Hydrogen Sulfide

Hydrogen sulfide is a highly toxic and flammable gas, but its presence in drinking water is not at a concentration to pose a health threat, and its presence is easily detectable by odor by most people. Because of its rotten egg odor, hydrogen sulfide is removed from water for aesthetic reasons. Hydrogen sulfide in water is not regulated by the EPA.

Hydrogen sulfide in water is typically the result of sulfur-reducing bacteria that can be found naturally in water. These bacteria use sulfates and sulfur compounds found in decaying plant material, with hydrogen sulfide formed as a by-product.

If the hydrogen sulfide odor is only noticeable in hot water, its origin is likely to be in the hot water heater; a magnesium rod in the heater present to inhibit corrosion can produce hydrogen sulfide from sulfates present in the water.

As to its removal from water, the chlorine added as disinfectant to water will convert the hydrogen sulfide to sulfates, but the chlorine cost will run very high for more significant concentrations (it typically requires 7 parts chlorine per part H2S for chlorine oxidation of H2S).

In public water systems, aeration is often used to address hydrogen sulfide contamination. The hydrogen sulfide removal is mostly due to the air stripping out hydrogen sulfide from the water; the oxygen in air can convert hydrogen sulfide to sulfate, but the reaction is slow. Proper pH control during aeration is necessary to get high percentage removal of the sulfides, with removal efficiency improving with lower pH. At a pH of 5, 98% of H2S can be removed by stripping, while at a pH of 8, less than 10% of H2S can be removed by stripping. High alkalinity also limits stripping performance. With proper pH control, the bulk of the hydrogen sulfide can be removed by aeration, with the chlorine addition downstream of the aeration eliminating the remaining hydrogen sulfide, at a much reduced chlorine cost than without aeration.