The pellet plant, built in 1990, treats and dries sludge from the city's wastewater treatment plant and converts it to fertilizer pellets.
Wheelabrator Hagerstown senior project manager Clyde J. Harris said that in March 1996, the plant stopped using chemicals and started using organic enzymes to kill odor-causing bacteria in sludge.
"The enzymes did fine for six or seven months, even through the summer," Harris said. "Then we started getting odor problems toward the end of October. We don't know what happened."
Assuming the problem was related to use of the enzymes, Harris and city officials on Sunday night started using chemicals again in an attempt to solve the problem. Only time will tell if it works.
City Administrator Bruce Zimmerman, Water Pollution Control Manager Rick Thomas and Harris said they hoped the switch back to chemicals would stop the odor, but they still don't know what caused it.
"It's like a mystery novel without a last chapter," Thomas said.
Harris said air samples are taken from the plant's stack every two hours while it's in operation. "We open them and breathe the air," he said. "We don't smell anything."
Thomas said he thinks cool weather might cause condensation of stack emissions, resulting in the release of odor in areas away from the plant.
A consultant hired by the city has identified the Hagerstown wastewater treatment plant and wastewater treatment plants at 1st Urban Fiber and Good Humor-Breyer's Ice Cream Inc. as sources of odor.
Harris, Thomas and Zimmerman said they've smelled the smells, and sympathize with local residents who have had to endure them.
Thomas said the city, Wheelabrator, 1st Urban Fiber and Good Humor-Breyer's are working together to clear the air. They take turns staffing an "odor patrol," and have jointly hired consultant TRC to sniff out the problem and recommend solutions, he said.
Results should be ready by the end of January. Preliminary findings are that the pellet plant is "one of the most prevalent" sources of foul odor in the city, Thomas said.