[ header = Composting intro ]

What is compost?

Compost is organic material that can be used as a soil amendment or as a medium to grow plants. Mature compost is a stable material with a content called humus that is dark brown or black and has a soil-like, earthy smell. It is created by: combining organic wastes (e.g., yard trimmings, food wastes, manures) in proper ratios into piles, rows, or vessels; adding bulking agents (e.g., wood chips) as necessary to accelerate the breakdown of organic materials; and allowing the finished material to fully stabilize and mature through a curing process.

Natural composting, or biological decomposition, began with the first plants on earth and has been going on ever since. As vegetation falls to the ground, it slowly decays, providing minerals and nutrients needed for plants, animals, and microorganisms. Mature compost, however, includes the production of high temperatures to destroy pathogens and weed seeds that natural decomposition does not destroy.

Did You Know That Compost Can...

  • Suppress plant diseases and pests.
  • Reduce or eliminate the need for chemical fertilizers.
  • Promote higher yields of agricultural crops.
  • Facilitate reforestation, wetlands restoration, and habitat revitalization efforts by amending contaminated, compacted, and marginal soils.
  • Cost-effectively remediate soils contaminated by hazardous waste.
  • Remove solids, oil, grease, and heavy metals from stormwater runoff.
  • Capture and destroy 99.6 percent of industrial volatile organic chemicals (VOCs) in contaminated air.
  • Provide cost savings of at least 50 percent over conventional soil, water, and air pollution remediation technologies, where applicable.
  • Why Compost?
Composting makes sense. Instead of sending organic matter to a landfill, it can be transformed into a useful additive which can even be sold.

What can compost be used for?

  • Farmers use compost for enhancing crops and for sod farms.
  • Landscapers use compost as a soil amendment and for decorative purposes at properties, golf courses, and athletic fields.
  • Landfill operators use compost to cover landfills and carry out reclamation projects.
  • Nurseries use compost for enhancing plant and forest seedling crops in reforestation projects and to prevent certain plant diseases such as root rot.
  • Public agencies use compost for landscaping highway median strips, parks, recreational areas, and other public property and remediating contaminated or eroded sites.
  • Homeowners use mature compost to enrich gardens, improve the soil around trees and shrubs, use as soil additive for house plants and planter boxes and as a protective mulch for trees and shrubs.
Organic Materials

Yard trimmings and food residuals together constitute 23 percent of the U.S. waste stream, as documented by EPA. An estimated 56.9 percent of yard trimmings were recovered for composting or grasscycled in 2000, a dramatic increase from the 12 percent recovery rate in 1990. Accompanying this surge in yard waste recovery is a composting industry that has grown from less than 1,000 facilities in 1988 to nearly 3,800 in 2000. Once dominated by public sector operations, the composting industry is increasingly entrepreneurial and private-sector driven, led by firms that add value to compost products through processing and marketing. Compost prices have been as high as $26 per ton for landscape mulch to more than $100 per ton for high-grade compost, which is bagged and sold at the retail level.

While yard trimmings recovery typically involves leaf compost and mulch, yard trimmings can also be combined with other organic waste, such as food residuals, animal manure, and biosolids to produce a variety of products with slightly different chemical and physical characteristics. In contrast to yard trimmings recovery, only 2.6 percent of food waste was composted in 2000. The cost-prohibitive nature of residential food waste separation and collection is the primary deterant to expanding food waste recovery efforts. Yet in many communities, edible food residuals are donated to the needy, while inedible food residuals are blended into compost or reprocessed into animal feed. In some areas, composting operations are working with high-volume commercial and institutional food producers to recover their food byproducts, saving these firms significant disposal costs.


MNN Public Information from the U.S. Environmental Protection Agency

[ header = The composting process ]  

Understanding the composting process

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.

MNN Public Information from the U.S. Environmental Protection Agency

[ header = Methods of composting ] 

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:

MNN Public Information from the U.S. Environmental Protection Agency

[ header = Backyard or onsite composting ]  

Backyard or onsite composting:

Backyard or onsite composting can be conducted by residents and other small-quantity generators of organic waste on their own property. By composting these materials onsite, homeowners and select businesses can significantly reduce the amount of waste that needs to be disposed of and thereby save money from avoided disposal costs. Learn how to create your own compost pile.

