Hazards in California Dairy Waste Structures

Dairy lagoons, settling ponds, and separator vaults can produce hazardous levels of hydrogen sulfide and methane.

CALIFORNIA leads the nation in milk production. Its approximately 1,200 dairies produced more than 32 billion pounds of milk in 2000.1 In the past 10 years, the industry has seen a declining number of dairies but large increases in herd size and increasingly sophisticated milking facilities and production management. Taking the state as a whole, the average dairy milks about 1,000 cows two to three times per day. In the San Joaquin Valley region of the state, average dairy sizes are closer to 3,500 cows, with some dairies approaching 8,000 head.

Dairy farming is a 365-day-per-year, 24-hour-per-day enterprise. Many of the dairy farms are operated by multi-generation families of Dutch and Portuguese lineage. The overwhelming percentage of dairy workers are Hispanic; many are mono-lingual, speaking only Spanish. As of this date, most dairy operations are non-union. A dairy's largest ongoing cost is for feed, and consequently some dairies have substantial farming operations to produce hay and silage.

Historically dairy farms have had relatively low accident frequencies but are prone to occasional severe injuries. Most injuries come from working in close proximity to large (1,000 pounds+) and unpredictable animals. Modern milking parlors usually involve an elevated and sometimes rotating milking floor, which offers the milkers a degree of separation and protection from the animals--but crushing injuries, lacerations, and contusions persist.

Artificial insemination is the method of choice for breeding the cows, but a certain number of bulls are run with the milking stock for "clean-up" purposes. Bulls are larger, can be more aggressive, and produce a large share of the fatal injuries in the industry. Other serious injury sources are strain/sprain injuries from feeding operations and machine-related injuries.

Confined space hazards exist on all dairies, although injury/illness incidents are rare. Dairy workers and managers literally live with these exposures. This can produce a false sense of security and disregard for proper precautions and safeguards, in some cases. This is particularly true for confined space hazards that can be overlooked.

There are several areas within each dairy where the classic combination of difficult entry/egress can combine with poisonous or oxygen-deficient atmospheres. Milk and feed storage tanks are two examples, but of much greater concern are manure transfer and storage structures. This was tragically confirmed by the occurrence of a well-publicized incident in Gustine, Calif., on Feb. 22, 2001. On that day, Enrique Ariaza entered a 30-foot-deep, 4-foot-wide wastewater standpipe to repair a malfunctioning piece of equipment. He was overcome by hydrogen sulfide gas. Attempting a rescue, Jose Alatorre also entered the standpipe. Both men were overcome and drowned in the wastewater.

No confined space precautions had been taken. The owner of the dairy was subsequently charged with two counts of involuntary manslaughter but was acquitted on Sept. 20, 2004.

Dairy Confined Space Issues
Even the smallest commercial dairies produce large amounts of animal wastes. Most of them would qualify as "concentrated animal feeding operations" (CAFOs) and use a combination of flushing systems, solids separators, settling ponds, and evaporation lagoons to handle and store the animal waste. In California dairies constructed in the past 10 years, there is a significant commonality of design of the physical plant, as well as in the design of waste handling systems. "A system of piping or culverts conducts wastewater from daily operations--such as milking parlors, holding pens, and, for some facilities, the alleys--to a central point so that the liquid wastes can be stored and treated or applied to cropland."2 It should be noted that these flushed "liquids" are a viscous mix of water, urine, feces, and uneaten roughage.

As animal waste decomposes, a variety of gases are produced by the aerobic and anaerobic microbial action of a range of bacteria. Of particular concern is the production of hydrogen sulfide (H2S) and methane (CH4). H2S is recognized by the typical "rotten eggs" odor. At concentrations above 50 ppm, olfactory fatigue can mask the presence of H2S. The NIOSH recommended exposure limit is 10 ppm with a ceiling of 10 minutes. The revised IDLH (Immediately Dangerous to Life and Health) of 100 ppm reflects the potentially fatal consequences of even short-term exposures. Levels of 600 ppm have been associated with fatalities, and ACGIH has reported that a dose of 1,000 ppm can cause immediate respiratory arrest. Methane, although flammable, is primarily considered a simple asphyxiant, displacing the oxygen in a confined space as it accumulates.

Regulatory Reaction
In the case of 2000 and 2002 fatalities, it is hypothesized the workers were initially overcome by high levels of H2S in standpipes that were also oxygen deficient. The vertical standpipes are normally about a yard wide and 10 feet or more deep. Although they are uncovered, air movement is poor at the bottom of the pipe where valves and other equipment are located. Access is usually by means of a fixed metal ladder.

