GUIDE

Why Force Mains Make Hydrogen Sulfide, and What Stops It

A force main is a pressurized pipe that pumps sewage from a lift station to the next point in a collection system rather than letting gravity carry the flow. It runs completely full, with no air space above the water, and can hold wastewater for hours between pump cycles while a station waits to reach its next start level. That combination, no air and long detention, gives the bacteria in the wastewater time to use up dissolved oxygen, then nitrate, then sulfate, which is what produces the dissolved sulfide that becomes hydrogen sulfide (H2S) gas. Because the main runs under pressure the whole way, that gas does not release until the flow reaches the receiving manhole, which is usually where the odor complaint and the corrosion both show up, well downstream of where the chemistry actually happened. This guide covers what a force main is, why it builds hydrogen sulfide, how to tell a force main is the source, what to measure, and how the control options compare.

The gas forms along the pipe and shows up at the far end.

A force main is a pressurized pipe that moves wastewater from a lift station to the next point in a collection system rather than relying on gravity flow.
A force main runs completely full, with no air space above the wastewater the way a gravity sewer has.
Wastewater in a force main can sit still for extended periods between pump cycles while it waits to be pushed forward.
Bacteria in wastewater use dissolved oxygen first, then nitrogen compounds such as nitrate, and turn to sulfate only once both are exhausted.
Dissolved sulfide stays in solution inside a pressurized pipe and converts to hydrogen sulfide gas where the flow depressurizes and turns turbulent, typically at a receiving manhole.
Hydrogen sulfide gas reacts with moisture on concrete and steel above the waterline to form sulfuric acid, which corrodes the crown of a pipe and a manhole structure over time.
The Occupational Safety and Health Administration publishes an airborne exposure ceiling for hydrogen sulfide that applies at a receiving manhole regardless of what produced the gas.

Published September 24, 2026

What is a force main?

A force main is a pressurized pipe that pumps sewage from a lift station to the next point in a collection system, instead of letting gravity carry the flow. Because pumps push the water into it, a force main runs completely full, with no air space above the wastewater the way a gravity sewer has. Pumps cycle on and off, so water already in the pipe can sit still for hours between cycles, waiting for the next run to push it forward. A full pipe, no air, and long stretches of standing water: that combination is what makes a force main a common place for hydrogen sulfide (H2S) to build.

Why do force mains make hydrogen sulfide?

Wastewater carries dissolved oxygen when it leaves a building, and the bacteria living in it use that oxygen first. With no air space to replenish it and hours of detention time between pump cycles, a force main lets that oxygen run out quickly. Once it is gone, the same bacteria turn to nitrogen compounds such as nitrate, and once nitrate is exhausted they turn to sulfate, which is present in ordinary wastewater. Stripping oxygen from sulfate leaves dissolved sulfide behind, and because the main is pressurized, that sulfide has nowhere to escape and stays in solution rather than off-gassing inside the pipe. Only when the flow reaches the receiving manhole, where pressure drops and the water turns turbulent, does the dissolved sulfide convert to hydrogen sulfide gas. That same spot is where the gas meets wet concrete or steel above the waterline and, through a separate bacterial reaction, becomes the sulfuric acid that corrodes the pipe crown and the receiving structure. The smell and the corrosion both surface at the discharge end, even though the chemistry ran its course somewhere back along the main.

How can you tell a force main is the source?

A few patterns point back to a force main rather than the lift station or the gravity sewer downstream. Odor complaints cluster at the discharge manhole, not the lift station, because that is where dissolved sulfide finally converts to gas. Corrosion shows up on the pipe crown and the manhole walls above the waterline at that discharge point, not evenly along the route. Complaints also tend to follow the pump cycle, a burst of odor shortly after the station starts, as the oldest water in the main finally arrives. None of these alone is proof, but together they distinguish a force main problem from a gravity line issue, a wet well problem, or a blockage closer to the source.

What should you measure?

Confirming a force main as the source starts with a few measurements at the discharge end. Headspace hydrogen sulfide, read in parts per million (ppm) above the water at the manhole, shows how much gas is coming off the flow. Dissolved sulfide, measured in the water itself, shows how much is still in solution and available to off-gas downstream. Detention time, calculated from pump run times and the pipe’s known volume, shows how long the water sits between cycles, the variable driving how much sulfide has time to form. Temperature matters too, since warmer wastewater speeds up the reactions behind it.

What are the options?

The right approach depends on the main’s length, its detention time and what a utility already has in place.

