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Slurry gases. Risks and Precautions

Slurry gases

Slurry, like any organic matter that ferments, produces different gases. The toxic gases released during the fermentation of manure are hydrogen sulfide, ammonia, carbon dioxide, and methane.
When manure is stirred in confined spaces (pre-pit, tanker) the gases released can quickly reach harmful or even deadly concentrations. As soon as the manure is stirred, for example, by moving through the remaining layer in the pre-pit, fermentation gases are released, including ammonia, methane, carbon dioxide and, most importantly, the neurotoxic and potentially deadly hydrogen sulfide (H2S).
Carbon dioxide and hydrogen sulfide are heavier than air and remain stagnant near the surface of the manure, but even the slightest agitation, such as a person walking through the residual layer of manure at the bottom of a pre-pit, will activate their release into the surrounding space. Because hydrogen sulfide remains bound to the manure particles and does not release bubbles like other gases, it is only released in large quantities during mixing.
The majority of accidents occur in confined spaces with limited volume and insufficient ventilation, such as a pre-pit, underground sewage disposal tank, pit, spreading tank, or similar enclosed spaces.
Backflow of manure gas from the building to the pre-pit is also a potential cause of poisoning. Pumping and mixing of manure and headwinds contribute to increased backflow.
Frequently, a manure gas poisoning accident can result in more than one victim. In 40% of cases in Quebec, the secondary victim(s) assisted the first person poisoned.Which gases

Hydrogen sulphide (H2S)*.
An extremely dangerous gas, it is responsible for most poisoning accidents on Quebec farms. At certain levels of concentration it becomes odorless, causing loss of smell and consciousness. Hydrogen sulphide is rapidly absorbed by the respiratory tract. In humans, inhalation of lethal concentrations produces effects in an interval that can vary from a few seconds to a few minutes.

Ammonia (NH3)

Its very characteristic acrid and pungent odor makes it possible to identify its presence very quickly. Light gas it is found in the upper parts of the premises. A good ventilation allows to release it quickly. Serious poisoning with this gas is unlikely since its irritating odor serves as an alarm.

Carbon Dioxide (CO2)*

All living organisms, including bacteria in slurry and manure, produce carbon dioxide. Odourless and heavier than air, this gas is mostly found on the surface of manure. At certain concentrations, carbon dioxide can displace oxygen from the air, depriving the body of oxygen, which can cause unconsciousness. Carbon dioxide also acts as a powerful central nervous system depressant. A concentration of less than 6% oxygen can lead to rapid death.
* Also known as carbon dioxide, carbon dioxide or carbon dioxide.

Methane (CH4)

Methane is the main fuel gas produced by active micro-organisms in stored manure. Odourless and lighter than air, this gas can be found high up in rooms. Like carbon dioxide, this gas takes the place of oxygen in the air and affects the organism in the same way as carbon dioxide.


Training  farmers and his employees on how to avoid  the dangers of these gases is recommended as well as the implementation of certain safety procedures:

Before entering the confined space :

  • Have a supervisor nearby
  • Being able to contact emergency services on the spot in case of need
  • Prepare the space before entering (continuous ventilation, removal of residual slurry as much as possible)
  • Have protective equipment
  • Be equipped with a safety harness for a quick evacuation of the premises.

In order to reduce the risk of poisoning hazards, a safe facility complying with the Canadian Farm Building Code will provide :

  • a good ventilation system that includes a type of low exhaust ventilation allowing the recovery of gases ;
  • pumping and agitation equipment that can be removed from storage structures without having to enter them;
  • storage structures with a depression in the floor to allow them to be completely emptied, without leaving residues of slurry or manure;
  • interlocking systems for energy sources and manure supply lines
  • mechanisms to protect against the return of manure gas and manure in buildings;guardrails, covers or other fall protection in storage structures;
  • signage indicating the presence of deadly gases and precautions to be taken.


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