Reaction to the fatalities took a variety of forms. Cal/OSHA began an aggressive program of inspection and educational activities. The losses shook the industry and probably motivated some dairymen to initiate prevention activities. As attention was focused on wastewater standpipes, other dairy structures--such as settling ponds, separators, and lagoons--came under regulatory scrutiny. As of this date, no specific policies were determined for these structures, other than general recommendations to provide warning signs and fencing, if possible.

Genesis of the Study
As worker's compensation providers, Zenith Insurance Company had a compelling interest in determining the level of risk that existed in these other structures. Our experience with dairy operators has shown they would take proper actions if we could base our recommendations on factual knowledge and not guesswork. Unfortunately, there is not much data in the safety and agricultural engineering literature related to this situation.

The hazards of covered manure storage structures, particularly in the swine production industry, were well documented. Our original thought was that open air structures were probably safe because of a combination of ambient ventilation and aerobic digestion of the animal waste. Our research indicated H2S and CH4 production are related primarily to anaerobic processes found in confined or poorly ventilated environments.

Study Design
We determined to sample a variety of dairies in Tulare and Kings counties during a one-year period. We wanted to test a variety of environmental conditions and dairy structures. We found dairy operators were eager and cooperative participants and usually gave us unfettered access to the dairies.

We planned to take air samples at the surface of separator pits, settling ponds, and lagoons in both placid and agitated conditions. "Agitated" samples were taken by breaking liquid or crusted surfaces by mechanical means. "Placid" samples were drawn from the undisturbed surface.

We used a multi-gas monitor that was calibrated before and after each survey using a check kit at a flow rate of 1.5 liters per minute. Immediately before and after each sampling, a fresh air check was performed. The instrument performed within tolerances for the entire sampling period.

We took 112 samples of lagoons, evaporative ponds, and separators in July 2003, November 2003, and March 2004 from 25 different central California dairies. Temperatures ranged between 45 and 92 degrees Fahrenheit. Wind speeds ranged between 0.5 and 7.5 miles per hour, with humidity varying from 24 to 95 percent. We detected no significant H2S or CH4 exposures in any of the structures that had no "crust" or algae mat under placid or agitated conditions. In structures with crusted surfaces under agitated conditions, we recorded H2S readings of from 1 ppm to 50 ppm.

Methane levels of from 1 to 100 percent of the lower explosive limit were recorded. In some cases, immediate (within five minutes) re-sampling was done, and in all samples rapid gas dissipation occurred. Oxygen readings were within acceptable ranges in all samples at all times with only slight variations. Statistical analysis of the data was performed. Through regression analysis we seek to determine whether two variables are related. The r2 value tells us the percent of variation in a dependent variable Y (H2S, for example) that is accounted for by a variation in an independent variable X (surface crusts). Values are always between 0 and 1, with r2=0 indicating the two values are completely independent. The correlation coefficient (r, or square root of r2) describes the linear relationship between two values, as well as the slope (positive or negative) of that relationship. Values range from -1 (a perfect linear relationship with a negative slope) to +1 (a perfect linear relationship with a positive slope). In our analysis, the r2 value for an association between H2S and a crusted surface was .4053, indicating a clear though not definitive association. The same relationship produced a correlation coefficient of .1643, indicating a clear if not strong relationship. The same values calculated for methane were .502 and .252.

The data suggest actively operating separators do not pose a problem insofar as H2S or CH4 is concerned. The separators are large, open-air, three- or four-sided concrete vaults up to 15 feet deep where heavier solids are screened and removed and waste liquids are held temporarily before being pumped out. These structures could still conceivably pose an exposure problem at lower water levels or if any accumulated bottom sludge were being removed. We were not able to sample under those conditions or observe them at low volumes. In all of our samples the separator vaults were relatively "full."

The dairy waste lagoons and settling ponds we observed were most likely of the facultative type. We observed excavation taking place in two cases, but we could only estimate depth. We observed that settling ponds are generally smaller and shallower than the larger structures. Some of the deeper structures might meet the definition of an "anaerobic" structure, but that did not seem relevant to us in the context of this study. Even in a shallower facultative structure, anaerobic digestion takes place. "A facultative pond achieves treatment by a combination of aerobic, oxidation, photosynthesis and anaerobic digestion. It is typically 1-1.5 meters in depth and contains two major zones of microbial activity. The upper zone is aerobic, the oxygen being supplied by the photosynthetic algae, and the lower zone is anaerobic.

