Performance, resiliency, and optimization of intensified treatment wetlands for removal of micropollutants and biological effects from wastewater

Organic pollutants such as carbon and nitrogen are effectively removed through nature-based solutions, particularly treatment wetlands, research is increasingly focusing on the behavior of and effects to the environment caused by micropollutants. The release of these anthropogenic chemicals into the environment creates a complex mixture of chemicals that can cause quantifiable adverse biological effects. While resent studies indicate the ability of treatment wetlands to remove micropollutants, major questions regarding the removal efficacy of biological effects, the most efficient design aspects (aerated vs. non-aerated; planted vs. non-planted; single-stage vs. two-stage), the resiliency of a treatment wetland, and the potential for optimization are still unsolved. This dissertation investigated these topics by combining cutting-edge methods from engineering, analytical chemistry, and environmental toxicology. The existing knowledge gaps were addressed and the understanding of treatment technologies as well as the potential of treatment technologies to reduce micropollutants and biological effects was improved. In this research, seven treatment wetlands and a municipal wastewater treatment plant were examined over the course of an entire year, to identify the most effective design of the treatment plants in terms of biological effect removal. Intensified wetlands showed higher annual removal efficacy (41 - >99%) than the non-aerated (conventional) horizontal flow wetland (24 – 78%) for investigated micropollutants and biological effects. Removal efficacy for carbamazepine, which is considered to be resistant to aerobic biodegradation, was observed to be higher in the non-aerated horizontal flow treatment wetland than in the intensified wetlands (24 %, 0 – 3 %, respectively). Benzotriazole, diclofenac, activation of aryl hydrocarbon receptor, activation of peroxisome proliferator-activated receptor gamma, and oxidative stress response were removed to a greater extend through the intensified treatment wetlands than by the municipal wastewater treatment plant. Also, for the first time, the effluent qualities of the treatment wetlands were compared with recently proposed effect-based trigger (EBT) values for surface water. The effluent bioanalytical equivalent concentrations (BEQs) for all intensified treatment wetlands and the municipal wastewater treatment plant were close to or even below the surface water EBTs, except for estrogenicity. This indicates the great benefit of using nature-based solutions for water treatment because the treated effluent would not impose a negative ecological effect on a receiving water. Aeration and therefore an elevated oxidation reduction potential was identified as a key environmental condition for increased removal of micropollutants and biological effects. In particular, the two-stage system (aerated vertical flow constructed wetland followed by an unsaturated vertical flow sand filter) achieved the highest removal efficacies for conventional parameters (66 to >99%), micropollutants (94 – 99%, except carbamazepine: 3%) and biological effects (91 – 99%). But also, the less cost-intensive one-stage treatment wetlands (aerated horizontal flow and aerated vertical flow) achieved high quality effluents for the observed parameters. With different aeration strategies, aerobic and anaerobic zones were created in an aerated horizontal flow treatment wetland. The sharp increase of the oxidation reduction potential resulted in a higher mass removal for the moderately biodegradable micropollutants benzotriazole (> 91 %; test system with 50% aeration), and diclofenac (81 %; test system with 50% aeration). But the removal efficacy of biological effects decreased with the enlargement of the non-aerated zone in the treatment wetland. Nevertheless, the combination of aerated and non-aerated zones can reduce moderate micropollutants. The advantage of high removal efficacy for micropollutants and biological effects through aeration comes with the susceptibility to technical disturbances such as a power failure or an air pump failure. A simulated aeration interruption of six days resulted in a poorer water quality (comparable to that of a conventional non-aerated horizontal flow treatment wetland). After switching the aeration back on, the investigated system recovered within a few days for most of the conventional wastewater parameters (8 – 22 d) and micropollutants (3 – 22 d). Results showed that removal of biological effects are negatively affected for a longer period of time (>22 d) than what was indicated by classical parameters or micropollutants alone. These results mark the importance of the use of bioassays for future water quality assessment. With respect to the removal of biological effects, three treatment wetland designs can be recommended: the two-stage system (aerated vertical flow treatment wetland followed by an unsaturated vertical flow sand filter); and the less cost-intensive one stage systems: aerated horizontal flow treatment wetland and aerated vertical flow treatment wetland. The aerated horizontal flow treatment wetland is also resistant to an aeration interruption of a couple of days and recovers, even if it takes longer (indicated by the in vitro bioassays) than indicated through conventional wastewater parameters and micropollutants. The complementary approach of this dissertation contributes to the further understanding of treatment wetlands. Furthermore, the research work provides new insights into removal efficacy of biological effects under various treatment wetland design aspects. This dissertation is intended to help generate and better understand a new generation of treatment technologies