Hydrogen sulfide generated from sulfate-containing landfill waste presents safety, odor and emissions challenges that require engineered gas treatment solutions.
The United States ranks third on the list of most populous countries and is one of the largest generators of waste. According to the Sensoneo Global Waste Index 2025, the United States consistently ranks as the top producer of municipal solid waste (MSW) per capita and generates more than 2,000 pounds per person, more than any other country.1
Construction and demolition (C&D) waste accounts for a significant portion of landfill volume in the United States — some 600 million tons in 2018, according to the Environmental Protection Agency (EPA). C&D debris was more than twice the amount of MSW that year. A significant portion of the total waste is materials like concrete, wood, drywall and asphalt.
According to Transparency Market Research, citing EPA data, C&D waste is expected to exceed 2.2 billion tons by 2025. Most troublesome are gypsum drywall and other sulfate-containing materials that break down under anaerobic conditions to form hydrogen sulfide (H2S) gas.2
At low levels, H₂S smells like rotten eggs and can be irritating in humans. At its worst, its presence can manifest into serious health issues such as neurological conditions, gastrointestinal distress and, in extreme cases, unconsciousness or death. Upon entering the atmosphere, it oxidizes into sulfur dioxide (SO₂) and then sulfates, contributing to acid rain and atmospheric cooling.
Landfill emissions are primarily regulated on the federal level. The EPA regulates environmental emissions through the Clean Air Act wherein it sets ambient air quality standards, while the Occupational Safety and Health Administration (OSHA) sets permissible exposure limits in the workplace. While OSHA has set a ceiling limit of 20 parts per million (ppm) with workplace exposure not to exceed 15 minutes if there is no other exposure, the National Institute for Occupational Safety and Health recommends a 10-minute ceiling limit of 10 ppm.
The EPA has not set a specific, numerical maximum ppm ambient air standard for H₂S, but the organization acknowledges that it is a toxic air pollutant and requires reporting under the Toxic Release Inventory when thresholds are met. However, the EPA’s Standards of Performance for New Stationary Sources and National Emission Standards for Hazardous Air Pollutants require large MSW landfills to install gas collection and control systems.3
Historically, landfill hydrogen sulfide was controlled by installing active gas management systems that would extract and burn the gas. It was later recognized that, when burned, the highly flammable gas produces other toxic vapors and gases, such as SO₂. In a quest to identify and adopt the best transformative solution for treating H₂S, landfills took into consideration solutions used by the largest industrial sources of H₂S — oil and gas production. The oil and gas industry treats H₂S primarily through chemical scavenging, stripping and oxidation.
The most logical treatment that fits within landfills’ critical operational, environmental and regulatory constraints is a liquid reduction-oxidation (redox) system that uses a chelated iron solution to convert H₂S into solid elemental sulfur.
Landfill Desulfurization: A Solution For Removing H2S
Liquid redox technology, initially developed in the late 1950s for the removal of H₂S from gas streams in the oil and gas industry, has since undergone significant proprietary advancements. Iron-based chelating systems have emerged as the most widely licensed and implemented variants. Over the decades, these processes have been refined and adapted for broader applications, including landfill gas treatment.
The wet scrubbing liquid redox system is designed to avoid toxic chemicals and hazardous waste byproducts and offers an economically viable solution for H₂S removal in landfill applications. The liquid redox process operates as a continuous loop, using a proprietary aqueous iron-chelate catalyst solution to absorb H₂S. The gas stream containing H₂S enters an absorber, where it is dissolved into the alkaline chelated iron solution. Ferric iron (Fe3+) ions in the solution oxidize the sulfide ions to elemental sulfur (S°) and are reduced to the ferrous (Fe2+) state.
The ferrous solution then flows to an oxidizer or regeneration section within the same vessel or system. If it is the same vessel, it uses an autocirculation design, where air is sparged through the solution. Oxygen in the air reoxidizes ferrous iron to the active ferric state, preparing the catalyst for reuse. The treated H₂S gas and the air are then vented to the atmosphere. A direct treatment unit design is also available.
The elemental sulfur produced in the reactions forms as particles suspended in the solution. These particles are concentrated in a settler and then separated using a filter system. The resulting product, known as “sulfur cake” — which is typically 65 percent sulfur and 35 percent moisture — can be used in agricultural applications or disposed of in a non-hazardous landfill. The recovered filtrate is returned to the process.
