Safety Around Anaerobic Digesters

Introduction

Safety is essential in every industry and should be applied to the farm and anaerobic digester environment. Anaerobic digestion, which operates in the absence of oxygen to produce biogas (composed of methane, carbon dioxide, hydrogen sulfide, and other impurities), is often used to generate heat and electricity, vehicle fuel, and renewable natural gas. However, this process has potential hazards associated with its use, posing severe threats to human life if not properly managed.

Hazards and Risks Associated With A Digester

Care should generally be taken when around a digester, but certain areas require extra care and caution, including those with one or more hazards or risks.

Hazards

1. Hot Surfaces: Hot engine blocks, exhaust pipes, and hot water or steam pipes are commonly found around a farm digester. If proper care is not taken, contact with these surfaces can cause burns. Areas with equipment pipes conveying hot fluids should be avoided, and workers with less experience and visitors should be cautioned from going to those places. When practical, hot surfaces should be insulated, or a guard installed to reduce the likelihood of burns.
2. Feedstock and digestate spills, biogas leakage: Most digesters experience spills and leaks, which can be significant to the environment and may lead to serious injuries or an explosion in rare cases. Spills can occur when loading/unloading a digester. When a spill occurs, the source of the spillage should first be identified and controlled, after which a temporary containment structure should be placed around the affected area until the spill is cleaned up. The cause of the leakage should be fixed to reduce the loss of biogas and minimize the risk of explosion.
3. Ignition sources: Smoking near the digester can create a risk of ignition or sparks, so it should be prohibited. The nearest smoking area should be 500 feet away from the digesters. Safety signs should alert workers to exclude all potential ignition sources.
4. Noise levels: Noise from the generator sets can cause discomfort depending on the exposure, temporary or permanent deafness. Protective devices should be worn to prevent hearing loss or damage.

Risks

Several risks are common to farm digesters—it is important to be aware of them so that you can minimize their likelihood of causing problems. Risks around a digester include:

1. Asphyxiation: Due to the gases in the digester, asphyxiation (deprivation of oxygen) can occur in confined spaces connected to the digester. Safety signs should be posted in confined spaces to alert employees and visitors. Workers should undergo safety training before entering a confined space because hazards are not always obvious. 
2. Risk of fires and explosions: The danger of fire and explosion is especially significant near digesters and gas reservoirs (Majolaine, 2020). Explosions can occur in the digester if air mixes with the methane that a digester produces, air can seep into the digester during maintenance and tank draining.
3. Risks associated with rotating mechanical equipment: Equipment like pumps, electric motors, mixers, and shafts present risks not only to the operator but also to people around it; rotating shaft can ensnare a person or can unexpectedly eject objects, which can lead to injuries. The noise from the equipment can, in some cases, lead to hearing loss.
4. Risks associated with pathogens (diseases): The source of waste used in the digester might contain some pathogens, such as bacteria (Escherichia coli (E. coli) and Salmonella), viruses, and fungi. These pathogens might be biological hazards (Marjolaine, 2020).
5. Drowning and falling: Areas near liquid storage and ponds can be particularly hazardous due to their slippery nature; in such locations, it is essential to have safety equipment like ropes and ladders readily available. This is necessary when employees work on equipment close to the digesters, with a higher risk of drowning. Occasionally, when there might be a need to service or maintain equipment above the digester (usually a place of height), safe measures must be taken to prevent falls, such as guardrails.

Going back to the topic of confined space safety (which is a big deal for digesters as well as manure handling systems in general), safety procedures for confined spaces are described in paragraph (c)(5)(i) of the Permit-required of the Occupational Safety Health Administration (OSHA) document (OSHA, 2011). Although a farmer with less than ten employees is exempted from this standard, it is still advisable to follow these guidelines:

• A gas detector must be used to measure the atmosphere of the confined space before entry.
• There should be due monitoring of the personnel going in by another personnel from a safe distance.
• The employee must wear a device that can be activated by personnel on-site to pull to safety. 

