Alternative technologies and uses for manure
Gregory Beatty and Hank Zygmunt, Office of Wastewater Management, Environmental Protection Agency
Editor’s note: This article includes excerpts from an EPA report titled “Alternative Technologies/Uses for Manure.”
Trends in the livestock industry
As a result of domestic and export market forces, technological changes and industry adaptations, the past several decades have seen substantial changes in America’s animal production industries. Despite support from the U.S. Department of Agriculture (USDA) for sustainable agricultural practices, these factors have promoted expansion of confined production in the following ways:
• increase in the number of animals per facility
• growth of production facilities in existing and new livestock sectors
•integration and concentration of the livestock industry
• geographic separation of animal production and feed production operations
• concentration of large quantities of manure and wastewater in some watersheds
Overapplication of manure and wastewater
Traditionally, manure and wastewater produced at an animal feeding operation is applied to croplands and pasture to take advantage of their nutrient value as fertilizers. The consolidation of the animal agriculture industry has resulted in the production of more manure-derived nutrients than are needed to meet crop nutrient needs in some regions of the country.
Why consider other utilization options?
Due to the imbalances in the animal livestock industry, manure and wastewater from animal feeding operations have the potential to contribute pollutants such as nutrients, organic matter, sediments, pathogens, heavy metals, hormones, antibiotics and ammonia to the environment. Alternative technologies and uses of manure may be the only practical methods to address this problem.
This [article] is a compendium of technologies and uses that provide a snapshot picture of available manure use alternatives. The options discussed in this document describe uses for manure from animal feeding operations in lieu of land application for crops. It is based upon available information for the more common technologies and is not intended to address all potential options.
Treatment technologies
The purpose of a manure treatment process is to convert the manure to a more stable product. Treatment processes fall into three categories:
1. physical
2. chemical
3. biological
The treatment process may be designed to:
• solve odor problems
• recover nutrients or energy from the manure
• kill pathogens and weed seeds
• increase the fertilizer value
• reduce the volume
• decrease the pollution potential of the manure
• prepare the manure for export
Physical treatment
It is sometimes desirable to separate the solid and liquid portions of livestock manure. Solid separation converts the waste into a product that can be sold off the farm, given that a market has been developed. A short discussion of exporting manure follows this section.
Solid separation may be desired for the following purposes:
• reuse manure solids for bedding or refeeding
• improve the treatment efficiency of vegetative infiltration areas and leach fields
• use the liquids for flushing
• reduce the volume of waste to be hauled
Centrifuges increase the effect of gravity by spinning the manure at high speeds. Centrifuges are small and can produce a substance consisting of 15 to 40 percent solids. The equipment requires routine maintenance.
Advantages include:
• Centrifuges are a proven technology
• Centrifuges produce a drier material
Disadvantages include:
• Costs associated with a centrifuge are high and cannot be fully recovered from the sale of the final product.
• It may be difficult to process large quantities with the use of a centrifuge.
Fresh liquid manure can be transferred to a covered concrete tank where the solids are separated by settling. Conventional manure pumps are then used to agitate and remove the solids. Finally, the liquids are transferred from the concrete tank to an uncovered earthen manure storage.
Advantages include:
• Low-cost open storage, which has low nuisance potential, can be used for large volumes of liquids.
• Smaller volumes of highly concentrated manure, which contains the most offensive odors, can be placed in covered storage.
Disadvantages include:
• The effectiveness of the covered concrete tank method of separation for odor control has not been quantified.
Drying or dehydration is used to reduce the volume of manure by encouraging the water to evaporate, thereby concentrating the solids. Dehydration is used primarily for odor control. Drying systems must be covered to protect them from rainfall, and supplemental heat or forced air is needed to encourage rapid evaporation.
Advantages include:
• Dry manure does not support the growth of microorganisms or insects.
• Dry manure can be used as a soil conditioner in basically the same way that composted manure is used.
Disadvantages include:
• Costs associated with moisture removal are high and cannot be fully recovered from the sale of the final product.
Filtering and screening systems use a filter or screen to hold solids as the liquid passes through. The technique usually involves the use of gravity, vacuum or pressure.