The liquid catalyst readily adapts to variations in flow and concentration. Flexible operation enables a wide turndown in gas flow and H₂S concentrations. The units require minimal operator attention.
The Longevity Of Liquid Redox
In 1992, Hurricane Andrew made landfall as a Category 5 storm in the southeastern part of Florida before making a second landfall in Louisiana, resulting in $27.3 billion in total damage. At the time, it was considered the costliest and most damaging hurricane ever to hit the United States, a record it maintained for 13 years.4
A major waste-handling company operating a large landfill in Florida experienced a substantial increase in C&D waste following the storm. Consequently, H₂S concentrations rose significantly. The company evaluated multiple technologies for H₂S removal from landfill gas with concentrations as high as 5,000 parts per million by volume (ppmv) and sulfur generation rates of 2 to 3 tons per day in the landfill gas.
This presented an issue for the landfill owners, who were planning to burn the landfill gas to generate up to 11 megawatts of power from three to five turbine power plants. Among all combustion equipment, turbines have one of the lowest H₂S tolerances, with a maximum of 100 ppmv.
To address this issue, the owners implemented a proprietary liquid redox process for H₂S removal. The H₂S Oxidation System was designed and delivered to the landfill within 22 weeks. Commissioned in 1994, the unit has operated continuously, producing gas with H₂S concentrations below 50 ppmv.
Following Hurricane Andrew, modifications were made to the types of waste accepted at the site. Alongside the planned development of additional C&D waste facilities, projections indicated increased gas flow and H₂S concentrations exceeding the existing treatment capacity, necessitating an expansion of the liquid redox H₂S oxidation system.
In 2001, the landfill owners initiated an evaluation of options to expand treatment capacity. Considerations included transporting gas off-site for treatment and utilization, implementing alternative H₂S removal technologies to replace the liquid redox system, or expanding the existing system to accommodate increased loads.
The owners proceeded with a plan to expand the unit’s capacity for treating sour C&D waste gas. Multiple modeling exercises assessed gas and H₂S production, as well as current and projected waste types and quantities, to establish a design basis. Although several alternatives were evaluated, none matched the cost-effectiveness, operational experience and performance guarantees provided by the existing liquid redox unit.
Construction of the liquid redox expansion system commenced in 2002. The upgrade increased the sulfur-handling capacity from 2.3 to 10.8 long tons per day, enabling treatment of gas with H₂S concentrations up to 33,350 ppmv and reducing output to less than 50 ppmv. The expansion required integration with existing units and gas lines during a scheduled plant-wide maintenance turnaround. The tie-ins were completed successfully, and the expanded unit began operation in January 2003, achieving H₂S emissions below the anticipated outlet levels.
Desulfurization Projects Reduce Emissions, Improve Air Quality
Landfills are designed to safely and permanently dispose of solid waste while protecting public health and the environment. Although modern landfills are well-engineered and managed, H₂S is produced when naturally occurring bacteria break down sulfur-containing waste in a wet, oxygen-deprived environment.
Under specific environmental conditions, hydrogen sulfide can pose a health hazard to humans and may be toxic if inhaled at certain concentrations.
Removing H₂S from landfill gas is critical for worker safety, community protection and emissions control. Chemical oxidation is among the most effective removal methods. Liquid redox systems, operating at peak performance, achieve more than 99.9 percent removal efficiency and convert H₂S to elemental sulfur, improving environmental quality around landfills. Liquid redox is reliable, efficient, and cost-effective, and can be licensed with guarantees for removal efficiency, sulfur capacity and chemical consumption.
References:
- EnvironmentAmerica Research & Policy Center, Trash in America 2021 – https://environmentamerica.org/center/resources/trash-in-america-2/#:~:text=The%20U.S.%20produces%20more%20than,product%20is%20purchased%20or%20used.
- U.S. EPA, Sustainable Management of Construction and Demolition Materials – https://www.epa.gov/smm/sustainable-management-construction-and-demolition-materials#:~:text=600%20million%20tons%20of%20C&D,materials%20in%20the%20C&D%20debris.
- Regulations.gov, Standards of Performance for Municipal Solid Waste Landfills –https://www.regulations.gov/document/EPA-HQ-OAR-2003-0215-0210
- NOAA National Weather Service, Hurricane Andrew 1992 – https://www.weather.gov/lch/1992Andrew#:~:text=Hurricane%20Andrew%20caused%207%20deaths,%2427.3%20billion%20in%20total%20damages.
