Everything needs clean air. Improving air quality improves human and animal health and can help promote a positive image among the non-agricultural public.
Dr. Jason Oliver, PRO-DAIRY Dairy Environmental Systems, Cornell University, discussed the air quality impacts of manure and other sources of reduced air quality on the farm. “As farms have become more consolidated, they are often new and larger emission sources – large open manure pits, areas where manure is spilling into a pit, large feeding centers, fertilizer tanks, feeding sources and flares from anaerobic digesters,” he said.
Ammonia is a big player in odor management and can impact respiratory health for farmworkers, livestock and neighbors. Fresh manure, long-term manure storage and land application of manure are the most common sources of ammonia.
Particulate matter is generated by farm equipment and cattle movement and indirectly from ammonia emissions. Volatile organic compounds emanate from fresh manure, manure storage, silage and TMR.
Although agriculture is a small contributor to greenhouse gas (GHG) emissions, Oliver said the industry has a role in mitigation strategies through manure management. “Even though our agricultural systems take a lot of heat for impacting GHG emissions, we’re a minor player compared to national or state-level emissions,” he said. “In New York, dairy production is only 4% of the state’s source of greenhouse gas emissions.”
In some cases, on-farm improvements that benefit one area of the operation can be detrimental to others. Livestock facilities that have implemented solid/liquid separation systems and indoor tanks can become hazardous with the potential for manure spills, creating risks for workers and impacting air quality.
“I see a lot of manure and TMR that spill out in drip areas at the edge of barns,” said Oliver. “These areas are wet and can be sources of odor. New facilities are more mindful of that, and I’m an advocate for gutters and managing rainwater.”
Maintaining farmstead road surfaces can make a difference in air quality. Truck and equipment traffic moving throughout the farm produce substantial levels of particulate matter. When conditions are dry and excess dust is created, simply driving slower can substantially reduce particulate.
Storing manure improves nutrient management, which improves water quality and helps recycle nutrients for use on cropland. “These storages can become a source of odors,” said Oliver. “They’re a source of methane because they’re typically anaerobic, and also a source of ammonia emissions.”
A roof structure over manure storage can help reduce ammonia emissions because the structure slows excess microbial activity resulting from the sun’s heat. “It prevents wind from going across the surface and kicking up gases,” he said. “On facilities with high solids content, encourage crust formation on manure storage. A natural crust is known to reduce ammonia emissions, although when crusts are too big, they can impact nitrous oxide.” Manure additives can also help promote crust formation.
“The practice of separating solids has increased across the Northeast,” said Oliver. “It’s pretty common on larger dairy facilities. Many farms are using screw presses, which gives the farm a great opportunity to recycle solids as bedding, which can be a huge cost saver.”
This practice produces two manure streams to manage: separated liquids and separated solids. Ammonia and methane emissions are affiliated with the liquid portion, while nitrous oxide is in the solid portion. Screw press separation keeps volatile solids out of manure storage, resulting in lower GHG emissions. With more effective separation processes and with more fine solids captured, there’s further reduction of GHG emissions. However, separation doesn’t impact ammonia emissions.
In addition to separating solids for bedding, some farms also incorporate lime as a quick pasteurization of the material before it’s used for bedding. “Those facilities can generate substantial amounts of ammonia,” said Oliver. “Some levels are quite hazardous to farm employees. I’ve also seen a lot of separators generating solids into a three-sided shed, and when the wind blows, solids are drawn out and dispersed through the air.”
Farms that practice solid/liquid separation should be aware of potentially generating dust and bioaerosol.
Anaerobic digestion technology is well-established and is often coupled with separation. Another option for digesters is the addition of food waste for co-digestion. Food waste can be valuable to the farm in the form of tipping fees and additional energy but can be a significant source of odors if not managed properly.
While some farms’ gensets use a catalytic converter, others have a stack from which a flare is released. The flare combusts methane when the biogas usage system can’t handle it, which can result in excess emissions. Oliver described some digestate storages as “lively,” with bubbles and foam from active methanogens – the microorganisms that produce methane. As more farms become involved with renewable natural gas (RNG), upgraded systems that separate CO2 from the biogas can also generate methane.
Cover and flare systems can help mitigate methane by reducing ammonia. Heavy plastic covers secured over stored manure capture gases and direct them to a flare. This reduces methane emissions and also diverts rainwater, keeping excess moisture out of manure storage and retaining full capacity for manure.
“These covers significantly reduce odors and are known to retain a lot more nitrogen in the manure, which limits ammonia emissions,” said Oliver. “Plastic can be rewelded if it leaks.”
He added that cover design for cover and flare systems has improved and there’s now a standard for materials manufacturing.
“They’ve moved from green to black materials that are more UV resistant,” said Oliver. “They’ve improved some of the means by which manure is moved in and out, and water management on top.” However, such covers are expensive and not suitable for all farms.
Another manure management option that’s continually improving is biofilter systems designed to manage manure and food waste. Reception pits collect manure and food waste, then air is drawn off to maintain a vacuum. Odorous emissions are drawn upward through a pipe, then fed down into a biofilter where naturally immobilized bacteria and fungi “eat” the odorous gases.
“Biofilters are known to substantially reduce odor, hydrogen sulfide and ammonia,” said Oliver. “We’ve also seen some intermittent methane reduction, but we have opportunities to make improvements.”
by Sally Colby