May 2011, Vol. 23, No.5

Operator Essentials

What every operator should know about confined-space entry

Compiled by Steve Spicer


Click here  for a PDF of this article.



A practical consideration

Confined space

According to the U.S. Occupational Safety and Health Administration (OSHA), the term confined space  means any space that

  • is large enough and so configured that an employee can physically enter and perform assigned work,
  • has limited or restricted means for entry or exit, and
  • is not designed for continuous employee occupancy.

Examples of confined spaces that wastewater operators encounter at treatment plants include tanks, sludge pits, wet wells, disinfection basins, funnel-shaped process vessels, materials silos, headworks areas, and bar screens. Collection systems vary widely, depending on service areas, but the common spaces include interceptors, manholes, large-diameter water lines, air-release vaults, valve pits, and stormwater lines. Lift stations pose unique confined-space challenges and should be included in any evaluation of confined spaces.

Permit-required confined space

This type of confined space has one or more of the following characteristics:

  • It has or could have a hazardous atmosphere.
  • It does or could contain a material that could engulf an entrant.
  • It has an internal configuration such that an entrant could be trapped or asphyxiated by inwardly converging walls or by a floor that slopes downward and tapers to a smaller cross-section.
  • It contains any other recognized serious safety or health hazard.

The permit identifies the dangers of the space and describes a safe way to perform the entry. It is a written plan for safe entry stating who, how, and under which specific conditions certain procedures will be required.


All confined spaces are permit-required until testing proves otherwise. Put another way, “Better safe, than sorry.”


Maintenance or work activities being conducted in a confined space may create a permit-required confined-space entry.

Atmospheric testing

Before anyone enters a confined space, the internal atmosphere must be tested for oxygen content, flammable gases, and potential toxic air contaminants, in that order.


The personnel entering the space should be involved in the testing.


Detector tubes, alarm-only gas monitors, and explosion meters are examples of monitoring equipment that may be used to test permit space atmospheres.


Broad-range sensors are best suited for pre-entry testing where the hazards have yet to be determined, because they provide a broad overview of whether hazardous contaminants and conditions exist. However, once the contaminants and/or conditions have been identified, substance-specific sensors work better, since they provide actual measurements of specific substances.

Space ventilation

Sometimes, mechanical ventilation may be used to eliminate or prevent atmospheric hazards in confined spaces. In these cases, the space can be entered without a permit after testing.

Ventilation systems should blow in 100% outside air whenever possible. Opening additional manholes to allow circulation also helps. Do not operate any internal combustion engines near the intake for mechanical ventilation systems.


If there are other hazards present, ventilation alone is not enough to change a permit-required space to a nonpermitted space.


Oxygen concentrations below 19.5% or above 23.5% are hazardous.


Safe entry conditions regarding oxygen are when atmospheric monitoring shows concentrations between 19.5% and 23.5%.

Normal air has about 21% oxygen.


Lower oxygen concentrations produce increasingly severe reactions:

  • 16% to 12% — increased breathing rate, accelerated heartbeat, and impaired attention, thinking, and coordination;
  • 14% to 10% — faulty judgment, intermittent respiration, and exhaustion with minimal exertion;
  • 10% to 6% — nausea, vomiting, lethargic movements, and perhaps unconsciousness;
  • Less than 6% — convulsions and cessation of breathing, followed by cardiac standstill. (These symptoms occur immediately.)


Too much oxygen increases explosion hazards.

Carbon monoxide

Carbon monoxide is a toxic, flammable, odorless, colorless gas.


Safe entry conditions for carbon monoxide are when atmospheric monitoring shows less than 35 ppm (25 ppm in California).


The OSHA permissible exposure limit for carbon monoxide is 50 ppm for 8-hour time-weighted average.


Carbon monoxide is flammable in air at concentrations between 12.5% and 74%.

Carbon monoxide is toxic because it prevents oxygen from being absorbed by red blood cells.


