Every time a water treatment plant cleans its filters, thousands of liters are sent down the drain. Perfectly treatable water, sourced from stressed basins, is lost without the option of return. In a world where more than 2 billion people live under conditions of severe water scarcity, this invisible waste can no longer be part of the system. But what if that same water could return to the process? What if we could close the loop, improve efficiency, and reduce pressure on sources?
The project presented here turns that vision into reality. It recovers water generated during filter backwashing at an urban drinking water treatment plant supplied by the Imperial Canal of Aragón and the Yesa Reservoir. Instead of discarding it, the water is subjected to a line of physical treatment and chlorine-free disinfection that allows it to be reintroduced into the operational circuit. This seemingly simple gesture directly reduces withdrawals from natural sources, improves the system’s overall efficiency, and eases the city’s water footprint.
The scale of change is notable: the volume recovered equals the monthly consumption of more than 400 households and represents between 2% and 4% of the plant’s total intake, with potential to scale further. Through physical and digital traceability, the project ensures that every cubic meter recovered aligns with the principles of additionality, intentionality, and verification, fundamental in Volumetric Water Benefit Accounting (VWBA 2.0). Moreover, by eliminating the need to treat the same volume of newly captured water, the project reduces the use of chemicals and energy, generating a positive environmental impact in both quantity and quality.
This project does not merely optimize a technical process: it redefines how we should think about efficiency in the 21st century. Where others see inevitable losses, here we see an opportunity to regenerate. Because being Water Positive is not a distant goal, it is a bold, concrete, and measurable decision that starts with every cubic meter we stop losing.
The starting point is as simple as it is challenging: each backwash cycle of the filters at Zaragoza’s urban drinking water treatment plant generates a significant volume of discarded water, which historically has been sent to the sewer or surface waters without additional treatment. This loss not only represents wasted freshwater in a region where reserves fluctuate due to climate change and overexploitation, but also creates unnecessary economic and environmental impacts for the operator.
The technical intervention consists of recovering, treating, and reinjecting this water into the treatment process. The installed system includes a sedimentation stage, physical particle filtration, and advanced chlorine-free disinfection. This last feature is key to avoiding by-products and ensuring the treated water’s safety, keeping it suitable for its intended use. All recovered flows are measured using dedicated flow meters and recorded in a digital platform that enables continuous tracking of the benefit generated.
In volumetric terms, between 400 and 600 m³ are recovered daily, representing up to 180,000 m³ of potential annual savings in freshwater withdrawals. This volume translates into a VWBA benefit of reduced consumption, clearly quantifiable, reportable, and auditable. Implementation has been led by the municipal water operator, with support from experts in treatment, digital traceability, and external verification, ensuring a replicable model for other cities with similar infrastructure.
Acting now is crucial: European water efficiency regulations and growing pressure on public service operators demand immediate, measurable, and sustainable solutions. This initiative turns an obligation into an advantage: it allows the operator to improve ESG performance, reduce dependence on scarce resources, and position itself as a benchmark for innovation in urban water management. Any public utility, food industry, or urban complex with filtration systems can adopt this model with similar benefits. The difference is not in the technology; it is in the will to transform waste into value.
The project proposes the installation of an autonomous backwash water recovery system operating as a parallel unit to the main treatment process, with the goal of intercepting and treating the residual flow generated by the periodic cleaning of filters. This solution consists robust and reliable ceramic membranes enabling direct filtration of the backwash waste for maximize water recovery:
The treated water is reintegrated into the plant’s main mixing chamber or directly into the feed channel, partially replacing the volume that would otherwise be sourced from external systems. This represents an effective reduction in annual raw water intake, quantifiable under the VWBA 2.0 methodology, with a direct impact on water efficiency, operational resilience, and environmental performance.
The applied technology consists of a skid-type backwash water treatment unit, with components assembled in modular containers that allow for plug-and-play installation without major civil works. The system captures the water immediately after the backwashing cycles of the sand and activated carbon filters, intercepting the flow before it enters the sludge system.
The backwash wastewater is directly fed into a membrane system using sustainable and robust ceramic membranes able to operate at extremely high suspended solid loads thus enabling maximum recovery rate and reuse of that backwash water. A medium-pressure UV disinfection module follows, eliminating pathogens and microorganisms without generating organochlorine by-products.
The modules are equipped with advanced control instrumentation, including turbidity transmitters, free chlorine, pH, and conductivity sensors, all integrated into the DRINKING WATER TREATMENT PLANT’s SCADA system. This enables real-time monitoring, alarm generation, and automatic adjustments for deviations in water quality parameters. The treated water is reintegrated into the mixing chamber of the main treatment train.
The system operates continuously and autonomously, with automatic filter cleaning cycles and data logging. The Zaragoza City Council’s technical team supervises operation, supported by technical assistance from the supplier during the first year to ensure the learning curve and initial calibration of the system. This solution enhances operational efficiency, reduces external abstractions, and extends the lifespan of the plant’s primary equipment.
The DRINKING WATER TREATMENT PLANT, operational since 1965 and with several modernization phases, treats raw water from the Imperial Canal of Aragón and the Yesa reservoir.
The process involves sand and activated carbon filters that require regular cleaning through backwashing with clean water. This generates a significant fraction of dirty water which, until this project, was routed to the sludge line without effective recovery.
With the installation of the new recovery system, backwash water is diverted to a separate line where it undergoes primary sedimentation. It is then subjected to physical filtration through membranes or granular media and final chlorine-free disinfection to avoid unwanted residues.
Once treated, this water meets the quality standards required for reintegration into the treatment system.
The system operates autonomously, with real-time feedback from sensors and validation by the DRINKING WATER TREATMENT PLANT’s in-house laboratory. As a result of this intervention, a significant portion of water previously discarded is now safely reused, generating structural savings in freshwater abstraction. This recovery also reduces the solids load and volume in the sludge treatment line, improving the overall efficiency of the plant.
This intervention aligns with the VWBA 2.0 and contributes measurably to the city’s sustainability objectives, serving as a replicable example for other urban infrastructures in water-stressed basins.
