Wastewater treatment plant based on the combination of natural treatment systems, such asconstructed wetlands and ponds, following the concept of waterharmonica.Constructed wetlands are water purification systems that reproduce the processes ofcontaminant elimination which occur in natural wetlands. Therefore, constructed wetlandtechnology operates as a complex ecosystem made of the following elements:

• The water to be treated, which flows through the filtrating substrate and/or vegetation.
• The substrate, which is the support of the plants and has to retain the microbialpopulation (in the form of a biofilm), and is essential in most processes aimed atremoving the wastewater contaminants.
• The emerging aquatic plants (macrophytes), which supply surface area for the formationof bacterial films; they facilitate the filtration and adsorption of the wastewaterconstituents, they help to oxygenate the substrate and remove the nutrients as well ascontrolling the growth of algae by limiting the penetration of sunlight. Furthermore, thevegetation helps to integrate these treatment devices in the landscape.

Traditionally, there are two types of constructed wetland depending on the type of water flow:surface flow or subsurface flow. In Surface Flow Wetlands, SFW, or Free Water Surface Wetlands,FWS, the wastewater flows over the substrate whilst in Subsurface Flow Wetlands, SSFW, orVegetated Submerged Beds, VSB, the water flows underground through the interstitial spaces ofthe filtrating bed.