An article on "Confined Space Manure Storage Emergencies" by Penn State Extension provides best management practices for farmers to follow during emergencies. In addition, the document "Confined Space Manure Storage Hazards" provides more details on this topic. Figure 1 shows a picture of a confined space, while Table 1 details the safety limit of common gases associated with a digester.

Overall, minimizing safety hazards around a digester relies heavily on the anaerobic digester operator and the designer, but it also requires the collective effort of individuals around the digester. 

Safety Equipment For A Digester

Safety equipment includes protective gear or devices worn, used, or installed to prevent harm and accidents. It is designed to avoid injuries and life-threatening situations and is tailored to the specific requirements of different fields of work. Safety equipment can include:

Personal Protective Equipment (PPE): According to OSHA, PPE is the "equipment worn to minimize exposure to hazards that can cause serious workplace injuries and illness." Typical PPE includes safety boots, gloves, reflective jackets, helmets, and respiratory protectors (Figure 2). They should be used whenever a corresponding hazard exists. Farmers should have PPE available onsite and train employees on the proper use of PPE.

Employers should emphasize the use of this equipment by new and existing personnel (Farm Works) (AgSTAR, 2011). A respirator is a key protective equipment that can prevent inhalation of particles used on the farm. The Penn State Extension article "Farm Respiratory Protection" describes the best way to effectively use a respirator for protection and provides information on approved respirators, categories, and types.

First Aid Kit: A well-maintained first aid kit is an important piece of safety equipment for any farm digester. It should be in easily accessible areas, have adequate supplies, and be frequently inspected to replace expired and finished products. First aid kits come in many sizes and levels of complexity. A "Class A" kit is used for common workplace injuries, and a "Class B" kit is recommended for high-risk environments. Figure 3 shows an image of a first aid kit with emergency supplies.

Fire Extinguisher: This equipment is used to put out small fires that can be done without danger to the farmer and the farm. ABC extinguisher (Class A: Ordinary combustible, Class B: Flammable liquid, and Class C: Energized Electrical equipment) is classified as a multi-purpose extinguisher preferable to water-based ones. The facility should be shut down before fighting the fire. An emergency shut-down switch should be readily accessible near the digester. The fire extinguisher in Figure 4 is of class ABC.

Multi (gas) Detector: A multi-gas detector measures the concentration of gases like Hydrogen sulfide (H₂S), oxygen, and carbon monoxide (CO). They can be used to check the air condition in confined spaces before entering. An article titled "Use Personal Gas Monitors to Avoid Exposure to Toxic Hydrogen Sulfide" further provides information on the use of gas monitors, workday exposure limit for H₂S, symptoms that can occur when exposed to various ranges of H₂S, gas monitor manufacturers, and other helpful information.

Accident prevention signs and tags: Signs are temporary or permanent warnings of an existing or high-hazard likelihood. Tags are mainly used to notify personnel of temporary hazards or safety restrictions, such as when equipment must be kept off during maintenance. Some prevention signs and tags can be seen in the Figures below.

Practical Steps To Reduce Exposure To Risk

Precautions should be followed to ensure the safety of life and property. The following steps should be adhered to in an anaerobic digester environment.

1. Select Appropriate Building Material for Constructing Anaerobic Digester: Materials used in constructing anaerobic digesters depend on the type of digester:
   • Complete Mix or Continuous stirred tank reactors (CSTR): This type of digester is usually made of steel, reinforced concrete, or fiberglass.
   • Plug flow anaerobic digesters: They use buried reinforced concrete, fiberglass, or steel.
   • Covered lagoon anaerobic digesters: They are constructed with pond-linear materials and a fixed or floating impermeable membrane cover.

   Anaerobic digesters are mostly constructed with reinforced concrete, fiberglass, or steel, which can be prone to issues like corrosion, chemical degradation, mechanical wear, and biological growth. Routine inspection should, therefore, be carried out to help detect these issues earlier.

