The main objective of this one year study was to identify a group of covered and uncovered pig manure storage units in Minnesota and measure their odor concentration.
It was decided initially that three separate clusters or areas of the state (southeast, southwest, and west central) should be used when selecting storage units for this project. The main reason for the 3 clusters or groups, instead of evenly distributing them throughout the state, was to save on travel expenses during the collection of samples. A number of pork producer and extension educator contacts were solicited to help identify appropriate storage units. A total of 22 manure storages were selected for this study which are located on 16 different farms sites. Each manure storage systems in the pool could be classified as one of the following four types of units: indoor deep pits, outdoor concrete / steel structures, earthen basins, and 2nd stage earthen storage or lagoons.
Although numerous attempts were made to find storages with existing covers on them only a couple of units were found which had a natural "crust" on them. Because of this, two Minnesota companies were contacted who had expressed an interest in cooperating on the project by supplying a "cover" for an existing storage units to expand the number of covered storages. The two companies were Kinetico, suppliers of "macrolite" (clay like material which floats) and Aeromix Systems, which sell mechanical aerators. Both agreed to participate and installed this past fall a "macrolite" and "aerated" cover, respectively, on one of the cooperator's farms.
As principle investigator, I personally visited each of the farms this summer and early fall to communicate to the individual producers our intent and answer any questions that they had. For the most part, almost everyone viewed this as an opportunity to expand our knowledge about odor and its larger concerns. Only 1 or 2 individuals contacted decided not to participate in the project.
Between mid-September and the end of October, two air samples were collected from almost all the storage systems approximately 1 month apart. The air samples were collected in 10 liter tedlar bags immediately above the surface of manure with a special "flux" hood or chamber and a commercial "vacuum" box to avoid capturing any odors from the internal mechanisms of an air pump. A bag was first filled at a two liters/minute flowrate for five minutes then evacuated so as to coat the interior of the bag with the sample air. Then a one liter/minute flowrate was used to fill the bag with air for the final sample. Impermeable hoses were used to eliminate contamination from previous sampling. After the bags were filled they were placed in a kitchen garbage bag and stored in a vinyl storage container to protect them from physical damage.
The air samples were transported to the St. Paul campus of the University of Minnesota to be analyzed within 24 hours by an olfactometer. The olfactometer is a device which is used to determine the odor threshold of an air sample. It dilutes an odorous air sample with clean nonodorous air and then presents that mixture to a panel of people. The process involves presenting three different samples of air to the panel. Only one sample contains the mixture of odorous and clean air. The other two samples contain only clean air. Each panelist then must pick which of the three samples contain the odorous air. For these reasons the instrument is often referred to as a triangular (3 choices) forced-choice (must select one of the three) olfactometer.
The process begins by using such a small amount of odorous air which none of the panelist can smell. Odorous air concentration in the one air sample is then increased and another group of three samples is presented to the panelist. This process is continued until the panelist is able to detect a slight difference between the three samples. The odor value is then reported as the dilution ratio which is the ratio of the volume of clean air to the volume of odorous air. This number is referred to as odor unit (ou).
Odor panel selections were begun in July, 1996 and continued into August with 1/2 day training sessions. A pool of approximately 20 panelists was established by mid-September when samples were first collected and analyzed. Sample analysis was done during 2 to 3 hours odor panel sessions. From 8 to 10 samples were typically analyzed during each of these sessions by 7 to 8 panelists. During each odor measurement session, a N-butanol standard was used to establish each panelist's sensory condition which was compared to past performance.
It was decided that in addition to odor measurement the air which was collected would also be analyzed for hydrogen sulfide (H2S). This was done in response to concerns about ambient air H2S concentrations in Renville County which occurred this past year. Hydrogen sulfide measurements were added to this study to determine if a correlation exist between that gas and the odor numbers being recorded.
The measurement of hydrogen sulfide concentrations were done by use of the JeromeTM meter for a large majority of the samples. This electronic device can measure very low concentrations of airborne sulfides (down to 1 or 2 ppb), one of the instruments being used in Renville county. H2S samples were directly taken from the same tedlar bag samples tested for odor. The air inside the tedlar bags were first screened for H2S by the use of SensidyneTM colorimetric detector tubes to determine if the levels were less than 20 ppm. The JeromeTM meter was then used to determine the level of hydrogen sulfide.
To date, approximately 40 odor and H2S measurements have been made for this project. The preliminary data shows a wide range of both the odor dilution thresholds and H2S concentrations for all storage units sampled. Odors number range from a low of 30 up to 1250 while hydrogen sulfide value varied from 30 ppb up to 120 ppm.
Results from a companion project have been combined with data from this study to show individual manure storage's odor and H2S levels for the four different types of manure storage systems (Figures 1 - 4). Information presented in these four figures is data collected from the first or "September" sampling when ambient daytime temperatures were from 45 and 70 0F and odor values ranged from 20 up to 400 odor units.
Initial results collected from different manure storage systems commonly used in Minnesota does not reveal an advantage of one system over another when odor dilution thresholds or H2S concentrations are used as criteria. There also seems to be little or if any correlation between odor numbers and H2S levels, although insufficient data is available to statistically confirm this statement. The pig's diets may and no doubt does influence both odor and H2S levels from the storage units but no trend was apparent with this preliminary data. Finally, the large variations in the data for all types of storages and the lack of logical physical explanations for most of these large difference, suggests that management practices may play a larger role in odor and gas generation than originally suspected.
Of the 22 storages selected for this study, only 2 presently have either a natural or artificial cover with the third "aerated" cover becoming active next spring. I intend to contact several organizations this winter to hopefully get several other cooperator's manure storages covered with something like straw, a tarp, or other materials next spring so more evaluations of covers on real storage systems can be made. The data so far has shown only a slight reduction in odor due to a cover compared to similar storage units. There does seem to be a greater reduction in H2S concentration when a cover is in place.
Further odor and hydrogen sulfide measurements will be made during 1997 from the manure storages identified in this report. Air sampling will resume next spring and continue on a monthly basis, for the 22 units in this project, until at least June. Manure analysis will be done for each storage unit as well as an inventory of other manure storage management practices. After this is done, more information will become available which will help explain some of the differences between storage systems and within the same system. Also, possible seasonal effects will be able to be detected as will other factors such as temperature, animal diets, frequencies of pumping, and general management of the storage.
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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