Types of waste and waste generators
Backyard or onsite composting is suitable for converting yard trimmings and food scraps into compost that can be applied on site. This method should not be used to compost animal products or large quantities of food scraps. Households, commercial establishments, and institutions (e.g., universities, schools, hospitals) can leave grass clippings on the lawn—known as "grasscycling"—where the cuttings will decompose naturally and return some nutrients back to the soil. Backyard or onsite composters also might keep leaves in piles for eventual use as mulch around trees and scrubs to retain moisture.

Climate or seasonal considerations
Climate and seasonal variations do not present major challenges to backyard or onsite composting because this method typically involves small quantities of organic waste. When conditions change—for example, if a rainy season approaches—the process can be adjusted accordingly without many complications.

Environmental concerns
Improper management of food scraps can cause odors and also might attract unwanted attention from insects or animals.

Backyard or onsite composting requires very little time or equipment. Education is the most critical aspect of backyard or onsite composting. Local communities might hold composting demonstrations and seminars to encourage homeowners or businesses to compost on their own properties.

The conversion of organic material to compost can take up to two years, but manual turning can hasten the process considerably (e.g., 3 to 6 months). The resulting natural fertilizer can be applied to lawns and gardens to help condition the soil and replenish nutrients. Compost, however, should not be used as potting soil for houseplants because of the presence of weed and grass seeds


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[ header = Vermicomposting ] 


Through this method, red worms—not nightcrawlers or field worms found in gardens— are placed in bins with organic matter in order to break it down into a high-value compost called castings. Worm bins are easy to construct (they are also commercially available) and can be adapted to accommodate the volume of food scraps generated.

Types of waste and waste generators
Worms will eat almost anything you would put in a typical compost pile (e.g., food scraps, paper, plants). Vermicomposting can be ideal for apartment dwellers or small offices that want to derive some of the benefits of composting and reduce solid waste. It is frequently used in schools to teach children conservation and recycling.

Climate or seasonal considerations
Worms are sensitive to variations in climate. Extreme temperatures and direct sunlight are not healthy for the worms. The optimal temperatures for vermicomposting range from 55 °F to 77 °F. In hot, arid areas, the bin should be placed under the shade. By vermicomposting indoors, however, one can avoid many of the problems posed by hot or cold climates. The primary responsibility is to keep the worms alive and healthy by providing the proper conditions and sufficient food.

Vermicomposting has only a few basic requirements, among them: worms, worm bedding (e.g., shredded newspaper, cardboard), and a bin to contain the worms and organic matter. Maintenance procedures include preparing bedding, burying garbage, and separating worms from their castings.

One pound of mature worms (approximately 800-1,000 worms) can eat up to half a pound of organic material per day. It typically takes three to four months for these worms to produce harvestable castings, which can be used as potting soil. Vermicomposting also produces compost or "worm" tea, a high-quality liquid fertilizer for house plants or gardens.


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[ header = Aerated (turned) windrow composting ]  

Aerated (turned) windrow composting:

Organic waste is formed into rows of long piles called "windrows" and aerated by turning the pile periodically by either manual or mechanical means. The ideal pile height, which is between 4 and 8 feet, allows for a pile large enough to generate sufficient heat and maintain temperatures, yet small enough to allow oxygen to flow to the windrow's core. The ideal pile width is between 14 and 16 feet.

Types of waste and waste generators
This method can accommodate large volumes of diverse wastes, including yard trimmings, grease, liquids, and animal byproducts (such as fish and poultry wastes), but only with frequent turning and careful monitoring. This method is suited for large quantities, such as that generated by entire communities and collected by local governments, and high volume food-processing businesses (e.g., restaurants, cafeterias, packing plants).

Climate or seasonal considerations
In a warm, arid climate, windrows are sometimes covered or placed under a shelter to prevent water from evaporating. In rainy seasons, the shapes of the pile can be adjusted so that water runs off the top of the pile rather than being absorbed into the pile. Also, windrow composting can work in cold climates. Often the outside of the pile might freeze, but in its core, a windrow can reach 140 °F.

