Agriculture, water purification systems and a wide range of businesses all have wastewater to treat. A filtering marsh is a choice technic for decontaminating these water effluents. It’s sure to be of interest for plant lovers !

A filtering marsh is an artificial marsh that recreates all the functions of a natural marsh [1]. The aim is to use this construction to filter and decontaminate various types of wastewater [2]. The mechanism is divided into 4 processes: the physical process, the chemical process, bioaccumulation and biodegradation by microorganisms and sometimes transformation by plants [1][3].


Image by Carla Bosteder

Physical process  

As wastewater flows through the marsh, suspended solids are held back by roots and gravel, much like a strainer. In addition, heavier particles, due to their weight,  settle to the bottom [1].

Chemical process 
Certain elements (iron, aluminium, calcium and magnesium) contained in gravel can “attract” pollutants (such as phosphorus and heavy metals) preventing them from continuing their journey further into the water. It is the actions of adsorbing, retaining particles without absorbing them, and precipitating, forming solids in a liquid [1].

Plants have abilities to absorb contaminants from soil or water into their tissues and retain them in their body. Pollutants such as lead, copper and many others remain in the aerial parts of the plant. Contaminated plants must then be cut or removed [1].

Biodegradation by microorganisms 
The microorganisms present in the filtering marsh do most of the work. They fragment complex molecules to make them more easily assimilated by other microorganisms. The smaller molecules are then transformed by the metabolism of the microorganisms in different ways, depending on the nature of the molecule. [1]

Biodegradation by plants 
As well as accumulating pollutants, some plants can metabolize them. This phytoremediation technique is called phytodegradation. It is a transformation of pollutants in plant tissues [3].


Advantages of filtering marsh

Artificial filter marshes are used to decontaminate polluted water of various kinds: for residential applications such as municipal wastewater, commercial and industrial applications such as paper mill effluent, or agricultural applications such as runoff [1][4]. In Quebec, this technology has been in use since the 1990s, with some 350 systems in operation in 2018 [4].

Fauna and flora, image by Raymond Brettschneider

Filter marshes offer natural, low-cost processes,  contribute to the landscape (visual interest), as well as maintaining  animal and plant biodiversity [1] . They improve water quality, retain pollutants and reduce the quantity of organic substances and bacterias [2]. For example, one study revealed an 18% reduction in nitrogen and a 41% reduction in phosphorus following the installation of a filtering marsh to treat part of the water from the Walbridge stream in the Eastern Townships [5].


Nature really is an inexhaustible source of solutions!


[1] Société québécoise de phytotechnologie, Fiches techniques de la SQP. 1. LES MARAIS FILTRANTS. 28 mai 2014. www.phytotechno.com https://www.phytotechno.com/wp-content/uploads/2018/02/SQP_Fiche_MaraisFiltrants.pdf

[2] Rivard, G avec la participation du Ministère du Développement durable, de l’Environnement, de la Faune et des Parcs et du Ministère des Affaires minucipales, des Régions et de l’Occupation du territoire . (s.d.) Guide de gestion des eaux pluvialeshttps://www.environnement.gouv.qc.ca/eau/pluviales/guide-gestion-eaux-pluviales.pdf

[3] Société québécoise de phytotechnologie, Fiches techniques de la SQP. La phytoremédiation. 1er juin 2016. www.phytotechno.com https://www.phytotechno.com/wp-content/uploads/2018/04/fiches-Phytoremediation.pdf

[4] Marqis, D et Hénault-Ethier, L. (s.d) Quel avenir pour les phytotechnologies au québec ? https://fr.davidsuzuki.org/wp-content/uploads/sites/3/2018/11/Rapport-Phytotechnologies-DSF-SQP_FINAL_2018-11-22.pdf

[5] kroeger et al. (2009) Les marais filtrants : une solution pour restaure les cours d’eau agricoles. Agro solution. https://irda.blob.core.windows.net/media/2251/kroeger-et-al-2009_article_marais_filtrants.pdf


Written by Audrey Thériault