Two trapped workers were not injured after they were rescued Tuesday morning at the Napa Sanitation District plant south of Napa.
The individuals, who worked for a contractor, had been lowered through a 30-inch-diameter hatch into a 1.3 million-gallon digester for a routine inspection, the district said.
Once they were inside, a mechanical system failed, making them unable to lower or raise themselves.
Eight truck crews and 30 fire personnel from Napa, American Canyon and Carneros responded to the call at 9:35 a.m to the 1515 Soscol Ferry Road incident. Five firefighters used a high-angle rope system to rescue the two trapped workers, Cal Fire said.
There was no immediate threat to the safety of the workers, so area fire departments used the incident as an opportunity for training for confined space entry rescue, the district said.