Sewer gas: the warning system that switches itself off

TL;DR. Sewer gas monitoring means measuring the atmosphere inside a wet well, manhole or rising main chamber continuously, rather than sniffing it with a four-gas meter on the morning of an entry. The gas that matters most is hydrogen sulphide. It is produced by bacteria in the sludge layer, it paralyses your sense of smell at roughly the concentration where it starts to be dangerous, and it converts into sulphuric acid on the roof of the pipe. The safest chamber is the one nobody has to open.

Key takeaways

• Hydrogen sulphide has a workplace exposure limit of 5 ppm over 8 hours and 10 ppm over 15 minutes, set by HSE in EH40/2005.
• People lose the ability to smell hydrogen sulphide after roughly 2 to 15 minutes of exposure at about 100 ppm, according to ATSDR.
• Levels above 500 ppm can cause sudden unconsciousness or death, so smell is never a usable warning system.
• The Confined Spaces Regulations 1997 require that nobody enters a confined space if the work can reasonably be done without entry.
• Sulphate-reducing bacteria generate the gas in the sludge layer; other bacteria oxidise it to sulphuric acid on the pipe crown, which is why sewers corrode from the top down.
• Remote level and flow telemetry removes most routine reasons to open a chamber at all.

Last updated: 20 August 2026

What is sewer gas, and what is actually in it?

Sewer gas is the mixture of gases that collects in the headspace above the flow in a drain, manhole, wet well or rising main chamber. Sewer gas is not one substance. It is typically methane, carbon dioxide, ammonia, a depleted oxygen fraction and, the one that does the damage, hydrogen sulphide. The proportions shift with temperature, retention time and how much organic load is sitting still.

Hydrogen sulphide matters more than the rest of the sewer gas mixture because of a cruel piece of physiology. ATSDR records that people can usually smell it at concentrations from 0.0005 to 0.3 parts per million, which is far below any level that would hurt them. It is famously the smell of rotten eggs. But the same source records that people lose the ability to smell the gas after about 2 to 15 minutes of exposure at around 100 ppm.

Very high levels, above 500 ppm, can result in sudden unconsciousness or death. So the sequence a worker actually experiences is: a strong smell, then no smell, then nothing at all. The nose stops reporting at exactly the point the reading starts to matter. That single fact is the entire argument for instrumenting sewer gas rather than trusting human judgement.

What are the legal limits for hydrogen sulphide in the UK?

HSE sets workplace exposure limits for hydrogen sulphide in EH40/2005, the list used with the Control of Substances Hazardous to Health Regulations 2002. The long-term limit, averaged over 8 hours, is 5 ppm or 7 mg/m³. The short-term limit, averaged over 15 minutes, is 10 ppm or 14 mg/m³. Those are the numbers any sewer gas risk assessment has to work to.

Put the two sets of figures side by side and the gap is stark. The short-term legal limit is 10 ppm. The concentration that switches off the human warning system is around 100 ppm, ten times higher. There is no operating window in which a nose is a compliant sewer gas detector. HSE publishes the full table in EH40/2005 Workplace exposure limits, fourth edition, 2020.

The second piece of law is the Confined Spaces Regulations 1997. Regulation 4 is unusually blunt: no person at work shall enter a confined space to carry out work for any purpose unless it is not reasonably practicable to achieve that purpose without such entry. HSE defines a confined space as one that is both enclosed or largely enclosed and carries a reasonably foreseeable risk of fire, explosion, loss of consciousness, asphyxiation or drowning. A wet well hits at least three of those.

How does hydrogen sulphide form in a sewer?

The hydrogen sulphide fraction of sewer gas forms wherever wastewater goes anaerobic and sits. Sulphate-reducing bacteria live in the slime layer and the settled sludge below the waterline. With no dissolved oxygen available they use sulphate as the electron acceptor instead, and the by-product is dissolved sulphide. Warm, slow, long-retention conditions push the reaction along. A rising main that holds its contents between pump starts is close to a purpose-built reactor for it.

The dissolved sulphide then partitions out of the liquid into the headspace as a gas, especially wherever the flow is agitated: a pump discharge, a drop shaft, a manhole with a step in it. That is why sewer gas concentrations are rarely uniform along a network. They spike at the turbulent points, which are also the points people are most likely to be working on.

Once the gas is in the headspace a second group of bacteria takes over. Work published in Applied and Environmental Microbiology traces the succession of sulfur-oxidising bacteria that colonise corroding sewer concrete. They settle on the damp crown of the pipe, oxidise the hydrogen sulphide and produce sulphuric acid directly onto the concrete. The concrete does not corrode uniformly. It corrodes from the top down, above the waterline, in the part of the asset nobody sees during a routine flow check. A sewer can be structurally sound at the invert and losing its crown at the same time.