No mixing or mechanical aeration is employed, mixing of the contents being dependent on wind action and thermal gradients within the pond. The relatively quiescent conditions permit the suspended solids to settle out to the anaerobic layer at the bottom of the pond, from where desludging is done on a very infrequent basis."3

The anaerobic layer is where the H2S is primarily produced. In those ponds and lagoons without a crust, we suspect that both H2S and CH4 are constantly being produced, released, and dissipated, and we in fact observed gas bubbles in a number of the uncrusted lagoons.

It seems clear that in those structures with soft or hard crusts, gases may be trapped and accumulate. We are not aware of, nor could we find in the literature, any evidence these open-air structures had caused employee injuries or deaths. We did find isolated evidence of children and animals finding their way into these structures without ill effects (drowning is another matter). The rapid diffusion of the released gases after the crusts are broken seems to be the key.

One large caveat needs to be raised regarding gas production. We did not sample for specific bacteria involved in these structures. The rate of bacterial growth depends on a number of factors, including species, temperature, nutrients present, and aerobic versus anaerobic conditions. These factors multiply together and could produce a lognormal distribution, with H2S or CH4 readings much higher than those we obtained.

The structures sampled are found in areas of the dairy where workers rarely go. They are largely "automatic" or passive operations that don't require human intervention. This raises, in our view, the possibility of untrained and unprepared workers encountering these exposures. Work on these structures is often performed by independent contractors over whom the dairy manager may have limited control and under circumstances where a problem has occurred that needs immediate attention.

Sludge and sediment removal from these lagoons/ponds was observed and involved large front-end loaders. Given the distance of the operator from the surfaces and the ambient ventilation, this didn't strike us as a concern. However, in any situation where this is being done by hand or where equipment in the bottom of a vault or lagoon is being repaired or serviced, we recommend that permit-required confined space procedures be followed. In the case of any work of this type in standpipes or even box faults where the depth is a multiple of the diameter, similar procedures should be followed.

Several of the dairy operators had fenced off their lagoon areas. This was done out of a concern for vehicle and traffic safety. We have anecdotal evidence of fatalities associated with vehicle accidents when occupants could not extricate themselves and drowned. Given that children often reside on dairies, this would seem a prudent step to take. Many older dairies were built in rural areas that are now being encroached on by housing developments, which is another reason for fencing these structures. The areas certainly should be posted as hazardous in languages or pictures that workers clearly understand.

Dairy lagoons, settling ponds, and separator vaults can produce hazardous levels of hydrogen sulfide and methane. Potential peak levels of these gases depend on a number of factors and could exceed the values obtained in our field tests. These gases accumulate when crusts or algae mats form on lagoon or pond surfaces and trap them. They are released when the crusts are broken. Our results showed that these released gases are quickly dissipated and normally don't pose an acute hazard.

In areas where ambient ventilation is not as effective, confined space precautions should be taken. These areas include standpipes, vaults where depth is a multiple of the diameter of the vault, and even larger structures where workers may be exposed to low water levels or sludge. These structures also pose vehicle accident and drowning hazards, and fencing them is recommended.

1. Blayney, Don, The Changing Landscape of U.S. Milk Production, USDA Statistical Bulletin Number 978, June 2002.
2. Pepper, Ian, Gerba, Charles, and Brusseau, Mark, Pollution Science. Academic Press, 1996, Page 244.
3. Lester, JN, and Birkett, JW, Microbiology and Chemistry for Environmental Scientists and Engineers, E & FN Spon, 1988, Pages 297-298.
4. Sund, Evenson, Strevett, Nairn, Athay and Trawinski, Nutrient Conversions by Photsynthetic Bacteria in a Concentrated Animal Feeding Operation Lagoon System, Journal of Environmental Quality, 30:648-655 (2001).
5. Anaerobic Digestion at Dairy Farms, BioCycle, October 1999, Pages 41-42.
6. Davis, J., Koenig, R., and Flynn, R., Manure Best Management Practices: A Practical Guide for Dairies in Colorado, Utah and New Mexico, Utah State University Extension, October 1999.

This article appeared in the February 2005 issue of Occupational Health & Safety.

This article originally appeared in the February 2005 issue of Occupational Health & Safety.

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