At approximately 2:00 pm, a maintenance employee entered the wastewater treatment (WWT) room of a dangerous household waste (e.g., paint, cleaners, pesticides, and solvents) recycling company. As he approached the clarifier (an open-top tank with a conical bottom for settling solids), he noticed a rotten egg odour that became stronger as he moved forward. He suddenly felt unable to breathe, tried to flee the area, but was overcome by hydrogen sulfide (H2S) gas and collapsed. Fortunately, fellow employees found the victim a few moments later and pulled him to safety. He recovered, and there were no other injuries
The day prior, water-based waste containing various contaminants, including heavy metals, was received into the WWT area. The waste was sampled and analysed. An experienced waste treatment operator determined what treatment was required and began to process the material. Late that day or early the next morning, the operator sampled the treated liquid in the clarifier. Tests revealed that the mercury content was above the discharge permit limit (0.02 ppm into the municipal sewer).
A typical method of removing heavy metals from water-based waste streams is to add a chemical to react with the metal. The product of this reaction is a salt that precipitates (i.e., settles) out of solution so that the remaining liquid can be decanted and discharged. On the day of the accident, the waste treatment operator added 50 pounds of sodium sulfide (Na2S) flake to the water-based waste in the clarifier to precipitate mercury sulfite. Because the clarifier is not equipped with a mixer or agitator, the operator connected a plant air hose to the vessel to provide mixing (this action is typically referred to as air rolling). After decanting, tests showed that the mercury was within limits, but the pH was high (11.4) due to the alkalinity of Na2S.
The operator then added an acidic chemicalpolyaluminum chloride (PAC)to the clarifier. This was intended to bring the mercury salts together into larger, heavier clusters (i.e., to flocculate) and to adjust the pH toward neutral (the permitted range is 6 to 10). Three 55-gallon drums of PAC were added over a few hours. The air used in the clarifier did not provide sufficient mixing to completely dissolve the Na2S flake and distribute the strongly acidic PAC. This produced a localized condition under which the two chemicals combined to form H2S, which was released from the top of the open vessel. At approximately 2:00 pm on December 11, the facility compliance coordinator was alerted of the H2S smell. By the time he went to fetch a portable gas detector, the mechanic that entered in the room to retrieve a tool had collapsed. The operator estimated that 2 pounds of H2S were released.
Experts from the chemical safety board (US CSB) investigated the accident and pointed that the wastes were chemically treated in a vessel not designed for such use : the treatment tank (enclosed, vented to a scrubber, and equipped with a mixer) should have been used, and not the clarifier. They underlined the lack of effective management systems such as :
- written procedures for operating the WWT area (the operator relied on the knowledge of plant personnel with many years’ experience),
- formal employee training (e.g. risks of mixing incompatible chemicals, H2S hazards ).
- Communication between operators and staff (on normal operations, incidents, risks )
One detector had been installed following a previous incident, but the detector was not inspected, calibrated nor maintained and the sensor head was out of order.
Subsequent to the incident, the operator developed its safety management system, including written procedures and training and improved the ventilation and the detection devices.
