Constructed wetlands: purifying water with plants

What is constructed wetlands

The term constructed wetlands indicates that biochemical process by which water is purified in a natural way, this process goes to reproduce in a forced way the process of natural self-purification of water, typical of aquatic environments and wetlands. In recent years it has comprised the importance of these areas as natural biofilters, their protection and restoration is vital to the health of our earth.

The term phytodepuration is deceptive, the word literally means purification with plants (Phyto = plant), but the real stars of this process are the bacteria that live in symbiosis with plants; these, in reality, only create a habitat suitable for the growth of the bacterial fauna.
This process then takes place thanks to the combined action between the plants, the microorganisms in the purification tank, the permeable substrate (which may also be the only roots) and the wastewater; without the presence of one of these factors, the phytodepuration can not happen.

Features and functions of plants:

The main function of the plants in the process of phyto purification is to capture the oxygen from the atmosphere and transport it through the leaves and the stem, in the root system and in the substrate, making it available to aerobic bacteria present in the medium.
The bacteria that colonize the roots of these plants in fact are aerobic (organisms that use oxygen to live). They degrade the pollutants and the organic substance into inorganic substances available for the plants.
The plants also contribute to the purification by absorbing, albeit in small amounts, a part of the dissolved substances or substances produced by bacteria and give stability to the system of constructed wetlands, creating a network of roots in the substrate.
In warmer weather, the plants also have the function of to shade and also lower the temperature of the basin, high temperatures can interfere to the proliferation of beneficial bacteria and slow the metabolism.
Already a few months after the start of a constructed wetland, the roots of these plants have biofilm formation useful for purification and the system begins to fulfill its function.

In summary, the plants in a constructed wetland are used to:

  • supply oxygen necessary for the decomposition of organic matter by aerobic microorganisms.
  • aabsorb, albeit in small quantities, the nutrients that present in the wastewater.
  • make stable the substrate and slow down the water flow to
  • reduce solar radiation of the pelvis, regulate the temperature of the wastewater.

Sothe plants for constructed wetlands, should have two fundamental characteristics: living in aquatic or marsh environment (rooted macrophytes), and have an excellent translocation of the oxygen to the roots.

The zones of constructed wetlands that are unable to receive oxygen instead host of anaerobic bacteria, which are also useful if properly dimensioned, for the purification process.

Which plants choose:

As we explained in the previous section, not all plants have the characteristics to be considered and used as plants for constructed wetlands.

In the '80s, when this technique was in its infancy, were also used terrestrial plants unsuitable for this purpose such as rosemary (Rosmarinus officinalis), broom (Genista ssp, Cytisus spp ...), the cherry laurel (Prunus laurocerasus), Photinia (Photinia serrulata Lindley), oleander (Nerium Oleander) etc. 
These plants, when grown in moist soil, are to the limits of survival, form rootlets which stop a few inches above the level of the wastewater and rot as soon as the water floods the roots, the plants can not create roots support, resulting in danger but mostly do not are roots in water and then it does not happen the oxygenation process that is essential in constructed wetlands.

The plants most suitable for constructed wetlands are:

You can find the characteristics of these plants in CONSTRUCTED WETLANDS section of our site.  

This science is constantly changing and many universities are practicing studies about plants.

Continuous depth study and read:

LE TIPOLOGIE DEGLI IMPIANTI DI FITODEPURAZIONE