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H2S Removal and Odor Control

Iron Salts

Solutions of iron salts control H2S and associated odors by precipitating it from aqueous solution.  The precipitation reaction is shown in the following overall reactions:

Ferric Sulfate

Fe2(SO4)3  +  3H2S  ↔  Fe2S3(s)  +  3H2SO4

Ferric Chloride

2FeCl3  +  3H2S  ↔  Fe2S3(s)  +  6HCl

These reactions demonstrate that 1 mole of Fe+3 precipitates 1.5 moles of H2S.  The actual precipitation reactions will depend on what soluble form the sulfide is in, as hydrosulfide (HS-) or sulfide (S-2).

Recommended ferric salts dosage for H2S applications vary somewhat depending on the control strategy chosen by the end-user.  For example, if the treatment objective is to control the liquid phase sulfide concentration, then the recommended ferric salt dosage is 110 – 125% of the stoichiometric dosage (based on the difference between the existing and target liquid-phase sulfide concentrations).  Slight overdosing is recommended to insure achieving treatment objectives.  If the treatment objective is to control the vapor phase H2S concentration, then the recommended ferric salt dosage is 125% of the difference between the existing liquid phase sulfide concentration and an assumed “target” liquid phase sulfide concentration that will generate the target vapor phase H2S concentration.  Note that since these systems are open, there is no true equilibrium between the vapor and liquid phases.  Thus, an inexact relationship exists between the vapor and liquid H2S concentrations, making this a somewhat trial-and-error approach.  Other complicating factors include the presence of phosphorus, organics, and other entities which consume the available iron in the system.

GenClear N (Sodium Nitrite Technical Liquor)

Unlike iron salts that remove the already formed hydrogen sulfide via chemical precipitation, the GenClear N product acts to prohibit the formation of the H2S in the first place.  In collection systems, sulfate-reducing bacteria, which grows in a “slime layer” that coats the sewer’s wetted walls, produce H2S when the bacteria metabolize sulfate oxygen.  These bacteria use oxygen in the most readily available form: first, from elemental oxygen; then, nitrate oxygen; then, sulfate oxygen.  Hence, in anaerobic conditions, if dissolved nitrate oxygen is not present the oxygen source available for the bacteria is from sulfates.  The sulfur then combines with hydrogen and forms hydrogen sulfide.  The addition of GenClear N provides the bacteria with a source of oxygen and keeps them from reducing the sulfates into hydrogen sulfide.


Odor and corrosion is controlled by the CAL~FLO® slurry by increasing the pH to a level that prevents H2S gas formation and release.  The dosage is independent of the sulfide level present and adds alkalinity which may be beneficial in plant operations.

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