Hydrogen Sulfide Release from Dairy Manure Storages Containing Gypsum Bedding

Why Look at Potential Connections Between Gypsum and Hydrogen Sulfide?

Gypsum, a recycled product from the waste streams of the manufacturing and construction industries, provides ideal bedding for livestock because it absorbs moisture keeping the animals dry, is non-abrasive, and discourages bacterial growth as it is inert and non-organic. Gypsum is calcium sulfate (CaSO4•2H2O) and provides a sulfur source material that potentially increases H2S production from the manure storage due to the high generation of H2S levels. The goal of this research project is to compare H2S concentrations among Pennsylvania dairy farms that use traditional bedding, gypsum bedding and gypsum bedding with an amendment. Related: Manure Storage Safety | 2013 Waste to Worth conference proceedings by these authors “Gypsum Bedding – Risks & Recommendations for Manure Handling

Information from this gypsum bedding project was recently featured on a pre-conference webcast More…


What did we do?

Ten farms were observed during 19 fall and spring agitation events. Portable multi-gas meters (models MX6, M40 and Tango, Industrial Scientific, Pittsburgh, PA) were place around the perimeter of each manure storage, 10 meters downwind of the storage perimeter and attached to the operator for the duration of the event to monitor exposure. Each meter recorded gas measurements every minute during the first hour of manure agitation. Wind speed, wind direction and air temperature were recorded every minute during these events using a weather station (Kestral Communicator model 4500, Nielsen-Kellerman, Birmingham, MI). Manure storage design and manure handling practices were characterized. Manure was characterized according to general chemistry (temperature, pH and oxidation-reduction potential) as well as submitted for a full nutrient analysis content including total nitrogen, sulfur, calcium, percent solids and phosphorus source coefficient.

What have we learned?

Figure 1 shows cumulative H2S concentrations versus gypsum application rate. Gypsum and non-gypsum farms represented by the diamonds show a significant increase in cumulative H2S concentrations with increasing gypsum application rate. The observations depicted by the squares represent farms that use Vital™ Breakdown (manufactured for Homestead Nutrition, New Holland, PA), a treatment reported to reduce H2S emissions. One of the farms observed, also pointed out in Figure 1 by the triangles, uses OK-1000 (manufactured by Pro-soil Ag Solutions, Hawkins, TX) as a manure additive. Though it appears that these manure additives reduce cumulative H2S concentrations, the reduction was not statistically significant because not enough observations were recorded to provide statistical power.

Movement of manure prior to the storage location appears to allow H2S to escape before entering long term storage (shown in Figure 1 as prior agitation), lowering H2S concentrations released during agitation of the manure storage. Wind flowing into proximate buildings inhibited the dissipation of H2S thereby increasing cumulative H2S concentrations as shown by the observation pointed out by the image of wind direction in Figure 1. Four observations of operator exposure (out of 19 observations including non-gypsum, gypsum and gypsum with additive) yielded H2S concentrations above 20 ppm (above the Occupational Health and Safety Administration’s (OSHA) recommended exposure limit. Of the four operators above 20 ppm H2S, three were working over the rim of the storage, which increased risk of exposure. Downwind H2S concentrations above 20 ppm ten meters from storage occurred for eight out of the fourteen observations from farms that use gypsum, showing that children and animals within ten meters of the storage are at risk.

Figure 1. cumulative hydrogen sulfide concentrations for first 60 minutes of agitations versus gypsum application rate

Future Plans  

Bench scale work will be performed in hopes of finding a manure additive that reduces H2S production from farms that use gypsum. A new product (Dri Mat) formulated from its precursor, VitalTM Breakdown, will be mixed with manure and analyzed to determine the efficacy to reduced H2S emissions. Iron oxide, a by-product of acid mine drainage passive treatment lagoons will be one of the treatments for this work as well.


Hile, Michael, Ph. D. Candidate in Agricultural and Biological Engineering (ABE) at Penn State (PSU) mlh144@psu.edu

Dr. Eileen E. Fabian, Professor in ABE at PSU, Dr. Herschel A. Elliott, professor in ABE at PSU, Dr. Robin Brandt, Senior Lecturer in ABE at PSU, Dr. C. Alan Rotz, Agricultural Engineer at USDA-ARS and Dr. Ray Bryant, Soil Scientist at USDA-ARS.


