Safety and health hazards should be addressed using a hierarchy of control measures — eliminate the hazard, reduce through management controls and finally, adequate protection.
This article is adapted from the soon-to-be-released second edition of the On-Farm Composting Handbook, published by NRAES (Natural Resource, Agriculture, and Engineering Service) at Cornell University in 1992.
Composting as an organic waste management method involves mechanical equipment, physical labor and handling of diverse biological materials. Although there are a few general safety and health guidelines for any type of composting operation, the specifics vary, depending on scale, feedstocks handled, composting methods, types of equipment, climate, worker skills and training, hours of operation and level of management. Each variable can affect the specific safety and health hazards encountered and the associated levels of risk. Table 1 provides an overview of the general hazards that composters may encounter in different facets of an operation.
Safety and health hazards should be addressed using a hierarchy of control measures (Figure 1). Ideally, the source of the hazard should be eliminated first, e.g., by altering the process, redesigning equipment, changing tools, installing ventilation, isolating the machine or adding machine guards. If the hazard can’t be eliminated, it should be reduced through management practices such as improving working procedures or establishing administrative controls (e.g., job rotation or reduced work time). To guard against any potential hazards that remain, the next step is adequate protection.
In the U.S., the Occupational Safety and Health Administration (OSHA) oversees and establishes occupational health and safety regulations on a federal level. In addition, individual states establish their own worker safety regulations, although most states tend to follow OSHA’s lead and adopt most of its requirements.
GENERAL EQUIPMENT AND SITE SAFETY
Equipment ordinarily employed in composting includes bucket loaders, skid loaders, tractors, turners, trucks, excavators, grinders, chippers, turners, mixers, screens, conveyors, forklifts and bagging devices. All types of equipment have at least two common features — moving parts and power units that move those parts. Several common safety concerns arise from the operation and maintenance of mechanical devices.
• Moving parts of equipment can grab or entangle hair, clothing and people during operations or maintenance. The moving parts require guards, shielding and good work practices. To prevent clothing and extremities from being caught by moving parts, guards or shields should be installed and maintained wherever pinch points or scissors points exists. Loose clothing, long unbound hair and hanging jewelry should not be worn when operating equipment with exposed shafts, chains, gears and other moving parts that can grab.
• Unprotected power-drive lines, such as on augers or the power take-off (PTO) shafts, are potential hazards. PTO shafts historically have had little or no shielding. As shields can get “in the way” or become bent or broken, people often remove them. Recent models have totally-shielded shafts, which are less likely to be removed. Both open drive lines or partially covered drive lines (e.g. “U” shaped shield) should be replaced by totally-shielded shafts.
• Wet and oily feedstocks, wet weather or ponded leachate can create slippery conditions at a composting facility. These conditions can occur on ground surfaces and on equipment platforms. Nonskid shoes or boots should be worn to reduce slipping.
• Some composting equipment — screeners, grinders, chippers, windrow turners — can throw objects. Eye protection such as shatter-resistant glasses with side shields or goggles are needed to protect against potential impact and injury.
• Workers should be appropriately protected from dust. Dust can also be a potential fire hazard and should be kept from accumulating in welding areas, on engine manifolds, mufflers and on other equipment components that get hot. Dust during screening is a particular problem because materials tend to be relatively dry when screened. Loader cabs should have a properly operating air filter. When working outside of cabs, operators should wear safety glasses with side-shields or goggles and an N95 dust respirator or a respirator with particulate cartridges.
• Be aware of overhead power lines and the possibility of electrical shock. When power lines are located over composting operations such as windrows and storage piles, the elevated part of the vehicle should be kept a distance of at least 10 feet from the power lines to prevent equipment from becoming energized and shocking the operator.
• Identify and permanently mark the location of underground utilities (electric, gas, water, cable). Notify the utility company when digging or trenching on site, especially in the vicinity of known underground utilities.
• Foreign objects delivered in some composting feedstocks are damaging to machinery and potentially hazardous to human operators. Examples include gas cylinders, containers with chemicals, cables, chains, rope, metal strapping and large or sharp metal items. Procedures should be in place for sorting and discarding them before they come into contact with equipment.
