After updates to the effluent limitation guidelines (ELG) Final Rule from the U.S. Environmental Protection Agency (EPA) went into effect on Dec. 14, 2020, coal-fired plants throughout the power industry face a need for new projects to maintain compliance. Biological treatment systems, specifically, will need to address how process-generated hydrogen sulfide (H2S) gas is mitigated.

Where Does H2S Come From?

Biological treatment reduces oxidized nitrogen species and metals through an anoxic, or oxygen-deficient, process. This removal process is controlled by managing the conditions in the biological system to achieve a targeted oxidation reduction potential (ORP). As the ORP is lowered, nitrates and nitrites are reduced and removed, and then selenium species are reduced and removed. If ORP is further lowered, sulfates are reduced, which then produces H2S gas. Because the biological process involves living organisms, it’s not possible to achieve a uniform, exact ORP. Therefore, biological systems operate withing a range of different ORP values, almost like a bell curve around the target ORP.

While the targeted ORP is focused on selenium removal, some portion of the system has a lower ORP, resulting in the formation of H2S gas. This gas will exit the system by remaining in the liquid, off-gassing at the liquid surface or releasing during a backwash. These potential emissions may need to be mitigated.

An Effective H2S Mitigation Design

A closed-top tank can be designed to accommodate a vacuum system for H2S mitigation. This vacuum system will constantly pull air from the building, through the tanks, into the piping system and then through the treatment process of two carbon-filled vessels in succession. The H2S-laden air will pass through the carbon bed, where the H2S gas is adsorbed onto the carbon.

Analyzers between the two carbon beds will indicate when H2S gas is detected at the outlet of the first carbon bed, but before the second carbon bed. When this occurs the carbon bed must be changed out, so the previous second carbon bed becomes the primary and the newly refreshed carbon bed will become the secondary scrubber. This process repeats every time a carbon bed is saturated and changed out to uphold the system’s H2S mitigation capabilities.

By implementing an effective H2S mitigation solution, biological treatment systems can maintain compliance with the new ELG rule updates.


The Final ELG Rule also addresses flue gas desulfurization wastewater and bottom ash transport water. Learn the essential dates, limits and requirements associated with this requirement.

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Doug Randall is a chemical engineer working on air and water pollution control systems for power generation projects at Burns & McDonnell.