Floating treatment wetlands locally remove nutrients and micropollutants from polluted surface water
Many waters are coping with eutrophication, poor ecological condition, and pollution by pesticides and drug residues. Together, the Vallei & Veluwe Water Authority and Wageningen University researched floating treatment wetlands as a potential solution. They appeared to be particularly suitable for slightly polluted water.
Constructed wetlands (helophyte filters) are nature-based systems that are used to remove nutrients and micropollutants from treated wastewater. Moreover, they reduce the ecotoxicity of the water and are known as sustainable, environmental-friendly, inexpensive, and easy to manage [1, 4]. This way, besides purifying water, they contribute to many of the current climate, energy, and biodiversity challenges.
Zijdewetering
The Zijdewetering in Veenendaal is a slow flowing drainage canal on sand that is mainly fed by effluent from the wastewater treatment plant in Ede. Despite the high removal efficiency of this wastewater treatment plant for nitrogen, the water remains too nutrient-rich, which causes excessive plant growth. The nocturnal oxygen concentrations are therefore low, and the ecological status is poor. Constructed treatment wetlands would be ideal for this location to treat the effluent water for multiple contaminants and improve local ecology at the same time [4]. However, physically there is not enough land area for this.
Figure 1. Removal of nutrients and micropollutants by floating treatment wetland.
Floating helophyte filters
Where there is a shortage of land area, floating treatment wetlands can be used in existing waterways. The choice of the substrate in these wetlands determines their treatment performance, and the use of a bio-based substrates contributes to their sustainability. However, there is not enough research into the effect of floating wetlands with bio-based substrates on water quality.
Within this study, floating treatment wetlands with jute or mycelium substrate were studied. This was done in a controlled mecocosm setup with water from the Zijdewetering. Oxygen dynamics were monitored together with the removal of nutrients and micropollutants. The results were scaled up for the whole Zijdewetering with calculations. In addition to the mesocosms, a field test was carried out with floating treatment wetlands in the Zijdewetering to monitor their development (in light of water currents and the weather).
Method
The research setup consisted of 39 48-litre mecocosms, each with a miniature floating wetland and 12 plants in various combinations (reed, reed mannagrass, and yellow iris). The substrates used were mycelium, jute or ‘no substrate’ (control). The mycelium was made from Ganoderma fungus cultivated on wood waste. Every 5 days, half of the water was replaced.
The removal efficiency was assessed 24 hours after replacing the water (equal to the retention time of water in the Zijdewetering). Ammonium, nitrogen, and phosphor were measured using Hach Lange tests; nitrite, nitrate, phosphate, and sulphate using ion chromatography; and for micropollutants, we used liquid chromatography. For every sample, the pH, temperature, electric conductivity, and oxygen concentration were determined.
Nutrients
Looking at nutrient removal in constructed wetlands with jute, mycelium, and ‘no substrate’, it is evident that mycelium has a lower purification yield for phosphate, ammonium, and total nitrogen. Furthermore, for half of the mycelium wetlands, we see an increase in phosphate (figure 3). This was caused by gradual breakdown of the mycelium, which released nutrients. The mycelium breakdown also has negative effects on nitrogen removal: mycelium wetlands removed nitrogen significantly less compared to jute- and the control wetlands.
Jute wetlands, however, remove phosphate efficiently compared to control mecocosms. Jute absorbed phosphate within 8 hours, which increased the removal efficiency from 70% (control) to 100%.
Figure 2. Sample mecocosm with mycelium substrate planted with reed mannagrass (photo Hazel van Waijjen)
Micropollutants
Of the twenty micropollutants studied, sixteen were present above the detection limit of 50 nanograms per litre. Their removal was better than expected: for ten substances, more than 70% was removed (figure 3). The removal efficiency of mycelium wetlands was again relatively low. This is likely caused by the presence of easily degradable organic material in these wetlands. Bacteria that break down organic micropollutants prefer to ’eat’ mycelium first. removal efficiency in jute wetlands was comparable to the control treatment.
Oxygen
The breakdown of mycelium and jute in the mecocosms required a great deal of oxygen. All mecocosms contained less than 1 milligram of oxygen per litre, which also led to lower ammonium removal with jute and mycelium, since complete nitrification is not possible without oxygen.
Healthy oxygen dynamics are essential for healthy water ecology as well as for ammonium removal. Submerged water plants use oxygen at night, which can lead to almost anaerobic conditions in case of too much vegetation. Border plants such as reeds, however, give off oxygen via their roots [1], also at night. The hypothesis is that floating wetlands with emergent water plants outcompete part of the submerged water plants and thus realise more robust oxygen dynamics. This will be further researched.
Seasonal dependency
The removal efficiency for micropollutants had halved by the end of the summer, compared with mid-summer. During the growing season, plants contribute to pollutant removal by direct absorption and by stimulating the microbiome, e.g. by giving off oxygen and signal substances. Aging of plants reduces these contributions. In addition, the breakdown of plants at the end of the summer leads to a surplus of easily digestible plant residue, which decreases the breakdown of micropollutants. Just like with the mycelium substrate, the bacteria then prefer ‘eating’ the plant residue.