This project would not have been possible without the support from Natural Resources Conservation Service’ (NRCS) Conservation Innovation Grant (CIG) program, USA Gypsum and Industrial Scientific.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.

Gypsum Bedding – Risks and Recommendations for Manure Handling

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Note: This topic was included in a poster at the 2015 Waste to Worth conference and was part of a pre-conference webinar on gypsum bedding & practical considerations for manure handlers.


Gypsum products created from construction industry waste streams provide low-input cost bedding. Some dairies report decreased somatic cell counts in milk with its use.

Recently, several incidents involving human and livestock death or injury have highlighted the possible creation of dangerous gases at farms using gypsum bedding. Human lives were lost at two separate events. In a third incident, a 2-year old and 4-year old were found unconscious adjacent to a manure storage where gypsum was present. In the European Union (EU) several agencies have forbade the use of gypsum as bedding based on losses of livestock as well as previous policies that restricted gypsum from landfill disposal.

Gypsum is a common term for hydrated calcium sulfate (CaSO4¬-2H2O). It is suspected that under the right manure storage conditions anaerobic bacteria convert the sulfur (S) in gypsum to hydrogen sulfide (H2S), a gas that can be deadly. Movement such as agitation of manure can lead to large H2S fluxes and localized dangerous levels of the gas.

While this is concerning, there remain many farms that utilize gypsum without incident. Data on this subject are lacking.

The goal of this symposium presentation is to update attendees on this ‘current event’ in manure management. Some laboratory studies are expected to complete between the time of this abstract composition (October 2012) and the symposium date. A general outline of the presentation includes:

  • Recap of cases leading to concern with this product
  • Policies of the EU and US
  • Industrial standards for dangerous H2S levels (OSHA and other)
  • Biological and chemical avenues of H2S production
  • Research review of gypsum use in manure
  • Recommendations for safety, management and education.

Why Are We Concerned About Gypsum Bedding on Dairies?

The goal of this ‘current event’ presentation is to increase national awareness of several deaths and severe injuries that have occurred recently in the Mid-Atlantic area involving manure gases. Several dairies where incidents occurred use gypsum from recycled drywall as low cost bedding material. There is great concern that gypsum increases dangerous hydrogen sulfide emissions from manure storages at these farms.

What Did We Do?

Recent deaths and severe injuries near manure storages highlight the importance of understanding and outreach needs. An overview of incidents involving manure gases at dairies that bed with gypsum will be given. Concerns and risks will be discussed, followed by recommendations on how to prevent incidents.

What Have We Learned?

In a true first step to determine gas productions associated with gypsum in manure preliminary bench-top scaled comparisons of manures with and without gypsum are ongoing in Pennsylvania and Wisconsin. A status update on progress on this early work will be discussed.

Future Plans

Literature and base knowledge on this subject are lacking. More work is needed to assess the actual risk to workers around manure storages where gypsum is present. There are countless factors that can contribute to gas production from manure storages. Identification of key factors that may lead to production of hydrogen sulfide when gypsum is present is needed. Further outreach to manure handling industries is warranted.


Robert Meinen – Senior Extension Associate, Penn State University Dept. of Animal Science rjm134@psu.edu

Davis Hill – Senior Extension Associate, Agricultural Safety and Health, Penn State University Dept. of Agricultural and Biological Engineering.

Rebecca Larson – Assistant Professor: Bio-waste, University of Wisconsin Dept. of Biological Systems Engineering.

Asli Ozkaynak – Post Doc Researcher, University of Wisconsin Dept. of Biological Systems Engineering.

Dennis Murphy – Distinguished Professor, Agricultural Safety and Health, Penn State University Dept. of Agricultural and Biological Engineering.

Eileen Fabian Wheeler – Professor, Animal Welfare and Agricultural Emissions, Penn State University Dept. of Agricultural and Biological Engineering.

Robin Brandt – Lecturer, Land-based treatment/recycling systems, Penn State University Dept. of Agricultural and Biological Engineering.

Herschel Elliot – Professor, Fate and Control of Pollutants in Soils and Water, Penn State University Dept. of Agricultural and Biological Engineering


The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2013. Title of presentation. Waste to Worth: Spreading Science and Solutions. Denver, CO. April 1-5, 2013. URL of this page. Accessed on: today’s date.

