Composting methods & technologies

What is composting

Decomposition is an essential and continuous process where organic, or carbon-based, materials exposed to the elements of nature, particularly air and water, are broken down into smaller compounds by microorganisms. Decomposition produces a nutrient-rich organic matter that can be readily digested by soil microbes that make the nutrients available for uptake by plants. However, producing this high quality, marketable product in a timely fashion requires giving nature a bit of a boost with the right mix of air flow, temperature and feedstocks.

Composting is the actively managed process of decomposition of organic residuals in the municipal solid waste stream. The end product, compost, is organic material that can be used as a soil amendment or as a medium to grow plants. Mature compost contains a stable, carbon-rich material called humus that is dark brown or black in color with a soil-like, earthy smell. Learn more on the uses of compost here. Finding the right technology for composting your community’s organics depends upon land availability, material volumes, population density, and regulatory constraints. For more on available technologies in your area and the regional challenges or opportunities of each approach, check out this list of state and regional organics recycling coordinators.

Organic waste sent to commercial composting facilities undergoes several pre-treatment steps before composting. Materials are screened for contaminants, often chopped or shredded to smaller particle sizes for faster decomposition, and may be blended with other organic streams or bulking agents for optimal density, carbon to nitrogen ratio, and moisture levels. For more on the science and engineering of composting, check out Cornell Composting Science and Engineering.

Backyard or Onsite Composting

Backyard or onsite composting offers the highest environmental benefit among organics management strategies because of the decrease or elimination of transportation impacts. There’s a wealth of information on how to get started with backyard composting and how to encourage it in your community. Here’s a list to get you started.

Vermicomposting

Vermicomposting uses red worms to digest organic matter and produce nutrient-rich castings used as soil amendments. Vermicomposting is very versatile and can be suitable for apartment dwellers up to large-scale food generators. Castings can be used in backyard gardens, harvested and sold, or brewed to produce compost tea, a liquid soil amendment. Worms thrive in moderate temperatures (55-77ยบ F) and require moist bedding in which to live. Worms eat a wide variety of organic materials such as paper, manure, fruit and vegetable waste, grains, coffee grounds, and ground yard wastes. Meat and dairy products can be consumed as well, but lead to potential odor and pest problems. Vermicomposting operations are often exempted from solid waste facility regulations but are subject to health and safety codes.

Windrow Composting

Windrow composting involves spreading organic materials into long, semi-circle shaped piles which are mechanically turned by heavy equipment to maintain even decomposition. Piles generally range from 4-8 feet in height and 14-16 feet in length. The relative simplicity of windrow composting makes it a prime candidate for processing high volumes of materials, particularly yard trimmings, on larger amounts of land. Moisture may be added in dry climates or a cover installed over the pile to prevent evaporation. Onsite leachate must be managed, as well as blowing debris and odor.

Aerated Static Pile (ASP) Composting

Aerated static pile composting maintains decomposition by pushing or pulling air through the pile. Piles are created in a trapezoidal shape and receive little to no mechanical turning. Bulking agents, typically wood chips or shredded newspaper, are added to the pile to enhance the flow of air and add porosity to the pile. If piles are covered, often by heavy duty plastic bags or tarps, a negative air system can pull air through the pile and then through a biofilter to reduce odors. This is known as enclosed ASP. Finished compost or yard trimmings may also be applied on top of the piles as biofilters. Animal byproducts or grease from food processing industries are not suitable for ASP; the technology was originally developed for sewage sludge but is also suitable for source-separated organics. Aerated piles may be housed indoors or outdoors, and the use of the blowers and fans allows for larger piles, reducing land needs compared to windrow composting. Because there is little to no mechanical agitation, homogenous particle size is more of a necessity. ASP is often used in conjunction with other composting techniques to cure the pile and manage odors.

Invessel Composting

In-vessel composting involves treating organics within a rigid enclosed container varying in size from a 55-gallon drum to a silo. Temperature, moisture and aeration are closely controlled to minimize leachate and odor production. Compost production happens very rapidly in these contained quarters under properly controlled conditions, taking as little as a few weeks, but the materials must then be stabilized and cooled for several weeks or months before being sold as a finished product. Units are often modular in nature so additional vessels can easily be added when volumes increase. In-vessel composting requires an expensive capital investment but require less land and labor than static piles or windrows. Air and moisture are pulled through the materials and then through a biofilter, as in ASP, and little turning takes place. Materials may be ready in as little as 4 weeks, but then require curing via windrows or ASP. Preview available in-vessel composting units.

Anaerobic Digestion (AD)

Anaerobic decomposition takes place in absence of oxygen. Instead of generating only carbon dioxide as the gaseous byproduct, as is the case with aerobic decomposition, anaerobic decomposition produces both carbon dioxide and methane in about a 1:1 ratio. This biogas is captured and used to produce energy or electricity. Anaerobic digesters are net producers of energy and can be used to power or heat surrounding infrastructure, such as at a wastewater treatment plant, or sold back to the grid for power. Once gas generation peaks and declines, the partially stabilized organic matter can be aerobically cured and used as compost. Anaerobic digesters are substantial more expensive than aerobic composting methods, and while proven throughout Europe, the North American market is still developing.

Sources: