Skip to content

Environmental Enterprises Hydrogen Sulfide Release

Overview

On December 11, 2002, at Environmental Enterprises, Inc. in Cincinnati, Ohio, one maintenance employee was overcome after inhaling hydrogen sulfide gas released from a waste processing vessel. The CSB final report concluded that the exposure was caused by using the incorrect vessel for waste treatment.

Incident Snapshot

Field Value
Facility / Company Environmental Enterprises, Inc. (EEI)
Location Cincinnati, OH
Incident Date 12/11/2002
Investigation Status The final report on this investigation, a CSB Case Study, was approved on September 17, 2003.
Accident Type Hydrogen sulfide release / exposure
Final Report Release Date 09/17/2003

What Happened

  • On December 10, water-based waste containing various contaminants, including heavy metals, was received into the WWT area, sampled, analyzed, and an experienced waste treatment operator determined what treatment was required.
  • Late that day or early the next morning, the operator sampled the treated liquid in the clarifier and tests revealed that the mercury content was above the discharge permit limit (0.02 ppm).
  • On December 11, 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 ("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 chemical—polyaluminum chloride (PAC)—to the clarifier.
  • Three 55-gallon drums of PAC were added over a few hours.
  • At approximately 2:00 pm on December 11, the facility compliance coordinator was alerted to an H2S smell, entered the WWT area, noticed the strong odor, and left immediately to get a portable gas detector.
  • A few minutes later, a maintenance employee entered the WWT area to retrieve a tool and noticed the odor of H2S.
  • The mechanic proceeded toward the tool cage, felt as if the breath was suddenly sucked from his lungs, then felt burning and pressure in his chest, stumbled back toward the door, and collapsed in a main pathway within 20 feet of the clarifier vessel.
  • When the compliance coordinator returned with a hand-held gas detector, he discovered the mechanic lying unconscious on the floor and not breathing.
  • The gas detector began to alarm.
  • The supervisor checked the meter within the hour; the indicator showed an H2S level of approximately 85 ppm.
  • The supervisor and the compliance coordinator pulled the mechanic from the room.
  • The victim began breathing on his own; rescuing employees gave the victim oxygen until emergency responders arrived.
  • He was taken to a local hospital for evaluation and released; he reported no lingering effects.

Facility and Process Context

  • Environmental Enterprises, Inc. (EEI) is a privately held company that has operated at its present location on Spring Grove Avenue in Cincinnati for 30 years.
  • EEI processes 35,000 drums of household hazardous waste per year, recycling more than 90 percent.
  • EEI employs a staff of 80 at the Spring Grove facility.
  • The EEI hazardous waste treatment and storage facility receives laboratory, light industrial, and household hazardous waste.
  • The aqueous portion of this waste and wastewater material is treated and filtered before being discharged to the municipal sewer.
  • EEI provides hazardous waste collection and transportation, hazardous waste emergency response, and onsite hazardous waste remediation services.
  • The hazardous waste treatment facility at EEI is a permitted treatment, storage, and disposal (TSD) facility as defined and regulated by the U.S. Environmental Protection Agency (EPA).
  • The treatment area consists of a series of tanks and filters that receive, store, chemically treat, filter, and settle solids out of water-based waste streams.
  • The wastewater treatment process is the final stage of hazardous waste treatment and is regulated by EPA under the Clean Water Act and by the City of Cincinnati Municipal Sewer District.
  • At the time of the incident, the facility had no written procedures for operating the WWT area.
  • Facility staff included the plant manager, a chemical engineer with waste treatment experience; a compliance coordinator; and a staff chemist responsible for lab-pack operations.

Consequences

  • Fatalities: 0
  • Injuries: 1 person was injured.
  • Environmental release: EEI estimates that approximately 2 pounds of H2S was released.
  • Facility damage: Not reported.
  • Operational impact: The victim was taken to a local hospital for evaluation and released; there were no other injuries.

Key Findings

Immediate Causes

  • wastes were chemically treated in a vessel not designed for such use.
  • The clarifier vessel is open-topped and was not equipped with a scrubber to minimize toxic gas releases and did not have an agitator to ensure adequate mixing.
  • 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.