Option	How it works	Falls short
Iron salts	React with dissolved sulfide to form a solid, pulling it out of solution before discharge	Treats sulfide only after bacteria have already made it; dosing must track a shifting load
Nitrate	Gives sulfate-reducing bacteria oxygen to use before they turn to sulfate	Passes downstream with the flow rather than being used up
Air or oxygen injection	Adds oxygen directly to the pipe through diffusers or a compressor along the route	Needs equipment and power along the route, which most force mains do not have
Supplying usable oxygen at the lift station	An inorganic compound carries oxygen to the bacteria already in the wastewater, dosed at the station so the whole main downstream becomes the treatment zone	Depends on a dosing point with water and power at the station; a biological program, not a fix for a failing check valve or an undersized main

Iron salts and nitrate work at the point they are applied, and air or oxygen injection can reach a main with the right access points along its length. Where a main is fed from a single lift station and injection equipment along the route is not practical, dosing usable oxygen at the station lets the whole main downstream act as the treatment zone instead of treating the discharge end after the fact.

What do we do?

We dose DRP Accelerator, an inorganic compound that carries usable oxygen to the bacteria already living in the wastewater, as far upstream as water and power allow, which for a force main is usually the lift station feeding it. We add no bacteria and no enzymes. With oxygen available, the bacteria take it first instead of stripping it from sulfate, so the hydrogen sulfide that would otherwise reach the discharge manhole is not formed. We call dosing this far upstream AerobicShift: the force main becomes the first stage of treatment instead of a pipe that only carries the problem downstream. The accelerator is consumed as the bacteria use its oxygen, so nothing of it is left in the water by the time the flow reaches the discharge end. It is not an oxidizer and does not attack metals. It is a monthly service we run and measure, with no chemical additives and no capital purchase. For the lift station side of the review, see the lift station H2S control page; for hydrogen sulfide control across a collection system, see H2S removal.

Summary

A force main runs full and under pressure, holding wastewater for hours between pump cycles while bacteria use up dissolved oxygen, then nitrate, then sulfate.
The resulting sulfide stays dissolved under pressure until the flow reaches the receiving manhole, which is why the odor and the crown corrosion both show up at the discharge end.
Iron salts, nitrate and air or oxygen injection each address part of the problem at the point they are applied; dosing usable oxygen at the lift station lets the whole main downstream become the treatment zone.

COMMON QUESTIONS

Straight answers.

What is a force main?+

A force main is a pressurized pipe that carries wastewater from a lift station to the next point in a collection system, whether that is another lift station, a headworks channel or the treatment plant. Pumps push the flow through it under pressure, so it runs completely full with no air space above the water, unlike a gravity sewer that flows partly full with open air at the top of the pipe.

Why do force mains produce hydrogen sulfide?+

Force mains produce hydrogen sulfide because holding wastewater for hours between pump cycles, with no air space to replenish it, lets the dissolved oxygen run out, and the bacteria in the water turn to nitrate, then to sulfate once both are gone. Stripping oxygen from sulfate leaves dissolved sulfide behind, and because the main is pressurized that sulfide stays in solution until the flow depressurizes at the receiving manhole and releases as hydrogen sulfide gas.

Why does the smell show up at the discharge manhole instead of along the pipe?+

The smell shows up at the discharge manhole because dissolved sulfide has nowhere to off-gas inside a pressurized, completely full pipe, so it travels the length of the force main in solution instead. Only when the flow reaches the manhole, where the pressure drops and the water turns turbulent, does the dissolved sulfide convert to hydrogen sulfide gas, which is why the odor and the corrosion concentrate there rather than along the route.

How can you tell a force main is producing hydrogen sulfide?+

A force main problem tends to show a consistent pattern: odor complaints cluster at the discharge manhole rather than at the lift station itself, corrosion appears on the crown of the pipe and the manhole walls above the waterline at that same discharge point, and odor spikes often follow the pump cycle as the oldest water in the main finally arrives. Together these signs point back to detention time in the main rather than to the lift station or the gravity sewer beyond it.

What should be measured to confirm a force main is the source?+

Confirming a force main as the source of hydrogen sulfide takes a handful of measurements at the discharge end: headspace hydrogen sulfide read in parts per million with a calibrated meter, dissolved sulfide in the water itself, detention time calculated from pump run times and the pipe's known volume, and wastewater temperature. Together these show how much gas is forming and how much time the main is giving the bacteria to make it.

What are the options for controlling hydrogen sulfide in a force main?+

The options for controlling hydrogen sulfide in a force main include iron salts, which bind dissolved sulfide after it has formed, nitrate, which gives the bacteria a source of oxygen to use before they turn to sulfate, and air or oxygen injection, which adds oxygen directly at points along the route where equipment can reach. Dosing usable oxygen at the lift station keeps the sulfate reduction reaction from starting in the first place, so the whole main downstream becomes the treatment zone instead of the discharge end.

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