Composting Materials

The major categories of substrates potentially suitable for composting are the following:

• Municipal biosolids

• Industrial sludges

• Food and agricultural wastes

• Special wastes

Many different materials are suitable for composting organisms. The most important parameter for composting is the C/N ratio (Table 12.1). Some materials contain high amounts of carbon in the form of cellulose, which is required by the bacteria for their energy and other materials contain nitrogen in the form of protein, which provide nutrients energy exchanges. Suitable ingredients with relatively high carbon content include:

Table 12.1 C/N ratio of some raw compost materials (in: Periurban Vegetable Project:

Nitrogen-rich waste

C/N ratio

Carbon-rich waste

C/N ratio

Liquid manure




Chicken dung




Grass cuttings


Legume straw


Kitchen waste


Corn cobs/rice straw


Cow/pig/horse dung




Feathers hair




• Dry, straw-type material, such as cereal straws

• Autumn leaves

• Sawdust and wood chips

• Paper and cardboard, such as corrugated cardboard or newsprint with soy-based inks

Ingredients with relatively high nitrogen content include:

• Green plant material (fresh or wilted), such as crop residues, hay, grass clippings, weeds

• Manure of poultry and herbivorous animals, such as horses, cows, and llamas

• Fruit and vegetable trimmings

The most efficient composting occurs by seeking to obtain an initial C/N mix of 25/30 by dry chemical weight (Haug 1993). Grass clippings have an average ratio of 10-19 to 1 and dry autumn leaves from 55-100 to 1. Mixing equal parts by volume approximates the ideal range (Haug 1993).

Poultry manure provides much nitrogen, but with a ratio to carbon that is imbal-anced. If composted alone, this results in excessive N loss in the form of ammonia - and some odor. Horse manure provides a good mix of both, although in modern stables, so much bedding may be used to make the mix to carbonaceous. A light dusting of agricultural lime (not on animal manure layers) can curb excessive acidity, especially with food waste. Seaweed meal provides a ready source of trace elements. Finely pulverized rock (rock flour or rock dust) can also provide minerals, while clay and leached rock dust are poor in trace minerals. Composting in the form of bioremediation can break down petroleum hydrocarbons and a variety of toxic compounds. This is the bacterial and some cases fungal content of the compost, which possess the enzymatic properties to de-polymerize the complex man-made molecules. Some materials are best left to high-rate, a thermophilic composting system, as they decompose slower, attract vermin, and require higher temperatures to kill pathogens than backyard composting. These materials include meat, dairy products, eggs, restaurant grease, cooking oil, manure and bedding of non-herbivores, and residual from the treatment of wastewater and drinking water. Meat and dairy products can be recycled using fermentation method. Human waste can be composted by industrial, high-heat methods and also by composting toilets, even though most composting toilets do not allow the thermophilic decomposition that is believed to be necessary for rapid killing of pathogens.

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