Hydrogen Sulfide Removal with Ozone

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Background

Hydrogen sulfide is sometimes found in ground water. It has the odor of rotten eggs and has a threshold of 0.0011 mg/l. It can also cause water to have characteristic and unpleasant tastes. Water supplies with as little as 1.0 ppm (part per million) hydrogen sulfide are corrosive, may tarnish copper and silverware, and occasionally release a black material that stains laundry and porcelain.

Hydrogen sulfide is formed by sulfur bacteria that may occur naturally in water. These bacteria use the sulfur in decaying plants, rocks, or soil as their food or energy source and as a by-product produce hydrogen sulfide. The sulfur bacteria do not cause disease, but their presence in water can cause a bad taste or odor. Since small amounts of hydrogen sulfide can be detected in water by smell and taste, laboratory testing is not needed to detect its presence. Laboratory tests for hydrogen sulfide are possible if care is taken in collecting the sample. Special sample bottles must be used that contain a stabilizing chemical that prevents the hydrogen sulfide from escaping when the bottle is opened at the lab.

While aeration can be used to strip some of the hydrogen sulfide from the water, this can convert a water problem to an air pollution problem if further treatment is not applied. Hydrogen sulfide is easily oxidized by ozone, ultimately to form sulfate. The initial oxidation is to form elemental sulfur which is seen as a light colored colloidal suspension. Further oxidation dissolves the elemental sulfur to sulfite and continued oxidation produces sulfate. As a result, more ozone is required to produce sulfate from hydrogen sulfide than is required sulfur.

Design Considerations (1)

The theoretical dose to oxidize ozone to sulfate is 3:1, but in practice the ratio is 4:1. This will leave a small ozone residual in the water, 0.2-0.3 ppm. This residual can be used to ensure that the hydrogen sulfide is fully removed. In the case of variable hydrogen sulfide concentration, following the residual will allow for adjustment in the ozone dosage rate to maintain complete removal of the ozone.

Ozone should only be injected into the first chamber of contact vessel if fine bubble diffusers and a multiple chambered contact vessel are employed. If ozone is injected in later chambers the off gas might strip hydrogen sulfide from the water.
This can foul the catalyst used to destroy excess ozone in the off gas. The proper injection point should allow for a long (2 year) catalyst lifetime.

Spartan provides ozone generators and associated equipment that can be used in hydrogen sulfide removal.

Rakness, K.L. 2005. Ozone in Drinking Water Treatment: Process Design, Operation and Optimization. AWWA Denver, CO