Environmental concerns
Leachate is liquid released during the composting process. This can contaminate local ground-water and surface-water supplies and should be collected and treated. In addition, windrow composting is a large scale operation and might be subject to regulatory enforcement. Samples of the compost should be tested in a laboratory for bacterial and heavy metal content. Odors also need to be controlled. The public should be informed of the operation and have a method to address any complaints about animals or bad odors. Other concerns might include zoning and siting requirements.

Windrow composting often requires large tracts of land, sturdy equipment, a continual supply of labor to maintain and operate the facility, and patience to experiment with various materials mixtures and turning frequencies.

This method will yield significant amounts of compost, which might require assistance to market the end-product. Alternatively, local governments can make the compost available to residents for a low or no cost.


MNN Public Information from the U.S. Environmental Protection Agency

[ header = Aerated static pile composting ]  

Aerated static pile composting:

In aerated static pile composting, organic waste is mixed together in one large pile instead of rows. To aerate the pile, layers of loosely piled bulking agents (e.g., wood chips, shredded newspaper) are added so that air can pass from the bottom to the top of the pile. The piles also can be placed over a network of pipes that deliver air into or draw air out of the pile. Air blowers might be activated by a timer or a temperature sensor.

Types of waste and waste generators
Aerated static piles are suitable for a relatively homogenous mix of organic waste and work well for larger quantity generators of yard trimmings and compostable municipal solid waste (e.g., food scraps, paper products), which might include local governments, landscapers, or farms. This method, however, does not work well for composting animal byproducts or grease from food processing industries.

Climate or seasonal considerations
Like windrow composting, in a warm, arid climate, aerated static piles are sometimes covered or placed under a shelter to prevent water from evaporating. In the cold, the core of the pile will retain its warm temperature, but aeration might be more difficult in the cold because this method involves passive air flowing rather than active turning. Some aerated static piles are placed indoors with proper ventilation.

Environmental concerns
Since there is no physical turning, this method requires careful monitoring to ensure that the outside of the pile heats up as much as the core. One way to alleviate bad odors is to apply a thick layer of finished compost over the pile, which can help maintain high temperatures throughout the pile. Another way to deal with odor, provided that the air blower draws air out of the pile, is to filter this air through a biofilter made from finished compost.

This method typically requires equipment such as blowers, pipes, sensors, and fans, which might involve significant costs and technical assistance. Having a controlled supply of air enables construction of large piles, which require less land than the windrow method.

This method produces compost relatively quickly—within 3 to 6 months.


MNN Public Information from the U.S. Environmental Protection Agency

[ header = In-Vessel composting ] 

In-Vessel composting:

Organic materials are fed into a drum, silo, concrete-lined trench, or similar equipment where the environmental conditions—including temperature, moisture, and aeration—are closely controlled. The apparatus usually has a mechanism to turn or agitate the material for proper aeration. In-vessel composters vary in size and capacity.

Types of waste and waste generators
In-vessel composting can process large amounts of waste without taking up as much space as the windrow method. In addition, it can accommodate virtually any type of organic waste (e.g., meat, animal manure, biosolids, food scraps). Some in-vessel composters can fit into a school or restaurant kitchen while others can be as large as a school bus to accommodate large food processing plants.

Climate or seasonal considerations
In-vessel composting can be used year-round in virtually any climate because the environment is carefully controlled, often by electronic means. This method can even be used in extremely cold weather if the equipment is insulated or the processing takes place indoors.

Environmental concerns
In-vessel composting produces very little odor and minimal leachate.

In-vessel composters are expensive and might require technical assistance to operate properly, but this method uses much less land and manual labor than windrow composting.


Conversion of organic material to compost can take as little as a few weeks. Once the compost comes out of the vessel, however, it still requires a few more weeks or months for the microbial activity to stabilize and the pile to cool.

Compost is organic material that can be used as a soil amendment or as a medium to grow plants. Mature compost is a stable material with a content called humus