By Lingying Zhao, Ohio State University associate professor and Extension agricultural engineer
Mitigate gas emissions from animal facilities
As animal farms evolve toward larger and more concentrated operations, animal barns and manure storages become significant sources of carbon and nitrogen gas emissions. These air emissions include odor, ammonia (NH3), hydrogen sulfide (H2S), methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O).
Ammonia emission results in rising environmental and health concerns. Methane and nitrous oxide emissions are known as greenhouse gases (GHGs) causing climate change concerns. The U.S. Environmental Protection Agency (EPA) estimates that 27% and 76% of the total anthropogenic methane (CH4) and ammonia (NH3) emit from agricultural animal production activities. To achieve sustainable animal production, effective technologies to mitigate or recover ammonia and greenhouse gas emissions from animal facilities are needed.
In reviewing the existing air emission abatement technologies, impermeable covers and bio-digesters are used to collect and produce methane from manure storages, biofilters are studied to effectively reduce odor emissions from swine buildings and manure storages, and wet scrubbers are developed to recover ammonia emission from animal buildings and manure storages.
Manure storage covers and bio-digesters
Effective air emission mitigation technologies for manure storages include anaerobic lagoons, manure storage covers and methane anaerobic digesters. There are many kinds of manure covers: natural or man-made, permeable or impermeable. Impermeable covers with flexible synthetic materials are commonly used for large liquid manure storages. Anaerobic digesters are devices to create a series of mechanical and biochemical processes to break down biomaterials, such as manure, using microorganisms in the absence of oxygen.
Among the mitigation options for manure storage, covers are the most effective and economical solution for abatement of air emissions. Covers create a physical barrier at the liquid-air-interface, which helps retain more gases and organic compounds in the liquid phase and minimizes emissions to the atmosphere. They cause reduced ventilation over the manure surface, and liquid turbulence is minimized. The odor and gas emission reduction efficiencies of an impermeable cover can be as high as 80% to 95%. In addition, covers that prevent ammonia volatilization conserve nitrogen. Therefore, covers can result in multiple benefits in addition to control of odor and gas emissions from manure storages.
To help Ohio producers explore the cover and carbon credit opportunity, Ohio State University (OSU) Extension has organized Manure Storage Cover Workshops in past years. Visit airquality.osu.edu/workshops/ for details.
A biofilter is a filter of organic material with microbes that breaks odorous gas into carbon dioxide and water. The organic materials can be a mixture of compost and wood chips or shreds. A biofilter can reduce odor by as much as 95% and ammonia emissions by up to 80%. However, biofilters saturate easily and pressure drops across them range from 25 Pa to 250 Pa, making it difficult for them to be adopted by animal facilities.
Difficulty also rests with the stringent moisture and temperature requirements for the process and more frequent media replacement. Furthermore, research shows that nitrogen accumulation in the biofilter material causes the release of nitrogen oxides and nitrous oxide, which are greenhouse gases.
According to research conducted at Minnesota University, construction costs of biofilters range from $100 and $150 per 1,000 cfm of exhaust air, and the operation costs of a biofilter are approximately $3.00 per 1,000 cfm per year. For more information, please see a biofilter factsheet at www.manure.umn.edu/assets/biofilters.pdf.
Wet scrubbers are devices that use a liquid, usually water-based, to remove gases by absorbing them onto wet surfaces or into liquid droplets or films. Typically, two types of wet scrubbers are suitable for agricultural applications: packed wet scrubbers and spray-type wet scrubbers. In Europe, pack-type wet scrubbers are commercial products and widely used to reduce ammonia emissions from animal facilities by up to 96%.
However, U.S. farms cannot readily adopt the pack-type wet scrubbers because many U.S. animal barns are very large in scale and use mechanical ventilation systems with axial exhaust fans. Packed wet scrubbers create significant pressure drop, which cannot be overcome by the axial exhaust fans. Spray type wet scrubbers with liquid droplet spray do not cause large pressure drop and can be used on barns and manure storages with axial exhaust fans. OSU is working to develop and field test spay-type wet scrubbers. The optimized spray-type wet scrubbers can remove ammonia emission from 70%-90% at different ammonia concentrations. Since sulfuric acid solution is used as the scrubbing liquid, effluent of the spay-type wet scrubber is mainly ammonium sulfate solution, which is a fertilizer, not waste water.
The European pack-type scrubbers cost $42 per pig or $1.3 per broiler. The annual operating cost was $14.82 per pig and $0.27 per broiler. Chemical costs were 14% and 37% of operating costs, respectively. The operating costs included fan power, pump power and scrubbing liquid consumption. Current work will estimate the cost of a spray-type wet scrubber.