Unit 7.5: Occupational Health and Safety

AD can pose a number of occupational hazards including those related to chemical, thermal, biological, ergonomic, occupational stress, physical, trauma, and violent hazards.  The US Department of Labor (DOL) Occupational Safety and Health Administration (OSHA) establish and enforce standards for occupational health and safety.  The OSHA standards that apply to AD are included in 29 CFR 1928 agricultural standards as well as 29 CFR 1910 general industry standards. It should be noted that the Occupational Safety and Health Act of 1970 Section 5. A.1 requires each employer to furnish to each of his employees employment and a place of employment free from recognized hazards that are causing or likely to cause death or serious physical harm to his employees.  Since the agricultural standards do not address many worker safety issues with AD, the best practice is to use the 1910 general industry standard for reference.  Currently, there are 22 states with state OSHA plans.  These state plans have standards at least as effective as the federal standards.  It is important that AD manager assess their operation for the standards that apply whether in a state under federal OSHA or a state plan OSHA.

AD’s contain confined spaces associated with the digester itself and the collection systems up and downstream of the digester. Confined spaces allow entry by an employee but have limited ventilation which can lead to a build-up of toxic or flammable gases. These confined spaces are permit required spaces due to the atmospheric hazards associated with digesters. The biogases produced as part of the AD process include gases that could asphyxiate a worker or explode and cause injury to a worker. The best practice to reduce exposing a worker to risk of confined space entry is to remove the hazard by engineering design. Consideration should be given during digester design to place equipment, particularly that which may require regular maintenance (e.g. pumps, mixing apparatus), outside the digester or alternatively designing the digester system to allow maintenance to be performed from outside the digester. This would allow for equipment maintenance without the worker making a confined space entry. If confined space entry is required, the employer must assess the potential hazards to workers upon entering the space. Confined space entry into a digester system component is permit required by OSHA. Prior to entry numerous factors need to be evaluated including atmospheric hazards, ventilation, fall protection, control of hazardous energy, potential for engulfment, mechanical hazards, thermal hazards and rescue considerations. A written plan is required prior to workers making entry. Workers must also receive training on known hazards and responsibilities including cancelling the permit and rescue procedures. OSHA 29 CFR 1910.146 Confined Spaces outlines the specific requirements for entry into permit-required confined spaces. ANSI can also be referenced as an additional standard.

Worker safety also needs to address 1910.1200 Hazard Communications as the gases produced have chemical characteristics known to cause health hazards. Biogas contents can lead to serious health problems or death with acute or chronic exposure over recommended limits. Four primary gases associated with waste handling and biogas production, methane, carbon dioxide, hydrogen sulfide, and ammonia should be addressed. In addition, carbon monoxide levels may also require attention if equipment used in or around the digestion system produces this gas. In order to reduce a worker’s exposure to these gases, permissible exposure limits (PEL) have been established. They are listed in Table 1.

Table 1: Air Contaminant Exposure Limits. Source: Becky Larson


Ventilation must be provided (part 1910.252) to maintain exposure limits below that listed in Table 1.  This may be required for additional areas where gas or fumes may be present including locations with electrical generators.  Based on monitoring, if ventilation does not adequately address the atmospheric hazards for worker entry, atmospheric respirators must be used.  Two types of atmospheric respirators include self-contained breathing apparatus (SCBA) or air-supplied respirators.  Prior to use of these respirators by a worker, the individual needs to be properly evaluated, fit tested and trained on respirator use.

Additional safety factors requiring evaluation of risk to workers relate to daily digester maintenance and operational requirements.  Working surfaces may include working at different elevation levels that may require fall protection consideration.  Ladders, staircases, guardrails or other elements of fall protection need to be evaluated based on the task and risk to the employee.  Electrical hazards and other energy control hazards during general maintenance should be addressed as part of a Lockout/Tagout program.   Warning signage must be visible and placed wherever possible safety concerns are present.  Signage should be in languages appropriate for the workforce.   Safety procedures for emergency situations (spills, fires, etc.) should be developed prior to operation of the system.

These are only the primary regulatory issues associated with digester health and safety.  Many additional OSHA regulations can be identified on the OSHA website, www.osha.gov, and any number of additional steps should be taken to ensure safety regardless of the regulatory issues for any identified hazards.


The general regulatory issues outlined in this module are intended to introduce the user to the wide variety of regulatory procedures required by federal, state, and local agencies.  These regulations are intended to protect the public, employees, employer, and the environment during construction and operation of a digester.  A site-specific comprehensive regulatory search should precede digester construction and operation.  As permitting and regulatory compliance can require extended periods of time, application and compliance measures should be undertaken early in digester planning stages to reduce the risk to human and environmental health and ensure compliance.


Additional Resources

AgSTAR |USEPA  Federal Regulations and Specific State Permitting


Authors and References