2. Provide Training: This is a vital safety aspect; all personnel, visitors, and farm workers should be trained before going close to the digesters or areas around it. The personnel working with the digester should be trained in properly handling and storing the biogas, emergency shutdowns, and gas leak detection. Signs and symbols should be placed in confined spaces, flammable and sensitive areas, which can be obtained from safety stores located in the vicinity.
3. Develop an Emergency Action Plan (EAP): An emergency action plan should be created to guide employees in emergencies associated with the digester. It should provide vital information to aid prompt response in emergencies, including emergency phone numbers of the digester operator, farm manager/owner of the farm, local emergency responders, utility company associated with the digester, safety-trained personnel on the farm, facility addresses, and exact location of the digester on site, and a detailed site map. The document is to be located and clearly labeled at the digester. OSHA regulations allow organizations with less than ten (10) employees to communicate the EAP orally. However, a well-written Emergency Action Plan can be helpful for any farm.
4. Establish A Tag-Out and Lock-Out System as A Safety Strategy: This safety strategy ensures the complete shutdown of equipment before its repair and maintenance to prevent an unforeseen accident. OSHA provides the safety strategy process under standard 29 CFR 1910.147. Penn State Extension's "Lockout/ Tag out to reduce Ag injuries and Fatalities" article includes comprehensive information on this strategy.
5. Maintenance and upkeep of the digester: Regular maintenance is a crucial safety strategy for an anaerobic digester because it helps to prevent potential malfunctions and safety hazards. Proper maintenance ensures that the digester operates efficiently and reduces the risk of accidents or equipment failure, thereby protecting personnel and the environment. The following maintenance and safety tasks can be a starting point for your farm's maintenance schedule (adapted from the USDA NRCS Conservation practice standard for anaerobic digester Code 366).

• For the digester, ensure the influent has the correct loading rates and total solids content.
• Adjust the farm's nutrient management plan to account for each feedstock's nutrient impact.
• Accurately operate the digester according to its instructions.
• Estimate the amount of methane in biogas, its output, and its potential for energy recovery.
• Give an overview of the scheduled startup processes, standard operating procedures, safety precautions, and routine maintenance.
• Describe backup plans for when equipment breaks down.
• Provide guidelines for using biogas safely and for flaring it.
• Describe in detail how the digester and other parts are maintained.
• Explain troubleshooting carried out on the digester.
• Provide a monitoring plan that includes frequently recording and measuring the amount of biogas produced, operational temperatures, digester inflow, and other pertinent data.
• Depending on the kind and construction of the digester, maintain the interior temperature.
• Maintain the temperature of mesophilic digesters between 95°F and 104°F, with 100°F being the ideal range. Keep daily temperature variations to less than 1°F.
• To avoid unintentional spills or discharges into the gas collecting system, the digester should be designed with a suitable freeboard and equipped with overflow or automated shutdown systems.
• Create and save an emergency contact list to speak with trained professionals.

Conclusions

Safety is an important aspect of every workplace and farm. Keeping the workers safe should be the utmost priority of the farm owner, especially if the farm has an anaerobic digester on it. Digesters used in the production of biogas on a farm to generate energy can include potential hazards and risks, but this can be well managed if designed and operated safely, and the awareness of the potential dangers associated with its use. The use of the right safety equipment and the maintenance and upkeep of the digester should be well communicated. Farmers and farm workers are encouraged to conduct more detailed reading and research, starting with the references provided. 

References

Agstar. (2011). Common Safety Practices for On-Farm Anaerobic Digestion Systems.

Anonymous. (2014, October 8). Material For Biogas Digesters. Energypedia.

Anonymous (2020). E3A: Anaerobic Digester Applications for The Farm or Ranch 2. Fact Sheet Em705. University Of Missouri Extension, Columbia, Missouri.

Kirk, D. (2011). Stay Safe in and Around Anaerobic Digesters. Progressive Dairy Magazine (Online resource):  Accessed January 2025.

Anonymous.(2024). Module 7: Manure Pit Safety. Virginia Tech University (Online resource): Accessed January 2025.

