TUESDAY, JAN 17, 2023: NOTE TO FILE
Module 2-7
The Rhizosphere in Wetlands Wastewater Treatment
‘Rhizo’ (from the Greek means ‘root’. The rhizosphere, therefore, is the area in and around the roots of plants. A rhizome is a horizontal stem growing underground and putting up shoots and putting down roots from the stem. Ginger, ginseng, arums and many of the plants used in wetlands, such as Typha, Phragmites and Irises produce horizontal rhizomes below the surface. We too often forget that so many of the essential functions supporting life are happening in the invisible world below ground.
Source: Gardenseeker.
All of the processes described above are microbial. In order for the necessary microbes to thrive, appropriate habitats must be created within the treatment systems. Plants and artificial media (gravel and sand) are used in natural treatment systems to create the media necessary to support rich microbial communities. Material with a high surface to volume ratio, like pumice or plant roots, offer a lot of attachment area for the microbial communities.
Nitrification of ammonia to nitrites occurs in the biofilms which attach to the media. A biofilm is effectively a symbiotic relationship between microbial communities which stick together by excreting a sort of sticky slime. They can attach to living and non-living substances and are found everywhere. For example, in humans, they form in the mouth overnight (up to 600 microbial communities in a single biofilm); they are also found hiding in hospitals as biofilms of antibiotic-resistant microbial communities. Most of them, however, are not pathogenic for humans.
In the for purifying water shown below, the roots of plants will provide ample space for biofilms. The choice of plants with root nodules for bacteria is also crucial for nitrification and denitrification.
Source: Biomatrix Water, https://www.biomatrixwater.com/floating-ecosystems/
These root systems remove excessive nutrient, suspended solids, some heavy metals (zinc and copper) and ammonia and oxygenate the freshwater bringing it to life. Source: https://deiceair.ca/
To see an enlarged version of the infographic, follow this link http://www.biomatrixwater.com/wp-content/uploads/2019/03/brochure_new-logo_web.pdf
The extensive root network and its rhizosphere in the planted ecologies provides the structure and nutrient support for the diverse microbial communities. Materials from the plant roots are exuded into the surrounding rhizosphere. These materials include hormones, antibiotics, metal chelators, nutrients, humic compounds and sugars (Todd et al, 1996).
By constantly extending their root networks, plants are also forming new pathways in the , through which water can filter. This keeps the wetlands open to the flow of water. Some plants also possess the ability to transfer oxygen from their aerial stems to their roots. This continuous exposure of organic matter to air promotes the decomposition and oxidation of the organic matter and enhances nitrification. In the aerobic treatment phase, the oxygenation of the water in contact with the biofilms is crucial for the effective functioning of treatment systems.
Attaching biofilms to surfaces significantly increases their stability and efficiency. They are also grazed by micro-animals and the number of dead bacteria that settles as sludge is minimized.
Plants
Typical plants used in Subsurface Flow Constructed Wetlands (SSFCWs) are the Cattail (Typha latifolia) and Reeds (Phragmites australis). Other plants that are locally available and robust can be added for biodiversity and colour. Typical examples are the Lily and Iris families. They are macrophytes including four types namely emergent macrophytes, floating-leaved macrophytes, submerged macrophytes and freely floating macrophytes. We can easily recognise many of these from garden ponds, or from natural wetlands but these are the most common in SSFCWs.
Module 2, lesson 8
