Department of Biosystems and Agricultural Engineering
University of Minnesota
St. Paul, MN 55108
During 1997, eight styles of covers were analyzed for their effectiveness in reducing odor and hydrogen sulfide emissions from swine waste. Twenty-one 750-L polyethylene tanks were filled with 600-L of swine waste on April 11, 1997. The experimental design consisted of a randomized block design containing three blocks (replications) of seven treatments. The seven treatments for the first study included:
Air samples were collected using a hood completely covering the tank top. The samples were tested for odor strength using a dynamic olfactometer and tested for hydrogen sulfide concentrations using a Jerome meter. Odor strength, defined in odor units, is a measure of dilutions-to-threshold or the maximum amount of dilution of filtered air mixed with the collected air sample that will allow a trained odor panelist to detect the presence of an odor.
During spring and early summer, air-sampling procedures were established and tested. Findings from this period indicated that the original swine waste (collected from a deep pit) was not of sufficient strength to generate enough emissions to measure reductions due to the covers. The decision was made to periodically add waste to generate enough emissions. In addition, the biodegradable plastic mat was determined not to have sufficient longevity to meet the criteria of this study. Upon termination of the first study, the 21 tanks were completely emptied.
The Permalon cover and Macrolite balls were reused for the second part of the experiment from the previous study; however, fresh straw and oil were used to make new covers for these treatments. The biodegradable plastic cover was replaced with a geotextile membrane (Typar 3201C).
Swine waste (collected from a shallow gravity drain gutter) was added to the tanks at four different intervals or collection periods:
|
Period 1 |
July 29 |
150 L added to an empty tank |
|
Period 2 |
August 19 |
300 L added |
|
Period 3 |
September 9 |
300 L removed, 300 L added |
|
Period 4 |
September 30 |
300 L removed, 300 L added |
Air samples were collected 24 and 48 hr after each waste addition. A total of 168 air samples were collected.
Statistical analysis indicates that all three main effects(cover treatment, collection period, and time after waste addition(significantly (5% level) affected odor and hydrogen sulfide emission. Therefore, the statistics were re-run, splitting out the collection period and time effects.
For all four collection periods, there was no significant difference (5% level) in either odor or hydrogen sulfide level between cover treatments and the control for air samples collected 48 hr after waste was added to the tanks. Two possible explanations are that within 48 hr, the majority of gases have volatilized from the waste and/or the control tanks may have developed a thin crust, reducing emissions so there was no difference from the other six treatments. Averaged over the four collection periods and seven cover treatments, odor units decreased 37% and hydrogen sulfide concentrations decreased 46% from the 24 hr collection to the 48 hr collection.
There was no significant difference (5% level) in cover treatments compared to the control for the fourth collection period. This may be explained by the colder temperatures which were observed during early October and resulted in lower emissions of odors and hydrogen sulfide from all tanks.
Thus the significant findings occurred during the first three collection periods within the first 24 hr after waste was added to the tanks. Detailed results are shown in Figures 1, 2, 3, and 4. In comparing the effective reduction from the cover treatments, both the 5% and 10% significant levels are indicated. Even though some cover treatments did reduce emissions on the average, they were not significant indicating that there was more variation within the treatment among the air samples collected than between the treatment and the control. Table 1 contains the percent reduction in odor and hydrogen sulfide emissions for those cover treatments significantly reducing the emissions.
For the first two collection periods, the straw cover significantly reduced both odors and hydrogen sulfide. One possible reason that straw did not significantly decrease odors and hydrogen sulfide during the third collection period is the possible decay or breaking down of the integrity of the floating mat. Mixing oil with the straw appears to increase the longevity of the cover.
Although odor and hydrogen sulfide emissions were reduced during the third collection period with the oil cover, using this type of cover is not recommended. The oil layer, when mixed with the waste, produced a distinctively offensive non-swine odor as observed by the individuals collecting the air samples. This may be due to the high carbon concentration of the oil mixing with the high nitrogen concentration of the waste and resulting in a different growth or type of microorganism at the surface which emitted a different odor.
The Permalon cover reduced hydrogen sulfide emission better than the other covers. For the first and third collection periods, the significant difference at the 10% level from the control could have been due to odors escaping around the edges of the cover.
The Macrolite clay balls did not perform as expected. Reducing the diameter of the balls to reduce the void volume and increasing the thickness of the mat layer may help to significantly reduce emissions.
A geotextile membrane may be a possible cover choice since the fabric is self-floating and a biofilm was growing on the mat that might self-seal the cover. This study might have been terminated prematurely for this cover, not allowing a thicker biofilm to develop to significantly reduce emissions. Alternatively, a thicker geotextile fabric should be studied.
Straw (possibly mixed with oil) and Permalon covers appear to be the most effective covers in reducing both odors and hydrogen sulfide. Oil should not be used as a cover. More studies are needed to assess the use of other types of Macrolite clay balls and geotextile fabrics.
Table 1. Percent reductions for covers that significantly reduced emissions at both the 5% and 10% levels.
|
Odor Units |
Hydrogen Sulfide |
|
|
Collection Period 1 |
||
|
Straw |
72% |
82% |
|
Straw/Oil |
83% |
|
|
Permalon |
71% |
90% |
|
Macrolite |
68% |
84% |
|
Collection Period 2 |
||
|
Straw |
84% |
94% |
|
Straw/Oil |
79% |
94% |
|
Permalon |
85% |
93% |
|
Macrolite |
64% |
|
|
Ceotextile Membrane |
68% |
71% |
|
Collection Period 3 |
||
|
Oil |
63% |
88% |
|
Straw/Oil |
84% |
88% |
|
Permalon |
63% |
88% |
|
Macrolite |
63% |
Page URL: http://www.bbe.umn.edu/extens/manure/programs/covercjc.html
Last updated July 1, 1998 by David Schmidt
For questions and further information, send email to David Schmidt
at: schmi071@maroon.tc.umn.edu
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