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 (S2-), 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.