Anaerobic Digesters and Biogas Safety

When manure is anaerobically digested, the biogas produced is primarily composed of methane and carbon dioxide, with lesser amounts of hydrogen sulfide, ammonia, and other gases. Each of these gases has safety issues. Overall, biogas risks include explosion, asphyxiation, disease, and hydrogen sulfide poisoning.

Image: US Municipal Supply Company.

Extreme caution is necessary when working with biogas. Adequate ventilation, appropriate precautions, good work practices, engineering controls, and adequate personal protective equipment will minimize the dangers associated with biogas. Wherever possible, digester-associated tasks and maintenance should be performed without anyone having to enter confined spaces, including pits. Systems should be initially designed so that confined space entry is not required to perform maintenance.

The information presented here is for reference purposes only. No liability is implied.

Biogas Hazards


Methane, approximately 60% of biogas, forms explosive mixtures in air. If biogas is diluted between 10% and 30% with air, there is an explosion hazard. In 2003, several explosions on Canadian swine farms were thought to have been caused by the methane in biogas exploding (Choinière, 2004). Hydrogen sulfide and ammonia are also potentially explosive.

Because of the explosion hazards, no open flames should ever be used near a digester. Also, equipment such as large engines and electric generators must be suitable to the environment so a spark will not ignite the gas. Explosion-proof equipment and electrical service, as well as non-sparking tools, should be used around digesters and biogas. There must be no smoking near the digester or related biogas lines and equipment.


Asphyxiation from biogas is a concern in an enclosed space where manure is stored. Osbern and Crapo (1981) report one case of three people who died from asphyxiation created by swine manure gas in an enclosed space. Even open-topped manure pits can generate methane at a sufficient rate to push out the air above the manure and render the space oxygen-deficient.

Never enter a facility where manure is stored or where there is a suspected biogas leak as natural ventilation cannot be trusted to dilute the explosion hazard sufficiently. Airing out a facility does not impart safety, as some of the gases produced are heavier than air. If a person is found unconscious in such a facility, do not enter the facility because you may be overcome as well. Contact emergency services so that firefighters wearing self-contained breathing apparatus (SCBA) can safely retrieve the victim.


Animal manure contains bacteria, viruses and, possibly, parasites. Biogas is generated by the anaerobic digestion of manure, which occurs because of the bacteria present in animal wastes, some of which can produce infection. When handling waste material, exercise appropriate precautions by using personal protective equipment to avoid contact with manure. Washing after working around the digester is recommended. It is particularly recommended to wash hands before eating and drinking and before touching the eyes or other mucous membranes.

Keeping the digester facility clean will reduce disease hazards as well as the spread of odors and fly populations in the digester facility.

Components of Biogas

Biogas consists mainly of 60% methane and 40% carbon dioxide, with low levels of hydrogen sulfide and other gases. Each of these gases can displace oxygen.


Methane is lighter than air and will collect toward the upper spaces of the building. It is explosive at 5% to 15% concentrations. While methane is not a toxic gas, it displaces air so that, in a confined space, it creates an oxygen-deficient atmosphere. This is how it kills.

Carbon Dioxide

Carbon dioxide is an odorless gas that is heavier than air. In a quiescent space, carbon dioxide can layer near the floor. Slightly elevated levels of carbon dioxide increase heart rate and respiration rate. Higher levels displace oxygen supply in the bloodstream, which can cause unconsciousness and death.

Hydrogen Sulfide

Hydrogen sulfide is a highly toxic gas that is heavier than air. At very low levels, it smells like rotten eggs and can produce eye irritation. At dangerous levels, it destroys the sense of smell and produces respiratory paralysis. Thus, at dangerous and fatal levels, where one can literally drop dead, there is no odor to warn of its presence.

The following table shows the health effects of hydrogen sulfide at different concentrations.

Parts per million (ppm) Possible health effects
0.01-0.3 Odor is detectable
1-10 Moderate to strong odor
Sleep loss
10-150 Irritation of the eyes

Irritation of the lungs

150-750 Severe health effects
Death becomes more likely
>750 Death may occur in minutes


Ammonia is a gas that is lighter than air, has a pungent odor, and can irritate the eyes and respiratory tract. Ammonia can displace oxygen in the bloodstream.