The addition of activated carbon or biochar to the wetlands can compensate for this lower removal efficiency. Biochar and activated carbon have a considerable absorbing capacity [2, 4], as a result of which micropollutants are retained during the winter to be broken down during the growing season. As a result, a natural version of a BODAC system emerges: a biologically enhanced activated carbon filter that absorbs pollution, which is then broken down aerobically by microbes [4].
The seasons also have a considerable influence on the removal of nutrients. In spring, plants absorb most nutrients, and release them again in autumn. In order to guarantee a high removal, biomass must be harvested twice per year, in early summer and early autumn. The great advantage of floating wetlands is that roots, where most nutrients are stored, can also be harvested [1, 3].
For that matter, problems with water quality are also seasonal. Most problems occur during the summer due to high biological activity and poisonous algal blooms.
Figure 3. Removal of phosphate, ammonium, total nitrogen, and micropollutants by containing floating treatment wetlands after 24 hrs of treatment (23 August). Removal efficiency of nutrients shown per mecocosm (njute= 14, nmycelium= 10, ncontrol= 15) . Removal of 16 micropollutants per mecocosm (njute= 224, nmycelium= 160, ncontrol= 240).
Stability of floating helophyte filters
In the field test with planted jute and mycelium wetlands in the Zijdewetering, the mycelium started to break down within a month. The jute, which was fastened around a floating PVC frame, also teared within a month. In a later six-month test, floating PVC frames covered with gauze and filled with jute were used. These appeared to be sturdier and provided excellent support for plant growth.
Added value
Floating wetlands are multifunctional; they not only remove nutrients and micropollutants, but also remove pathogens, reduce ecotoxicity, and work as an ecological buffer. In addition, they are adaptive, robust, and aesthetic, as a result of which they are suitable for application in urban areas. There, they contribute to mental health, have a cooling effect, and create natural habitats for city wildlife [5]. In short, floating wetlands are an integral ‘no regret’ solution to several problems[1, 4].
Conclusions and advice
Floating treatment wetlands can be an enrichment to (urban) ponds thanks to their positive effect on water quality and ecology. However, the removal efficiency for nutrients is too low for considerable eutrophic water or for controlling (blue-green) algae nuisance. Too many wetlands would be needed for an adequate effect. They are extremely suitable for the treatment of slightly polluted water, for the revitalisation of urban water bodies, or for ecosystem services and additional nature value.
The advice is to choose a robust and sustainable design of recycled materials (such as second-hand PVC or gauze), filled with the bio-based substrates jute and biochar (which retains micropollutants during winter). Our research shows that the type of plant species have less effect on the water quality (data not presented). Therefore we advise endemic fast-growing flowering plants, thanks to their positive effect on insects. This vegetation must be harvested twice yearly for efficient nutrient removal, whereby the harvested material can serve for the production of energy or materials (biogas, fibres, reed roofs, etc.)
Thanks
This research was carried out as an internship at the Vallei & Veluwe Water Authority, and was made possible by SIGN (thanks to Peter Oei, Laila Kestem, and Dewi Hartkamp), and Wageningen University Environmental Technology.
Hazel van Waijjen
(Vallei & Veluwe Water Authority)
Frans de Bles
(Vallei & Veluwe Water Authority)
Anita Buschgens
(Vallei & Veluwe Water Authority)
Katarzyna Kujawa-Roeleveld
(Wageningen University)
Background picture:
Field study with floating treatment wetland in the Zijdewetering (photo by Richard Huinink).
Summary
In this study, we researched the effect of floating treatment wetlands on the water quality of surface water. Two bio-based substrates were applied: mycelium (made from wood waste) and jute. Mycelium showed to break down quickly, and nutrients were released instead of removed. However, jute resulted in excellent phosphate removal and good nitrogen removal. During the summer, the jute wetlands were very effective in removing micropollutants, but this effect decreased considerably towards autumn. This can be remedied by adding biochar or activated carbon in order to retain contaminants for breakdown during the following summer. All in all, floating treatment wetlands are robust and very suitable for cleaning up and revitalising slightly polluted (urban) water bodies.
References
1. Shen, S., X. Li, and X. Lu, (2021). Recent developments and applications of floating treatment wetlands for treating different source waters: a review. Environ Sci Pollut Res Int, 28(44): p. 62061-62084. https://doi.org/10.1007/s11356-021-16663-8 26
2. Lei, Y., et al., (2021). Sorption of micropollutants on selected constructed wetland support matrices. Chemosphere, 275: p. 130050
3. Kadlec, R., J. Vymazal, (2005). Vegetation effect on ammonia reduction in treatment wetlands, in Natural and Constructed wetlands: Nutrients, Metals and Management. Leiden. p. 233-260
4. Van de Beuk, J. et al., (2022). Study of natural purification systems for removal of organic micropollutants (Verkenning natuurlijke zuiveringssystemen voor verwijdering van organische microverontreinigingen). STOWA: Amersfoort
5. Semeraro, T. et al., (2021). Planning of Urban Green Spaces: An Ecological Perspective on Human Benefits. Land, 10(2) https://doi.org/10.3390/land10020105
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