Constructed wetlands as a nature-based treatment solution for emerging contaminants in wastewater: chemical fate and ecotoxicological impacts

: (Alternative Format Thesis)

Abstract

Constructed wetlands offer a nature-based treatment option to improve the quality of secondary wastewater effluent before it is discharged to the wider environment. These biological treatment systems allow for the biogeochemical processing of nutrients and organic matter, as well as the retention and potential degradation of trace organic compounds which pose a risk to the receiving environment, such as pharmaceuticals and personal care products (PPCPs). Future environmental regulation of these emerging contaminants in the UK is likely, following a proposed extension of environmental quality standards in the EU to include PPCPs. The UK water industry is committed to expanding the use of nature-based solutions to improve the quality of treated effluents due to their comparative low cost, low carbon footprint, and opportunity for net biodiversity gain compared to traditional grey infrastructure approaches. Research into the functioning of full scale constructed wetlands with regards to emerging contaminants is therefore required to support the optimal design and maintenance of similar systems in the future.

A full scale integrated constructed wetland (ICW) receiving secondary effluent from a rural water recycling centre (WRC) with 2,000 population equivalent (PE) in Gloucestershire, England, was monitored over the first two years of its operation. This study utilised a range of analytical techniques to enable an in-depth performance assessment of the ICW with respect to emerging chemical contaminants. These techniques included: quantitative analysis by ultra-high performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) for 131 pharmaceuticals, metabolites and personal care product ingredients; dissolved organic matter (DOM) characterisation by direct injection high resolution mass spectrometry (DI-HRMS); and an assessment of the teratogenicity of organic chemical mixtures before and after treatment, through application of a fish embryo acute toxicity (FET) assay.

Analysis by UPLC-MS/MS identified a wide range of PPCPs entering the ICW in all seasons, with more than 80 of 131 chemical targets detected in the wetland influent in each monitoring campaign. Pharmaceutical metabolites contributed most of the wetland influent load in all seasons, highlighting the importance of monitoring these species alongside their parent compounds in the environment. The average total mass load removal of the detected targets through the ICW was between 44 – 64 % across all seasons monitored, with the lowest treatment performance in September of 2020, after only 150 days of operation. Where WRC influent was analysed, tertiary treatment by the ICW improved overall treatment efficiency of the system from 68 ± 2% to 88 ± 1%, and from 89 ± 2% to 95 ± 1% in the summer of 2021 and 2022 respectively. For individual species, highly efficient removal through the ICW (i.e., >70%) was consistently observed for fluoroquinolone and tetracycline antibiotics, cardiovascular medicines, beta-blockers, and some metabolites. However, consistently poor removal was observed for some compounds, including clindamycin, nicotine, and parabens. Overall, the wetland performed well for PPCP removal in both winter and summer, operating to remove compounds which were poorly removed by the WRC under the winter operating conditions, and to reduce high concentrations of analytes present in the secondary effluent in summer due to low dilution.

Biodegradation was considered the most important removal mechanism for most PPCPs monitored through the ICW, supported by decreasing parent / metabolite ratios through the treatment system. Different regions of the wetland appeared to serve different roles in the removal of PPCPs. More readily biodegradable compounds were typically removed through the first 4 cells (shallow, densely planted, and faster flowing), whereas the later cells provided higher removal rates for more recalcitrant compounds and those removed by abiotic mechanisms, likely due to the increased hydraulic retention time (HRT) through this section. Variable HRT along parallel treatment lines in the summer of 2021 identified the importance of HRT in PPCP removal, with consistently higher removal achieved under a longer HRT. Fractionation of the chemical mixture by solid phase extraction identified proportionally higher removal efficiency for basic species, followed by weak acids, zwitterions, and neutral species. This was supported by DI-HRMS analysis which showed a greater shift in the DOM composition through the ICW for basic extracts, and least change for the neutral / hydrophobic and acidic fraction.

Environmental risk assessment identified a reduction in risk following tertiary treatment by the ICW. This was based both on the traditional risk quotient (RQ) approach, and the application of a novel combined exposure-effect protocol utilising the zebrafish embryo developmental toxicity assay (ZEDTA). Risk quotients identified an overall reduction in environmental risk in the wetland effluent, as well as a reduction in the complexity of the mixture in all seasons monitored. The ZEDTA identified a reduction in the teratogenicity of the wetland effluent based on the assay’s morphology score protocol, which assessed sub-lethal impacts to the exposed embryos. Through exposure to fractionated chemical mixtures, the highest ecotoxicity potential was identified for the neutral / hydrophobic and acidic fraction. In line with DI-HRMS analysis of wetland extracts, the greatest reduction in the teratogenic response after treatment was for the basic fraction, with a lesser impact on neutral / hydrophobic species. Compounds which showed consistently poor removal through the ICW and were subsequently identified as high risk in wetland effluent (i.e., RQ >1) were clindamycin, sildenafil, and bisphenol A. Variable removal efficiency was observed for ibuprofen and macrolide antibiotics, with improved removal of macrolides in the second year of monitoring, but a reduction in treatment performance for ibuprofen.

The studied ICW reduced PPCP concentrations and mixture complexity in all seasons, and reduced the teratogenicity of the treated effluent. As a result, it can be concluded that integrated constructed wetlands are a promising advanced treatment option to improve the quality of secondary wastewater effluents with respect to emerging chemical contaminants. From a regulatory perspective, the ICW reduced concentrations of compounds which were poorly removed or formed during primary and secondary treatment, including those identified for proposed future regulation in surface water (e.g., diclofenac and azithromycin). This provides evidence for the UK water industry that nature-based solutions such as constructed wetlands can be considered amongst other advanced treatment options. Variable treatment performance for other proposed priority substances (i.e., ibuprofen, macrolide antibiotics, and BPA) was observed across the 2-year monitoring period. More data are required to assess the behaviour of these compounds in constructed wetland systems, and how their removal may be optimised.

The effect-based monitoring approach in this thesis identified that the positive environmental impact of advanced treatment via integrated constructed wetlands could be increased primarily through improving the removal efficiency for neutral and acidic species, with implications for future design and maintenance of similar systems. A 7-day 24-hour time composite monitoring strategy identified highly variable influent loads for some PPCPs, highlighting the importance of quantifying flow rate and HRT in future monitoring of constructed wetland systems, in order to reduce uncertainty in the behaviour of moderately or poorly removed compounds.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Barbara Kasprzyk-Hordern (Supervisor), Jannis Wenk (Supervisor), Charles Tyler (Supervisor), Richard Evershed (Supervisor) & Ruth Barden (Supervisor)