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Pacific Southwest, Region 9

Serving: Arizona, California, Hawaii, Nevada, Pacific Islands, Tribal Nations

Dairy Manure Management:
Technologies for Treating Dairy Manure

Liquid Waste Treatment to Remove Nutrients and Salts

Crop Nutrient Management Anaerobic Digestion/ Thermal Conversion Composting Solid-Liquid Separation Dairy
Blue boxes indicate processes. Green boxes indicate products with economic value. *Current practice on
California dairies or pilot project in place. Flow Diagram, PDF (1 pg, 12K) About PDF

The liquid fraction of the waste steam after solid-liquid separation contains dissolved nitrogen, potassium, and other dissolved solids (salts), and some phosphorous depending on the effectiveness of the separation process. Dairies with sufficient cropland to assimilate the nutrients and salts may be able to use the liquid fraction directly for irrigation and fertilization. If the cropland available to a dairy is insufficient to assimilate the nutrients and salts in the waste stream without contaminating surface or groundwater, further treatment can remove the excess nutrients and salts. The different constituents require different treatment processes: precipitation (likely at the solid-liquid separation stage) for phosphorous, nitrification/denitrification for the nitrogen, and reverse osmosis for nitrate-nitrogen, ammonium-nitrogen, potassium, and other dissolved solids. Taking another approach, one pilot project has investigated the feasibility of using fast-growing algae in place of land-based crops for capturing nutrients from animal feeding operations. The algae can then be composted or fed to fish and livestock.

Nitrification/Denitrification

One of the greatest challenges facing dairies is avoiding the uncontrolled release into the environment of the nitrogen in dairy manure (Reference 1). Conventional manure management practices result in much of the nitrogen in dairy manure being converted to ammonia (NH3 or ammonium ion NH4 + when in solution) and to other nitrogen compounds. The various forms of nitrogen can volatilize into the air (Reference 2), provide fertilizer for crops, or pollute surface water and groundwater (Reference 3).

One method for addressing the "nitrogen problem" is to convert all of the reactive nitrogen compounds in the dairy waste into the chemically inert form of diatomic nitrogen gas (N2). The process used to make that conversion is referred to as "nitrification/denitrification." For dairies, nitrification/denitrification is best applied to the liquid-waste stream, usually after the bulk of the solids have been removed from the manure slurry, which reduces the energy cost of nitrification. Removing the solids greatly reduces the cost of energy needed to adequately mix the manure during the nitrification/denitrification process. The solids must be appropriately handled in order to minimize or avoid the pollution problems that may result from the excess nutrient loads from this part of the dairy waste stream.

As the name suggests, nitrification/denitrification is a two-step process. In the first step, aeration of the liquid allows certain bacteria to oxidize organic nitrogen and ammonia to nitrate- and nitrite-nitrogen. In the second step, other bacteria in an anaerobic environment convert nitrate and nitrite to inert nitrogen gas, which joins the 80% of the atmosphere that is N2. Considerable energy is required to aerate the liquid in the first step and to move the liquid from one step to the next. The result is a loss of the fertilizer value of the nitrogen, but this may be advantageous in some situations. The denitrified water could be used to water cows or crops. EPA and the Tennessee Valley Authority funded construction of such a system for testing at California Polytechnic - San Luis Obispo.

Reverse Osmosis

Reverse osmosis is an energy-intensive process that forces pre-filtered water through a barely permeable membrane that removes a large percentage of the remaining ions. Biogas from an earlier anaerobic digestion or thermal conversion stage could offset the required energy. The concentrated filtrate would contain salts as well as crop nutrients but might retain some value as part of a comprehensive soil nutrient management system. The filtered water could be used directly for stock water or irrigation or to dilute a portion of the liquid waste stream to agronomic nutrient concentrations, or it could be further processed for potable uses if permitted.

References

  1. Burton, C.H., and C. Turner. 2003, Manure Management: Treatment Strategies for Sustainable Agriculture, 2nd edition, Silsoe Research Institute, Silsoe, UK, 451 pp.
  2. National Research Council, 2003, Air Emissions from Animal Feeding Operations: Current Knowledge, Future Needs, The Ad Hoc Committee on Air Emissions From Feeding Operations, Committee on Animal Nutrition, Board on Agriculture and Natural Resources, Board on Environmental Studies and Toxicology,Link to EPA's External Link Disclaimer National Academies Press, 286pp.
    • Also: Akiyama, H. and H. Tsuruta, 2003, "Nitrous Oxide, Nitric Oxide, and Nitrogen Dioxide Fluxes from Soils after Manure and Urea Application", J. Env. Quality 32:423-431.
  3. USGS National Water-Quality Assessment Program (NAWQA), "Water quality and nonpoint sources in agricultural watersheds" Link to EPA's External Link Disclaimer. And "Nutrients National Synthesis, Publications about Nutrients"Link to EPA's External Link Disclaimer.

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