Water pollution is a great threat to human health and biodiversity itself. In addition to (certainly necessary) system solutions on a large scale, good water condition can also be ensured from the bottom up – for example, by building floating treatment wetlands. It is a simple, cheap, completely natural and effective solution. It helps to easily clean the ponds and small water reservoirs, while activating us to engage in environmental protection.
What are floating treatment wetlands?
Plant (or hydrophyte) floating treatment wetlands are a relatively recently popularized, natural way to purify waters. The assumption is not complicated – such a wetland is simply appropriately selected filter plants, mounted on a buoyantant wooden structure. Such an island is moored and drifts on the surface of the water. Hydroponic mats constructed this way support the flow of water below and through the plants, and their root system acts as a natural filter .
Floating treatment wetlands work perfectly everywhere it is difficult or impossible to maintain swamp vegetation that acts as a natural filter. Therefore, such filter islands are great for tanks with large fluctuations in water level, without a shallow water zone or with banks that are too steep for plants to grow .
How do hydrophyte islands purify water?
Floating treatment wetlands improve water quality in several ways. Firstly, they take in nutrients from the upper layers of water, which largely come from surface runoff (rainwater flowing down the ground to the water reservoir). Such runoffs flush out a lot of agricultural and other anthropogenic pollution and poison reservoirs. In the presence of islands, the effects of such pollution are less noticeable. The filter islands perform the function of phytoremedians, or they pull heavy metals out of the water, which become trapped in plants’ tissues.
In addition, such plant constructions are an excellent habitat for bacteria useful for the aquatic ecosystem (which also contribute to water treatment) .
Floating filter plants support biodiversity
Water purification from excess nutrients and toxins is the main purpose of this type of construction, but not their only advantage. In addition, the filter islands shade the reservoir, which is very useful if the water heats up too much (which, in turn, directly and indirectly threatens aquatic organisms) . Thanks to the release and transport of oxygen, they also contribute to the oxygenation of the tank or, for example, phytotoxin oxidation .
The roots of plants that form filter islands begin to form dense, long “forests”. It is an excellent shelter for fry and a haven for insects. Over time, the root-rhizome zone of the wetland may grow to such an extent that support in the form of a frame and buoyancy elements will be unnecessary . Plants on the island can also add a valuable variety to the species composition of vegetation in a given reservoir, potentially attracting a greater variety of animals. Some of the floating treatment wetlands are eagerly occupied by water birds.
Which plants are suitable for floating treatment wetlands?
Plants from which filter islands can be created are so-called macrophytes, i.e. relatively large aquatic plants (which are often indicators in the assessment of the waters’ ecological status). There can be several of the recommended species on one island:
- marsh calla (Calla palustris);
- yellow iris (Iris pseudacorus);
- flowering rush (Butomus umbellatus);
- lesser pond-sedge (Carex acutiformis);
- common sedge (Carex nigra Reichard);
- great manna grass (Glyceria maxima);
- creeping jenny (Lysimachia nummularia);
- tufted loosestrife (Lysimachia thyrsiflora);
- water mint (Mentha aquatica);
- bogbean (Menyanthes trifoliata);
- water forget-me-not (Myosotis scorpioides);
- lakeshore bulrush (Schoenoplectus lacustris);
- simplestem bur-reed (Sparganium erectum);
- dwarf bulrush (Typha minima).
To create floating treatment islands, it’s best to use a hydroponic plant or acquire them thanks to the courtesy of another pond owner. Plant substrate from container crops may, for example, contain fertilizers – undesirable in water bodies.
Some of the graphics used in the article come from the On Water foundation’s website. This Polish foundation establishes water gardens and floating treatment wetlands.
-  Colares G., Dell’Osbel N., Wiesel P., Oliveira G., Lemos P., Silva F., Lutterbeck C., Kist L., Machado Ê., 2020. Floating treatment wetlands: A review and bibliometric analysis. Science of The Total Environment, 714.
-  Fundacja Sendzimira: Roślinne Wyspy Filtracyjne [received 25.06.2020].