In the past few weeks, there have been several incidents in Minnesota and Iowa where explosions or flash fires have occurred in livestock buildings with manure pits. So far, the explosions have mainly resulted in building damage, with few animal losses and no personal injuries or fatalities reported. Agricultural engineers, animal scientists and an industry consultant have drawn up recommendations to help producers deal with this potential danger.

When liquid manure is agitated to suspend the settled solids so that the slurry can be pumped, many gases are released into the air. Some of these are hazardous to people and animals (hydrogen sulfide and others), but methane, which is very flammable, is also released. If the methane concentration in the barn or pit reaches its explosion threshold of 4 percent to 5 percent (or 40,000 to 50,000 ppm) and there is an ignition source (such as a pilot light on a heater) in the area space, an explosion will likely occur.

Here are some key suggestions to help address this problem: 

  •  Provide continuous ventilation to prevent a gas build-up. Increased ventilation during pit agitation to quickly dissipate released gases. Sufficient ventilation or exchange of air in the barn is essential in all cases to keep the methane concentration below its explosive threshold.
  • What is sufficient air exchange in a barn while agitating and pumping its manure pit? It's at least 2 to 3 times the minimum ventilation rate (or about 10 air changes per hour) for the barn. If the pit is full or nearly full, do not rely just on pit fans to supply this airflow rate, since these fans may be severely restricted. In fact, it may be better to use only wall fans to supply air exchange while agitating or pumping the barn's manure pit since methane gas is lighter than air.
  • Make sure your normal ventilation inlets are open and operating properly to ensure good air distribution in the barn. This is also important in preventing animal deaths (if animals must be present in barns) while the manure pit is agitated and pumped.
  • To keep from igniting an explosive methane concentration, turn off heater pilot lights and other non-ventilation electrical systems (such as the feeding system) that might produce an ignition spark. Not providing supplemental heat in the barn may be problematic for cases when there are no animals in the barn or there are only small animals that require warmer inside temperatures. It may require that you only pump manure from the barn to warmer days or a warmer part of the day.
  • When pumping pits that are close to being full, pump without agitation until manure is about 2 feet below the slats. This will allow pit fans (if available and used) to perform properly during agitation and provide more dilution space for methane and other gases that are released. 
  • Foaming in manure pits is a growing and significant concern that may be related to the explosion incidents. Some of the recent cases have reported "foaming" or extensive bubbling on the manure surface prior to the explosions. According to some reports, several feet of foam has developed in a matter of days. Some barn manure pits will foam while others do not. Currently there is no consistent solution to controlling this foaming action in livestock manure pits and we do not understand all of the factors (diet, manure pH, others) that cause this problem. More relevant information is being collected in addition to preliminary field measurements in producers' barns to see what techniques and products are effective at reducing foam levels.

    Additional information on this issue can be found at the University of Minnesota Extension's swine Web site; an Iowa State University press release; and the Minnesota Pork Board Web site.

    Source: Larry Jacobson, agricultural engineer, University of Minnesota Extension.