ON THE MOVE
THE HEAVY TRUTH ABOUT HYDROGEN SULFIDE Due to its weight compared to oxygen, hydrogen sulfide can accumulate close to the ground or in confined spaces and becomes a threat to humans and animals. by Robert Meinen
H
ydrogen sulfide (H2S) is undoubtedly the most dangerous gas associated with manure storage and handling. The United States Occupational Safety and Health Administration (OSHA) lists acceptable exposure concentration limits for H2S as 20 parts per million (ppm) with a 10-minute exposure maximum peak over an 8-hour shift as 50 ppm. OSHA lists concentrations as low as 700 ppm as the level that can cause immediate collapse and death within minutes, and 1,000 ppm is the level that can cause nearly immediate death.
Heifer barn
NORTH
An unfortunate by-product Hydrogen sulfide can be produced as a microbial by-product of manure degradation, with production of the deadly gas exacerbated when anaerobic manure storage conditions favor proliferation of sulfate-reducing bacteria. Higher levels of H2S can be produced in manure when sulfur inputs into the manure are elevated from sources such as gypsum bedding, by-product feed ingredients, or sulfur-containing water sources. Temperature also impacts H2S production, since microbial activity increases in warmer manure. Higher H2S concentration will lead to greater emissions. Emissions can occur one molecule at a time when H2S moves across a gradi-
This dairy’s heifer barn blocked air movement when winds shifted to the southeast during agitation, elevating hydrogen sulfide to a dangerous level between the manure storage and the barn. ent from concentrated solutions at the manure surface to low concentrations in the air above the manure. However, since H2S is only slightly soluble in water, the molecule “wants” to come out of solution as concentrations grow. In deeper anaerobic areas of a manure storage, where microbial production continually supplies H2S, concentrations of gas can elevate beyond their solubility limit and escape the solution by coming together to
16 | Journal of Nutrient Management | May 2022
F3 16-17 May 2022 On the Move.indd 2
form bubbles and release from manure through ebullition processes. Other gases produced from microbial degradation of manure, such as methane and carbon dioxide, also form bubbles. Bubbles can contain a mixture of different gases. Disturbing manure with processes such as agitation are expected to intensify H2S release. Violent manure movement brings higher concentrations found in the depth of manure to the jofnm.com
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