Organic Materials Composting: Overview
Compost is decayed plant and organic material that is commonly used as a soil amendment to improve plant growth. Composting is an aerobic process, i.e. it requires oxygen, that depends on bacteria and other living organisms to speed the process of decomposition. This is the reason that soild waste at the lower layers of landfills do not decompose quickly - they lack the levels of oxygen necessary for bacteria and other necessary living organisms to thrive.
The 5 main variables that must be controlled during the composting process are:
- Feedstock and nutrient balance: Compost piles need both "green" materials (grass, food scraps, manure) to provide nitrogen and "brown" materials (branches, wood chips, dried leaves) to provide carbon necessary for the feeding of the microorganisms
- Aeration / Oxygen Flow: Proper aeration supports life for micro-organisms and allows decomposition to occur faster
- Particle size: The size of the waste material in the compost pile will affect the rate of decomposition. The pile has to be dense enough to reach the required temperature, while being coarse enough to allow proper oxygen flow
- Moisture content: The micro-organisms that breakdown organic material require moisture to survive. However, too much moisture can retard their growth
- Temperature: The micro-organisms perform optimally at temperatures between 130-150 degrees Fahrenheit. The heat also destroys pathogens and weed seeds. At lower temperatures, anaerobic decomposition, or simple rotting, occurs
The 5 main methods of composting are:
- Onsite Composting: Commonly referred to as Backyard Composting when conducted on residential sites. Typically used for yard trimmings and small amounts of vegetative food scraps
- Vermicomposting: Use of composting earthworms to speed decomposition. Earth moving earthworms (e.g. night crawlers) ingest soil to extract nutrients; however, composting worms (e.g. red wrigglers or red worms) feast on the microorganisms associated with decaying food matter. The worms eat the microorganisms that grow on organic solid waste, and in so doing, eat the material itself which is excreted as a rich soil-like material.
- Aerated Windrows: Long rows of regularly turned organic material that can be used to decompose large volumes of diverse organic wastes — including grease and animal byproducts
- Aerated Static Composting: Bulking agents (such as wood chips) or ventilated pipes allow for aeration without turning. Can be used for municipal solid waste, but cannot include grease or animal byproducts
- In-Vessel Composting: Organic materials are placed in an environment-controlled drum or container with an agitating mechanism. Handles large volumes of any type of organic waste material and can operate in any climate year round. Used for municipal and large volume operations
Municipal composting programs are typically conducted in parallel with general solid waste collection operations. How the waste material is collected is a key factor in a community’s composting efforts.
The 3 main types of municipal organic waste collection are:
- Source Separation: Requires residents to separate designated materials from their solid waste for curbside collection or drop-off. Generally produces a clean stream of organic material. Requires intensive public education and outreach, efficient collection, and adequate processing facilities.
- Mixed or Single Stream: Does not require residents to separate their solid waste in order to isolate organic materials. Requires more intensive processing to remove all non-compostable items.
- Onsite / Backyard Composting: Allows residents to operate on-site composting of organic waste. Avoids the need for a separate sorting and collection, but requires resident training, and typically involves subsidized bin distribution.
RATIONALE
The U.S. Environmental Protection Association (EPA) estimates that yard trimmings and food waste together make up 24 percent of the U.S. waste stream. In 2000, approximately 56.9 percent of yard trimmings were recovered for composting, but only 2.6 percent of food waste was composted. As such, composting offers a viable means of reducing the size and cost of landfills.
EFFORT REQUIRED
Mixed waste collection of compostable material requires the least effort from residents. However, local governments must then invest in additional processing facilities to sort the incoming waste stream to remove compostable material.
Source-separated curbside collection and residential onsite composting programs both require effective public education campaigns for proper compliance and participation.
BENEFITS
- Divert organic waste materials from landfills
- Reduces the production of methane in landfills
- Improves local soil quality and remediates contaminated soil
- Creates revenue as a marketable commodity
RISKS
- Upfront investment in containers, hauling vehicles, and processing facilities
- Improper sorting of materials, lack of customer participation
- Improper pile management causing unwanted effects (odors, pests, slow decomposition, etc)
ACTION AGENT(S)
- Environmental Management Department
- Solid Waste Management / Recycling Department
- Public Works Department
- Parks Department
COST
Tipping fees at composting facilities are generally cheaper than tipping fees at landfills. The key to effective cost management is to implement a collection program for organics that does not add to total program costs. The EPA found the cost of municipal composting to range from $11 to $102 per ton, and the avoided landfill disposal fees ranged between $5 and $137 per ton.
In residential programs that already have a weekly yard trimmings pickup, adding food waste collection can increase landfill diversion without adding significantly to costs.
Additional start-up costs may be incurred public education campaigns and providing basic equipment (worm bins, kitchen scrap pails, etc) to residents. These costs can be recouped through ongoing savings in disposal fees. Compost prices have been as high as $26 per ton for landscape mulch to more than $100 per ton for high-grade compost, when bagged and sold at the retail level.
Waste disposal sites for solid waste from human activities.Charged by a landfill for disposal of waste, typically quoted per ton.A mixture of decayed plants and other organic material that is used to enrich soil with nutrients.The collection, reprocessing, marketing, and use of materials that were diverted or recovered from the solid waste stream.The overall flow of waste from consumers to a landfill, incinerator, or other disposal site.
