Background
Liquid manure storage can be a significant odor source. The anaerobic nature of manure stabilization can cause offensive odors and release of hydrogen sulfide and ammonia along with other gases during storage, agitation, and subsequent land application. The Minnesota Pollution Control Agency (MPCA) currently regulates feedlot air emissions/odor through the state hydrogen sulfide (H2S) ambient air standard. This standard limits H2S emissions to a 30-minute average of 30 ppb (parts per billion) no more than twice in five days, or to a 30-minute average of 50 ppb no more than twice per year. MPCA currently addresses ammonia (NH3) emissions from livestock production facilities during the environmental review portion of the feedlot permitting process. Consideration of ammonia emissions is a result of the proposed Minnesota Department of Health (MDH) chronic and acute inhalation health risk values for NH3. MPCA began testing for the presence of ammonia emissions from feedlot facilities during the 1998 field season.
A logical method to reduce the odors being emitted from open manure storage units is to contain the gases under an impermeable cover (impermeable plastic, concrete lid, etc.) or to place some type of floating cover on the manure surface. Floating covers can be made with a variety of materials and are usually permeable. Natural floating covers are those formed by the fibrous material in the manure (e.g., crust). Artificial floating organic covers, also called biocovers, include straw, chopped cornstalks, sawdust, wood shavings, rice hulls, or other organic materials. Polystyrene foam, plastic mats, air-filled clay balls like Leca and Macrolite, Permalon, PVC membranes, and geotextile membranes and mats have also been used as floating covers. By covering an outside manure storage pit or tank, the mass transfer of H2S, NH3, and other volatile organics from liquid to gas phase is reduced. In practice, a physical barrier is created at the liquid-air interface and more volatile chemical compounds are retained in the liquid phase.
Both barley and wheat straw can be used to form organic floating covers. The straw is applied to manure storage tanks using a straw chopping/blowing machine. Cover durability, plus the cost and labor to install and maintain the covers, are very important issues. Straw covers (12-in. thickness) have been observed to float from two to four months after one application on earthen pig manure storage basins in Minnesota. Six straw-covered manure storage units were monitored in 1998 and half of them reported no problem with manure agitation and pumping. One operator did not try to break up the straw cover and the other two experienced some difficulty in chopping up the straw. Successful agitation and pumping of straw-covered storages can be accomplished by using appropriate equipment (i.e. chopping pumps).
Geotextile materials, with a layer of straw on top, might provide a better cover than straw alone by keeping the straw from sinking. However, geotextile or geotextile+straw covers create some new challenges. So far, on-farm use of geotextile- or geotextile+straw-covers has resulted in limited agitation of manure before pumping and land application. There have also been problems getting the cover to float again after winter if the basin is completely emptied the previous fall. The straw that is placed over the geotextile gets wet after thawing, making the whole cover system heavier than if it is dry.
Methods
In an attempt to answer some of the questions that came up after 1998-99 field trials, a controlled study was carried out at University of Minnesota. Swine and dairy manure were placed in 32 individual PVC columns (4.6 ft-deep and 15-in internal diameter) to simulate deep pit systems. Manure was added on a bi-weekly basis (1 gal per addition), increasing the manure depth by 1-in. with each addition. Manure characteristics are given in Table 1.
Table 1. Swine and dairy manure characteristics (mean ± one standard deviation).
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Parameter |
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Total Solids, TS (%) |
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Volatile Solids, VS (% of TS) |
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Total Kjeldahl Nitrogen TKN (mg/L) |
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Ammonia Nitrogen NH3-N (mg/L) |
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Sulfide, S2 (mg/L) |
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The manure in the columns was covered with straw and/or a geotextile material. Straw thicknesses were 4, 8, and 12 in. and geotextile thicknesses were 0.3, 1.1, and 2.4 mm (1/100, 1/25, and 1/10 in.). Air samples (total of 320) were taken and measured for odor, H2S, and NH3. A total of 16 cover combinations were tested with each type of manure, including a control column with no cover.
Results
Percent odor and gaseous reductions were not affected by specie (swine or dairy). Mean results are given in Figures 1 to 3.
Figure 1. Percentage odor unit reduction.
Figure 2. Percentage of hydrogen sulfide reduction.
Geotextile Cover Alone
As shown in Figures 1 to 3, a geotextile cover alone had only a
slight effect on odor and gaseous emissions. Odor reductions varied
from 10 to 45%, depending on geotextile thickness. Hydrogen sulfide
reductions for the no straw treatments were between 13 and 35% and
ammonia reductions between 8 and 27%. Note that ammonia reduction was
highest with the thinnest geotextile cover. It appears that the
thinner membrane allowed NH3 to pass through the entire
exposed surface area whereas the thicker geotextile membranes forced
the volatile and gaseous compounds to pass around the membrane next
to the pipe wall to escape into the atmosphere.
Figure 3. Percentage of ammonia reduction.
Straw Cover Alone
A 4-in. layer of straw alone gave 60%, 69%, and 61% reductions on
odor, H2S, and NH3, respectively. Reductions in
odor and gaseous emissions of more than 80% were achieved with a
12-in. layer of straw. Thicker layers of straw alone (8 and 12 in.)
resulted in even better odor and gas percent reductions (70% or more)
than for a 4-in. straw treatment, with the exception of the value
obtained for ammonia reduction with an 8-in. layer of straw (about
60%).
Geotextile Plus Straw
Putting straw on top of the geotextile covers resulted, in general,
in lower percent reductions of odor and gaseous emissions than with
straw alone. It seems that this may be due to the different type of
surfaces that are in contact with manure. It is hypothesized that the
geotextile membranes, which are smoother and flatter than straw,
allow more gases to move sideways. In field conditions, the gases
need to move longer distances to escape around the cover and may get
trapped, thus the improvement in odor and gas emissions. Also,
dispersion of gases which pass through the cover to the atmosphere is
enhanced by wind action. With straw alone, the interface is rougher,
and more gas moves up into the straw layer instead of moving
sideways. When the gases get trapped in the straw, then there may be
a chance for the compounds to be oxidized by microorganisms,
resulting in less odor.
Conclusion
The above data supports the concept that the reduction of odor and gas emissions from the manure surface involves both a liquid-to-air interface exchange and a biofilter effect. The results also would indicate that the biofilter or biological effect can be more significant than the physical barrier effect since the straw provided larger reductions in odor and gaseous emissions than the geotextile cover. The geotextile cover may be useful as a barrier between the manure and the straw. A 0.3-mm geotextile membrane with either 8 or 12-in. of straw on top was able to reduce odor, H2S, and NH3 emissions by more than 70% in this pilot study. The membrane might also keep the straw from sinking under some specific conditions (from early spring until late fall or until snow starts falling). If the specific weight of the cover system and porosity of the straw material change (i.e. become greater than the manure specific weight), then the system will likely sink.
The different results for this pilot study and the field study are not easily explained. Performance of geotextile covers on field storage units seems to be acceptable based on limited observation and monitoring. However, accurate measurements from these sites are difficult to make. More data should be taken from field units, possibly with improved methodology, to accurately access performance. Meanwhile, larger tanks (375 gal) will be used for further research to evaluate how the covers perform in wet/dry conditions and during freeze/thaw cycles.
Acknowledgments
Funds for this research were provided by the Minnesota Legislature through the Minnesota Department of Agriculture.
The University of Minnesota is an equal opportunity educator and employer.
This page is part of the Biosystems and Agricultural Engineering Department web at http://www.bae.umn.edu/