Why is a four-gas meter on entry day not enough?

A portable detector tells you what the sewer gas is doing in the minutes around an entry. It does not tell you what it did at 4am on the hottest night of the year, or what it does every time the pump downstream starts. Sewer gas is event-driven, and the events are exactly the ones nobody is standing there to see.

That gap has three practical consequences. Odour complaints get investigated after the fact, with no sewer gas record for the period being complained about. Corrosion is discovered at the point of structural survey rather than predicted. And entry risk is assessed from a single sample rather than from a profile, so a chamber that is benign at 10am and hostile at 2am is classified on the 10am reading.

How does continuous monitoring change the work?

Continuous sewer gas monitoring changes the question from what is in the chamber right now to what this chamber normally does. Once you have a time series, a spike has a shape, a time of day and a correlation with pump starts or rainfall. That is enough to act on the cause rather than the symptom.

The bigger prize is the one regulation 4 points at. Most routine visits to a wet well are made to find out a number: how high is it, is the pump running, is it surcharging. Those numbers can be measured from outside the atmosphere. The AQUAIOT Radar is a 60 GHz non-contact level sensor accurate to within ±2 mm that reads through foam, vapour and condensation, with ATEX variants for hazardous environments. It never touches the liquid and it never needs anyone in the chamber to take a reading.

That is the honest version of the safety case. Instrumentation does not make a confined space safe. It removes the reason to enter it. AQUAIOT’s sewer monitoring service applies the same radar sensing across wet wells, rising mains, manholes and combined sewer overflows, with encrypted cellular and LoRaWAN telemetry and threshold alerts through the AQUAIOT Cloud, so the number arrives at a desk instead of being fetched from a chamber.

Where should you monitor first?

Start where the sewer gas is being generated and where it is being released, not where the complaints are loudest. In practice that means four asset types, in this order.

1. Rising mains with long retention between pump starts, because that is where the sulphide is produced.
2. The receiving manhole at the end of a rising main, because that is where the gas comes out of solution.
3. Wet wells and pumping station chambers, because that is where people go.
4. Drop shafts and any chamber with a fall in it, because turbulence strips sulphide into the headspace.

On an estate rather than a public network, the same logic applies to private pumping stations, package treatment plants and interceptor chambers. The asset is smaller. The physiology is identical.

Frequently asked questions

What is a safe level of sewer gas?

HSE sets a long-term workplace exposure limit of 5 ppm averaged over 8 hours and a short-term limit of 10 ppm averaged over 15 minutes in EH40/2005. Those are exposure limits for workers, not a description of a safe atmosphere. A chamber can be well above them and still smell of nothing at all, because the sense of smell fails around 100 ppm.

Why can you not smell sewer gas at dangerous levels?

The hydrogen sulphide in sewer gas is detectable by smell from about 0.0005 ppm, but it deadens the olfactory nerve with continued exposure. ATSDR records that people lose the ability to smell it after roughly 2 to 15 minutes at around 100 ppm. The strong smell disappearing is not a sign the gas has cleared. It is often a sign the concentration has risen.

Does sewer gas monitoring remove the need for a confined space permit?

No. Sewer gas monitoring does not change the classification of the space or replace a permit system, and regulation 4 of the Confined Spaces Regulations 1997 still applies. What continuous sewer gas monitoring does is remove routine reasons to enter, because level, flow and surcharge data can be read remotely. Fewer entries, each better informed, is the realistic outcome.

Why do sewers corrode at the top rather than the bottom?

Because the damage is done by a gas, not by the flow. Sulphate-reducing bacteria produce hydrogen sulphide below the waterline, the gas moves into the headspace, and a second group of bacteria on the damp pipe crown oxidises it into sulphuric acid. The acid attacks the concrete above the flow line, so the crown thins while the invert stays sound.

The point of the exercise

Sewer gas is a problem with two victims: the asset it corrodes and the person sent to look at it. The asset is being eaten from the crown down by an acid that forms in the air, and the person is being asked to assess an atmosphere with a sense that switches off at the wrong moment. Neither problem is solved by visiting more often.

If you are opening chambers to find out a level, that is the first thing to stop doing. Talk to an AQUAIOT engineer about instrumenting the wet wells and rising mains on your site so the reading comes to you.

By GP, Digital and IoT lead at AQUAIOT.