Sewers, drainage and stormwater

Why do sewers produce hydrogen sulphide, and what does it damage?

• It is a residence time problem. The longer sewage sits without oxygen, the more sulphide forms.
• Rising mains are the classic source. A full, sealed pipe with a long retention time is close to ideal conditions.
• The damage happens above the waterline. Bacteria on the crown convert H2S to sulphuric acid, which attacks concrete.
• Smell is an unreliable detector. H2S deadens the sense of smell at higher concentrations, so the warning fades as the hazard rises.

Where it forms, and why rising mains are the worst case

Fresh sewage arrives with dissolved oxygen in it. While that oxygen lasts, the bacterial community stays aerobic and no sulphide is produced. Once it is exhausted, sulphate reducing bacteria in the slime layer on the pipe wall begin converting sulphate into dissolved sulphide.

That makes retention time the controlling variable. A gravity sewer running briskly and part full keeps reaerating at the surface, so it tends to stay aerobic. A rising main is the opposite case, because it runs full, sealed from the atmosphere, and holds its contents for as long as the pumping cycle dictates. A station that fills slowly and pumps infrequently gives the bacteria the longest possible window.

The gas then comes out of solution wherever the flow becomes turbulent, which is why the discharge point of a rising main, and the chamber immediately downstream of it, are usually where the corrosion and the odour complaints concentrate.

What it damages

The corrosion mechanism is indirect and that is what makes it easy to underestimate. H2S gas itself is not what destroys the pipe. The gas reaches the damp concrete above the waterline, where a different group of bacteria oxidise it into sulphuric acid directly on the surface.

The result is that the crown and the exposed walls of a chamber degrade while the invert, permanently submerged, can look sound. Concrete loses surface material and softens, exposed reinforcement corrodes, and manhole steps and metalwork deteriorate. Because the damage is above the normal flow line it is easy to miss on a survey that focuses on the pipe itself.

There is a safety dimension as well. H2S is toxic, heavier than air, and therefore collects in exactly the low, confined spaces that people are asked to enter. Confined space entry into chambers is a controlled activity for this reason.

Why smell is a poor detector

At low concentrations H2S has a very distinctive rotten egg odour and people detect it readily. That is precisely why it gets treated as self alarming, and why that assumption is dangerous.

At higher concentrations H2S causes olfactory fatigue, deadening the sense of smell. The practical effect is inverted from what anyone would want: the warning weakens as the concentration rises, so the absence of smell cannot be read as the absence of gas.

Continuous gas monitoring in the chamber removes the reliance on human perception entirely. Read alongside level and pump run data it also becomes diagnostic rather than just protective, because a rise in H2S that tracks longer pumping intervals points straight at retention time as the thing to change.

What reduces it

Because it is a residence time problem, the most durable fixes shorten the time sewage spends without oxygen. Reviewing pump control to cycle more often with smaller volumes, removing unnecessary storage, and correcting the I&I that dilutes and slows a network all help.

Where the hydraulics cannot be changed, chemical dosing is the usual route, whether that is nitrate to give the bacteria an alternative to sulphate, or an iron salt to lock sulphide up as a precipitate. Dosing is only as good as the data behind it, which is why it pairs with continuous monitoring rather than replacing it.