Symptoms of acute exposure include headache, flushing, nausea, vertigo, weakness, irritability, unconsciousness, and — in persons with pre-existing heart disease and atherosclerosis — chest pain and leg pain.

Hydrogen sulfide

Hydrogen sulfide is a colorless, flammable, extremely hazardous gas with a “rotten egg” odor.


Safe entry conditions for hydrogen sulfide are when atmospheric monitoring shows less than 10 ppm.


The OSHA permissible exposure limit for hydrogen sulfide is 20 ppm.


The gas is highly flammable and explosive at concentrations between 4% and 45% in air.

This is the main hazardous gas found in wastewater systems. It is heavier than air and tends to sink and pool in pipes and tanks.


Even though hydrogen sulfide can be detected by the nose at concentrations as low as 0.5 ppb, it quickly renders your nose unable to detect it. Do not rely on smell to detect this gas.


Low-level exposure symptoms are irritated eyes, nose, and throat.


High-level symptoms are shock, convulsions, coma, and death.


Isolation refers to the process by which a permit-required confined space is removed from service and protected against the accidental release of energy and/or hazardous material into the space.


Isolation alone is not enough to change a permit-required space to a nonpermitted space.

Double-block and bleed isolation is commonly used in wastewater lines. This means that two valves in-line and upstream from the confined space have been closed, the section between them emptied of flow, and a valve or vent within the emptied space has been locked open.


Line blanking also achieves isolation by bolting a plate and gasket over the end of a pipe. This plate must be capable of meeting the maximum intended pressure of the pipe. Displacement also can be an effective isolation method: Flow is simply physically diverted away from the confined space.


Closing a single valve and placing locks and tags on the valves controlling flow into a space are not enough to isolate a space.

Lockout and tagout

This is the process by which electrical and mechanical equipment within a confined space is removed from service during the entry.


Lockout and tagout alone are not enough to change a permit-required space to a nonpermitted space.

Where isolation deals with material entering the space, lockout and tagout deal with electricity, mechanical hazards, and a wide range of other forms of stored energy.


You can lock out and tag out pumps and motors or, sometimes, you can remove the chains or drive belts that transfer the energy to the moving parts.


Every worker entering a confined space must wear a full body harness attached to a lifeline that is secured outside the space.


A mechanical device must be available to retrieve personnel from vertical permit spaces more than 1.5-m (5-ft) deep.

Lifelines enable workers outside the confined space to rescue workers inside without entering a potentially hazardous atmosphere. And lifelines may help prevent falls from simple slips when descending into manholes or other ladder-accessible spaces.

Attendant duties

At least one worker, the attendant, must remain immediately outside the space to monitor those within the space.

Attendants must be well-trained and able to identify behavioral symptoms of entrants in confined spaces. They must know all of the possible hazards that may be faced during the entry, and they must constantly monitor the entrant. If there is a problem, they must be able to remove the entrant immediately.


The attendant must first try to remove the entrant using his or her lifeline. The attendant may only enter the space if approved by the employers’ confined-space rescue policy; if fully trained, equipped, and qualified to perform entry rescue operations; and only after calling 911 and having a second/backup attendant onsite.

Topside safety

Open manholes or tank lids pose hazards to workers or passersby not entering the confined space. Erect railings or other barriers to prevent inadvertent entry of people, tools, or equipment.

In addition to protecting people topside, this step also prevents objects or people from falling on workers in the confined space.


Also, keep in mind that when hazardous gases are being blown out of the confined space, consideration must be given to the dispersion of those gases.

Doug Prentiss , president of Doug Prentiss Inc. (Alachua, Fla.); Stephen Davenport, safety coordinator for the City of Stillwater, Okla.; and John Bannen, Western region health and safety advisor for Severn Trent Services (Fort Washington, Pa.) contributed to this article.


©2011 Water Environment Federation. All rights reserved.