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Understanding the Composting Process

pile of organic materials

One of the most important steps for evaluating composting options is to become familiar with how the composting process works. Before you begin composting or start a composting program, you should understand the five primary variables that must be "controlled" during composting. These include the following:

  • Feedstock and nutrient balance. Controlled decomposition requires a proper balance of "green" organic materials (e.g., grass clippings, food scraps, manure), which contain large amounts of nitrogen, and "brown" organic materials (e.g., dry leaves, wood chips, branches), which contain large amounts of carbon but little nitrogen. Obtaining the right nutrient mix requires experimentation and patience and is part of the art and science of composting.

  • Particle size. Grinding, chipping, and shredding materials increases the surface area on which the microorganism can feed. Smaller particles also produce a more homogeneous compost mixture and improve pile insulation to help maintain optimum temperatures (see below). If the particles are too small, however, they might prevent air from flowing freely through the pile.

  • Moisture content. Microorganisms living in a compost pile need an adequate amount of moisture to survive. Water is the key element that helps transports substances within the compost pile and makes the nutrients in organic material accessible to the microbes. Organic material contains some moisture in varying amounts, but moisture also might come in the form of rainfall or intentional watering.

  • Oxygen flow. Turning the pile, placing the pile on a series of pipes, or including bulking agents such as wood chips and shredded newspaper all help aerate the pile. Aerating the pile allows decomposition to occur at a faster rate than anaerobic conditions. Care must be taken, however, not to provide too much oxygen, which can dry out the pile and impede the composting process.

  • Temperature. Microorganisms require a certain temperature range for optimal activity. Certain temperatures promote rapid composting and destroy pathogens and weed seeds. Microbial activity can raise the temperature of the pile's core to at least 140 °F. If the temperature does not increase, anaerobic conditions (i.e., rotting) occur. Controlling the previous four factors can bring about the proper temperature.

Methods of Composting

Composting takes on many forms, from simple and inexpensive backyard or onsite composting methods to more expensive and high-tech methods such as in-vessel composting. Composting varies as much in its complexity as in the range of organic materials recovered. The most common composting methods are listed in order of increasing costs and levels of technology required and are described in greater detail on the following pages:

After reviewing the science of composting above, select an appropriate method or combination of methods that will best meet your needs. Will backyard composting suffice for reducing residential volume, or should you invest in equipment and labor for larger volumes from restaurants or other businesses? Selecting the right composting equipment at an affordable price also requires careful research. Hundreds of vendors sell composting equipment and there are many variations on each type of equipment.

cupped hands holding compost
Photo courtesy of Jepson Prairie Organics

Composting Challenges

Challenges for the composting industry as a whole include a lack of consistent product quality, market research and planning, investment, accepted national compost specifications, and sophisticated product marketing. In addition, compost end uses range from city and county landscaping to niche markets such as soil remediation. Government agencies could play a larger role by increasing purchases and promotion of compost products. New technologies allow compost companies to tailor their products to specific end-uses, increasing the market value of the material. In fact, more and more compost producers are engineering multiple compost products for applications as diverse as bioremediation of contaminated soil and erosion control at construction sites. Many composting companies are packaging and marketing compost in home repair, garden center, and other retail outlets. Some companies use compost to control odors through new process technologies such as biofilters, while still others are using compost as a filter in water treatment systems.

To Learn More About Environmentally Safe Ways to Compost

Please see Chapter 7 of EPA's Decision-Maker's Guide to Solid Waste Management (PDF, 58 pages, 1.7 MB, About PDF), Second Edition (EPA530-R-95-041, September 1997) for more guidance on environmentally safe ways to compost.

U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, NW
Washington, DC 20460