Overview

A by-product is any material, substance, or output that is produced incidentally during the manufacture of a primary product or the performance of a process. By-products arise across industries and biological systems: manufacturing plants, food processing, agriculture, chemical production, and energy systems all generate secondary outputs. Some by-products are discarded as waste, others are recovered and reused, and a subset can be hazardous. How a material is classified—as a by-product, a co‑product, or waste—depends on technical, economic and legal factors.

Types and common examples

By-products vary widely in composition and value. Common categories include inert solids, organic residues, sludges, gases and heat. Typical examples are listed below:

  • Sawdust and wood chips from timber processing.
  • Whey and lactose from cheese making.
  • Spent grain from brewing and distilling industries.
  • Fly ash, slag and kiln dust from combustion and metal smelting.
  • Glycerol from biodiesel production and soap manufacture.
  • Wastewater sludges from municipal and industrial treatment.

Environmental and health concerns

Many by-products pose environmental or health risks when unmanaged. Some are biologically degradable and create nuisance problems such as odors or nutrient pollution; others contain toxic compounds, heavy metals, or persistent organic pollutants and require controlled handling. Where harmful substances are present, they can contaminate soil, water and air or create risks to workers and communities. In extreme cases, toxic by-products have long-term ecological impacts and require specialized treatment before reuse or disposal—particularly when they cannot be safely directed to a landfill or conventional treatment plant or when they contain known hazardous constituents (see toxic materials).

Valorization and reuse

Economic demand and environmental policy have encouraged turning by-products into resources. Strategies include recycling, material recovery, energy recovery (combustion, anaerobic digestion), chemical conversion and use as raw materials for other sectors. Examples of valorization: converting spent grain into animal feed or bio-based products, processing glycerol into chemical feedstocks, using fly ash as an ingredient in cement, and capturing waste heat for district heating. These approaches reduce disposal costs, conserve resources and can create new revenue streams.

Animal by-products

By-products from animal slaughter and processing include fat, bones, blood, offal, and other tissues that are not intended for direct human consumption. Such materials are commonly rendered, processed into pet foods, fertilizers, technical fats, or used in animal feed chains after treatment. Because they can carry pathogens or present food‑safety issues, animal-derived by-products are subject to specific hygiene and handling practices in many jurisdictions. They are distinct from products made for animal welfare or medical uses and are often incorporated into non-human feed and industrial applications.

Not every secondary output is treated the same. A co-product is produced intentionally and has market value alongside the primary product; a by-product is secondary and often of lower value; waste is material discarded without plan for recovery. Legal definitions differ by country and sector, influencing obligations for reporting, storage, transport, treatment and disposal. Because classification affects liability and permitting, producers and regulators carefully assess whether a material qualifies as a by-product or as waste under applicable rules.

Outlook and notable developments

Interest in circular economy models has raised the profile of by-products as feedstocks for new supply chains. Industrial symbiosis—whereby one industry's by-products become another's inputs—reduces raw material needs and environmental footprint. Advances in biotechnology and green chemistry are also expanding options to convert organic residues into fuels, chemicals and materials. Nevertheless, careful assessment of safety, life-cycle impacts and economic viability remains essential to ensure that by-products are managed in ways that protect health and the environment while creating value.

Further reading: See resources on waste management, industrial processing and animal by-product handling for detailed guidance. For more information about specific practices and regulations consult sector publications and local authorities.

More on waste | Landfill practices | Toxic by-products | Animal processing | Non-human uses