Rising H2S in a Sewage Wet Well With a Cycling Pump
A 4.2 m deep wet well at a Hong Kong sewage pumping station has a 600 mm access opening. Two workers are to clear a rag jammed at the submersible pump. The pump restarts automatically on a float control every 8 minutes. Pre-entry readings taken 0.5 m above the sludge are O2 20.6%, LEL 0%, H2S 3 ppm, CO 2 ppm. Two minutes after the pump starts, H2S rises to 12 ppm, O2 falls to 19.8%, then both drift back. The permit-to-work states '30-minute entry; continuous forced ventilation'. The blower is a 300 mm axial fan 15 m from the manhole, with flexible duct discharging at the manhole rim rather than into the well. The standby person is a 19-year-old labourer with no confined-space training, no gas monitor, and only a personal mobile phone. The entry supervisor wants to 'go now while the reading is low'.
1. Should the competent person allow entry while the reading is low? Justify your decision and state the immediate actions before any entry.
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No. The key evidence is the change with the pump cycle: H2S rises from 3 ppm to 12 ppm and O2 falls from 20.6% to 19.8% within two minutes of the pump starting. That is an active, repeating source, not a transient blip; the pump mixes sludge and releases gas from the wet well. The 'low' reading is also taken at only one point, 0.5 m above the sludge, and may not represent the bottom or the headspace near the pump. Therefore the pre-entry assessment no longer matches the permit.
Immediate actions:
- Stop and lock out the pump and its float control so the cycle cannot restart.
- Keep the manhole open but barricade it.
- Ventilate with the duct discharging near the bottom, not at the rim.
- Re-sample at top, middle and bottom after ventilation and again after the pump is proved isolated.
- Set action levels, e.g. O2 19.5-23.5%, LEL 10%, H2S 5 ppm, and a continuous-monitoring regime.
- Revise the risk assessment and permit; only then consider entry with full standby and rescue arrangements.
2. The supervisor says the 12 ppm H2S is 'only for a minute' and the 30-minute permit is still valid. How should the competent person respond, and what is wrong with the ventilation and standby arrangements?
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The competent person should reject the argument. A permit is valid only for the conditions assessed; a repeatable 12 ppm peak and falling O2 are a material change. H2S also deadens the sense of smell and can rise quickly when sludge is disturbed. The 30-minute permit cannot cover a pump cycle that restarts every 8 minutes.
Ventilation: an axial fan 15 m away with duct at the manhole rim short-circuits fresh air across the opening and does not purge a 4.2 m well. The duct should reach near the bottom, the fan intake should be in clean air, and the exhaust should discharge downwind. Ventilation must be continuous and verified by monitoring, not assumed.
Standby: a 19-year-old untrained labourer with no monitor, no harness or tripod and only a mobile phone cannot perform standby duties. The standby must be trained, remain at the entry, maintain communication, monitor the entrants, operate retrieval equipment and summon the rescue team. The standby must not enter.
3. What specific monitoring and emergency arrangements should be written into the revised permit?
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Monitoring: continuous personal multi-gas monitoring for O2, LEL and H2S/CO at the entry point and at the worker's position; pre-entry sampling at three depths; alarm set at O2 19.5% low and 23.5% high, LEL 10%, H2S 5 ppm or the company action level, CO 25 ppm; any alarm means stop work, withdraw and re-assess.
Emergency: a site-specific rescue plan with a tripod/winch and harness on each entrant, a rescue line, trained rescuers on site before entry, a standby with radio and direct access to 999, first-aid arrangements and a briefing or drill. The permit should record pump isolation, ventilation details, entry duration, names and signatures, and the evacuation trigger.