Why Water Reuse Matters

Water scarcity is a critical challenge of the 21st century. Population growth, urbanization, and climate change are putting increasing pressure on limited freshwater resources. The Mediterranean basin, including Catalonia, is especially vulnerable to recurrent droughts and structural water stress.

Water reuse—treating wastewater to a quality suitable for beneficial uses—is a key pillar of sustainable water management. By transforming a liability (wastewater) into a resource, reuse reduces pressure on rivers, aquifers, and reservoirs while increasing resilience against drought.

Types of Water Reuse

Different applications require different treatment levels and quality standards:

• Agricultural irrigation

The largest global use of reclaimed water. Properly treated effluent can safely irrigate crops, reduce extraction from aquifers, and provide nutrients that lower fertilizer demand.

• Industrial processes

Reclaimed water can be used for cooling towers, process water, washing, and even boiler feed after advanced treatment, reducing dependence on potable water and lowering operating costs.

• Urban non-potable uses

Includes landscape irrigation (parks, golf courses, green roofs), street cleaning, toilet flushing, and firefighting. These uses can significantly cut municipal drinking water consumption.

• Environmental enhancement

Reuse can help maintain ecological flows in rivers, restore or sustain wetlands, and support aquatic and riparian ecosystems, especially during dry periods.

• Indirect potable reuse (IPR)

Highly treated water is introduced into aquifers, reservoirs, or surface waters and later abstracted and treated again for drinking. This creates an environmental buffer and enhances water security.

Treatment Technologies

Modern reuse systems combine multiple treatment steps to achieve the required quality for each use case. Typical advanced processes include:

1. Membrane Bioreactors (MBR)

Integrate biological treatment with membrane filtration (microfiltration or ultrafiltration). MBRs produce a high-quality effluent with low suspended solids and pathogens, ideal as a feed for further advanced treatment.

1. Reverse Osmosis (RO)

A pressure-driven membrane process that removes dissolved salts, nutrients, pharmaceuticals, and many micropollutants. RO is central to high-grade industrial reuse and indirect potable reuse.

1. UV Disinfection

Uses ultraviolet light to inactivate bacteria, viruses, and protozoa without adding chemicals or forming disinfection by-products. Often used as a final barrier.

1. Advanced Oxidation Processes (AOP)

Combine oxidants (e.g., ozone, hydrogen peroxide) with UV or catalysts to generate hydroxyl radicals, which break down persistent organic compounds, trace pharmaceuticals, and some industrial chemicals.

These technologies are typically integrated into treatment trains tailored to the target application and local regulatory requirements.

Regulatory Framework in Spain

Spain is one of Europe’s leaders in water reuse. Royal Decree 1620/2007 established the national legal framework, defining:

• Permitted uses of reclaimed water
• Quality criteria (microbiological, physical-chemical) for each use
• Monitoring and control requirements

The framework has been updated to align with EU Regulation (EU) 2020/741, which sets minimum requirements for water reuse in agricultural irrigation. Key elements include:

• Mandatory risk management plans

Operators must identify hazards, assess risks, and define mitigation measures along the entire reuse chain.

• Regular monitoring

Continuous or periodic control of microbiological indicators (e.g., E. coli), turbidity, and relevant chemical parameters.

• Traceability

Clear documentation from the treatment plant to the end user, including volumes, quality data, and distribution routes.

• Public transparency

Information on reuse schemes, performance, and safety must be accessible to authorities and citizens to build trust.