• Equipment jams are a regular occurrence at composting facilities, in part because of the diverse and difficult materials encountered. Conveyors, grinders, shredders, chippers, mixers, screens and even turners are prone to jams. Operators have been known to climb inside the tub of a grinder with a sledge hammer and pickax to break up the jam. Climbing into such equipment should be avoided; wherever possible, the task should be performed from outside.
• When removing a jam, the equipment first should be disconnected from the power source. If the unit is electric, power should come from an outlet equipped with a ground-fault interrupter (GFI) to stop electrical current if a short occurs in the system. The moving parts are then blocked and chocked. It is possible that parts may still be able to move under their own weight once the jam is removed. Some equipment may have residual energy held back by the jam. After the jam is removed, the mechanism may thrust forward or blades, augers and paddles could rotate. A block and chock helps to prevent injury. If the jammed mechanism is driven by a PTO, the PTO should be disconnected.
Size Reduction – Grinding, Shredding and Chipping
Because size reduction is an especially aggressive operation involving high-powered machinery and very fast rotational speeds, it deserves particular respect in regard to safety. Grinders, chippers and shredders present common hazards but also some that are specific to the type of equipment in use. Ones not discussed in the previous section include:
• Hoppers and loading chutes for grinders and shredders often are open on the top or sides. Guards or shields may be impractical because the operator needs to continually feed material. Because they potentially can be pulled into moving parts, operators should stay clear of open top hoppers and feed chutes. As much as possible, avoid manually feeding material into hoppers and chutes. Recognize that there are feed mechanisms (e.g. rollers, chains, conveyors) operating behind hanging curtain deflectors.
• Operators should wear either safety glasses with side shields or goggles to protect eyes from flying objects, as well as a hardhat.
• Size reduction is an especially loud operation. Hearing protection is needed. Noise levels can vary with the model and whether it is powered by gas or electric.
• Like screening, size reduction is an active operation with multiple vehicles and mechanisms working simultaneously.
• Because grinders and shredders typically are open on the top or sides, flying debris is possible. Several precautions can be taken to reduce risk of accidents from projectiles:
√ Keep the tub loaded with feed material when the grinder is operating to intercept items ejected from the hammermill chamber.
√ Establish a zone around the grinder where no one but the grinder/loader operator is allowed. The radius of the zone depends on the grinder model and should be based on the manufacturer’s recommendations. Ask the manufacturer to specify the grinder’s “thrown object zone.”
√ Erect a tall screen to catch flying debris. Objects can be ejected from tub grinders in any direction but they predominantly fly toward the direction that the hammers rotate at the top of the chamber.
√ Install covers, partial covers, deflectors or other safety devices designed to contain thrown objects.
• Wood chipping equipment presents specific hazards because many models are fed manually. The following safety precautions should be observed with chipping equipment:
√ The chipper should be thoroughly inspected each day before start-up. The hood should completely cover the chipper knives, and workers should ensure that knives come to a complete stop before opening the hood.
√ Use a long branch to push shorter material into the chipper. If shredding leaves, use the tamper on the unit to push leaves into the shredding chamber.
√ If purchasing a chipper, consider those with an interlock system where the chipper hood cannot be opened while the cutter disk is turning. Chipper-shredders should have a certification symbol of safety from the Outdoor Power Equipment Institute (OPEI) and the American National Standards Institute (ANSI).
√ Personal protective equipment is mandatory when operating chippers due to close proximity of workers to chippers and the danger from flying debris, blowback from the hopper, and the potential for entanglement of clothing in moving parts. The personal protective equipment recommended includes hard hat, eye protection (either safety glasses with sideshields or goggles), hearing protection, safety boots, gloves and close-fitting outer clothing.
Additional Safety Procedures
Other categories of safety procedures at composting operations include moving vehicles, process and windrow pile monitoring and fire protection.
• Visibility can be occasionally obstructed by steam rising from windrows, fog and piles of material. Operators of bucket loaders and skid loaders, and those working near loaders, must be especially diligent as loaders maneuver at a rapid pace and frequently change direction. Each day an operator should start with an understanding of the general whereabouts of coworkers and visitors on that day and presume that they will remain there.
