This project is part of Zaragoza’s efforts to consolidate itself as a resilient city in the face of water stress by optimizing the urban water cycle. At the Drinking Water Treatment Plant, the treatment process includes pretreatment, sedimentation, filtration (via sand and activated carbon filters), and disinfection, each with specific cleaning needs. The backwashing cycles used to regenerate the filters generate a significant flow of wastewater that, without intervention, is discarded as part of the sludge stream.
The proposed initiative introduces a compact modular technology for treating backwash water, developed for urban and continuous operation. This technology uses robust ceramic membranes for direct filtration, eliminating the need for sedimentation of the backwash waste. It offers a compact design, maximizes water recovery for optimizing water footprint, and provides complete removal of bacteria through a reliable physical barrier, ensuring safe, high-quality output with a simple, single-step process.
The system is designed to be fully autonomous and scalable according to the plant’s needs. It includes inline sensors for critical parameters such as turbidity, pH and conductivity, enabling automated control and integration with SCADA platforms. By reintegrating recovered water into the treatment train, this technology directly reduces the volume of water drawn from external sources such as the Yesa reservoir or the Imperial Canal. This not only improves operational efficiency but also contributes directly to climate resilience objectives, water footprint reduction, and compliance with accounting frameworks such as VWBA 2.0.
The Zaragoza DRINKING WATER TREATMENT PLANT treats approximately 6 m³/s of water, distributed across several treatment lines involving physical pretreatment, sedimentation, filtration with sand and activated carbon beds, and final disinfection stages. The 14 sand filters and 20 activated carbon filters are critical components, and their maintenance requires frequent backwashing to avoid loss of retention capacity, clogging, and solids carryover into treated water.
These backwash cycles, conducted with clean water and accounting for a significant volume (between 1% and 5% of the total treated flow), produce an effluent with high concentrations of suspended solids, colloids, organic matter, trapped metals, and biofilms, as well as physicochemical alterations such as increased turbidity and residual chlorine content.
Without a recovery system, this wastewater is diverted to the sludge treatment line, causing two critical operational and environmental consequences: (1) the structural waste of a technically recoverable volume of water, increasing pressure on water abstraction sources; and (2) a higher hydraulic and pollutant load on the sludge line, raising operating costs, energy consumption, and waste generation.
In addition, there is growing pressure on the abstraction systems, Yesa reservoir and the Imperial Canal, due to climatic and demographic factors. Projections of reduced runoff, increased temperature, and lower recharge in the headwaters of the basin require urban plants to improve their internal water efficiency coefficient. In this context, decoupling the plant’s efficiency from raw water dependency through safe and controlled recirculation, as proposed in this project, represents a high-impact sustainability strategy.
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.
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