Gas emissions from dairy farms using sand
Minnesota businesses, including dairy farms, must comply with Minnesota's ambient air quality standards. These standards set limits on hydrogen sulfide and eight other air contaminants at a business's property line. The hydrogen sulfide ambient air standard allows two 30 part per billion (ppb) hydrogen sulfide concentrations in any five-day period and two 50 ppb hydrogen sulfide concentrations per year.
New and expanding dairy operations in Minnesota must demonstrate that they can meet the State's ambient hydrogen sulfide property line standard as part of the permitting process. They can use MNSET, a free downloadable EXCEL spreadsheet available online at www.manure.umn.edu, to do a preliminary screening. In cases where MNSET results indicate that the hydrogen sulfide property line standard may not be met, additional modeling and environmental assessment may be required. Limited emissions data for common sources on Minnesota dairy operations that use sand bedding makes it difficult for dairy producers to demonstrate whether their operation could meet the State's ambient hydrogen sulfide property line standard.
Sand bedding is popular with many dairy producers because well designed and managed freestalls bedded with sand can provide a comfortable place for cows to lie. Sand laden manure collected from manure alleys can be either stored before being applied to cropland to recycle the nutrients or it can be separated so that the sand can be recycled as bedding and the manure stored before being land applied. Since the mid 90s mechanical separators and sand lanes have been developed to separate sand from sand laden manure for sand recycling.
In 2009, the University of Minnesota Rapid Agricultural Response Fund funded a small project to measure gas flux rates from key sources on eight dairy farms that used sand bedding. The key sources included manure alleys, cow lanes and holding areas, liquid and solid manure storage basins, manure reception pits, recycled sand piles and sand lanes. The gases measured were hydrogen sulfide and ammonia.
Gas flux rates are the amount of gas emitted into the air per square foot of emitting surface. Emission rates, the amount of chemical per unit time emitted into the air, are calculated by multiplying the flux rate times the emitting surface area. Emission rates depend on both the flux rate and the amount of emitting area. Large emissions can come from sources with low flux rates and very large emitting areas or sources with high flux rates and small emitting areas. Obviously sources with both large flux rates and large emitting areas will have very large emissions. The project, after peer review and publication, will provide emissions data for a few sources on dairy operations using sand bedding that can be used in MNSET to screen for the potential to exceed Minnesota's hydrogen sulfide ambient standard.
Hydrogen sulfide and ammonia flux rates were measured three times on eight dairy farms that used sand bedding in 2010. Flux rates were measured using a wind-tunnel, gas sample bags and gas analyzers. Two farms did not recycle sand, two farms used mechanical sand separators, two farms had continuously running sand lanes, and two farms had sand lanes that ran intermittently when the manure alleys were scraped.
The wind-tunnel was a portable device that covered a small portion of an emitting surface and blew air slowly across the covered surface. Differences in gas concentrations of air entering and leaving the wind tunnel, the known area covered, and the airflow rate through the tunnel were used to determine flux rates. Flux rates were measured from cow manure alleys, recycled sand piles, solid and liquid manure basins, sand lanes, and reception pits. Emissions rates from the rooms that housed the mechanical separators were determined too.
Results of the measurements indicated that hydrogen sulfide and ammonia emissions flux rates and emissions rates varied from source to source and farm to farm. Hydrogen sulfide flux rates were the highest from the intermittent and continuous flow sand lanes and the reception pits at the ends of the continuous flow sand lanes. Cow manure alleys and piles of recycled sand had the lowest hydrogen sulfide flux rates.
Sources with the highest ammonia flux rates included the manure basins on farms that did not recycle sand, liquid manure basins on farms with continuous flow sand lanes, and manure basins on farms with intermittent sand lanes.
Manure storage facilities were generally the sources with the largest emissions rates of hydrogen sulfide and ammonia because they had large emitting surface areas. Both the continuous flow and intermittent flow sand lanes were important sources of hydrogen sulfide emissions. They had relatively small emitting surface areas compared to other sources but some of the larger flux rates. Cow manure alleys were also important sources of ammonia emissions.
Wind tunnel sampling from a manure alley.
Wind tunnel sampling from a manure storage basin.