Contributing Factors

  • Operating procedures and operator training were inadequate.
  • The WWT operator did not have written instructions or training on the proper treatment of wastes, the importance of using a properly designed treatment tank, or the consequences of treating waste in the open-topped clarifier.
  • Management had not implemented an incident investigation program to communicate lessons learned; the operator was unaware of the enforcement order from the city that prohibited adding Na2S flake.
  • Management had not implemented controls to limit access to the WWT area during treatment or to notify facility personnel that WWT operations might present a hazard.
  • Communication was inadequate to inform facility personnel of the hazards of H2S.
  • Employees did not respond appropriately when they smelled the characteristic odor because they did not fully appreciate the dangers of H2S.
  • Employees were not warned when waste treatment operations had the potential to release H2S.
  • EEI had not implemented a mechanical integrity program to provide for calibration, inspection, and maintenance of the H2S detector.
  • The detector was not functioning on the day of the incident and failed to warn employees of a dangerously high H2S concentration in the WWT area.
  • The clarifier was not equipped with a scrubber to minimize toxic gas releases and did not have an agitator to ensure adequate mixing.
  • The H2S detector had not been calibrated for 2 to 3 months prior to the incident.
  • Calibration attempts following the incident revealed that the detector was not functioning because of a bad sensor.

Organizational and Systemic Factors

  • EEI relied on the knowledge of plant personnel with many years’ experience in waste treatment to take appropriate actions.
  • The operator had no formal training in waste treatment or chemistry; he relied on his prior experience to determine waste treatment protocols.
  • Facility staff, maintenance and administrative employees, and laboratory personnel were not trained on the hazards of the WWT process or on the properties and hazards of H2S.
  • Employees assumed that someone knowledgeable in WWT operations would alert them if they were in danger and needed to evacuate.
  • Offensive odors were considered part of the business at EEI.
  • All facility personnel were accustomed to strong odors and the characteristic rotten egg smell of H2S.
  • EEI did not have a formal system for investigating incidents and communicating findings to employees.
  • New employees were not made aware of the specifics of the city order.
  • The facility did not implement procedures or assign responsibilities for calibrating, inspecting, and maintaining the H2S detector.
  • The operator or mechanic performed calibration on an irregular basis, and no records were kept.
  • Lacking written operating procedures and oversight, the WWT operator relied solely on his experience and judgment to perform treatment.
  • None of the onsite personnel or degreed chemists available at the EEI analytical laboratory were consulted on the treatment protocol in use on the day of the incident.

Failed Safeguards or Barrier Breakdowns

  • no written procedures for operating the WWT area
  • no written instructions to warn operators of the hazards of adding treatment chemicals to the clarifier
  • no procedures specifying what to do in the event that a waste failed to meet discharge limits after treatment
  • the clarifier is not equipped with a mixer or agitator
  • the clarifier was not equipped with a scrubber to minimize toxic gas releases
  • the wall-mounted H2S detector did not alarm on December 11
  • the detector had not been calibrated for 2 to 3 months prior to the incident
  • the detector was not functioning because of a bad sensor
  • no procedures or responsibilities for calibrating, inspecting, and maintaining the H2S detector
  • no formal system for investigating incidents and communicating findings to employees
  • no controls to limit access to the WWT area during treatment
  • no notification to facility personnel that WWT operations might present a hazard
  • no training on the hazards of the WWT process or on the properties and hazards of H2S
  • no signs at entrances to the WWT area prior to post-incident remediation

Recommendations

  • None listed in the provided incident JSON.

Key Engineering Lessons

  • Use the proper vessel for waste treatment; the clarifier was not designed for the treatment being performed.
  • Do not rely on air rolling in an open-top vessel when adequate mixing is required to prevent localized reactions.
  • Provide engineering controls to minimize toxic gas releases, including a scrubber where H2S can be generated.
  • Maintain H2S detection through a formal calibration, inspection, and maintenance program.
  • Ensure written operating procedures and training address the hazards of the specific waste treatment chemicals and the consequences of treating waste in the wrong vessel.
  • Communicate incident lessons learned and restrictions such as the city order prohibiting addition of Na2S flake.

Source Notes

  • Consolidated from the final report (source_priority 1) and supporting documents.
  • Priority 1 findings override the supporting-document phrasing where there were overlaps.
  • The supporting documents were used only to supplement context and recommendations; no facts were added beyond the provided extracts.

Similar Incidents

Incidents sharing the same equipment, root causes, or hazard types.

Same Equipment

Same Root Cause

Same Hazard

← View in Knowledge Graph