In brief
In manholes, wells and sewers, the atmosphere changes constantly as organic matter in the wastewater decomposes; hydrogen sulphide can build up, oxygen can fall and methane can form an explosive mixture. So gas testing at different depths before entry, mechanical ventilation and continuous monitoring throughout the work, a standby person who never leaves the surface and a tripod arrangement ready for rescue from outside are essential.
Why is manhole work safety treated separately?
Manholes, wells and sewers are typical examples of confined spaces: places with narrow access, naturally inadequate ventilation and not designed for continuous occupancy. We explained the general rules of confined space work in our article on working in confined spaces. Manhole work safety, however, differs from a tank or silo in three respects:
- The atmosphere renews itself. Once a tank has been emptied and cleaned, its atmosphere stays largely constant. In a sewer, wastewater and sludge keep producing gas, and gas coming from another point on the line can also reach the work point.
- The flow can change suddenly. Rain, a pumping station upstream starting up or a factory’s discharge can raise the water level in the sewer within minutes.
- The work area is often on the road. An open manhole cover creates a risk both for the worker inside and for pedestrians and drivers.
Annex 1 of the Regulation on Occupational Health and Safety in Construction Work lists sewer, drain, well and channel work among construction work; Annex 2 lists work involving a risk of drowning and well work among work posing special risks.
H2S poisoning: do not trust your nose
Hydrogen sulphide (H₂S) forms when organic matter in wastewater decomposes without oxygen. When wastewater is agitated, for example at drop structures, pumping stations, points where pressure mains discharge, or when sludge is stirred, the H₂S in the water passes rapidly into the air. Because the gas is heavier than air, it accumulates at the bottom of manholes and wells, at the level where the worker bends down.
Under Annex 1 of the Regulation on Health and Safety Measures in Work with Chemical Substances, the 8-hour limit value for hydrogen sulphide is 5 ppm (7 mg/m³) and the 15-minute short-term limit value is 10 ppm (14 mg/m³).
H2S poisoning cases have one thing in common: the worker treats the smell as a warning. At low concentrations, the rotten egg smell of H₂S is distinct; but as the concentration rises, the sense of smell quickly tires and is paralysed. When the smell fades, people think “the gas has gone”, whereas the opposite may be true. At high concentrations, H₂S causes sudden loss of consciousness and respiratory arrest. The most tragic aspect of these accidents is that the second and third persons who go in to rescue collapse in the same way.
Oxygen deficiency and other gases
In sewers, bacteria consume oxygen, and gases such as methane and carbon dioxide displace the air. In normal air, the oxygen content is about 20.9%. In international practice, for example in the confined space standard of OSHA, the US occupational safety administration, below 19.5% by volume is considered oxygen-deficient. Oxygen deficiency impairs judgement without warning signs; the worker weakens without being able to recognise the danger.
| Gas | Source | Where does it accumulate? | On the detector |
|---|---|---|---|
| Hydrogen sulphide (H₂S) | Decomposition of organic matter without oxygen, agitated wastewater | At the bottom, close to the sludge and water surface | Read in ppm |
| Oxygen deficiency | Consumption by bacteria, other gases displacing the air | At every level, especially in stagnant sections | Read in % |
| Methane | Decomposition of sludge | In upper sections, under the cover | % of the lower explosive limit |
| Carbon monoxide | Exhaust from a generator or vehicle placed next to the manhole | Near the entrance and the ventilation intake | Read in ppm |
| Fuel and solvent vapours | Fuel or chemical discharges entering the sewer | At the bottom; most vapours are heavier than air | LEL sensor or special sensor |
The mixture methane forms with air can ignite; so spark-producing tools, open flames and smoking are not used around the manhole.
Ventilation in sewer work
Natural ventilation is not relied on before sewer work. Leaving the cover open does not provide an air change; heavy gases stay at the bottom.
- Set up mechanical ventilation. Bring fresh air down to the working level with a fan and flexible ducting. Place the fan intake on the side where the air is clean, away from exhaust gases, the generator and the roadway.
- Consider neighbouring covers. Opening a cover upstream and downstream may help the airflow; every cover opened must also be protected against traffic and falls.
- Keep ventilation running without interruption. The fan runs throughout the work; because the atmosphere renews itself, ventilation before entry alone is not enough.
- Do not supply oxygen. Ventilation is done only with clean air; supplying oxygen to the space increases the fire risk.
