Constructed wetlands for removal of metals from contaminated effluents: Studying the role of plants, microorganisms, and substrate

Anaerobic digestion biotechnologies are globally expanding to manage the high organic waste generation and address rising energy demands. However, challenges remain in valorising digestate due to regulatory constraints and potential environmental pollution risks. Reclaimed water reuse is imperative for effective water resource management to address the increasing pressure on freshwater. The implementation of nature-based solutions, such as constructed wetlands, for the treatment of industrial wastewater for subsequent irrigation purposes can promote a circular bioeconomy. This thesis explores integrating constructed wetlands as a post-treatment of anaerobic digestion to reuse the liquid fraction of the digestate for irrigation. The focus is on removing metals, antibiotics, and antibiotic resistance genes, and producing biomass for energy recovery. A novel configuration using Sparganium erectum for phytoremediation was tested with four liquid digestate compositions (spiked with oxytetracycline, sulfadiazine, or ofloxacin, or without antibiotic spiking). Lab-scale systems demonstrated high removal efficiencies of organic matter, ammonium and phosphate ions, metals, antibiotics, antibiotic resistance genes, and potential pathogens, showing its potential in liquid fraction of the digestate treatment. Despite the antibiotics dosing, at the tested concentrations, no effect on the removal efficiencies was observed. However, after three months of antibiotic dosing, the microbial communities of the rood bed substrate, and the endosphere of the roots, showed significant differences as compared with the control, especially systems treating sulfadiazine and ofloxacin. Additionally, strategies to enhance the methane production from S. erectum biomass, harvested from constructed wetlands systems, were investigated through three pretreatment methods, storage, aqueous ammonia soaking, and hydrothermal treatments. Each pretreatment altered biomass properties, with aqueous ammonia soaking and hydrothermal treatments having notorious effects in biomass morphology and physicochemical properties. All pretreated biomass improved methane yields, with hydrothermal carbonisation at 200°C for 60 minutes showing the most substantial increase. Moreover, dry storage emerged as the most feasible option, while aqueous ammonia soaking and hydrothermal treatments require further optimisation for economic feasibility. Overall, coupling constructed wetlands with anaerobic digestion contributes to closing the loop of AD products' lifecycles, and promotes circular economy principles, facilitating synergy between resource utilisation and environmental conservation. Finally, the thesis also proposes two educational outreach STEM activities to motivate students and promote meaningful learning, engaging them with environmental disciplines, contributing to expand circular economy beyond Academia.

Funder

European Union

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)