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What is Composting?
Composting is a natural process whereby micro-organisms transform organic waste materials into a soil- like product called humus (pronounced "hue-mous"). Kitchen scraps, leaves and yard waste, paper, wood, food-processing wastes, as well as agricultural crop wastes and animal manures, are excellent organic waste materials that can be composted.

Composting has a wide range of benefits: it helps to reduce the amount of waste going to landfills and produces a valuable product, compost, of benefit to soil health & vitality in addition to many other attributes such as water and soil conservation. It is estimated that about 40 to 60 percent of the total waste stream could be composted!

For the composting process to work best, it is important that the micro-organisms have a continuous supply of food (i.e., organic wastes), water and oxygen. As well, managing the temperature of the composting material is important to make the process work.

Although most organic wastes supply all of the nutrients necessary for the micro-organisms to grow, they grow best with certain levels of carbon (C) and nitrogen (N). Paper, leaves and wood are high in carbon, while grass clippings and vegetable trimmings are high in nitrogen. The materials in the composting "recipe" need to be mixed in the correct combination to create the right C:N ratio.

Types of Composting
Composting can be done in many different ways. Types of composting range from residential or backyard composting to mid-scale and central municipal or industrial composting systems. Selecting the most suitable method depends on the amount and type of organic materials to be composted.

Residential or backyard composting means that an individual household composts most of its food and yard waste in a container located outside the home. Worm composting is a viable option to compost kitchen waste indoors. Composting at home is the simplest and most cost-effective method because collection, transportation and costs are avoided. People benefit directly from their own efforts by producing a valuable additive for their own garden soil.

However, not all food and yard waste can be managed so simply. Organic material from commercial sources, such as restaurants, supermarkets, apartment buildings and food manufacturers, needs to be managed differently. This is where mid-scale and centralized composting fits in.

Both mid-scale and centralized composting involves significantly larger quantities and a larger variety of organic wastes.

Mid-scale composting is the on-site management of organic waste generated by a group of people, such as in an apartment complex, office building or hospital. This avoids the transportation of organic waste. Centralized composting involves the collection and transportation of organic materials to a special facility where it will be prepared and processed into compost.

Centralized Composting Facilities
The design and set-up of a centralized composting site must take into account such factors as the type and volume of organic waste, waste collection methods, sorting, storage factors and the end use for the finished compost. Quality organic waste and good operating procedures ensure the production of high-quality compost.

Among the most common centralized composting process technologies, in order of increased technology, are:

Organic materials are placed in long triangular rows called windrows. Windrows are turned and watered occasionally to ensure that the micro-organisms get an adequate supply of oxygen and that any clumps of organic material are broken up.

This method is commonly used for composting leaf and yard waste, commercial food wastes, or such "specialty items" as shredded Christmas trees.

Static Aerated Pile
Organic waste materials are formed into windrows over perforated pipes. Rather than the windrows being turned, air is supplied to the micro-organisms through the pipes.

Almost all municipalities own the necessary equipment required for centralized composting operations using windrows or static aerated piles. This means that the capital costs are relatively low.

In-vessel systems are either fully or partially enclosed, and can handle more material in a smaller space than windrows or static aerated piles. However, they tend to be more costly. These systems provide better control of aeration, temperature and the moisture in the organic materials being composted, all of which result in faster decomposition.

If necessary, water can be added to maintain the correct moisture level, and air can be pumped in to provide oxygen and to control the temperature.

Although different in-vessel systems are available, they are generally of three basic types: channels or troughs, containers and rotating drums (sometimes called tube digesters).

Channels (or Troughs)
The composting process takes place in long rectangular troughs or channels. The organic waste materials are mixed so that the clumps are broken up and the material is aerated.

Composting takes place in closed containers that are supplied with air. Excess moisture and exhaust air are removed from the containers to maintain ideal conditions for the micro-organisms throughout the process.

Rotating Drums (Tube Digesters)
Organic waste materials are added to a drum which is continuously rotating. The rotation ensures that the micro-organisms are constantly supplied with the oxygen they need and that all of the organic waste materials are exposed to them. The material remains in the drum for three to five days and is then transferred to windrows for final curing.

Anaerobic Digesters
Organic waste materials can also be digested in an oxygen-free, or anaerobic, environment by micro-organisms that do not need oxygen. The length of time required to digest the organic waste material varies according to the individual technology usually between two and twenty days. The process produces humus, methane and carbon dioxide. The methane is captured and converted into energy. Following digestion, the humus can be transferred to windrows or another method for final composting.

Excerpts from Centralized Composting Helping To Complete The Carbon Cycle, written by Susan Antler of The Composting Council of Canada on behalf of Environment Canada.

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