Innovadores Naturales Innovation Sostenibilidad

Willow trees for wastewater treatment


Valoriza Agua, in collaboration with IMDEA Agua and the Consorcio de Aguas de Asturias (water board of Asturias), is implementing a pilot project in the La Franca (Asturias) treatment plant that aims to regenerate wastewater via intensive green filter technology. The willow trees planted in the treatment plant, along with improved soil, optimise the quality of treated wastewater and reduce solid nutrient content (nitrates and phosphates), organic matter and even emerging compounds. 

By Patricia Terrero and Elena Campos

In the circular economy and given the growing scarcity of water, the European Commission is promoting reuse of safely treated wastewater. The R&D initiative being developed by Valoriza Agua in the La Franca treatment plant (Asturias) is encompassed within this framework. In collaboration with IMDEA Agua, it is trying to resolve the problem of tertiary treatment of wastewater in small towns that cannot assume the cost of high-efficiency technologies.

These places need simple, cheap schemes with a low environmental impact, something for which green filter technology is a good alternative. A water treatment technology that can also be used to regenerate and reuse treated water from a secondary or tertiary process, with the advantage of removing nutrients and emerging compounds, and produce biomass, capture CO2, contribute to increasing groundwater resources from irrigation return flows and produce a positive visual impact.

With all of this in mind, a pilot study has been carried out by installing a Green Filter at the La Franca wastewater treatment plant, with which it intends to regenerate the treatment plant effluent, reducing the concentration of nutrients (nitrates and phosphates) and eliminating emerging compounds.

The facilities have a surface area of 100m2 and a planting density of 10,000 plants/hectare of willow trees (Salix Purpurea), adding to the original soil a vegetable mix with wood shavings of the same species. Once the green filter is installed, the secondary treatment effluent is applied via drip irrigation, evenly across the orchard.

The results

The green filter has been operating for a year, during which weekly samples of input and drained water have been collected in order to study its performance. These samples show that the nitrates and phosphates in the water at the green filter outlet have fallen compared to the input, an average 48.4 and 60.6%, respectively. It has been observed that the wood shavings used to improve the soil favour the removal of nitrates and emerging compounds, although they do not have a vital role in phosphate retention.

The results show that the green filters are capable of reducing the concentration of nutrients and emerging compounds, representing an alternative and viable solution for regenerating treated wastewater in small towns.

The full article is available at

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