Where hydrogen sulfide is found

Hydrogen sulfide forms in two main ways. Biologically, bacteria that live without oxygen, called sulfate-reducing bacteria, use sulfate as a substitute for oxygen and give off hydrogen sulfide. They thrive in sewage, manure, sludge, stagnant water, wet organic waste and the bottoms of tanks. Geologically, over millions of years, reactions in deep reservoirs between hydrocarbons and sulfate minerals, along with bacterial action, have produced sour oil and gas. Industrial processes also create or release it: pulp making, leather tanning, petroleum refining, viscose rayon production and some mining and metal refining operations. Knowing the source tells you where the gas will be released and when.

Oil and gas

In Canada, sour gas is concentrated in the Western Canada Sedimentary Basin, especially in Alberta and northeastern British Columbia, where some deep reservoirs contain more than 20 per cent hydrogen sulfide and a few contain much more. Saskatchewan's heavy oil and conventional oil fields also produce sour gas and oil, and solution gas from oil batteries can be very sour. Gas processing plants strip hydrogen sulfide from the gas and convert it to elemental sulfur, which is why large yellow sulfur blocks and piles are a feature of the Alberta landscape.

Hydrogen sulfide in oil and gas operations can appear at almost every stage:

Drilling: sour zones can release gas into the drilling fluid, which brings it to surface at the shale shakers, mud tanks and flare line. Well testing and completions: drill stem tests, swabbing, flowback and fracturing operations bring reservoir fluids to surface. Production: separators, treaters, tanks, compressors and pipelines carry sour gas, oil and produced water. Trucking: drivers load and unload sour crude and produced water and open hatches to gauge tanks. Gas processing: plants concentrate hydrogen sulfide in acid gas streams before converting it to sulfur. Maintenance: opening vessels, changing filters and pigging lines can release trapped gas and iron sulfide.

Detailed well-site and facility procedures are covered in HSE-331 and HSE-332. For awareness purposes, the most common exposure points in production are tank hatches, sample points, vents, open-ended lines, filter housings, pig launchers and receivers, and any vessel being opened for maintenance.

Sewers and wastewater

Sewage contains sulfate from drinking water and organic sulfur compounds from human waste. When sewage sits without oxygen, sulfate-reducing bacteria in the slime layer on pipe walls produce hydrogen sulfide. Long force mains, where pumped sewage sits in a full pipe for hours, are notorious. Warm weather speeds the process. The gas dissolves in the sewage and comes out where the flow becomes turbulent.

Typical release points in a collection system and treatment plant include: wet wells at lift stations, especially when pumps start; the discharge end of force mains; drop manholes and hydraulic jumps; siphons and junction chambers; headworks and screening rooms at treatment plants; grit chambers; sludge holding tanks, thickeners and digesters; and dewatering buildings where sludge is pressed or centrifuged.

Hydrogen sulfide also causes major damage to sewers. Bacteria on the moist crown of concrete pipes and manholes convert the gas to sulfuric acid, which eats away the concrete, a process often called crown corrosion. Workers may see soft, pasty concrete and exposed reinforcing steel. The same chemistry corrodes electrical equipment and instruments in pump stations, which can disable fans, alarms and gas detectors.

Agriculture: manure storage

Liquid manure stored in pits under barns, in outdoor tanks and in lagoons generates hydrogen sulfide, methane, ammonia and carbon dioxide. Concentrations in the air above stored manure are usually modest while the manure sits undisturbed. The danger comes when the manure is agitated before pumping out. Agitation can release hydrogen sulfide in a burst, driving concentrations in a barn or pit from a few ppm to hundreds or thousands of ppm within minutes. Livestock and people in the barn above an under-floor pit have died during agitation.

Manure gas incidents often involve family members and neighbours on small farms, where formal confined space programs may not exist. The farm safety associations in every province and state publish guidance with the same core rules: never enter a manure pit or tank without testing, ventilation, breathing apparatus and a rescue plan; ventilate barns and keep people and animals out during agitation; and never attempt a rescue without breathing apparatus.

Pulp and paper

In the kraft pulping process, wood chips are cooked in a solution containing sodium sulfide. Hydrogen sulfide and other reduced sulfur compounds, such as methyl mercaptan, are released in digesters, blow tanks, evaporators, recovery boilers and the collection systems that gather these gases. The characteristic smell of a pulp mill comes from these compounds. Workers can be exposed during maintenance on the gas collection system, in the sewer and effluent treatment areas, and during upsets. Mills that use sulfite processes or treat sludge in lagoons have similar hazards.

Other industries

Landfills deserve a note. When gypsum drywall is buried in wet conditions, the sulfate in it feeds bacteria that produce hydrogen sulfide. Construction and demolition landfills have had odour complaints and occasional high readings in trenches and leachate systems because of this.

When releases happen

Across all these settings, hydrogen sulfide exposures cluster around certain activities. Recognizing these triggers is often more useful than memorizing a list of industries:

Opening: hatches, manways, valves, flanges, filter housings, sample points and bleed valves. Agitating: mixing manure, stirring sludge, cleaning tanks, running pumps or jetting sewers. Pumping and transferring: loading and unloading trucks, emptying wet wells, draining vessels. Heating: cooking, steaming out vessels, warm weather. Changing chemistry: adding acid to sulfide-containing waste, which releases hydrogen sulfide very quickly. Entering: confined spaces, pits, trenches, vaults and holds. Equipment failure: leaks, ruptures, failed seals, power failures that stop ventilation.

Acid and sulfide: a dangerous mix

One trigger deserves special attention. Many wastes and process streams contain dissolved sulfide that stays in solution while the liquid is alkaline. If acid is added, the sulfide converts to hydrogen sulfide gas almost instantly. Deaths have occurred when acidic waste was dumped into a sewer or tank containing sulfide waste, and when cleaning chemicals were mixed in drains. Tanneries, chemical plants, oil field waste handling and vacuum truck operations are at particular risk. The rule is simple: never mix unknown wastes, keep acids away from sulfide-bearing wastes, and check compatibility before transferring liquids into a tank or truck.

# Fraction of dissolved sulfide present as H2S gas-forming species at different pH
# (first dissociation constant pKa1 about 7.0 at 25 C; second step ignored).
pKa1 = 7.0
for pH in (5, 6, 7, 8, 9, 10):
    frac_h2s = 1 / (1 + 10 ** (pH - pKa1))
    print(f"pH {pH:2d}: {frac_h2s*100:5.1f} % of dissolved sulfide as H2S")

pH  5:  99.0 % of dissolved sulfide as H2S
pH  6:  90.9 % of dissolved sulfide as H2S
pH  7:  50.0 % of dissolved sulfide as H2S
pH  8:   9.1 % of dissolved sulfide as H2S
pH  9:   1.0 % of dissolved sulfide as H2S
pH 10:   0.1 % of dissolved sulfide as H2S

The calculation shows why pH matters so much. At pH 9 or 10, almost all dissolved sulfide is in an ionic form that stays in the water. At pH 7, about half is hydrogen sulfide. Below pH 6, nearly all of it is hydrogen sulfide, ready to escape into the air. A small amount of acid in a large volume of sulfide waste can turn a low-odour liquid into a lethal gas source.

Building an inventory

A good hydrogen sulfide program starts with an inventory of where the gas could be present and what activities could release it. For each location, the inventory should record the source, the expected concentration range from monitoring data or process information, the release triggers, the controls in place and the training and equipment needed. The inventory feeds the hazard assessment, the detector placement plan, the confined space list from HSE-211 and the emergency response plan. Lesson 12 builds such an inventory for an invented site.