The high concentration of Hydrogen Sulfide (H2S) gas was caused by the use of biodegradable cleaners and the chemical reactions and resultant H2S byproduct involved in breaking down oils in an anaerobic (low oxygen) environment like that found in a full slop tank.
In this case, vessel officers had detected significant H2S concentrations (>200 PPM) in the vessel’s engine room bilge holding tank. Two hundred (200) PPM of H2S is considered Immediately Dangerous to Life and Health (IDLH). This situation had existed for a long time as the crew had previously reported smelling rotten eggs near the slop tank, which is an indicator of the gas.
Another similar case involved dangerous levels of H2S gas concentrations found on several tug boats. Initially, H2S gas was discovered when personnel began to transfer waste ashore from a pressed up and nearly full slop tank containing typical oily bilge slops.
In order to mitigate and prevent this from happening, USCG recommends the following:
- Review machinery space cleaning practices; clean machinery spaces with detergents when the ship is in port, using fresh water; do not add the washings to the oily-water holding tank but pump them directly to a shore-based disposal facility; avoid the use of biodegradable detergents at sea to prevent their addition to the holding tank.
- Review drainage pathways of all nutrient-bearing "non-oily" wastes and ensure they do not enter the oily-water waste system.
- Keep the sea-water content of the oily-water waste as low as possible; use fresh water for cleaning operations when it is available; handle "emulsified" oily-water waste apart from "non-emulsified" oily-water waste so that separation equipment can be used effectively.
- Be aware of the chemical composition of cleaning agents and other substances entering bilge; use biodegradable detergents sparingly; avoid the addition of phosphates to the oily-water waste; DSTO advice should be sought before new cleaning agents.
- Prevent the oily-water waste from becoming anaerobic; remove bilge from machinery spaces frequently; keep the contents of the holding tank well aerated.
- Do not store oily-water waste on a ship unnecessarily, dispose of it at every available opportunity.
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