Where does hydrogen sulphide occur and how is it formed?

H2S is a putrefaction gas.

It is formed when organic material breaks down in an oxygen-poor environment. We call that anaerobic conditions. Think of manure cellars, sewage sludge, digestion tanks or standing water. 

Organic material to think of includes: manure and slurry, sewage sludge, food residues, plant waste, paper pulp, animal remains, biological residual streams in digesters,  organically polluted surface water. All these materials contain carbon compounds that micro-organisms use as an energy source.

And sulphate is an oxidised form of sulphur. Chemically: SO₄²⁻.
It occurs naturally in: groundwater, seawater, soils, manure, industrial waste water and sulphur-bearing minerals In water treatment plants and sewer systems sulphate is therefore almost always present. That forms the basis for H2S formation. And that is the gas we measure when we measure hydrogen sulphide.

In an oxygen-poor environment bacteria switch to electron acceptors other than oxygen. You can see that in the so-called redox ladder in the image below. Formation takes place through microbial processes. Sulphate-reducing bacteria use sulphate as an electron acceptor. In doing so they convert sulphate into sulphide. This is a redox reaction. The end product is hydrogen sulphide gas.

That is why you often see H2S together with methane. Methane is formed by methanogenic micro-organisms in the same anaerobic environment. Both gases are released during digestion, putrefaction and biological breakdown. But as you can see, methanogenesis sits lower on the redox ladder than sulphate reduction. That means micro-organisms such as bacteria gain more energy when they reduce sulphate. So: as long as sulphate is present, sulphate reduction will be energetically more favourable than methanogenesis. In other words: only once the sulphate has run out does methane formation increase sharply. That is why in many systems you first see H2S formation and then dominant methane production. And measuring H2S can be done perfectly well with an electrochemical sensor, whereas we measure methane with an LEL sensor.

Typical locations where H2S (and therefore also methane) is released:

• Sewage pumping stations
• Water treatment plants
• Manure storage
• Biogas plants
• Oil and gas extraction
• Refineries
• Tank storage

Because the gas is heavier than air, it can accumulate at working height in confined spaces. That makes measurement beforehand and during the work crucial.

H2S is toxic.
Methane is explosive.

Together they form a combination of asphyxiation hazard, poisoning and explosion risk. For the reasons above, hydrogen sulphide is also best measured at various heights. It may well be that at the opening of a sewer low values are measured at the top. However, the further down you climb, the higher the readings rise, because it is a heavy gas. Measuring H2S is therefore not simply a matter of wearing a device and getting on with the job.

Dutch limit value for H2S

In the Netherlands clear rules apply to exposure to hydrogen sulphide. The aim is to protect employees against the harmful effects of H2S, both in the long and the short term.

Time-weighted average (TWA-8 hours)
The maximum average exposure over an 8-hour working day is 2.3 mg/m³ (~1.6 ppm). Do you work for longer periods in a manure cellar, sewer, tank or at a waste water plant? Then you must not exceed this limit. A gas detector will not only measure hydrogen sulphide on the basis of acutely dangerous concentrations. The device will also alarm when you have been exposed to too high a daily quantity.

Short-term exposure (STEL)
For short peaks there is a guide value of approximately 10 ppm (14 mg/m³) over 15 minutes. Higher concentrations can be dangerous or fatal within minutes. Every gas detector will measure a STEL for H2S, so that you know whether you have had too high an exposure over the last 15 minutes.

Why measuring is mandatory
Because H₂S can deceive the nose, smelling is not a safety strategy. The gas can accumulate low down near the ground or in confined spaces. Only a reliable gas meter gives certainty where measuring H2S is concerned .

Practical points for employers and field workers regarding H₂S

• Always check the air before entering a pit, cellar or tank. Hydrogen sulphide can in fact also be measured with a pumped gas detector. That way you can know the H2S concentration before you enter a space. 
• Use personal H2S detectors.
• Document the measurements for the Dutch Working Conditions Act and internal safety.

Tip: for temporary work or occasional checks it is handy to rent an H2S detector. That way you have professional measuring equipment available without any investment, and you meet all statutory requirements. In many cases a standard 4-gas detector such as the Riken Keiki GX-3R is the best choice. That way you are immediately protected against methane (CH4), oxygen (O2), and explosion hazard (LEL). Measuring H2S with a pump? Then you can opt for an additional Riken Keiki RP-3R, or for example a WatchGas POLI.