- Test again after ventilation. The entry decision is based on the test after ventilation.
Gas testing: before entry and throughout the work
Testing is part of the confined space entry permit; the general operation of the permit system is in our article on the permit-to-work system. The steps specific to manholes and wells are:
- Test from outside first. When the cover is opened, test with a pumped detector and a sampling hose without leaning in.
- Test at different depths. Since the upper section is critical for methane and the bottom for H₂S and heavy vapours, repeat the test at upper, middle and lower levels; wait for the hose delay time.
- The person inside carries a personal detector. A detector continuously measuring oxygen, flammable gas, H₂S and CO is clipped to the collar throughout the work; on an alarm, exit without waiting.
- The work changes the atmosphere. Sludge removal, clearing a blockage or a drop in the water level can suddenly increase H₂S; renew the test before these tasks.
- Test the detector daily. A device that fails the bump test is not used; you can track calibration dates with the OHS calendar.
For sensor types, alarm thresholds and test records, see our article on confined space gas testing.
Duties of the standby person at the surface
In well and manhole work, the standby person waiting at the surface is the lifeline of the person inside. The standby person’s duties should be in writing, and they should not be given any other work:
| The standby person does | The standby person does not |
|---|---|
| Stays at the surface from entry to exit | Go in to rescue the person inside |
| Records how many people are inside and who they are | Leave their post, even briefly |
| Maintains continuous contact with the person inside by voice, radio or rope signals | Help with another job, for example carrying materials |
| Monitors detector alarms and changes in air and flow | Treat an alarm as a “device fault” and carry on working |
| Protects the area around the cover with barriers, keeps pedestrians and vehicles away | Leave an open cover unprotected |
| In danger, calls those inside out, calls 112 and starts rescue from outside | Attempt an unplanned intervention before the rescue team arrives |
In work inside a well, the worker wears a full-body harness attached to the winch line on a tripod; so if they lose consciousness, the standby person can pull them up without going in. Before work inside a well begins, the tripod must be set up, the winch tested and the line attached. Our article on confined space rescue guides you on the rescue arrangements.
For work on the road, the area around the cover should be marked and cordoned off so that drivers can see it from a distance; for the form of signs, see our article on health and safety signs.
Wastewater plant safety: biological risks and hygiene
Wastewater plant safety is not limited to gas. Wastewater and sludge carry micro-organisms that can cause infections such as hepatitis A, leptospirosis and tetanus. The Regulation on the Prevention of Risks of Exposure to Biological Agents requires this exposure to be addressed in the risk assessment; for risk groups, see our article on biological agents and risk groups.
- Use waterproof gloves, boots and work clothing; set aside a place for showering and changing clothes after work.
- Wash hands before eating, drinking and smoking; cover wounds with waterproof plasters.
- The workplace physician decides which vaccinations are needed according to the risk assessment; for planning, see our article on the workplace vaccination programme.
- At treatment plants, provide guardrails and lifebuoys at the edges of aeration and settling tanks; falling into a tank carries a risk of drowning.
For infrastructure contractors in Bursa and neighbouring provinces, companies working for municipalities and water authorities, and industrial businesses with their own treatment plants, we set up the permit, testing and standby arrangements for manhole and well work in our OHS specialist service. Our approach to construction work is on our construction page, and the general framework of the regulation is in our article on the OHS regulation for construction work.
Frequently Asked Questions
Which gases should be tested before entering a manhole?
At least oxygen, flammable gas (percentage of the lower explosive limit), hydrogen sulphide and carbon monoxide should be tested. Testing is done with a pumped detector without leaning in, at upper, middle and lower levels, and continued throughout the work with a personal detector.
If the H2S smell has gone, is the space safe?
No. At high concentrations, H₂S paralyses the sense of smell; not smelling it may indicate not that the gas has gone but that the concentration has risen. The decision is made only on the basis of testing.
What should the standby person do if the person inside collapses?
They do not go in. They call for help, call 112 and try to pull the person up from outside with the tripod winch. Only a trained rescue team with suitable breathing apparatus and rescue equipment goes in.
Is a filter mask enough for sewer work?
No. A filter mask does not supply oxygen, and its protection is limited at high H₂S concentrations. The atmosphere should be made safe by ventilation; if that is not possible, air-supplied breathing apparatus should be used.
A certified specialist matched to your hazard class: site inspections, approved logbook, training plan, annual plan and report follow-up.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.