• Moving equipment can generate dust. In vehicles, exposure is best reduced by use of an enclosed cab with well-sealed, air-conditioned and heated cabs with filtered air intakes. Filters should be removable and washable or replaceable and employees should be instructed to remove and clean or exchange filters frequently.
• Safety procedures must be established for employees monitoring the composting process, taking temperatures, recording data, inspecting feedstocks, performing maintenance, sampling materials and generally assessing operations.
• It is possible for workers walking on windrows, piles or bins to drop down into the material and be engulfed — trapped or buried alive. This problem is particularly risky when tall piles are loosely packed or have sufficient surface moisture to experience “crusting,” causing them to mistakenly look and feel solid underfoot. Furthermore, the interior of such piles can be very hot and cause burns, even if a person is only partially immersed. This situation has occurred on farms with bins or silos of corn or other grain, as well as in other industries. On a composting site, relatively free-flowing bulk materials like wood chips and dry compost have the potential to collapse. If there is no alternative to walking on windrows or piles, a coworker should be observing from a safe stable point (e.g. not also on top of the windrow). Preferably a body harness should be worn with retrieval line attached to a fixed point.
• Piles and bins that are burning internally (i.e. spontaneous combustion) can collapse due to the voids created inside as the material burns. A pile that is smoldering, or suspected of burning, should never be walked or driven on for any purpose.
• Conveyor belts are used for many tasks at composting facilities, by themselves and as integral components of other equipment such as grinders, screens and turners. Conveyor chute openings should have covers or guard rails to protect workers from falls. Guards need to be in place to protect from pinch and shear points such as terminals, drives, take ups, pulleys, and snub rollers. Critical points occur where a belt changes direction including where belts wrap around pulleys, at the discharge end of the belt, on transfer and deflectors and at take-ups. Warning signs should be posted around conveyor belts — especially if guards are not practical. Operators should be trained on general rules for working around conveyors including:
√ Only authorized maintenance personnel can make adjustments to conveyors.
√ Lockout the conveyor before working on it.
√ Make sure that the emergency stop is reachable, secured and in working order.
√ When shoveling on a belt, always face the opposite direction from the belt movement.
Gases, Fumes, and Confined Spaces
Proper ventilation, and protection from toxic fumes and gases, are essential to worker health and safety at composting facilities. For example, welding gases and fumes are a complex mixture which may include: ozone and nitrogen oxides; metal vapors and other gases from the substrate being welded or from the welding rod; and carbon monoxide, carbon dioxide and other gases from coatings and oils on the substrate (carbon dioxide may also be used as a shielding gas). If welding is done in a confined space, gases can collect and increase to hazardous levels. When welding inside a building, sufficient ventilation is extremely important to dilute and remove gases and fumes. At the benchtop, the best arrangement is a slot hood connected to a blower that draws the gases and fumes to the outside. General building ventilation is not as effective. Welding curtains and other barriers can help to reduce drafts and improve the capture of air contaminants. If sufficient ventilation is not possible, a respirator with cartridges for welding fume and oxidizing gases could be worn.
The topic of worker safety protocols for confined spaces was covered in a recent BioCycle article (see “Worker Safety In Confined Spaces,” November 2011). That article describes a confined space as “a space that has limited or restricted means of entry or exit, is large enough for a person to enter and is not designed for occupancy.” It also suggests designing new facilities to minimize the potential hazard. The OSHA standard on confined spaces provides excellent work practices for any confined workspace (e.g. storage bin, container).
To determine if the atmosphere in any confined space is safe (e.g., before and during a welding task), use an air tester with a direct readout that checks the air for levels of oxygen, explosive gases and toxic gases (typically hydrogen sulfide and carbon monoxide). By monitoring the air during the task, an alarm can alert workers to exit the space if the atmosphere becomes unacceptable.
Biological and Chemical Concerns
Composting workers are subject to the hazards generally associated with performing physical labor near large equipment in an outdoor environment including noise, physiological stress, extreme heat and cold and fatigue. In terms of biological concerns, composters are exposed to the resident organisms involved in decomposition, and their components (e.g. spores, endotoxins), plus the volatile compounds and dusts generated during composting. When these biological and chemical elements become airborne (usually with dust particles and mists), they are referred to as bioaerosols.
