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Things You Need to Know about Manure Gas

26 August 2011

Robert Chambers P Eng, Engineer Swine and Sheep Housing and Equipment of OMAFRA offers advice on avoiding tragic accidents resulting from toxic gases in the latest Pork News and Views from Ontario's Ministry of Agriculture.

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 (and a multitude of minor gases). The four principle gases are hydrogen sulphide, carbon dioxide, methane and ammonia. While ammonia can build up to levels to be an irritant, it does not accumulate to lethal levels in barns. All are by-products produced from the anaerobic decomposition of organic matter (liquid manure). Their production is influenced primarily by temperature, pH and time. Always assume they are present in lethal quantities in the liquid manure that is stored more than a week.

The gas that is responsible for the vast majority of deaths is hydrogen sulphide: one breath is lethal at concentrations above 1,000 ppm. It is 1.2 times the weight of air. Its smell is similar to rotten eggs, detectable at 5ppb, though it becomes undetectable by smell at levels greater than 500ppm, giving the false sense that it is no longer present. The average daily exposure level is presently set at 10ppm (over eight hours). It is soluble in the liquid portion of the manure and is released when shaken, instantly. During agitation when the manure jet breaks the surface, values of 1,600 to 1,700 ppm were measured. If you hear a splash, then gas being released.

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

Methane is the third gas of concern with liquid manure systems. Though it possible to be asphyxiated at levels above 50 per cent, it is an extreme rarity for this to occur. The principle danger with this gas is its explosive and flammable properties. It is explosive in the range of 5.3 per cent to 15 per cent by air volume. At concentrations greater than 15 per cent, it will not explode but tends to burn and ignite any combustibles present. At 0.555 times the weight of air, it can easily accumulate in unvented head-spaces over the manure storage. Its production increases with the temperature of the manure although it will always be produced, even in cold temperature, though at a lower rate. 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. As both 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 back in the hole after the manure drains from the pit, the fan then pulls air from the pit as this is the easiest air for it to access. These gases can then 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 plenty 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 placed back in the hole immediately after the liquids have drained.

When shutting down barns or rooms, especially in winter, there is the temptation to completely shut off the ventilation system and to tighten up the room to keep the heat in. The issue is that 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 the range of five to 15 per cent and comes into contact with a pilot light from a heater or a spark from a light switch, an explosion and or fire can result. The solution is to completely empty the pits and always leave some ventilation on to achieve at least 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.

Although the exact cause of foaming manure is still unknown, we do know that the foam traps methane, carbon dioxide and hydrogen sulphide that would normally be removed by the ventilation system. The foam consists of about 60 per cent methane; remember it is explosive in an atmosphere between five per cent and 15 per cent. As long as the methane remains in the foam, it is fine but the problem is when it is released suddenly from the foam by pressure washing or aggressive agitation. If you notice foam build-up in your pit, seek advice.

Gas detectors are an economic method to ensure that the manure gases present are within acceptable limits and an early warning to possible dangers. As an absolute minimum is a hydrogen sulphide detector. A good recommended upgrade is a multi-gas detector including hydrogen sulphide, carbon monoxide (a by-product of incomplete combustion from heaters), oxygen and LEL (Lower Explosive Limit), which is good for methane. For hydrogen sulphide concerns, wear as low as possible; for methane, wear it high.

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 is present.

OMAFRA has a factsheet on this subject available by clicking here.

August 2011

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