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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

Composting

Crop Nutrient Management Removal of Nutrients &
 Salts Anaerobic Digestion/ Thermal Conversion 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

Composting is the aerobic decomposition of organic material by microorganisms under controlled conditions. During composting, microorganisms consume oxygen while breaking the chemical bonds of organic material to obtain energy for growth. In this process, the amount of humus increases, the C:N ratio decreases, pH neutralizes, and the exchange capacity of the material increases.

Composting generates considerable heat and releases CO2 and water vapor into the air, potentially amounting to half the weight of the initial materials. Composting may also release nitrogen gas and ammonia to the air. Nevertheless, most nutrients in the starting material remain in the compost as humus and within the bodies of living and dead microorganisms. The final product has a low rate of microbial activity (Reference 1).

Essential factors for composting include aeration, nutrients, C:N ratio, moisture, pile structure, pH, temperature, and time. Aeration provides the large amount of oxygen needed for composting and removes heat, water vapor, and other gases from within the composting materials. The required rate of aeration for heat removal can be ten times greater than that for supplying oxygen (Reference 1).

The primary nutrients required by the microorganisms involved in composting are carbon, nitrogen, phosphorus, and potassium. An optimal carbon to nitrogen (C:N) ratio will prevent nitrogen loss and ensure rapid composting. A C:N ratio of around 30:1 usually ensures that the other required nutrients are present in adequate amounts. Water is necessary to support the metabolic processes of the microbes, providing the medium for chemical reactions, transporting nutrients, and allowing the microorganisms to move throughout the composting material.

Pile characteristics -- structure, porosity, texture, and physical properties of the starting material -- affects the composting process, primarily by influencing aeration. The composting process is relatively insensitive to pH within the range commonly found in mixtures of organic materials, largely because of the broad spectrum of microorganisms involved. The length of time required to transform manure into compost depends upon many factors including the temperature, moisture, frequency of aeration, and user requirements for the finished compost.

Composting systems, in order of increasing operating costs, range from static piles to turned windrows to positive-aerated-static-piles to negative-aerated-static-piles to enclosed facilities with biofilters and scrubbers. Each has their inherent attributes. In general, as operating costs increase, so also does efficiency and environmental protection.

Co-Composting Dairy Manure and Urban Green Waste

The Merced County Landfill, with technical assistance from Sustainable Conservation and funding from EPA's OSWER Innovations Pilot Project, successfully implemented co-composting of manure from a dairy with municipal green-waste at their existing composting site. The final report is available at Sustainable Conservation Exiting EPA (disclaimer).

References

1. On-Farm Composting Handbook, Natural Resource, Agriculture, and Engineering Services (NRAES) Cooperative Extension, Ithaca, NY, 1992.

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