Journal of Environmental Sciences study reveals insights into passive water purification by constructed wetlands
Elizabeth Chang
Background
Constructed Wetlands, or CWs, are eco-friendly solution to removing pollutants from micro-polluted waters. The issue with traditional treatment processes, such as Wastewater Treatment Facilities (WWTPs), is that although they are effective at treating highly concentrated polluted waters, they often miss low-concentration pollutants in micro-polluted waters. CWs use biolife and plants to naturally remove pollutants and purify micro-polluted waters.
Peer-reviewed article
Micropolluted water has slowly been finding its way into food and human bodies, and is widely known to possibly lead to many different potential health risks (1). Sources of micropollution divide into three major categories: run-off, WWTPs, and polluted rivers or waters.
Run-off comes largely from agriculture and urban areas when run-off exceeds that ecosystem’s carrying capacity. These waters are mainly polluted with high levels of nitrogen and phosphorus. Agricultural run-off holds high levels of pesticides, while urban run-off holds high levels of heavy metals such as Cadmium and Lead. High precipitation could also lead to more heavy metals from urban areas, and because run-off is caused by excess water escaping an ecosystem, rainfall leads to a higher volume of run-off, making run-off volume a very seasonal cycle
WWTPs, as discussed earlier, are great ways to treat highly concentrated polluted waters, but unfortunately, they fail at filtering out low-concentration micropollutants and even contribute further to micropollution. They bring in various forms of nitrogen, trace amounts of pesticides, and trace amounts of heavy metals.
The last of the three micropollution sources is polluted rivers and waters. These are bodies of water made up of mixed sources of pollution, including agricultural run-off ubran run-off and wastewater from treatment facilities. These rivers and lakes are very nutrients dense with total nitrogen concentrations (TN) ranging between 1.7 and 47 mg/L, total phosphorous concentrations (TP) exceeding 0.1 mg/L, and organic matter - measured in carbon-oxygen density (COD) between 20 and 250 mg/L (2). Like run-off, polluted rivers and waters fluctuate with season and rainfall. Additionally, pathogens, such as coliform bacteria, also form in these bodies of water as it sits untreated.
Figure 1: The removal process in a CW (2)
Constructed wetlands, as shown in the figure above, rely on innate physical, chemical, and biological mechanisms to remove contaminants from the water. The wetland plants directly absorb nitrogen and phosphorus as nutrients, and release oxygen and provide roots to be used as sites for microbial attachment, which majorly contributes to the removal of pollutants. The medium of plant-growth – written as the substrate layer in the figure – can also serve as a biofilm, which plays a role in the absorption, filtration, and biodegradation of nutrients and other pollutants such as pesticides and heavy metals.
Nitrogen purification proves to be a challenge because it has its own cycle. In fact, the TN removal efficacies of any of the three sources – run-off, WWTPs, and polluted rivers – never exceed 50%, being at 44.20%, 47.95%, and 41.42% (2). Mainly, nitrogen is removed in CWs from the extra vegetation in the CWs that creates a nitrogen sink and allows nitrogen to be taken up and harvested in those extra plants.
Pesticides can be removed in CWs by many processes, but there is photolysis, microbial degradation, and plant uptake to name a few.
Many of the processes are influenced by the weather. There is usually a decrease in productivity in winter due to low plant life and low temperatures. In fact, pesticide removal efficacy sees an almost 40% reduction, going from 84.96% in the summer to 49.99% in the winter (3).
In order to really use CWs efficiently, extra nutrients such as vegetation and compressed oxygen are put in. For example, Phragmites australis and Iris pseudacorus (2) are put into CWs because they raise contaminant removal efficacy. Compressed oxygen can serve as an electron donor, largely contributing to the nitrogen cycle and overall TN removal efficacy. Biochar is also a large example as a helpful addition to CWs, as it serves as an electron donor and has a high absorption capacity for heavy metals and other contaminants.
Additionally, there are many types of CWs. Mainly, they divide into two: free-water surface flow CWs (FWSCWs), and subsurface-flow CWs (SSFCWs). FWSCWs were found to be more effective in COD, NH4+-N, and TP removal, while SSFCWs were more efficient in TN removal.