Manufacturer Warnings

Failure to heed manufacturer warnings may result in death or serious injury. Contact the manufacturer for maintenance and service requirements and availability of service.

Safety Walk-Throughs

A safety walk-through can help you determine potential hazards and preventative measures. Cornell University developed a comprehensive self-assessment guideline for farmers. It is intended to be used by farm owners and managers or farm staff who are responsible for the operations and/or maintenance of anaerobic digesters and their related processes. It provides guidance for process and job evaluation with suggestions based on typical potential hazards for farm digester systems and their associated preventative measures.

Gas Sensors

Explosion, suffocation, and poisonous gas hazards may be detected using gas sensors. These sensors include both disposable and electronic sensors. Electronic sensors need testing regularly, and these sensors may have a disposable component that needs periodic replacement. Only qualified people should use these sensors to determine if an area is safe.

Personal Protective Equipment

An area where manure is stored should never be entered without the appropriate personal protective equipment, which may include a self-contained breathing apparatus (SCBA). The use of protective equipment such as an SCBA is covered by OSHA regulations, and the operator must be certified in its use with equipment-fit testing and medical clearance.

For More Information

  • Pennsylvania State University: Biogas Safety. This site also has biogas and anaerobic digestion information and links.
  • Pennsylvania State University Manure Pit Safety. This site has educational videos that demonstrate manure storage hazards, the importance of monitoring manure gas levels before entry, and recommendations for the design and installation of ventilation of manure storages, emphasizing the importance of a positive pressure system for forcing fresh air into the storage. The site also has fact sheets that address manure storage hazards, monitoring for gases and oxygen, ventilating manure gases, and emergency rescue procedures. Information for accessing ANSI/ASABE S607, Ventilating Manure Storages to Reduce Entry Risk, and ASABE EP 470, Manure Storage Safety, are provided.
  • Respiratory Protection, Personal Protective Equipment. OSHA Regulation 1910.134 that applies to some anaerobic digester facilities; all operators should refer to these standards in an advisory capacity.


Brown, N.J. 2007. Conducting a safety walk-through on a farm: hazards of the manure handling system, anaerobic digester, and biogas handling system – a self-assessment guideline for farmers. Manure Management Program. Cornell Dept. of Biological and Environmental Engineering. Ithaca, NY.

Martin, J.H. 2008. A New Method to Evaluate Hydrogen Sulfide Removal from Biogas. M.S. Thesis. Raleigh, N.C.: North Carolina State University, Department of Biological and Agricultural Engineering.

Osbern, L.N., and R.O. Crapo. 1981. Dung lung: a report of toxic exposure to liquid manure. Ann. Intern. Med. 95(3):312-4.

Choinière, Y. 2004. Explosion of a deep pit finishing pig barn, investigation report on biogas production. In Proc. ASAE/CSAE Meeting. Ottawa, Ontario, Canada.

Wright, Peter. 2001. Overview of Anaerobic Digestion Systems for Dairy Farms. Natural Resource, Agriculture and Engineering Service:NRAES-143. Ithaca, NY.

Contributors to This Article


Peer Reviewers

Manure Storage Safety

Manure storage structures are an integral part of livestock feeding operations. They allow for manure containment until conditions are favorable for land application or other treatment. Manure may be stored in enclosed structures, near or directly below the animal housing facility or it may be stored in open structures such as above ground tanks or storage ponds/lagoons.

Recommended Resources:

Archived webinar: Manure Safety & Transport

Penn State Extension Manure Pit Safety Fact Sheet Series

The risks differ for each type of manure handling system. Enclosed structures are commonly associated with asphyxiation hazards related to gases produced during manure decomposition. Open structures are most often associated with drowning. Most systems include pumps or mechanical components that present the risk of entanglement. It is critical that producers take the time to assess the risks on their operation and evaluate ways to prevent hazardous situations and to develop emergency response plans. Related: Manure Storage Safety fact sheet

Enclosed Structure Hazards

Enclosed structures, especially those located below ground, are potentially the most hazardous for people and livestock. While being stored, manure undergoes decomposition. As a result, many potentially harmful or fatal gases are produced, but most remain at low levels or are adequately diluted by the ventilation system. Situations in which these gases can become deadly include:

  • Agitation of the liquid manure prior to or during pumping from the pit
  • Power outage or other failure of the ventilation system
  • Buildup of flammable or explosive gases

Manure Gases and Their Associated Safety Concerns

Hydrogen Sulfide


A sign like this “Danger: Liquid Manure Storage” should posted for all types of liquid manure storage, enclosed or open. Consider posting this message in other languages if there are non-English speakers living or working on or near the farm.

