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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:[1]

... 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).

Major by-products

Animal sources

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Minerals and petrochemicals


  • sludge – from wastewater treatment
  • waste heat - from electricity production and usage
  • CO2 - process of burning

See also


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