However, from a technical perspective, they can be divided into two basic types: freewater surface-flow CWs (FWSCWs) and subsurface-flow CWs (SSFCWs).
HCWs were a mixed system containing different CWs. These were optimal to use as the make-up of HCWs could be changed between environments and locations to better fit the needs of the population and the concentrations of pollutants in local waters. However, it is worth note that HCWs are much more expensive to invest in.
Figure 2: Removal of different types of contaminants in different types of CWs.
The largest point for this article centered around the issue that these CWs, while amazing, cannot be employed large scale yet, as they cannot account for all the variations that happen in larger environments. Currently, CWs are highly efficient at the purification of micro-polluted waters in smaller environments and are efficient at identifying pollutants on larger scales. However, purification on a larger scale still seems to be off the table. The article explores a few different further exploratory studies – such as emerging pollutants, substrate selection, and predictive modeling.
News Article
The news article, “Journal of Environmental Sciences study reveals insights into passive water purification by constructed wetlands” starts with a brief explanation of the makeup of micropolluted water, mainly concentrating on organics such as Nitrogen, Carbon, and phosphorus. It explains what a CW is and the shortcomings of it in the current day. The author interviews the researcher, Professor Haiming Wu, about the paper and summarizes the factors that contributed to the removal of pollutants in a CW system. Lastly, they briefly pointed out one of the three avenues of further research the paper suggested.
Although the article does a good job of writing for a lay audience, I give this article a 7/10 because it seems to gloss over topics. For example, the article hardly mentions heavy metals, only having it twice in the article, when, although not an overwhelming point, it was a significant pollutant that the paper delved into. It also points out only one of the three points of further research suggested. Overall, maybe it is due to the order in which I read the two, but after reading the paper, the news article seemed very superficial – like scratching the top of an iceberg.
Sources:
[1] https://www.uchealth.org/today/microplastics-are-everywhere-from-water-to-food-what-are-the-health-impacts/
[2] Ning, Q; Yan, P.; Zhao, L.; Lin, Z.; Zhang, J.; Guo, Z.; Qu, H. Recent advances on micro-polluted water remediation by full-scale constructed wetlands: Pollutant removal performance, key influencing factors, and enhancing strategies, Journal of Environmental Sciences, V. 159, 2026, pgs. 565-576, https://doi.org/10.1016/j.jes.2025.03.
[3] Chen, C.; Luo, J.; Bu, C.; Zhang, W.; Ma. L. Efficacy of a large-scale integrated constructed wetland for pesticide removal in tail water from a sewage treatment plant, Science of The Total Environment, V. 838, pt 4, 2022, https://doi.org/10.1016/j.scitotenv.2022.156568.
[4] https://www.eurekalert.org/news-releases/1103241
Hi Elizabeth! Great analysis-- this is such an interesting topic that really speaks to the capabilities of natural ecosystems. I was particularly interested in how the constructed wetlands are not able to be expanded to a large scale due to their inability to account for all the varying pollutants. I see how this could be a significant issue, but I'm curious if there is a major enough source of micro-pollutants that a large-scale CW could still make a difference? I imagine that a large-scale CW would still have overall positive effects on reducing pollution, so I'm curious if there is a cost-benefit side of choosing not to implement them on large scales. On that note, did the peer-reviewed article mention anything about the cost of constructing wetlands? I can imagine that it would be incredibly expensive, especially on larger scales, but I'm curious what aspects of constructing wetlands contribute the most to costs/if the varying kinds of constructed wetlands have different costs and benefits? Finally, what kind of locations are the most in need of CWs? Is it mostly urban populations?
ReplyDeleteHi Eliza,
DeleteIt seems the issue with large scale CWs don't lay in an impeded source of micro-pollutants. Rather, the number of unaccountable variables like variations in wind, weather, seem to be the core reason CWs are not as viable an option large scale. For your question about cost, the peer-reviewed article did go a little into the costs of different types of CWs. By far, the HCWs were the most expensive as it combines several CW types together. As for locations, I'm actually not too sure. I know urban areas lead to more heavy metals whereas agricultural can lead to more pesticides, but not too sure if there is a large difference in need of CWs.
