The scarcity of water resources is a chronic problem in many southern regions of the European Union which, according to the forecasts of water resources models that incorporate the effects of climate change, will worsen and will spread to the northernmost regions.

These water resources maintain the environmental quality of rivers and wetlands, whose loss due to human activity is being very rapid, as reported in forums such as the Ramsar Convention. Therefore, prioritizing the environmental health of our aquatic ecosystems, as well as the multiple services they provide, is a strategy that could give good results in the medium and long term.

The generation of good quality water resources from unconventional sources in a sustainable and economically viable way is an option that is gaining more and more strength.

Wastewater treatment is an unconventional water resource that, with proper post-treatment, can guarantee a quantity and quality of water suitable for environmental uses. In fact, this is one of the uses provided for in Spanish legislation on the reuse of reclaimed water (RD 1620/2007, Use 5.4).

The European proposal for the reuse of water only considers the use for agricultural irrigation (COM / 2018/337).

The Urban Wastewater Treatment Directive (91/271 / EEC, UWWTD) is the framework for ensuring minimum quality requirements for effluents from wastewater treatment plants (WWTP).

However, the competent water authorities must ensure compliance with the environmental objectives established in the Water Framework Directive (2000/60 / EC, DMA), including the achievement of good condition of surface waters. In this sense, it is evident that the requirements for the discharge of phosphorus established in the UWWTD (1 or 2 mg P / l), are insufficient when considering the use of treated water to feed wetlands or other lentic water bodies, due to their vulnerability to eutrophication processes.

In addition, human activity generates a large amount of solid waste, the management of which is inefficient. When drinking water is produced, one of the main wastes generated is the physical-chemical water treatment sludge (DWTS). The management of this waste carries a cost that could be reduced if a reuse is found.

Regarding DWTS, it could be thought that the adsorbent capacity of the coagulant used had not been exhausted and, therefore, it could have a “second life”, functioning as a phosphorus sorbent in wastewater post-treatment.

The use of nature-based solutions (NBS) such as vertical flow constructed wetlands (VFCW) with DWTS as a filter medium to supplement an existing WWTP to achieve concentrations below 1-2 mg P / l, may be more sustainable than increase the use of coagulants in the same WWTP. By linking this NBS to a conventional WWTP, the effluent quality can be significantly improved, achieving not only a significant reduction in phosphorus, but also nitrogen, priority substances, emerging pollutants, pathogens or microplastics.

In addition, if a free-water surface artificial wetland (FWSCW) is added, effluent renaturation can be achieved, enhancing the biological biodiversity of plankton, macroinvertebrate fauna, amphibians and insects. Both CWs provide other benefits such as landscape integration and the generation of wetland habitats, which have been in significant decline for centuries.

Therefore, these BSS can constitute an important value for the transition zones between WWTP and the natural environment.


The general objective is to demonstrate that it is possible to obtain reclaimed water from WWTP
effluents through the combination of NBS and industrial waste, in order to produce high quality water suitable for environmental uses, such as the recovery / conservation of wetlands.

This general objective can be broken down into the following specific objectives:

  • Demonstrate the viability of reusing DWTS, produced in one stage of the urban water cycle, in another stage, assess this waste and transform it into raw material.
    Optimize this reuse by using it as fill material for the vertical underground flow constructed wetlands (VFCW), thus favoring the biological processes that occur in the roots of the plants.
    Demonstrate that the combination of VFCW, whose substrate is the DWTS, with a FWSCW that host various environments (free surface areas and areas with vegetation) gives better environmental results than if there is only one FWSCW available at the exit of the WWTP.
    Improve the quality of WWTP effluents, reducing their concentration of nutrients, emerging pollutants, priority substances and pathogens. Consequently increasing its biological biodiversity: aquatic invertebrates, amphibians and insects; and thus improving the biodiversity of the environment, providing ecosystem services, such as pollination and insect reservoir for pest control in agriculture. 5 Demonstrate that these NBS can become sanctuaries for the introduction of endangered species and instead for educational use.
    Develop specific design and operation guidelines for this type of renaturation systems, to be used by the competent authorities to include them among the improvements proposed in the bids for WWTP operation and maintenance contracts.
    Carry out a life cycle analysis (LCA) and the cost-benefit ratio of this type of facility to demonstrate the suitability of its implementation.
    Establish water quality criteria for environmental uses, contributing to the development of Spanish legislation and European proposal on water reuse. This will also contribute to meeting the goals of both the WFD and the Habitats Directive (92/42 / EEC).
    The project integrates the principles of the circular economy as it seeks to generate new resources with high natural value from two waste streams (wastewater and DWTS), such as a stream of naturalized water and a high-biodiversity habitat as a transition zone between the area. “Industrial” (WWTP) and the receiving natural environment.