Hydrogen Sulfide Review

Wastewater influent is commonly tested for sulfate (SO₄^-2) to asses its potential for odor formation and the treatability of sludge. Hydrogen sulfide results from the reduction of sulfate to hydrogen sulfide gas by bacteria under anaerobic conditions as shown in Equation 1.

Equation 1: Sulfate Reduction

Hydrogen sulfide is the most common (and most easily identified by the public) odorous gas found in sanitary sewers and wastewater treatment systems. This corrosive, toxic gas is soluble in wastewater and has a characteristic rotten-egg odor at low concentrations. H₂S is colorless, flammable, and explosive. It paralyzes the respiratory system and lessens the sense of smell as it increases in concentration. The specific gravity of H₂S is 1.19 so it concentrates down low though it can be at a higher elevation if the air is hot and humid. The effects of hydrogen sulfide at various concentrations is shown in Table 1.

Table 1: Effects of Hydrogen Sulfide

The concentration of oxidized sulfur compounds in the influent to an anaerobic digester is important, as high concentrations can have a negative effect on anaerobic treatment. Sulfate-reducing bacteria compete with the methanogenic bacteria for COD and thus can decrease the amount of methane gas production so critical to a cogeneration project. While low concentrations of sulfide (less than 20 mg/L) are needed for optimal methanogenic activity, higher concentrations can be toxic as previously stated.

Because un-ionized H₂S is considered more toxic than ionized sulfide, pH is important in determining H₂S toxicity. The degree of H₂S toxicity is also complicated by the type of anaerobic biomass present (granular versus dispersed), the particular methanogenic population, and the feed COD/SO₄^-2 ratio. With higher COD concentrations, more methane gas is produced to dilute the H₂S and transfer more H₂S to the gas phase.

Hydrogen sulfide exists in aqueous solution as either the hydrogen sulfide gas (H₂S), the ion (HS‾), or the sulfide ion (S^2-), depending on the pH of the solution. At a pH of approximately 9, more than 99% of the sulfide dissolved in wastewater occurs in the form of the non-odorous hydrosulfide ion (HS‾). Therefore, odorous amounts of hydrogen sulfide gas will not be released if a pH above 8 is maintained. Below this pH value, hydrogen sulfide gas is released from the wastewater. In anaerobic digesters the optimal pH is between 6.5 and 7.5, a range that allows the release of H₂S of up to almost 70 percent as shown in Figure 1.

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Figure 1: Hydrogen Sulfide vs. pH

The Sulfur Cycle

The mechanisms by which sulfates are converted to hydrogen sulfide are shown graphically in Figure 2.

Figure 2: The Sulfur Cycle

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