Understanding the Difference Between Sulfur-Oxidizing and Sulfur-Reducing Bacteria in Groundwater

If you've ever noticed a rotten-egg smell in your groundwater or well water, you're not alone—and you're probably dealing with sulfur-related bacteria. But not all sulfur-related bacteria are the same. In fact, two very different types—sulfur-oxidizing bacteria (SOB) and sulfur-reducing bacteria (SRB)—play very different roles in the chemistry and condition of groundwater and wells.

Let’s break it down:

1. Sulfur-Reducing Bacteria (SRB)

What they do:

Sulfur-reducing bacteria are anaerobic—they thrive in oxygen-poor environments, such as deep in aquifers or behind well scale or slime layers. They consume sulfate (SO₄²⁻), a naturally occurring compound found in many groundwater sources.

What they produce:

Their metabolic byproducts include:

• Hydrogen sulfide gas (H₂S) – causes that familiar rotten-egg smell

• Organic acids – contribute to corrosion of metal well components

Where they grow:

• Inside well screens and casing where oxygen is limited

• Under slime or scale layers

• In stagnant water zones

Why they're a problem:

• Hydrogen sulfide corrodes metal pipes, pumps, and casing

• Biofilms formed by SRB can clog screens and reduce water flow

• H₂S gas causes odor and staining (black residue)

2. Sulfur-Oxidizing Bacteria (SOB)

What they do:

Sulfur-oxidizing bacteria are typically aerobic, meaning they require oxygen to live. These bacteria oxidize hydrogen sulfide or elemental sulfur into sulfuric acid or sulfate.

What they produce:

• Sulfuric acid (H₂SO₄) – highly corrosive

• Elemental sulfur – can accumulate as yellow, slimy deposits

Where they grow:

• Near the water table, where oxygen and hydrogen sulfide meet

• In pressurized or aerated systems like filters or holding tanks

• On wet surfaces exposed to air

Why they're a problem:

• Acidic byproducts can damage plumbing, pumps, and well linings

• They form slime layers that trap particles and bacteria

• Yellow sulfur deposits can clog filters and foul taste and odor

Summary: Key Differences

Feature	Sulfur-Reducing Bacteria (SRB)	Sulfur-Oxidizing Bacteria (SOB)
Oxygen requirement	Anaerobic (no oxygen)	Aerobic (needs oxygen)
What they use	Sulfate (SO₄²⁻)	Hydrogen sulfide (H₂S), sulfur
What they produce	H₂S gas, organic acids	Sulfuric acid, elemental sulfur
Common signs	Rotten egg smell, black sludge	Yellow slime, corrosion
Where they thrive	Deep wells, low-oxygen areas	Aerated systems, near surface

What Can You Do?

Both types of bacteria can lead to:

• Reduced water flow

• Equipment damage

• Poor water quality

Regular monitoring and well maintenance are essential. Techniques like shock chlorination, acid cleaning, or professional well rehabilitation (chemical and mechanical) can help keep bacterial growth under control.

Need Help?

If you're noticing signs of sulfur-related issues in your well or irrigation system, contact Aquabiotics Industrial. Our team specializes in water well cleaning, rehabilitation, and maintenance.