Composting

Technology
Technology details
Name:	Composting
Category:
Feedstock:	Biowaste in general, Food and kitchen waste, Garden and park waste (wood, leaves)
Product:	Compost

Composting is a biological process in which micro-organisms convert organic matter such as plant and animal scraps into soil-like material called compost. Compost is easier to handle than manure and other raw organic materials, stores well and is odor-free. Composting is an ancient technology, practiced today at every scale from the backyard compost pile to large commercial operations.

Feedstock

Composts can be made from most organic by-products. Common feedstocks are poultry, hog and cattle manures, food processing wastes, sewage sludge, municipal leaves, brush and grass clippings, sawdust, and other by-products of wood processing.

Ideally, several raw materials should be mixed together to create the "ideal" range of conditions, which are as follows:

Condition	Ideal
C:N ratios of combined feedstocks	25-35:1
Moisture content	45-60 wt.%
Available oxygen concentration	>10% or more
Feedstock particle size	Variable
pH	6.5-8.0
temperature	54-60°C

Process

Composting occurs through the activity of micro-organisms naturally found in soils. Under natural conditions, earthworms, nematodes and soil insects do most of the initial mechanical breakdown of organic materials into smaller particles. Under controlled conditions, composters break down large particles through grinding or chopping. Once optimal physical conditions are established, soil bacteria, fungi, actinomycetes and protozoa colonize the organic material and initiate the composting process. These mesophilic organisms function best at warm temperatures (10-45°C). As temperatures in the compost pile increase, thermophiles (i.e., micro-organisms that thrive at temperatures above 45°C) take over. In the active "thermophilic" phase, temperatures of 54-65°C are reached which is high enough to kill pathogens and weed seeds and to break down phytotoxic compounds (i.e., organic compounds toxic to plants). After the active composting phase, temperatures gradually decline to around 37°C. The mesophiles recolonize the pile and the compost enters the "curing phase". During curing, organic materials continue to decompose and are converted to biologically stable humic substances (i.e., the mature or finished compost). There is no clearly defined time for curing. Common practices in commercial composting operations range from one to four months.

Product

The final product is a valuable soil resource. Compost can replace materials like peat and topsoil as seed starters, container mixes, soil amendments, mulches and natural fertilizers.