GUIDE

Why Wastewater Bacteria Need Oxygen: Anaerobic, Aerobic and Facultative Explained

Wastewater bacteria fall into three groups defined by how they use oxygen: aerobic bacteria need it, anaerobic bacteria live without it, and facultative bacteria use it when it is present and switch to another electron acceptor when it runs out. That switch is not random. Bacteria draw down dissolved oxygen first, then nitrate, then sulfate, then carbon dioxide, in a fixed order that each step makes slower and less productive than the one before it. Hydrogen sulfide (H2S), the gas responsible for the rotten-egg smell and the corrosion in a sewer system, is what sulfate-reducing bacteria leave behind once oxygen and nitrate are both gone. A treatment plant uses this order on purpose, in zones built for it. A collection system, a pipe network with no such zones designed in, gets the same chemistry by accident. This guide walks through the three bacteria types, the order they draw down electron acceptors, why a plant wants some of that chemistry and a sewer never does, and what supplying usable oxygen upstream changes about it.

Bacteria do not choose an electron acceptor. They use whatever is left, in a fixed order.

Aerobic bacteria require dissolved oxygen to break down organic material and cannot survive for long without it.
Anaerobic bacteria do not use oxygen and instead draw energy from other electron acceptors such as nitrate, sulfate or carbon dioxide.
Facultative bacteria use dissolved oxygen when it is present and switch to another electron acceptor once it is depleted.
Bacteria draw down electron acceptors in a fixed order: dissolved oxygen first, then nitrate, then sulfate, then carbon dioxide.
Sulfate-reducing bacteria convert sulfate into sulfide, which becomes hydrogen sulfide gas once it reaches the air.
Denitrification and biological phosphorus removal are treatment processes that use a deliberately oxygen-free zone as part of a plant's design.
Water can dissolve only a limited amount of oxygen regardless of how much air is mixed into it, which is a physical limit on aeration rather than an equipment limit.

Published September 24, 2026

What are aerobic, anaerobic and facultative bacteria?

Every bacterium in wastewater falls into one of three groups, defined by whether it needs oxygen. Aerobic bacteria need dissolved oxygen to survive and break down organic material, and go dormant or die off without it. Anaerobic bacteria never use oxygen: they get their energy from other electron acceptors, the substances a cell uses in place of oxygen. Facultative bacteria sit between the two, using dissolved oxygen when it is present and switching to an alternate acceptor the moment it is gone.

In a healthy system, most of the working population is aerobic or facultative, and those two groups are the fastest at breaking down fats, oils, grease and organic solids, because oxygen-based metabolism is the most efficient reaction available. Strictly anaerobic bacteria are slower by comparison, which matters once dissolved oxygen runs out, as it does routinely in a sewer line, a wet well or a lagoon bottom.

What order do bacteria take oxygen in?

Bacteria do not pick an electron acceptor at random. They draw down whatever is available in a fixed order, because each option yields less energy than the one before it, and a cell always uses the most efficient option on hand. Dissolved oxygen goes first, used exclusively as long as it lasts. Once it is gone, bacteria turn to nitrate, a condition called anoxic, and denitrifying bacteria strip oxygen from it and release nitrogen gas.

Once nitrate is also exhausted, bacteria turn to sulfate, an ion present in ordinary wastewater. Sulfate-reducing bacteria strip oxygen from sulfate and leave sulfide behind, which becomes hydrogen sulfide gas the moment it reaches air, usually where the water turns turbulent: a pump discharge, a drop structure or a wet well. If sulfate also runs out, the last acceptor is carbon dioxide, and methane-producing bacteria take over. Each step is slower and yields less energy than the last, which is why anaerobic sludge digests so much more slowly than the same material would aerobically.

Why do treatment plants sometimes want zones without oxygen?

A treatment plant can put the anoxic step to work. Denitrification uses a deliberately oxygen-free zone with nitrate present, so facultative bacteria strip nitrogen out of the water as nitrogen gas rather than releasing it downstream. Biological phosphorus removal uses a separate anaerobic zone earlier in the process, conditioning bacteria to take up phosphorus once oxygen returns. Both are designed, controlled zones, sized and timed for the reaction a plant wants, with the water moving on to an aerated zone right after. Neither is the condition that produces hydrogen sulfide: the nitrogen removal step stops at nitrate and never reaches sulfate.

Why does nobody want a collection system to go anaerobic?

A sewer line, a force main or a lift station has nothing designed into it the way a plant’s anoxic or anaerobic zones are designed. It is simply a pipe, with no basin, no timed cycle and no engineered stopping point. Left long enough, wastewater runs through the same electron acceptor order a plant’s zones use on purpose, but with no control over where it stops: oxygen first, then nitrate, then the sulfate step that produces hydrogen sulfide. In a pipe, that step removes nothing useful. It only makes gas, sulfuric acid on exposed concrete and steel above the waterline, and conditions that let fats, oils and grease settle rather than break down. A collection system going anaerobic is not a designed process; it is oxygen-starved wastewater sitting long enough for the worst step to take over.

Why does aeration have a ceiling?

