Hydrogen sulfide (H2S) is often identified as a potential culprit of odors and nuisance complaints near municipal solid waste (MSW) landfills. Some base their complaints on information found on the Internet as fact. As experts, let’s start by saying data from other landfills or pulled from an AI browser summary online will not provide accurate answers. H2S concentrations vary widely and are unique to individual landfills.
How is H2S generated in an MSW landfill, and why do concentrations vary?
Calcium sulfate (CaSO4•2H2O, aka gypsum), the primary ingredient in wallboard (aka drywall), can be biologically converted to H2S under select and somewhat rare conditions. Specifically, seven conditions are required for the biodegradation of gypsum to H2S. See (Gypsum Association, Industry Technical Paper: Treatment and Disposal of Gypsum Board Waste (Jan. 1991); Gypsum Association, Treatment and Disposal of Gypsum Board Waste, Part II, Technical Paper (Mar. 1992).
Condition 1 – Liquid Water. The biological conversion of sulfate to H2S occurs in the aqueous phase—i.e., sufficient free liquids must be present, and sulfates must dissolve into the free liquids. Modern landfills with leachate collection systems may experience intermittent perched and discrete zones of saturation within the waste mass, particularly following periods of extended precipitation. Low-permeability confining layers (e.g., clay or clay-like soil used for intermediate cover) may temporarily trap water/leachate in discrete pockets within the landfill.
Condition 2 – Source of Soluble Sulfate. Gypsum, having the chemical formula CaSO4•2H2O, is a source of soluble sulfate. Gypsum sources include wallboard (aka drywall), flue gas desulfurization (FGD) material from coal-fired power plants, and some industrial wastes. Sulfates and sulfur compounds can also be present in lower concentrations in other waste streams, depending on what the MSW landfill accepts.
Condition 3 – Sulfate-reducing Bacteria. Sulfate-Reducing Bacteria (“SRB”) use dissolved sulfate as an electron acceptor in the oxidation of carbon. Primary SRB include Desulfovibrio and Desulfotomaculum. These SRBs, as well as many other bacteria, are commonly present in MSW landfills. However, the presence of SRB within a landfill may not be ubiquitous, and may be limited to regions where the other required conditions favor their existence and survival.
Condition 4 – Organic Material. SRBs use organic material as a food source to multiply and degrade sulfate to H2S. Carbon serves as a source of energy for the bacteria. Typical MSW has a high organic content due to a wide variety of organic materials such as wood, paper, cardboard, food, vegetative waste, and fabrics. Many communities with recycling programs help divert these waste materials for reuse and recycling.
Condition 5 – Anoxic Environment. SRBs thrive under anoxic (without oxygen) conditions. The presence of oxygen can kill SRBs. While anoxic conditions are typically not present in areas where MSW was recently disposed, they are typical in portions of MSW landfills where organic wastes have been present for at least a few months and decompose to produce methane and carbon dioxide.
Condition 6 – Appropriate pH Range. SRB reduction of sulfate to H2S is reportedly optimum within a pH range of about 7 to 8, and does not occur outside a pH range of about 4 to 9. The pH range within a typical MSW landfill falls within this activity range.
Condition 7 – Appropriate Temperature Range. SRB reproduction and H2S generation are reportedly optimum within a range of about 30 °C to 38 °C (86 °F to 100 °F). Many MSW landfills are within or a little above this optimum range. Studies of SRB in geologic environmental settings found reduced activity above about 60 °C (140 °F), and no activity above about 80 °C (176 °F). Similarly, SRB activity ceases in freezing conditions.
In summary, although the necessary conditions for H2S generation are likely intermittently present within some discrete pockets within many MSW landfills, the conditions are not likely ubiquitous throughout the waste. MSW landfill conditions and waste composition are typically highly heterogeneous with respect to both location within the landfill and time. Thus, there are zones within landfills where many, but not all of the seven required conditions are present, and H2S generation does not occur. For example, there are undoubtedly many regions within landfills where free liquids (i.e., saturated conditions) are not present and, therefore, SRB conversion of sulfates to H2S does not occur, despite the presence of the other six conditions.
Similarly, a landfill may have pockets where bulk sulfate-containing waste has been disposed of but where the internal portion of the pocket is not exposed to moisture, organics, or SRB—each a necessary condition for converting sulfate to H2S.
Considering these seven conditions and heterogeneous landfill conditions, there are too many variables to provide a reliable and defendable quantitative model for H2S generation at all MSW landfills.
Monitoring and Treating Landfill H2S Conditions
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About the Author: Jeff Marshall, PE, is a Vice President of SCS Engineers and our National Expert on Emerging Contaminants (e.g., PFAS) and Innovative Technologies. He has over four decades of experience emphasizing environmental chemistry (e.g., hydrogen sulfide generation at MSW landfills), environmental permitting and compliance (e.g., fumigation facilities), hazardous materials/waste management, site assessment/remediation, treatment technologies, and human health risk issues. Hydrogen sulfide experience includes dozens of facilities, including landfills, coal-fired power plants, and paper mills.
EPA updated its Compliance and Emissions Data Reporting Interface (CEDRI) for the electronic reporting of air emissions under the NESHAP related to MSW landfills. Three new reporting templates were added on October 15, 2024, each linked to its corresponding Excel spreadsheet template. These include the
The Semi-Annual report is the most significant because MSW landfills have 90 days to begin using the Excel template. Reports due January 13, 2025, or any time after that must include this electronic filing.
EPA’s color-coded template provides a bit of instruction.
The gray tab (Company Information) contains general information likely to be unchanged from report to report. After completing the gray tab, you may save the workbook as a site-specific template to use in subsequent reports to limit subsequent data entry.
Complete the green tabs (Certification, CMS Info, Description of Changes, Exceedances, and Number of Exceedances) as appropriate to complete the semi-annual report.
Complete the blue tabs (Deviation Detail, Deviation Summary, CMS Detail, and CMS Summary) if deviations or CMS out-of-control periods or downtime periods occur according to §63.10(e) and as defined in §63.1990.
The orange tabs (Well Expansion, Operational Statements, Site Specific Treatment, Enhanced Monitoring, Bypass CDT Not Operating, and Corrective Action Analysis) cover information required by the semi-annual report requirements of §63.1981(h); be sure to complete the requisite tabs.
Professionals at SCS Engineers will post more guidance but plan to continue preparing our clients’ semi-annual reports as we do now, and completing and submitting this spreadsheet. Please work with your air emissions specialist or project manager, or contact us for support.
Additional Resources:
A revision to AP-42 regarding Municipal Solid Waste (MSW) Landfills was finalized on August 15, 2024. AP-42 is the Compilation of Air Pollutant Emission Factors guidance developed by the U.S. Environmental Protection Agency (EPA) to evaluate air pollution emissions from various sources. EPA drafted new emission factors for the MSW Landfill portion of AP-42 on January 12, 2024 and offered a 60-day public comment period that ended March 12, 2024.
Highlights of this final action include:
For additional information, please contact SCS Engineers, or visit the EPA Air Emissions Factors and Quantification website.
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