Matamoros, V., Rodriguez, Y., and Bayona, J. (2017). Mitigation of emerging contaminants by full-scale horizontal flow constructed wetlands fed with secondary treated wastewater. Ecological Engineering 99: 222-227. DOI: https://www.sciencedirect.com/science/article/pii/S0925857416306772.
What’s in your wastewater?
In today’s society, people’s daily routines are incorporating an increased number of lifestyle products, such as detergents, personal care products, and pharmaceuticals. While one may believe that such lifestyle products are confined to their own bodies and households, this is not the case. In fact, these products are being washed down our drains and toilets and are emerging as harmful contaminants introduced to the environment via wastewater. In many countries, wastewater is processed at wastewater treatment plants (WWTPs); however, conventional WWTPs are not currently equipped to treat/remove emerging contaminants. The wastewater then leaves the WWTPs and carries emerging contaminants directly into the environment. The harmful effects of emerging contaminants can especially be seen in surface waters, where fish, shellfish, and invertebrate diversity and populations are declining.
Wetlands to the rescue
Because conventional WWTPs are ill-equipped to remove many of the products being introduced into today’s wastewater stream, additional treatment options are being employed at many sites. While it is possible to incorporate advanced treatment methods within the WWTPs, one more cost-effective and eco-friendly solution is to construct wetlands that intercept the wastewater before it enters bodies of water. Natural wetlands consist of plants and soils that are capable of filtering out pollutants in water, thus overall improving water quality. Constructed wetlands (CWs) aim to mimic the treatment processes performed by natural wetlands, offering additional wastewater treatment in a controlled environment. Many CWs have been built as surface water wetlands; however, due to the harmful contaminants and pathogens that are often present in wastewater, surface water wetlands could pose as a risk to human health. Alternatively, CWs can be constructed that rely on water movement below the soil surface (horizontal subsurface flow) rather than surface flow. These alternative CWs are typically made of trenches lined with granular material and aquatic plants that are buried underground. Wastewater is then able to flow beneath the soil surface and interact with these wetland features, which work to remove emerging contaminants through physical and chemical processes.
Constructed wetlands vs. emerging contaminants
Researchers from Barcelona, Spain recently performed a study investigating the ability of 12 CWs relying on horizontal subsurface flow to remove emerging contaminants. All CWs being investigated were constructed along the River Besòs, adjacent to a conventional WWTP, over 10 years ago. The CWs utilize river gravel and the common reed plant (Phragmites australis) in order to treat the incoming wastewater. Once treated by the CWs, wastewater is then discharged directly into the River Besòs.
During the months of May and July, researchers obtained samples of the influent (wastewater that had left the WWTP and was about to enter the CWs) and effluent of each CW (wastewater that had been treated by the CWs) in order to quantify removal of the contaminants. Specifically, 16 emerging contaminants were investigated, consisting of pharmaceuticals and compounds associated with fragrances, personal care products, and flame retardants. In order to assess the ecological impact associated with these CWs, researchers also performed an aquatic risk assessment analysis on the Daphnia magna (a small crustacean found in freshwater environments) populations in each CW.
Analysis of the CW influent revealed that chemicals used in products meant to prevent corrosion (for example, benzotriazole), ones found in personal care products (such as 5-methylbenzotriazole), and two nonsteroidal anti-inflammatory drugs (ketoprofen and diclofenac) were present in greatest abundance. This suggests that these chemicals were the most difficult to remove at the WWTP using conventional technology. The CWs were able to assist in removing some, but not all, of these emerging contaminants. In some cases, CWs removed up to 92% of the contaminants, while other CWs were unable to remove any of the contaminants. On average, the addition of CWs as a treatment method removed about half of the contaminants being measured. Even though removal of the various emerging contaminants was not uniform across all CWs, it did reduce the ecological risk posed to Daphnia magna populations by 66%. This study revealed that without additional treatment of the wastewater processed by conventional WWTPs, many harmful contaminants are being directly discharged into the environment. While wastewater treatment processes can definitely be improved by coupling them with the natural processes of a CW, results suggest that we are still not yet in the clear when it comes to treating these emerging contaminants.