Examples of filtering and screening systems include:
• Liquid-solid separators, which use stationary and vibrating screens to remove solids from flushing water.
• Sand drying beds, which use gravity to carry the liquid down through the sand while the solids form a cake on top.
• Vacuum filters, which use cloth or wire screens to hold the solids as the liquid is drawn through.
• Presses, which use cloth or wire screens to hold the solids as the liquid is pushed through.
Freezing has been used to aid in dewatering manure, improving settling and filtering.
Incineration is an extension of drying. Manure is converted to an ash, requiring application or disposal. Self-sustaining incineration requires a waste of approximately 30 percent solids. Wetter manure with lower solids content requires supplemental fuel to continue incineration.
Settling uses gravity to separate the solids from the liquids. Livestock manure is placed in a still basin to allow solids to settle to the bottom. The separation process can take as little as 30 minutes. Solids must be removed regularly to maintain the treatment efficiency of the settling system and to recharge the storage capacity.
Examples of settling systems include:
• septic tanks installed ahead of leach fields
• settling basins used with vegetative infiltration areas
Chemical treatment
Manure can be chemically treated to improve solids removal, kill microorganisms, eliminate odors and limit the spread of disease. Adding coagulating agents such as ferric chloride, alum, lime and organic polymers can greatly improve the dewatering characteristics of manure. Coagulants bring manure solids together so they will settle more quickly. Bringing the small particles together also improves the removal of solids by filtration. Care should be taken when handling coagulants because some are corrosive and others are extremely slippery if spilled.
Manure can also be treated chemically by raising the pH to about pH 12 for 30 minutes. This treatment kills most of the microorganisms living in the manure, which eliminates odors and limits the spread of disease. Lime is typically added to raise the pH of livestock manure. A limitation of using lime is that ammonia is immediately lost from the manure. Lime should never be added to manure that is located in a poorly ventilated or confined location.
Biological treatment
Biological treatment uses naturally occurring microorganisms in manure to change the properties of the waste. Examples include biodrying, anaerobic digestion and anaerobic lagoons and aerobic lagoons. Biodrying of manure is accomplished by recycling dry compost.
Advantages include:
• Odor, volume, and weight are reduced.
• Equipment for solids handling is available on most farms.
• Storage of solids is safer environmentally than liquid storage.
• Materials may be marketed.
Disadvantages include:
• Costs of operation may be high.
• Material handling may be excessive.
• Additional amendment may be required.
• Winter operation may require closed buildings.
Anaerobic digestion, which is the decomposition of manure in an oxygen-free (anaerobic) environment, is used in on-the-farm anaerobic digesters and anaerobic lagoons. Digesters break down the manure into a biogas that can be collected and used for fuel or energy. Anaerobic lagoons can be covered to collect gas. Uncovered anaerobic lagoons are usually 100-times larger than anaerobic digesters.
Advantages include:
• Digesters reduce odors during treatment of manure.
• Biogas recovered from digesters can be used as an energy source.
Disadvantages include:
• Digesters require daily attention.
• Digester biogas is flammable and must be handled with care.
Aerobic lagoons stabilize the manure by adding oxygen. Aerobic digestion is a one-step process, in which microorganisms use the bacteria to convert manure to carbon dioxide and water. This process takes between one to three months. Aerobic lagoons are 10-times smaller than anaerobic lagoons. Aerobic bacteria are sensitive to the lack of oxygen, and must be aerated and mixed regularly to prevent the lagoon from going anaerobic, which results in strong odors.
Advantages include:
• Aerated and mixed lagoon limit odors.
• Aerobic lagoons are smaller than anaerobic lagoons.
Disadvantages include:
• Aerobic lagoons have a high energy requirement.
• Aerators requires routine maintenance.
Export
Farmers may sell or give away excess manure or litter to be used for its nutrient value on other farms, lawns or gardens, nurseries, reclaimed strip mines, revegetated landfills or highway right-of-way. Prior to exporting animal waste from the farm, one of the waste management practices or treatment technologies described earlier may be applied.
The technical and social barriers associated with export include demand, transportation, handling costs and nutrient needs at the intended site of application versus nutrient content of the manure. ANM
—Excerpts from EPA website
