Safety, Confined Spaces, and Hazardous Conditions
Permit-required confined spaces
Acceptable entry atmosphere: oxygen 19.5–23.5%, flammable gas below 10% LEL, hydrogen sulfide below 10 ppm (many programs use 5 ppm), and carbon monoxide below 35 ppm time-weighted average with a 200 ppm ceiling. Test from outside with a calibrated multi-gas meter, typically checking oxygen, combustibles, then toxics, then ventilate and retest. Roles are entrant, attendant, and entry supervisor; a rescue plan, retrieval system, and continuous monitoring are required. Never make a non-entry rescue into an entry.
Gas hazards
Hydrogen sulfide is heavier than air, accumulates in wet wells and low spots, and deadens the sense of smell at higher concentrations. Methane from digesters and sewers is explosive between about 5% and 15% in air. Chlorine gas is heavier than air with an IDLH of about 10 ppm. Carbon monoxide comes from engines, generators, and heaters.
Other hazards
Lockout/tagout for electrical and mechanical energy; arc flash and wet conditions; excavations 5 ft or deeper need a protective system and competent person; fall protection; slipping and tripping; traffic control at manholes; biological hazards addressed with gloves, eye protection, handwashing, no eating in work areas, and vaccination.
Traps
Trusting an odor as a test, trusting one meter reading taken in the wrong spot, or assuming an enriched oxygen reading is safe - it makes fire far more likely.
Rule of thumb
If a meter or tool drops into a wet well, do not go after it. Retrieve it from outside the space or accept the loss.