Hydrogen sulfide is the greatest danger to humans in enclosed manure storage areas. It has a characteristic “rotten egg” smell and is heavier than air, so it tends to collect in the lower levels of a structure and in corners of the storage area where air circulation is least available. It quickly desensitizes the sense of smell so that a person does not detect greater levels of the gas after breathing it. It can be rapidly released when manure is agitated. At low levels (10 ppm) it can irritate the eyes. Death can occur when it reaches levels of 500 ppm or greater.


Ammonia has a distinctive, sharp odor and is heavier than air. It becomes irritating to humans at around 50 ppm. If it reaches levels of 1000 ppm or more, it can be deadly, although most people are so uncomfortable at this level, they usually seek relief by leaving a building before it reaches dangerous concentrations. Prolonged exposure to high ammonia levels can also impact animal performance.


Methane is a concern because it is potentially explosive at levels above 50,000 ppm. It is lighter than air and odorless. In the fall of 2009, enough manure pit-related fires and explosions were reported to attract renewed attention to the safety concerns related to gas buildup. Other potentially explosive gases produced by manure decomposition are hydrogen sulfide (H2S) and phosphine (PH3, but both become lethal to animals and humans at concentrations far below that required for ignition. A literature review by Iowa State University (supported by the Pork Board) provides additional information on this topic. Deep Pit Swine Facility Flash Fires and Explosions.

In a properly designed anaerobic digester, methane production can be enhanced and possibly captured for use in electrical generation. For more information see Introduction to Biogas and Anaerobic Digestion.

Carbon Dioxide

Carbon dioxide is odorless, but can cause asphyxiation if it displaces enough oxygen in the air. It is heavier than air and tends to accumulate in the same areas as hydrogen sulfide.

Open structure hazards


This manure storage structure is fenced and has a life preserver in a prominent location. The concrete ramp by the gate provides an easy escape point for humans and wildlife that fall into the pit. Note that this fencing will discourage entry but will not prevent a determined child from exploring the pit. If young children live on or visit your farm, a chain link fence will provide a higher degree of deterrence than multi-strand wire fences and gates such as this.

Open manure storage ponds or above ground storage tanks also pose hazards, the most obvious of which is drowning. A storage pond may form a crust on the surface that appears solid and capable of holding a person’s weight. Unfortunately, this is not always the case. Children are also at risk of drowning in these structures and safety considerations must always include ways to prevent access to these areas, such as fencing, gates with locks and outside walls on concrete structures that preclude easy entry.

To prevent drowning, it is recommended that farmers purchase and install safety measures such as life preservers or life vests, throw ropes, and/or safety harnesses (with anchor points around the structure). This equipment can save lives; not only for the victim but rescuers who can safely assist without entering the structure themselves. This was illustrated in May, 2012 in a tragic incident in Maryland. A farmer and two of his teenage sons were drowned in a manure storage structure while attempting to pump the manure out for land application. There were no surviving witnesses, but the

likely scenario is that one of the three fell into the pit and the other two died trying to rescue him.

Recommended Reading

Case Studies

Authors: Chip Petrea, University of Illinois and Jill Heemstra University of Nebraska
Reviewers: Saqib Mukhtar, Texas AgriLife Extension; Jennifer Zwicke, USDA NRCS; Troy Chockley, USDA NRCS, Greg Martin, Penn State

Waste to Worth Preview: Gypsum Bedding Risks and Rewards

In a preview of the useful topics that will be discussed and presented via posters and informational sessions at the Waste to Worth Conference in Seattle, a group of professors and extension professionals present about the use of gypsum in dairy bedding. This presentation was originally broadcast on February 27, 2015. More… Continue reading “Waste to Worth Preview: Gypsum Bedding Risks and Rewards”