Things You Need to Know about Manure Gas

Educate yourself and your employees on the different manure gases and their properties to avoid a tragedy, extols Robert Chambers, Engineer Swine and Sheep Housing and Equipment in the latest Pork News & Views newsletter from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).
calendar icon 4 May 2012
clock icon 7 minute read

The danger of death or injury from manure gas exposure is real. Many human and even more swine deaths in the past few years have been attributed to manure gas exposure. If everybody that works in and around liquid manure understands what the properties and risks are with each of the manure gases the danger of a tragic incident can be reduced.

Manure gas consists of four principle gases (plus minor gases). These are hydrogen sulphide, carbon dioxide, methane and ammonia. While ammonia can be an irritant, it does not accumulate to lethal levels in barns. All are by–products of the anaerobic decomposition of organic matter. Their production is influenced primarily by temperature, pH and time.

Always assume they are present in lethal quantities in liquid manure stored more than a week. Some products claim they can reduce manure gas levels; none has been proven in scientific studies to be effective in eliminating the risks associated with manure gases.

The gas responsible for the majority of deaths is hydrogen sulphide; one breath is lethal at concentrations higher than 1,000ppm. It is 1.2 times the weight of air, smells like rotten eggs and detectable at 5ppb although it becomes undetectable by smell at levels over 500ppm giving the false sense that it is no longer present. The average daily exposure level is 10ppm (8 hours). Soluble in the liquid portion of the manure, it is released when shaken – instantly. During agitation, when the manure jet breaks the surface, values of 1,600 to 1,700ppm have been measured. If you hear a splash, then gas is being released.

Carbon dioxide is less toxic but still deadly. Unconsciousness is followed by death in a few minutes by asphyxiation at levels over 70,000ppm. Heavier than air (1.5 times), it is completely odourless. The average daily safe exposure level is below 5,000ppm. Soluble in the liquid portion of manure, it is easily released when shaken, instantly. The author has measured values of 175,000 ppm. Again, if you hear a splash, then this gas is being released.

Methane is the third gas of concern with liquid manure systems. Although it is possible to be asphyxiated at levels over 50 per cent, this is extremely rare. The principle danger is its explosive and flammable properties. It is explosive at 5.3 to 15 per cent by air volume. At concentrations greater than this, it will not explode but tends to burn, igniting any combustibles present. At 0.555 times the weight of air, it can easily accumulate in rooms or unvented headspaces over the manure storage. Its production increases with temperature though it will always be produced, even in cold temperatures but more slowly. It is only slightly soluble in manure, so major releases of this gas are not expected during agitation.

The main danger of hydrogen sulphide and carbon dioxide gas exposure occurs during manure handling such as pit agitation, pumping and pulling plugs to empty pits. Methane dangers occur mainly with double pitted barns (typically a four–foot deep pit that drains into an eight–foot deep pit directly below), when barns are shut down for a period of time, and foaming manure situations.

Any time manure is moved, handled or agitated, there is an increased danger of excessive gas exposure. Hydrogen sulphide and carbon dioxide gas are soluble and are immediately released upon excessive agitation; many of the incidents involving these gases were a result of the ventilation system pulling the gases into the barn space since these gases are only slightly heavier than air.

When the manure is being moved or agitated, it is critical to know where those fans are getting their air. During cool or cold weather, the ventilation inlets are reduced and room static pressure increases. This causes the fans to work harder and increases the air speed of the inlet air promoting proper mixing and improves the comfort of the pigs. If a pit cover is opened or a pull plug not placed immediately after the manure drains, the fan then pulls air from the pit, and these gases can be pulled up into the barn space.

Pigs, as they breathe close to the slats, are usually more susceptible than humans. If there is any unusual pig activity during any manure agitation or transfer, immediately stop and evacuate the building. With naturally ventilated barns, ensure that there is lots of wind movement and that the curtains are completely lowered. Be prepared to stop manure handling if the wind speed drops or at the first sign of any pig agitation.

The challenge with double–pitted barns is that the bottom pit is usually unvented. The bottom pit becomes a crude anaerobic digester and methane builds up in the air space. If the pit plug is pulled and left out, this near–pure methane rises into the barn space. If the ventilation system is shut off or at minimum, it can quickly build up to explosive or flammable levels. A pilot light from a box heater or a spark from a motor or light switch can result in explosion and/or fire.

The solution is to install a chimney and air inlet to ventilate the top of the bottom pit. Always ensure that the pull plug is replaced immediately after draining.

When shutting down barns or rooms, especially in winter, do not completely shut off the ventilation system to keep the heat in. The methane bacteria in the pit slow down but do not stop, even in the cold. If the ventilation system is shut off and the fan covers are on, then the gas builds up in the room. If it reaches five to 15 per cent and comes into contact with a spark, an explosion and or fire can result.

The solution is to empty the pits COMPLETELY and always leave a bit of ventilation on – more than one air change per day. If walking into a barn that has been closed up, bring a LEL (Lower Explosive Limit), oxygen and hydrogen sulphide detector and do not turn anything on until the atmosphere is proven safe.

The exact cause of foaming manure is still unknown but we do know that the foam traps methane, carbon dioxide and hydrogen sulfide that would normally be removed by the ventilation system.

The foam consists of about 60 per cent methane; remember that it is explosive in an atmosphere between five and 15 per cent. As long as the methane remains in the foam, it is fine. The problem is when it is released suddenly from the foam by pressure washing or aggressive agitation.

Gas detectors are an economical method to ensure that manure gases present are within acceptable limits and an early warning to possible dangers. An absolute minimum is a hydrogen sulphide detector. A good recommended upgrade is a multi–gas detector for hydrogen sulphide, carbon monoxide (a by–product of incomplete combustion from heaters), oxygen and LEL (Lower Explosive Limit), which is good for methane detection. For hydrogen sulphide concerns, wear as low as possible, for methane wear high. Safety suppliers such as Levitt Safety (+1 866 741 7101), Acklands-Grainger locations throughout Ontario and Agviro Inc. (519 836 9727) are some of the many suppliers of these types of detectors. These companies also supply training literature/courses on their proper use and maintenance.

In conclusion, to avoid a tragedy, educate yourself and your employees on the different gases and their properties. Be aware of what causes sudden releases of these gases. Always know where the supply air is coming from in the barn. Wear and use a hydrogen sulphide or multi–gas detector. Close pull plugs immediately after the pit is empty. Always be extra vigilant when manure is being moved. Make sure that the ventilation is on even if the room or barn is empty. And finally, remember if you hear a splash, gas will be present.

May 2012

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