By-product

A by-product is a secondary product derived from a manufacturing process or chemical reaction. It is not the primary product or service being produced. A by-product can be useful and marketable or it can be considered waste.

IEA offers the following definition for the purpose of life-cycle assessment:
 * ... main products, co-products (which involve similar revenues to the main product), by-products (which result in smaller revenues), and waste products (which provide little or no revenue).

Animal sources

 * blood meal – from slaughterhouse operations
 * poultry by-product meal – clean parts of the carcass of slaughtered poultry, such as necks, feet, undeveloped eggs, and intestines
 * chrome shavings – from a stage of leather manufacture
 * collagen and gelatin – from the boiled skin and other parts of slaughtered livestock
 * feathers – from poultry processing
 * feather meal – from poultry processing
 * fetal pigs
 * lanolin – from the cleaning of wool
 * leather - hides and skins from slaughterhouse operations processed via the leathermaking process
 * manure – from animal husbandry
 * meat and bone meal – from the rendering of animal bones and offal
 * poultry litter – swept from the floors of chicken coops
 * whey – from cheese manufacturing

Vegetation

 * acidulated soap stock – from the refining of vegetable oil
 * bagasse – the fibrous residue remaining after sugarcane or sorghum stalks are crushed to extract their juice
 * black liquor from the production of cellulose pulp using the kraft process
 * bran and germ – from the milling of whole grains into refined grains
 * brewer's yeast – from ethanol fermentation
 * cereal food fines – from breakfast cereal processing
 * corn steep liquor – from corn wet-milling
 * distillers grains – from ethanol fermentation
 * glycerol – from the production of biodiesel
 * grape seed oil – recovered from leftovers of the winemaking process
 * molasses – from sugar refining
 * orange oil and other citrus oils – recovered from the peels of processed fruit
 * pectin – recovered from the remains of processed fruit
 * sawdust and bark – from the processing of logs into lumber
 * soybean meal – from soybean processing
 * stover – residual plant matter after harvesting of cereals
 * straw – from grain harvesting
 * tall oil from the production of cellulose pulp using the Kraft process
 * vinasse – from the fermentation of sugar to ethanol fuel

Minerals and petrochemicals

 * asphalt – from the refining of crude oil
 * fly ash – from the combustion of coal
 * slag – from ore refining
 * gypsum – from flue-gas desulfurization
 * - from natural gas extraction
 * ash and smoke – from the combustion of fuel
 * mineral oil – from refining crude oil to produce gasoline
 * salt – from desalination
 * – from extraction

Other

 * sludge – from wastewater treatment
 * waste heat - from electricity production and usage
 * - process of burning