What Is Hydrogen Sulfide and Where Does It Come From?

Hydrogen sulfide (H2S) is a colorless gas with a distinctive rotten egg odor that is one of the most common aesthetic complaints associated with water quality. The human nose can detect hydrogen sulfide at extremely low concentrations, as low as 0.5 parts per billion (ppb), making it noticeable long before it reaches levels of health concern.

Hydrogen sulfide in water has three primary sources. Sulfate-reducing bacteria (SRB) are anaerobic microorganisms that convert sulfate ions to hydrogen sulfide in oxygen-depleted environments. These bacteria commonly colonize wells, water heaters, and stagnant sections of distribution systems. Naturally occurring hydrogen sulfide dissolves into groundwater from geological sources, including petroleum deposits, volcanic regions, and formations containing iron sulfide minerals such as pyrite.

A common household source is the reaction between sulfate in water and the magnesium sacrificial anode rod in water heaters. The warm, anaerobic conditions inside the heater create an ideal environment for sulfate-reducing bacteria, explaining why many consumers notice the rotten egg smell only in hot water.

Effects of Hydrogen Sulfide in Water

At concentrations commonly found in water supplies (typically below 1 mg/L), hydrogen sulfide is primarily an aesthetic concern. The objectionable rotten egg odor can make water unpalatable and unusable for cooking, bathing, and laundry at concentrations as low as 0.05 mg/L.

Hydrogen sulfide is corrosive to metals, particularly iron, steel, copper, and brass. It reacts with metal surfaces to form metal sulfides, causing black staining on plumbing fixtures and silverware. The corrosion of iron plumbing produces black water and can accelerate pipe deterioration.

Hydrogen sulfide interferes with chlorine disinfection by consuming free chlorine residual, reducing the effectiveness of disinfection. Each mg/L of H2S consumes approximately 2.1 mg/L of chlorine. This chlorine demand must be satisfied before effective disinfection can occur.

In confined spaces, hydrogen sulfide gas released from water can accumulate to dangerous levels. H2S is toxic at concentrations above 10 ppm in air, and concentrations above 100 ppm can cause loss of smell (olfactory fatigue), creating a dangerous situation where the gas is present but no longer detected. Well pits, pump houses, and enclosed water treatment areas require proper ventilation when treating H2S-bearing water.

Guidelines for Hydrogen Sulfide in Water

The EPA does not regulate hydrogen sulfide in drinking water because it is not considered a health risk at naturally occurring concentrations. However, the strong odor makes treatment necessary for consumer acceptance and to prevent corrosion damage to plumbing and equipment.

How to Test for Hydrogen Sulfide in Water

Hydrogen sulfide must be tested at the point of collection because it is a dissolved gas that rapidly escapes from water samples. On-site analysis is essential for accurate results. Standard Method 4500-S2- provides several analytical approaches including iodometric titration and the methylene blue colorimetric method.

Field test kits using lead acetate paper or colorimetric reagents provide quick screening measurements. For detailed analysis, sample preservation with zinc acetate to fix H2S as zinc sulfide allows laboratory analysis.

Testing should differentiate between hot water only, cold water only, or both, as this distinction helps identify the source (water heater anode vs. well or groundwater source).

Treatment Methods for Hydrogen Sulfide Removal

Aeration

Aeration systems strip dissolved hydrogen sulfide gas from water by exposing it to air. Packed tower aerators, spray aeration, and forced-draft aeration are effective for H2S concentrations up to approximately 2-3 mg/L. Aeration systems also oxidize a portion of the H2S to elemental sulfur, which is removed by subsequent filtration.

Oxidation and Filtration

Chemical oxidation using chlorine, potassium permanganate, ozone, or hydrogen peroxide converts hydrogen sulfide to elemental sulfur or sulfate, which are then removed by filtration. Chlorination is the most common approach: 2.1 mg/L of chlorine is required per mg/L of H2S. Oxidizing chemicals must be dosed upstream of a contact tank to ensure complete reaction.

Activated Carbon Filtration

Granular activated carbon (GAC) adsorbs hydrogen sulfide from water and catalytically oxidizes it to elemental sulfur on the carbon surface. GAC is effective for low to moderate H2S concentrations (below 1 mg/L) and provides simultaneous removal of taste and odor compounds. Carbon requires periodic replacement as its adsorptive and catalytic capacity is consumed.

Catalytic Carbon Media

Catalytic carbon media (such as Centaur carbon) have enhanced catalytic properties compared to standard GAC, providing greater capacity for H2S oxidation. These filtration systems are effective for moderate H2S concentrations and require periodic backwashing to remove accumulated sulfur deposits.

Manganese Dioxide Media

Manganese dioxide-based filter media (greensand, Filox, Pyrolox) oxidize hydrogen sulfide on contact and are also effective for combined H2S, iron, and manganese removal. These versatile media are widely used in residential and commercial applications for well water treatment.

Frequently Asked Questions

What causes the rotten egg smell in water?

The rotten egg smell in water is caused by hydrogen sulfide (H2S) gas. It can originate from sulfate-reducing bacteria in wells and distribution systems, naturally occurring in deep groundwater from geological sources, or from the reaction between sulfate in water and the magnesium anode rod in water heaters. The human nose can detect hydrogen sulfide at concentrations as low as 0.5 ppb.

Is hydrogen sulfide in water dangerous?

At concentrations typically found in drinking water (below 1 mg/L), hydrogen sulfide is primarily an aesthetic nuisance rather than a health hazard. However, it is corrosive to iron, steel, copper, and brass plumbing, causing black staining and tarnished silverware. At very high concentrations (above 250 mg/L in air), hydrogen sulfide gas released from water can be toxic.

Why does my water smell like rotten eggs only with hot water?

If the rotten egg smell occurs only with hot water, it is likely caused by a reaction between sulfate in the water and the magnesium sacrificial anode rod in the water heater. Sulfate-reducing bacteria can colonize the warm, anaerobic environment inside the heater and produce hydrogen sulfide. Replacing the magnesium anode with an aluminum or powered anode rod, or treating the water heater with chlorine, can resolve this issue.

Need to Remove Hydrogen Sulfide from Your Water?

ForeverPure provides commercial and industrial hydrogen sulfide removal systems, including aeration units, oxidation-filtration systems, activated carbon filters, and chemical feed equipment. Our engineering team designs solutions based on your H2S concentration, flow rate, and co-occurring water quality issues.

Contact ForeverPure for a customized hydrogen sulfide removal solution.