of Cedar Grove, he said.

At times in the past, when callers have complained, inspectors have traced the odor directly to Cedar Grove. At other times, they were unable to connect the complaint location to the smell but encountered the distinctive Cedar Grove aroma nearby, Pedroza said.

“The odor’s still there, the people still smell it,” he said.

Numerous complaints about a strong stench were received in the summer of 2008 in Marysville and north Everett. At least three times the Clean Air Agency traced the smell to Cedar Grove.

Over the winter of 2008-09, with prodding from the Clean Air Agency, the company spent several hundred thousand dollars on trees and on doors for a large building that holds raw compost. It also stepped up maintenance of a tank that holds drippings from the raw compost and added odor-control liquid to the material.

Complaints dropped last summer from the year before but many residents said they still smelled the same odor.

The agency has the power to levy fines. One complaint last Aug. 24 was traced to Cedar Grove and the company was fined $14,000, Pedroza said.

Cedar Grove was also cited last summer for two violations at its Maple Valley operation in King County, with fines totaling $22,000, he said.

The business’s volume increases dramatically in the spring with an influx of grass clippings and other yard waste, company officials have said.

They said little about the complaints on Thursday.

Some of the trees put in last year died from planting shock and will be replaced, Cedar Grove vice president Jerry Bartlett said in an e-mail.

Agency officials have talked of requiring the company to enclose parts of its operation, particularly the open-air machine that grinds the raw garbage into compostable material. This could cost up to $300,000, company officials have estimated, and the agency so far has stopped short of requiring it.

It has not been ruled out, Pedroza said.

“There’s still stuff that has to be done,” he said. “We have to figure out where and what and how.”

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