Application: used within deep-pit buildings
Pros
- Very little cost if included in original design prior to construction.
- Construction using wall fans in lieu of pit fans may be simpler.
- Fans are more easily cleaned and maintained.
Cons
- More wall surfaces are used for fan installation.
- Potential for more particulate matter to be released during winter.
| Figure 1. A fan mounted on an under-building manure pit access port to provide ventilation through the manure pit. |
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Description
Buildings with manure storage beneath the floor, often called “deep pit” buildings, generally have concrete annexes in the deep pit wall which are used for pumping manure from the building. Typically fans mounted on the covers of these annexes are used for minimum ventilation.This location is used because it is a relatively easy to mount a fan on the lid and there is a belief that gases will be drawn down through the flooring, thereby preventing gases in the manure storage from entering the animal zone.This is a fallacy disproven by research and, in fact, the pit fans tend to actually increase emission of gases.
Research shows that pit fans exhaust have proportionally higher emissions of several gases than do wall fans. Jacobson et al (2007, 2008) tested emissions from a deep pit swine building with and without pit ventilation.They found that the majority (75 to 80%) of NH3 and H2S emissions originated from the pit exhaust fans even though they only provided 20 to 30% of the barn’s ventilation air. Concentrations of particulate matter less than 10 microns (PM10) were the same in air leaving the wall fans as that leaving the pit fans with the except of winter. During winter pit fans had lower PM10 concentrations than did wall fans, presumably because dust particles collect more on the condensation on pit walls during cold weather.
Jacobson et al (2008) concluded that using pit fans for minimum ventilation shows limited value or benefit. In other words, the air quality within the building was nearly the same in buildings using pit fans to provide minimum ventilation compared to using wall fans to provide minimum rates. They noted that this may indicate that when emission reductions are desired that treating air from the pit ventilation with technologies such as biofilters or scrubbers result in reductions of emissions in excess of 50%. They also conclude that a reduction of 10 to 20% in pollutants could be expected by simply eliminating pit fan usage. This makes intuitive sense because air is pulled across the manure surface by pit fans, which likely warms the manure and accelerates volatilization of gases due to convection. In other words, leaving air in the pit stagnant would slow the release of gases to some degree.
To further understand why pit ventilation does not function as many people assume, it is important to understand the physics of pit ventilation, which have been explored by Nicolai and Hoff (2003). In order to get a downward movement through the floor so as not to allow gases to rise into the animal zone a pressure must exist across the floor between the animal zone and the manure pit. In most buildings, 6 to 8 inch slats are used with 1 inch slots which results in a floor opening of 11 to 14%. In order to create a downward draft between 36 and 44 cubic feet per minute per pig (cfm/pig) of ventilation is required. If this is compared to typical minimum ventilation rates for 60 lbs finishing pigs, which is 3 cfm/pig (MWPS, 1983), and weaned pigs, which is 1.5 to 2 cfm/pig, it can be further seen that it is not possible to cause a downdraft during minimum ventilation.
In summary, pit ventilation does not provide improved air quality for animals and increases the emission rate from the manure pit. A simple reduction of 10 to 20% of emissions can be realized by simply ventilating from wall fans instead of pit fans.
Effectiveness
| Component | Reduction | Notes |
|---|---|---|
| NH3 | 10 to 20% | |
| H2S | 10 to 20% | |
| Odor | 10 to 20% | |
| Particulate Matter |
Slight increase |
PM10 (<10 microns) |
| Volatile Organic Compounds (VOC) | 5% | not reported but likely similar to other compounds |
| Cost | $$ | little or no added cost if implemented during construction |
Cost Considerations
If designed into the original construction, no additional cost will be required to use wall fans instead of pit fans in deep manure pit buildings. A cost savings may actually be realized since installation of pit fans can be more expensive than wall fans. Pit fans also are more costly to maintain because of the harsh environment and often have a shortened useful life.
References
Jacobson, L.D., B.Pl. Hetchler and D.R. Schmidt. 2007. Sampling pit and wall emission for H2S, NH3, CO2, PM & odor from deep-pit finishing facilities. In: International Symposium on Air Quality and Waste Management for Agriculture, Proceedings of the 16-19 September 2007 Conference (Bloomfield, CO). ASABE Publication Number 701P0907cd, St. Joseph, MI.
Jacobson, L.D., B.Pl. Hetchler and D.R. Schmidt. 2008. Reducing H2S, NH3, PM, and odor emissions from deep-pit pig finishing facilities by managing pit ventilation.In: Mitigating Air Emissions from Animal Feeding Operations, Technology Summaries of the 19-21 May 2008 Conference (Des Moines, IA). p. 26.Iowa State University, Ames, IA.
MWPS. 1983. Swine Housing and Equipment Handbook. MWPS-8.Midwest Plan Service, Iowa State University, Ames, IA. pp. 112.
Nicolai, R. and S. Hoff. 2003. Ventilation requirements to prevent pit air up-drafting in a swine finishing barn. In: Swine Housing II, Proceedings of the 12-15 October 2003 Conference (Research Triangle Park, NC). p. 25-30. ASAE Publication 701P1303. St. Joseph, MI.