Subproductos

In modern materials and waste management, the concept of a by-product is central to the transition towards a circular economy. Unlike waste, which must be managed for recovery or disposal, a by-product is a substance or object resulting from a production process that, although it is not the main product, can be used lawfully as a resource in another process without complex additional treatment.

This concept makes it possible to reintroduce secondary flows into the economy, reduce the amount of waste generated and save natural resources and energy.

Difference between waste and by-product

Although the two concepts may look similar, the legal and practical difference is decisive:

• Waste: material that its holder discards or is obliged to discard. It requires authorised management.
• By-product: a usable material, generated unintentionally, that meets the legal conditions and can be used directly in another process.

The four conditions set out in Article 5 of the Waste Framework Directive (2008/98/EC) are: further use is certain; the material can be used directly without further processing beyond normal industrial practice; it is produced as an integral part of a production process; and its further use is lawful (it meets all relevant product, environmental and health requirements).

Declaring a material as a by-product in Spain

In Spain, classifying a material as a by-product follows a regulated procedure:

1. Application by the producer to the competent authority (usually the regional government).
2. Submission of documentation proving that the conditions are met (technical reports, characterisation, intended use).
3. Administrative assessment, which may include consultations with waste managers, users and environmental authorities.
4. Administrative decision: a positive decision classifies the material as a by-product; a negative one keeps it as waste.

Examples of by-products by sector

Agri-food

• Brewers' spent grain: used as animal feed.
• Olive stones: recovered as biomass for energy.
• Whey: used in the food industry to produce proteins and lactose.

Industrial

• Steel slag: used as aggregate in road construction.
• Power-plant ash: used in cement manufacturing.
• Marble cutting residues: used as mineral fillers in paints.

Forestry and agriculture

• Pruning and sawmill residues: used as pellets or chipboard.
• Nut shells: used in the cosmetics industry and as a natural abrasive.

Energy and biogas

• Digestate from biogas plants: used as an organic fertiliser in agriculture.

By-products and the circular economy

Using by-products is a clear example of the circular economy in action, because it fosters industrial symbiosis: what is surplus for one company becomes raw material for another. A classic example is steel slag, which would otherwise be waste but is used as aggregate in construction, closing a productive loop.

The Spanish Circular Economy Strategy (España Circular 2030) sets concrete goals, including reducing waste generation by 15% by 2030 compared with 2010 and increasing the reuse of industrial by-products in key sectors such as construction and agri-food.

Additional regulatory framework

Beyond Law 7/2022 and the Waste Framework Directive, other relevant rules include:

• Regulation (EU) 2024/1157 on shipments of waste (which replaced Regulation (EC) 1013/2006): it does not apply to by-products, which facilitates their trade.
• Sector-specific technical standards: for example, the use of by-products in cement is governed by UNE-EN standards.

The concept of end-of-waste is closely related but distinct: end-of-waste applies to a material that was waste and ceases to be so after a recovery operation, whereas a by-product never becomes waste in the first place.

Current challenges

1. Administrative burden: the procedure to declare by-products can be long and complex.
2. Lack of business awareness: many SMEs do not realise their surpluses could be recovered as by-products.
3. Lack of established markets: consumers ready to absorb certain flows do not always exist.
4. Technical standardisation: the need to standardise the use of by-products in sectors such as construction or food.

By-products and the Sustainable Development Goals

Promoting by-products aligns directly with the SDGs, in particular SDG 12 (responsible consumption and production), SDG 9 (industry, innovation and infrastructure), SDG 13 (climate action) and SDG 8 (decent work and economic growth).

In short, by-products are a strategic lever for the circular economy: they reduce waste, generate added value and help move towards a more sustainable and competitive production model.

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