Aeration, mixing air into water mechanically, is the most direct way to keep bacteria aerobic, and it works well wherever it reaches. But it has two hard limits. Water itself can dissolve only a limited amount of oxygen no matter how hard the blowers run, a property of water, not a shortcoming of the equipment. And aeration only raises dissolved oxygen inside the tank or basin where it is installed. A blower at a plant does nothing for a force main, a wet well or a lagoon bottom miles upstream, exactly where a collection system runs out of oxygen and starts down the sulfate step.

What does supplying usable oxygen upstream mean?

Supplying usable oxygen is a different approach: instead of mixing air into water where equipment happens to be installed, an inorganic compound carries oxygen the bacteria can already use, on the molecule itself, into wherever the water goes. We add no bacteria and no enzymes; the native bacteria do the work. As long as that oxygen is available, the same facultative bacteria keep using it instead of turning to nitrate and then sulfate, so the sulfate step and the hydrogen sulfide it produces are never reached. The compound is consumed as bacteria use its oxygen, so nothing of it remains downstream. We dose it as far upstream as a site’s water and power allow, the practice we call AerobicShift, letting a force main or a wet well stay aerobic instead of sliding down the ladder. It changes nothing about a plant’s own designed zones; those stay exactly what the plant built them to be. How the program is dosed and measured covers the delivery side.

The electron acceptor ladder

Step	Electron acceptor	Bacteria type	What it produces	Where it happens
1	Dissolved oxygen	Aerobic and facultative	Carbon dioxide and water	Anywhere dissolved oxygen is present, the fastest step
2	Nitrate (anoxic)	Facultative, denitrifying	Nitrogen gas	A zone with no dissolved oxygen but nitrate present, often designed on purpose
3	Sulfate	Anaerobic, sulfate-reducing	Sulfide, then hydrogen sulfide (H2S) gas	Oxygen and nitrate both used up: a force main, a wet well or a lagoon bottom
4	Carbon dioxide	Anaerobic, methane-producing	Methane gas	Deep sludge or a digester, once sulfate is also exhausted

Summary

Bacteria are aerobic, anaerobic or facultative, and most of the useful work in a healthy system is done by aerobic and facultative organisms while oxygen is available.
Bacteria draw down electron acceptors in a fixed order, oxygen, then nitrate, then sulfate, then carbon dioxide, and hydrogen sulfide only appears once the sulfate step is reached.
A treatment plant puts the nitrate step to work on purpose in designed anoxic zones; a collection system has no such design, so reaching the sulfate step there only makes gas, acid and grease.

COMMON QUESTIONS

Straight answers.

What is the difference between aerobic, anaerobic and facultative bacteria?+

Aerobic bacteria need dissolved oxygen to survive and break down organic material. Anaerobic bacteria do not use oxygen at all and rely on other electron acceptors such as nitrate, sulfate or carbon dioxide instead. Facultative bacteria can do either: they use dissolved oxygen when it is available and switch to an alternate electron acceptor once the oxygen is gone, which is why most bacteria in a wastewater stream are facultative rather than strictly one or the other.

Why do bacteria produce hydrogen sulfide?+

Bacteria produce hydrogen sulfide when they run out of dissolved oxygen and nitrate and turn to sulfate as their next available electron acceptor. Stripping oxygen from sulfate leaves behind sulfide, and dissolved sulfide becomes hydrogen sulfide (H2S) gas once it reaches air, typically at a point of turbulence such as a pump discharge or a wet well. Bacteria do not produce H2S when dissolved oxygen or nitrate is still available to them.

What is an anoxic zone in wastewater treatment?+

An anoxic zone is a section of a treatment process with no dissolved oxygen but with nitrate present, which lets facultative and denitrifying bacteria use nitrate as their electron acceptor instead of oxygen. Treatment plants build anoxic zones on purpose as part of the nitrogen removal process, separate from and well upstream of the sulfate-reducing conditions that produce hydrogen sulfide.

Why does dissolved oxygen run out in a sewer line or a lift station but not in a treatment plant's aeration basin?+

A treatment plant's aeration basin has blowers or mixers actively adding oxygen to the water the whole time it is there. A sewer line, a force main or a lift station has no such equipment: wastewater sits or moves slowly with only the oxygen it carried in, and bacteria use that supply up within hours, well before the water reaches a plant built to replace it.

What are electron acceptors in wastewater biology?+

An electron acceptor is the substance bacteria use to complete the chemical reaction that gives them energy from organic material, in place of oxygen when oxygen is not available. In wastewater, bacteria use dissolved oxygen first, then nitrate, then sulfate, then carbon dioxide, each one less efficient than the last, which is why sludge that has gone fully anaerobic digests so slowly.

Why doesn't a collection system just get more aeration to fix this?+

Aeration equipment raises dissolved oxygen only in the specific tank or basin where it is installed, and water itself can hold only a limited amount of dissolved oxygen no matter how much air is mixed into it. A force main, a wet well or a lagoon bottom sits well outside where any blower reaches, so more aeration at the plant does nothing for oxygen depletion that happens miles upstream.

How is supplying usable oxygen different from aeration?+

Aeration mixes air into water mechanically and is limited to wherever the equipment is installed. Supplying usable oxygen means dosing an inorganic compound that carries oxygen the bacteria can use, on the molecule itself, so it travels with the water into a force main, a wet well or a lagoon bottom that no blower reaches. It is consumed as the bacteria use it and is dosed as far upstream as water and power allow.

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