At 3:30 pm., 2 workers were killed and eight others injured when they inhaled deadly hydrogen sulfide gas leaking from an underground process sewer at a paper mill. Nine of the victims were employees of a construction company, who were working on a maintenance project. The tenth victim was a driver from a local trucking company. The gas was the product of a sudden, uncontrolled chemical reaction taking place in the sewer as the men worked above.
Sodium hydrosulfide (NaSH) solution was delivered to the mill periodically by tank truck, leading to the loss of liquid into a collection pit during each delivery. For the maintenance works, subcontractors had to stand in the collection pit. To assist them, a mill operator opened a valve to drain the contents of the pit into the wastewater system. Unfortunately, unknown to the operator, the collection pit drained directly into a sewer line where sulfuric acid was being added to treat the mill effluent. As soon as the NASH contacted the acidic contents of the sewer, it began reacting to form hydrogen sulfide gas, which leaked through the seal of a fiberglass manhole cover near the construction workers.
All ten victims were exposed to the escaping gas. Three workers were overcome almost immediately and fell to the ground. Instead of evacuating the area, three of the remaining workers attempted to drag the fallen men to fresh air. Two of them passed out in the course of assisting the others. Mill workers further away saw the victims collapsing and called in emergency help.
Looking into root causes, the US Chemical safety board (CSB) found the plant did not follow good engineering practices when in 1995 the drain from the collection pit was connected to the sewer line where acid was periodically added. No review of the potential hazards that might result from the mix had been carried out. The plant did not identify the unloading station as a hydrogen sulfide risk area and did not install any gas warning devices.
There had been earlier indications of problems with the fiberglass manhole cover from which the hydrogen sulfide escaped. Employees reported previous occasions where another toxic gas, chlorine dioxide, leaked through the seal of the manhole cover. However, these leaks were not reported or investigated as near-miss incidents. Maintenance personnel repaired the leaks but were unable to find a permanent solution to the problem. Had the earlier incidents been investigated, it is likely that management would have redesigned the sewer system opening.