Sustainable Management of Food

Types of Composting and Understanding the Process

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Composting Basics

There are five main areas that must be “controlled” during composting.

Feedstock and nutrient balance

Composting, or controlled decomposition, requires a proper balance of “green” organic materials and “brown” organic materials. “Green” organic material includes grass clippings, food scraps, and manure, which contain large amounts of nitrogen. “Brown” organic materials includes dry leaves, wood chips, and branches, which contain large amounts of carbon but little nitrogen. Obtaining the right nutrient mix requires experimentation and patience. It is part of the art and science of composting.

Particle size

Grinding, chipping, and shredding materials increases the surface area on which microorganisms 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 enough 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.

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Onsite Composting

Organizations that are going to compost small amounts of wasted food can compost onsite. Composting can significantly reduce the amount of wasted food that is thrown away. Yard trimmings and small quantities of food scraps can be composted onsite. Animal products and large quantities of food scraps are not appropriate for onsite composting.

Learn how to create your own compost pile

Things to think about

  • The climate and seasons changes will not have a big effect on onsite composting. Small adjustments can be made when changes happen such as when the rainy season approaches.
  • Food scraps need to be handled properly so they don’t cause odors or attract unwanted insects or animals.
  • Onsite composting takes very little time or equipment. Education is the key. Local communities might hold composting demonstrations and seminars to encourage homeowners or businesses to compost on their own properties.
  • Creating compost can take up to two years, but manual turning can speed up the process to between three to six months.
  • Compost, however, should not be used as potting soil for houseplants because of the presence of weed and grass seeds.
  • You can leave grass clippings on the lawn-known as “grasscycling.” These cuttings will decompose naturally and return some nutrients back to the soil, similar to composting.
  • You can put leaves aside and use them as mulch around trees and scrubs to retain moisture.

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Vermicomposting

Red worms in bins feed on food scraps, yard trimmings, and other organic matter to create compost. The worms break down this material into high quality compost called castings. Worm bins are easy to construct and are also available for purchase. One pound of mature worms (approximately 800-1,000 worms) can eat up to half a pound of organic material per day. The bins can be sized to match the volume of food scraps that will be turned into castings.

It typically takes three to four months to produce usable castings. The castings can be used as potting soil. The other byproduct of vermicomposting known as “worm tea” is used as a high-quality liquid fertilizer for houseplants or gardens.

What can be composted - vermiculture?

  • Food scraps
  • Paper
  • Yard trimmings such as grass and plants

Things to think about

  • Ideal for apartment dwellers or small offices.
  • Schools can use vermiculture to teach children conservation and recycling.
  • It is important to keep the worms alive and healthy by providing the proper conditions and sufficient food.
  • Prepare bedding, bury garbage, and separate worms from their castings.
  • Worms are sensitive to changes in climate.
    • Extreme temperatures and direct sunlight are not healthy for the worms.
    • The best temperatures for vermicomposting range from 55° F to 77° F.
    • In hot, arid areas, the bin should be placed under the shade.
    • Vermicomposting indoors can avoid many of these problems.

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Aerated (Turned) Windrow Composting

Aerated or turned windrow composting is suited for large volumes such as that generated by entire communities and collected by local governments, and high volume food-processing businesses (e.g., restaurants, cafeterias, packing plants). It will yield significant amounts of compost, which might require assistance to market the end-product. Local governments may want to make the compost available to residents for a low or no cost.

This type of composting involves forming organic waste into rows of long piles called “windrows” and aerating them periodically by either manually or mechanically turning the piles. The ideal pile height is between four and eight feet with a width of 14 to 16 feet. This size pile is large enough to generate enough heat and maintain temperatures. It is small enough to allow oxygen flow to the windrow's core.

Large volumes of diverse wastes such as yard trimmings, grease, liquids, and animal byproducts (such as fish and poultry wastes) can be composted through this method.

Things to Think About

  • 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.
  • 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.
  • 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.
  • Leachate is liquid released during the composting process. This can contaminate local ground water and surface-water supplies. It should be collected and treated.
  • Windrow composting is a large-scale operation and might be subject to regulatory enforcement, zoning, and siting requirements. 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.

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Aerated Static Pile Composting

Aerated static pile composting produces compost relatively quickly (within three to six months). It is 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), such as local governments, landscapers, or farms. This method, however, does not work well for composting animal byproducts or grease from food processing industries.

In aerated static pile composting, organic waste mixed in a large pile. 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 sensors.

Things to Think about

  • In a warm, arid climate, it may be necessary to cover the pile or place it under a shelter to prevent water from evaporating.
  • In the cold, the core of the pile will retain its warm temperature. Aeration might be more difficult because passive air flowing is used rather than active turning. Placing the aerated static piles indoors with proper ventilation is also sometimes an option.
  • 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.
  • Applying a thick layer of finished compost over the pile may help alleviate any odors. If the air blower draws air out of the pile, filtering the air through a biofilter made from finished compost will also reduce any of the odors.
  • This method may require significant cost and technical assistance to purchase, install, and maintain equipment such as blowers, pipes, sensors, and fans.
  • Having a controlled supply of air allows construction of large piles, which require less land than the windrow method.

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In-Vessel Composting

In-vessel composting can process large amounts of waste without taking up as much space as the windrow method and it can accommodate virtually any type of organic waste (e.g., meat, animal manure, biosolids, food scraps). This method involves feeding organic materials into a drum, silo, concrete-lined trench, or similar equipment. This allows good control of the environmental conditions such as temperature, moisture, and airflow. The material is mechanically turned or mixed to make sure the material is aerated. The size of the vessel can vary in size and capacity.

This method produces compost in just a few weeks. It takes a few more weeks or months until it is ready to use because the microbial activity needs to balance and the pile needs to cool.

Things to Think About

  • Some are small enough to fit in a school or restaurant kitchen.
  • Some are very large, similar to the size of school bus. Large food processing plants often use these.
  • Careful control, often electronically, of the climate allows year-round use of this method.
  • Use in extremely cold weather is possible with insulation or indoor use.
  • Very little odor or leachate is produced.
  • This method is expensive and may require technical expertise to operate it properly.
  • Uses much less land and manual labor than windrow composting.

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