Workers throughout the collection system, at lift stations, or within treatment plants are exposed to potential injury from both physical and environmental hazards.
Physical hazards may be chemical, electrical, mechanical, or even environmental. A rotating shaft on a pump, a high-voltage electrical panel, and work in a confined space each presents immediate and typically easy-to-identify physical hazards. On the other hand, the recognition of potential health hazards from exposure of toxic airborne contaminants, such as hydrogen sulfide or ammonia, may be more difficult to quantify.
Health hazards are defined as exposures that can cause a measurable change to the body. It is possible that plant operations may present an occupational exposure to hydrogen sulfide above allowable levels, and protective measures must be addressed. To protect against health hazards, employers should adopt a written hydrogen sulfide exposure control program to communicate the hazards likely found in the workplace, as well as the safe work practices that are needed to minimize or eliminate exposure.
Health hazards due to odors in the collection system mostly will be limited to exposure to hydrogen sulfide gas. Hydrogen sulfide has a distinctive rotten-egg odor, yet at levels at or above 50 ppm, the sense of smell can be lost with prolonged exposure. Exposures above 100 ppm can result in olfactory fatigue in 1 to 15 minutes.
Thus, the body’s physical warning system against this substance is gone, too. Put another way: If you smell the gas, you have already been exposed, and even if the smell goes away, it could still be present. Do not rely on sense of smell to detect hydrogen sulfide.
Because hydrogen sulfide is heavier than air, workers entering confined spaces or the bottom of an excavation during sewer work have a high risk of exposure. The continual presence of sulfides in the wastewater stream means the potential for a hazardous condition to develop is highly likely.
In a low-pH or low-dissolved-oxygen environment in the collection system, sulfur-reducing or sulfate-reducing bacteria will react with components in the water to release hydrogen sulfide gas. In an enclosed space, the hydrogen sulfide, which is heavier than air, will displace the oxygen and push it up and out of the space. Trap the hydrogen sulfide in an enclosed vault, wet well, or manhole, and the concentrations can rise quickly to lethal levels.
Understanding the potential for hydrogen sulfide to be present will enable workers to prepare, and either avoid the situation or mitigate the risk potential.
Putting this into numbers
So, at what point does hydrogen sulfide exposure become harmful? The U.S. Occupational Safety and Health Administration (OSHA) has defined a permissible exposure level (PEL) of 20 ppm for the time-weighted average (TWA), meaning over a worker’s 8-hour shift he or she cannot be exposed to a calculated average of greater than 20 ppm. (The calculation would take into consideration workers consistently working longer shifts, such as four 10-hour shifts instead of a standard 8-hour shift.) OSHA also has a maximum peak exposure limit of 50 ppm, not to exceed 10 minutes if there is no other measurable exposure once exposure occurs.
In addition to the OSHA PEL, which is the enforceable standard by law, the National Institute of Occupational Safety and Health (NIOSH) has issued a recommended exposure limit (REL) of not more than 10 ppm for 10 minutes. The NIOSH level also is what OSHA has included as the acceptable entry condition for hydrogen sulfide as published in Appendix E of the permit-required confined space standard (29 CFR 1910.146). Determining the level of exposure can be done using a toxic gas meter that has a hydrogen sulfide test cell or using colorimetric detector tubes.
Exposure to hydrogen sulfide can have both acute and chronic effects. Acute effects due to exposure at 20 to 50 ppm include irritation of the throat and respiratory passages, as well as blurred vision. Prolonged exposure at 250 to 500 ppm will affect the central nervous system and result in nausea, dizziness, and headaches.
At levels above 500 ppm, the risk of death increases dramatically. Exposure at these levels can lead to respiratory paralysis, irregular heartbeat, and death. Many confined space fatality studies discuss how the entrant was “immediately overcome.” This refers to the breath in excess of 500-ppm hydrogen sulfide the entrant took as he or she entered the contaminated area that caused respiratory paralysis. That breath would be the last he or she would ever take.
Workers also may be exposed to ammonia or other airborne contaminants, such as hydrocarbon fumes, which can emanate from sewers and confined spaces. Enforcement of pollution prevention programs by the local governing agency is the most effective way to control not only hazardous exposure to workers but also potential problems at the treatment facility.
Control of odors to reduce the offensive hydrogen sulfide smell for the public may result in the use of chemicals. In this case, ensure that workers consult the material safety data sheets (MSDS) for proper use and protective equipment, and safe work practices.
Keeping odors from escaping the treatment plant also has become a priority in recent years. The application of treatment systems specifically for odor control limits the exposure of workers in many facilities, yet maintenance activities or tasks in areas not connected to the odor scrubbing systems still present a potential hazard to workers.
Also, the operation and maintenance of the odor scrubbing equipment itself presents chemical and physical hazards to the workers. Many wet air odor scrubbing units use sodium hypochlorite, which presents a chemical hazard. Carbon absorption equipment has powdered carbon that presents inhalation hazards when changing or adding media. And workers may have to climb to an elevated platform to clear clogged spray nozzles, inspect equipment, or take samples, leading to fall hazards requiring structures be built or fall-arresting systems be used.
Finding more information
The U.S. Centers for Disease Control Workplace Safety and Health Topics Web page on hydrogen sulfide (www.cdc.gov/niosh/topics/hydrogensulfide) has a considerable amount of guidance information available to employers. This page also includes links to information on exposure limits, air sampling methods, and exposure control methods.
is Western region health and safety adviser for Severn Trent Services (Fort Washington, Pa.).
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