The effect of different water-saturation levels on micropollutants removal in constructed wetlands

Tongxin Ren

Doctoral thesis

Abstract:

Nowadays, the contamination of micropollutants (MPs) in surface water and wastewater has been paid more and more attention because they are resistant to the environment and cannot be removed completely by wastewater treatment plants. Constructed wetlands (CWs) as an economic, landscape-valued and sustainable treatment method are widely used. The natural structures of constructed wetlands including substrates, water and plants constitute an advantageous environment for the removal of MPs by multiple interrelated mechanisms. Several types of CWs with different characteristics, such as different water-saturated levels and various kinds of substrates can be used to treat wastewater containing MPs. So far, the effect of water-saturation level, especially the efficiency of partially-saturated systems, on the removal of MPs in CWs has been scarcely discussed in the literature. In addition, side-by-side comparison of the MPs removal efficiency in CWs with various saturation levels is rare. Therefore, this study explored the removal of organic MPs in CWs on the lab or mesocosm scale, investigating the influence of different water-saturated levels (unsaturated, partially-saturated and saturated) and different types of substrate amendments (woodchips, iron chips and their mixture) on the degradation efficiency. The main objectives of this study were to (1) investigate the effect of different water-saturation levels in CWs on MPs removal from household wastewater; (2) evaluate the impact of the introduction of woodchips and iron chips on the MPs removal; (3) explore the effect of various saturation levels and the amendments on the removal of standard contaminants, in particular total nitrogen. This doctoral research was divided into three experiments. The experiment 1 investigated the removal of bisphenol S (BPS), fipronil (FPN) and ketoprofen (KTP) under various saturation conditions: unsaturated, partially-saturated and saturated, and mimicked the conditions occurring in unsaturated, partially-saturated intermittent vertical-flow CWs and in horizontal-flow CWs, respectively. The experimental CWs were operated BPS and KTP exhibited contrasting behaviour against FPN in the CWs. Namely, BPS and KTP were almost completely removed in the unsaturated CWs without a considerable effect of plants, but their removal in saturated CWs was only moderate (approx. 50%). The plants had only a pronounced effect on the removal of BPS in saturated systems, in which they enhanced the removal by 46%. The removal of FPN (approx. 90%) was the highest in the saturated and partially-saturated CWs, with moderate removal (66.7%) in unsaturated systems. Noteworthy, partially-saturated CWs provided high or very high removal of all three studied substances despite their contrasting degradability under saturated and unsaturated conditions. The removal of the micropollutants in partially- saturated CWs was comparable or only slightly lower than in the best treatment option making it the performance all-rounder for the compounds with contrasting biodegradability properties. The experiment 2 evaluated the removal efficiency of BPS, diclofenac (DCF), fluconazole (FCZ), ketoprofen (KTP), sulfamethoxazole (SMX), sulfanilamide (SNM) and 5-amino-3-methylisoxazole (ISX) under unsaturated CWs and saturated CWs separately in the first stage of this experiment. In the second stage, the mechanism of SMX in anoxic biofilters supplemented with iron chips and woodchips was explored. It was hypothesized that the presence of reducing conditions (additionally imposed by woodchips) and Fe (II) (from iron chips) would promote the reduction of SMX within its isoxazole cleavage. To better understand the degradation mechanisms two analogues of the SMX subunits: SNM and ISX, were also added to separate influents. The influent was a simulated household wastewater. The experimental system consisted of up-flow columns filled with sand (negative control system), woodchips (10% v/v), iron chi

Abstract:

ps (10% v/v), and mixed woodchips and iron chips (10% v/v each). The presence of iron chips (also in a mixture with woodchips) improved the removal of SMX by 68% (from 28% to 96%). The removal of SNM was negligible in all the types of columns used and, on the other hand, the removal of ISX was almost complete in all the types of treatments. Non-target LC-MS/MS analysis confirmed cleavage of the isoxazole ring within the structure of SMX. The ecotoxicity tests showed the toxicity of the influent and effluents containing SMX towards the plant Sinapis alba. The removal of SMX was reflected in decreased toxicity reaching a stimulative effect. On the other hand, the influent and effluents with ISX exhibited the inhibition of Aliivibrio fischeri. In contrast to the effect of SMX, the removal of ISX only slightly decreased the toxic effect. The experiment 3 evaluated the effect of partially-saturated and unsaturated planted constructed wetlands on the removal of 26 MPs and standard contaminants. The presence of woodchips improved the removal of 17 MPs due to sorption or providing organic carbon source for the denitrification process involved in the degradation. The use of partially-saturated CWs improved the removal of nine MPs, but the overall effect was low. The partially-saturated CWs with woodchips, however, improved the removal of total nitrogen by 46%, which qualifies these systems to be a favorable solution for the treatment of household wastewater in terms of MPs removal and TN decrease. Overall, this study contributes to the insights into the removal efficiency of the selected twenty-eight MPs and highlights the role of partially-saturated CWs and substrates in optimizing treatment system performance.

Keywords

Contracted wetlands, Nitrogen removal, Organic micropollutants, Water-saturation levels, Wastewater treatment

Language used: English

Date on which the thesis was submitted / produced: 9. 2. 2024

Thesis defence

• Supervisor: prof. Ing. Jan Vymazal, CSc.
• Reader: Lukáš Trakal, doc. Mgr., Ph.D., Hana Brunhoferová, externi, Katarzyna Pawęska, externi

Czech University of Life Sciences Prague

Faculty of Environmental Sciences

Doctoral programme:
Applied and Landscape Ecology