Westenbroek, P., and Martin, J. (2019). Anaerobic Digesters and Biogas Safety. Livestock and Poultry Environmental Learning Community (Online Resource): Accessed December 2024.

Marjolaine. (2020). Risks And Safety Measures for Anaerobic Digestion: How Can You Make Your Plant Safer? Biogas World (Online resource): Accessed December 2024.

OConnor.(2021). Fire Extinguisher Placement Guide. NFPA Website (Online resource): Accessed November, 2024.

Anonymous. (2011). Permit Required Confined Spaces. Occupational Safety and Health Administration Website (Online resource): Accessed December 2024.

Anonymous. (2024). Personal Protective Equipment. Occupational Safety and Health Administration Website (Online resource): Accessed December 2024.

Steel, J., Murphy, D., and Manbeck, H. (2023).  Confined Space Manure Storage Hazards.  Penn State Extension Website (online resource): Accessed December 2024.

Brown, S., and Fetzer, L. (2023).  Lockout Tag Out to Reduce Ag Injuries And Fatalities. Penn State Extension Website (online resource): Accessed December 2024.

Fabian, E., Hofstetter, D., and Pate, M. (2023). Use Personal Gas Monitors to Avoid Exposure To Toxic Hydrogen Sulfide.  Penn State Extension Website (online resource): Accessed December 2024.

Fetzer, L., and Murphy, D. (2023). Farm Respiratory Protection. Penn State Extension Website (online resource): Accessed December 2024.

Hawkins, S. (2021). How To Manage Toxic and Suffocating Manure Gases (PDF). University of Tennessee Institute of Agriculture. Report #W1052.

Anonymous. (2020). Anaerobic Digester Biogas System Operator Guidebook United States EPA Document #EPA430-B-20-003.

Anonymous. (2017). Conservation Practice Standard, Anaerobic Digestion 366-Cps-1. USDA Natural Resources Conservation Service.

Knowledge Check 

Check your understanding of this article by answering the following questions (answers provided below). 

1.    Which of the following is not usually a safety concern at or around a farm digester?
A)    Drowning and falling
B)    Fire and Explosion
C)    Heavy lifting
D)    Ignition sources

2.    Can the following be sources of health or safety hazards associated with a digester? Mark all that apply:
A)    Gas poisoning
B)    Fires and explosion
C)    Pathogen
D)    Swimming pool

3.    What monitoring procedures should be in place for personnel entering a confined space? Mark all that apply
A)    Use video cameras
B)    Assign another person to monitor from a safe distance
C)    Provide walkie talkies
D)    Set up a time clock

4.    What steps should be taken to locate and contain spills while loading and unloading a digester?
A)    Conduct regular inspections and have containment materials ready
B)    Use larger containers and slower processes
C)    Increase the number of workers
D)    Decrease the amount of feedstock

5.    What general safety signs and protocols should be implemented to ensure overall safety in areas with digesters?
A)    Emergency exits and hazard warnings
B)    Promotional banners
C)    Company logos and branding
D)    Decorative lights and plants

6.    What kind of training is crucial for workers in an anaerobic digester plant?
A)    Customer service training
B)    Safety training specific to the risks in the plant
C)    Financial management training
D)    Marketing and sales training

7.    Why is it important to have clear safety signs in riskier areas?
A)    To decorate the facility
B)    To ensure regulatory compliance
C)    To promote the farm's branding
D)    To alert employees and visitors of potential hazards

Answers
1.    C       2.    B & C       3.    B       4.    A       5.    A       6.    B       7.    D

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The article was written by Mary Akingbasote, a graduate student at Penn State Department of Agricultural and Biological Engineering.  Reviewed by Florence Becot, Ph.D., Michael Lee Hile, Ph.D., and Daniel Ciolkosz, Ph.D.

Creation of this article was supported by Penn State Extension and the C-CHANGE Grass to Gas Project (USDA-NIFA award No. 2020-68012-31824, Consortium for Cultivating Human And Naturally reGenerative Enterprises)