The first step in minimizing the risks from biological and chemical substances is understanding their potential routes of entry into the body. Chemicals and biological agents enter or make contact with the body through eye contact, skin absorption, injection through the skin, or ingestion and inhalation. Common sense, hygiene and personal protective equipment block these routes of entry.
Biological health concerns include potential exposures to bacteria, endotoxins, fungi (molds and yeast), parasites (protozoa, protistans), worm cysts, and viruses. While all of these biological agents exist in the environment, they are likely to be present in higher concentrations at composting sites, and also farms, due to nature of the feedstocks and the fact that composting fosters biological decomposition. A composting facility is a source of bioaerosols — airborne particles including fungi, bacteria, and endotoxins. Elevated levels, sufficiently high to cause potential harm to workers, occur both upwind and downwind of activity areas, within a distance of about 90 feet of the source.
The greatest risk of bacterial infection appears to be in the initial stages of composting, and from raw feedstocks like manure, biosolids and postconsumer food scraps. They pose little risk in final stages because pathogenic forms die at the high temperatures of composting. Handling organic materials can expose workers to pieces of dead bacteria, known as endotoxins, that are potentially present in nearly any feedstocks and the composts made from them. They are heat-stable and not substantially inactivated by composting temperatures. They are most often associated with dust. This association plus the fact that endotoxins affect the respiratory system suggests that inhalation is the primary entry route of concern. Dust suppression and respirators minimize exposure.
Composting also fosters the growth of fungi, especially with woody feedstocks and in the latter stages of the process. While predominant fungal species vary, common organisms include various species of Aspergillus and Penicillium — organisms that produce tiny, easily airborne spores. Aspergillus fumigatus is one species that has received particular attention in regard to composting because it is common and a well-known allergen. Most fungi commonly encountered in the environment are unable to cause infectious disease. However, some species of fungi are considered opportunistic, and can cause infection in people whose immune system is compromised (i.e. immunodeficient) by other diseases or treatments for those diseases.
In general, viruses do not survive well beyond their host environment and appear to be inactivated readily during composting. Also, viruses tend to be retained well in the pile and are not readily airborne. The type of virus present primarily depends on the feedstocks and their sources. Most concern is for those in manure, biosolids and food.
A number of parasitic species of protozoa and worms (i.e. helmiths) can inhabit composting feedstocks, especially manure and biosolids. For example, workers at a biosolids composting project apparently were infected with Giardia from handling raw biosolids, probably via accidental ingestion.
Reducing Safety and Health Hazards
As shown in Figure 1, safety and health hazards can and should be reduced via a hierarchy of practices that seek to first eliminate a hazard, (e.g. process or engineering controls), second reduce the risk of or exposure to the hazard (e.g. administrative controls) and then protect workers from hazards that remain (e.g. protective equipment).
Process and engineering approaches to safety and health change the nature of the work in order to minimize and preferably eliminate a hazard. Examples include paving roads and other surfaces to reduce dust and standing water and providing enclosed environmentally-controlled cabs for equipment. Administrative controls are more procedural in nature and can affect process and site management as well as worker activities. Limiting employee work hours to reduce fatigue is an administrative control that reduces hazards in different ways. In particular, it is important to establish and encourage facilities to practice good hygiene and to have hygiene facilities available (e.g., showers, clean-up facilities) to reduce the potential for exposures to chemicals and diseases.
Improvements in safety and health can be identified through hazard evaluation tools such as job hazard analysis, vulnerability analysis and process hazards analysis. Specific programs include “Hazard and Operability Analysis” and “Hazard Analysis and Critical Control Points” (HACCP). HACCP, for example, systematically identifies the critical points in a production system responsible for potential hazards, sequentially corrects the problems at those points and then continues to monitor, test and improve the system at new critical points.
In situations where workers are exposed to hazards, personal protective equipment (PPE) provides an important line of defense. PPE includes visible clothing, hard hats, eye and ear protection and dust masks and respirators. In addition workers should wear appropriate clothing, gloves and footwear (e.g. steel toes).
Nellie Brown, M.S., is a Certified Industrial Hygienist and Director of the Workplace Health and Safety Program at Cornell University’s School of Industrial and Labor Relations.