Industrial production across nearly every sector results in more than one physical output. While the main product is the intended result of the manufacturing process, other materials are inevitably created alongside it. These secondary outputs have distinct classifications that determine how they are managed, valued, and utilized within a circular economy. Understanding these classifications is fundamental to modern resource management and efficiency.
Defining a By-Product
A by-product is formally defined as a secondary substance derived from a production process that is not the primary output the process was designed to create. Its generation is typically inherent to the manufacturing or chemical reaction necessary to produce the main commodity. For instance, in the process of refining crude oil, the primary goal is often gasoline, but other fuels and materials are concurrently produced.
Although a by-product is secondary, it is distinct from simple refuse because it retains commercial utility and value. The net realizable value (NRV) of a by-product is generally minor when compared to the main product, meaning it generates considerably smaller revenue. Despite this lower economic standing, by-products are frequently recovered, repurposed, or sold to reduce overall production costs and maximize resource utilization.
Distinguishing By-Products from Waste and Co-Products
The distinction between a by-product and other secondary outputs relies heavily on two factors: the intent behind its creation and its relative economic value. By-products are unintentional yet unavoidable, possessing some market value, while waste and co-products occupy opposite ends of the economic spectrum.
Waste
Waste, or scrap, is material that has no economic utility and is typically discarded, requiring specific disposal procedures. If a substance loses its utility or fails to meet environmental and health requirements for reuse, it transitions from a potential by-product into regulated waste. A by-product is certain to have further lawful use and can be utilized without additional processing beyond normal industrial practice.
Co-Products
Co-products are secondary outputs that are produced intentionally and possess comparable economic value to the main product. While a by-product is minor in quantity and revenue, co-products are jointly produced with the main item and contribute significantly to the total revenue of the process. For example, in meat production, the primary goal is muscle meat, but hides or specialized organ meats may be considered co-products due to their high, comparable market value.
Common Examples in Industry and Food
During lumber processing, the primary goal is cut timber, but the act of sawing inevitably generates sawdust and wood chips. These materials, which are by-products, are then recovered and sold for use in particleboard, animal bedding, or biomass fuel.
The food industry generates vast quantities of by-products that are increasingly being valorized. In dairy production, the main product is cheese, but the separation process leaves behind a liquid residue known as whey. Historically treated as waste, whey is now a major by-product, processed into protein powders and functional ingredients for the food fortification market.
Molasses is another common example, which is a viscous, dark syrup remaining after the crystallization and refining of sugar from sugarcane or sugar beets. While the primary product is refined white sugar, the molasses by-product is recovered and used in fermentation processes, animal feed, or certain food products. Similarly, in the production of vegetable oils, the seeds or fruits are pressed, leaving behind a protein-rich cake or meal that is repurposed as animal feed or ingredient flour.