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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

Energy Production

Crop Nutrient Management Removal of Nutrients & Salts 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)
Today's manure can equal tomorrow's profits.

Two processes can produce energy from dairy manure.

Anaerobic digestion is a natural biological process by which bacteria break down organic matter in an oxygen-free environment with moisture content of 85% or higher. The process produces "biogas," inorganic salts, and residual organic material. The biogas consists of CH4, CO2, and trace amount of other gases including hydrogen sulfide (H2S). Biogas can be burned to produce heat or to power an electric generator. The amount of biogas produced and the percentage of residual organic matter depends on the duration of the anaerobic digestion process and factors such as temperature, moisture, nutrient content, and pH. The residual organic material can be used for animal bedding or as a soil amendment.

Several types of anaerobic digester are available, including, in order of increasing complexity:

  • Covered lagoon
  • Plug-flow digester
  • Completely-stirred tank reactor (CSTR)
  • Upflow anaerobic sludge blanket (UASB), and
  • Anaerobic sequencing batch reactor (ASBR)

All produce biogas which can be burned for heat or to generate electricity, or, after refinement and compression, used as a vehicle fuel or injected into a natural gas pipeline for distribution and use. For example, one project in the San Joaquin Valley of California is converting cheese factory waste and dairy manure to a vehicle fuel, replacing diesel fuel for trucks converted to run on natural gas. It is also possible to compress and catalytically convert biogas to methanol, ammonia, or urea, though at a cost of two-thirds of the energy.

Digester Projects

The St. Anthony Farm Digester

The St. Anthony Farm Digester

In 2006, approximately 18 digesters were currently running or under construction at dairies in California (see map below). Support for these projects included $10M from SB5x through the California Energy Commission, administered by Western United Resource Development, Inc. In addition, section 9006 of the 1996 Farm Bill provided approximately $2M to support construction of three to five digesters on dairies near Elk Grove, California. The EPA AgSTAR program at headquarters provided technical assistance for both state- and federal-funded. Dairy operators, Sacramento Municipal Utilities District, the non-profit Sustainable Conservation, and USDA-NRCS' Environmental Quality Incentives Program also provided support in the form of grants and matching funds.

EPA's AgSTAR program summarizes several of these projects in a table on their web site. The California Energy Commission's web site provides reports on digesters (choose Renewable - Biomass in the Search list) at Eden-Yale, Van Ommering, Koetsier, Hillarides, Meadowbrook, Cottonwood (Joseph Gallo farms), Castelanelli Bros., and Blake's Landing (Straus) dairies.

Thermal Conversion

Technologies that burn waste to produce energy or treat waste to produce fuels are classified as "thermal conversion" and include direct combustion (burning with excess air to produce heat), pyrolysis (thermal treatment in the absence of air, resulting in the production of pyrolysis oil and a low-BTU gas), gasification (thermal treatment at higher temperatures in an oxygen-restricted environment to produce a low- to medium-BTU gas), and hydrothermal liquefaction (thermal conversion of solids in a liquid stream to oils and char for separation and use as a fuel). The fuels that are products of pyrolysis and gasification can be used in boilers and engines.

Most of the thermal conversion technologies (hydrothermal liquefaction may be an exception) are not suitable for raw dairy manure because of the high energy cost to dry the manure to acceptable moisture levels. Dairy manure as excreted is 10 to 12% solids; in 80% of dairies in the San Joaquin Valley it is diluted with flush water to less than 2% solids. Evaporating the water takes at least twice the energy content of the remaining solids. Therefore, this technology may be appropriate only for manure solids that have been separated from the liquid fraction or for air-dried solids collected from the open-lot areas.

The Inland Utilities Agency in Chino, California, funded by Farm Pilot Project Coordination, Inc., is testing an Agricultural Waste Solutions gasification system that reportedly captures 90% of the phosphorous and 60% of the nitrogen in the solids that are burned. It is installed at a "scrape" dairy that manages manure without flushing it with water into a lagoon, though the operator expects the system to work at "flush" dairies also.

For More Information

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