Hi Elizabeth, I found your article very interesting! It does seem counterintuitive that a wastewater treatment facility would contribute more to pollution. I love learning about natural processes that can be used as eco-friendly alternatives to current systems. I’d be interested to read about how further research is being done on creating large-scale CWs. Because this is a natural process, there are so many factors to consider that cannot be easily modified like temperature and plant growth in the colder months. What are the criteria the researchers found for efficient small CWs? The article states that constructed wetlands are not suitable for large-scale facilities, would it be a realistic solution to create many smaller wetlands to handle pollution more efficiently?
ReplyDeleteHi Talia,
DeleteYes, I would think creating a lot of smaller wetlands may help, but is probably not cost-effective. The peer-reviewed article does briefly mention WWTPs are used a lot because they are more wallet-friendly. As for the criteria researchers used, I wouldn't say there was a specific criteria to meet to be counted as an efficient CW, there was more of a caparison between different types of CWs and how well they removed certain pollutants relative to the concentrations from before and relative to how other CWs performed.
Hi Elizabeth! Thank you for sharing this interesting article! I enjoyed learning about constructed wetlands (CWs) as an alternative to wastewater treatment plants (WWTPs). I am interested in the CW plants. What types of plants are commonly use to make CWs? What are the advantages and disadvantages of them? I am curious how these plants, which uptake heavy metals and nutrients, impact the ecosystem and food chain. Do pollutant concentrations ever increase to the point that the plants must be disposed of? If so, how are they discarded? A pollutant of interest to me is per- and polyfluoroalkyl substances (PFAS). I know efforts are in place to remove PFAS from contaminated water in WWTPs. Could CWs remove PFAS? I am also interested in how flow rate impacts CWs. You mention two types of CWs: free-water and surface flow. How is flow rate related to the differences in pollutant removal efficiencies between them?
ReplyDeleteHi Shyleigh,
DeleteTo what I understand, CWs are a combination of vegetation and plant life - like it's own little ecosystem. The peer-reviewed article did mention a couple plants by name: Phragmites australis and Iris pseudacorus, but these were mentioned as plants that help raise efficacy of removing contaminants, not particularly as a common plant used in CWs. Different plants are better at different aspects, like some are better at removing heavy metals while others are more efficient at removing organic contaminants; it's really up to what is in the water and what the CW is trying to remove.
I would think any pollution too great could overwhelm a plant to the point that it is poisoned, but there wasn't anything in the article to back that. In my opinion, however, if the water in question is so polluted as to poison the CW plants, I would not personally categorize it as micro-polluted water, just polluted water. The article never mentioned PFAs, but I would think a realistic way to remove PFAs would be to remove them via WWTPs then treat micro-polluted water from the WWTP with another CW.
The types of CWs are not dictated by flow rate, rather they are the location of water flow. For example, free water flow CWs (FWFCWs) flow above ground and are more prone to temperature fluctuation, insects, and developing odor. On the other hand subsurface flow CWs flow, well, subsurface.
This is a really interesting article, and I liked your analysis! I never really considered that plants could be used to help remove contaminants from water, but it seems like a very interesting process, and I like that people are considering using natural processes in constructed wetlands to help remove pollutants. I also like that you discussed the differences in types of pollutants found in agricultural vs urban areas, which I thought was lacking in the news article. I thought the article did a pretty good job, and I appreciated that they included quotes from the author of the peer-reviewed article, and I thought they did a good job breaking down the experiments. However, I do agree that they glossed over some information that seems relevant from the paper. Does it seem likely that there will be widespread use of these constructed wetlands? Is it possible and feasible for these to be constructed in urban areas?
ReplyDeleteHi this is a really interesting article and something that I think could definitely see having an impact being residents within Michigan. I found the diagram that showed how CWs are able to almost cleanse the wetlands. I found it interesting that different CWs are able to be effective for different pollutants this could definitely be optimize in a further study or a even a large scale deployment. I was wondering where CWs come from, and if we genetically produced them? Do you think that genetic modification could be good or bad for improving CWs at substrate affinity. Maybe enzymatic modification of existing wetlands plants could lend itself to improving these environments.
ReplyDelete