Aaron Coyle

Project Title: EVAPOTREAT – Willow evapotranspiration systems as nature-based solutions for on-site wastewater treatment in areas with low permeability soils

The research project, EVAPOTREAT, will evaluate the applicability of nature-based, on-site wastewater treatment systems in areas with low permeability soil, where solutions specified in the current legislation (i.e., the 2021 Code of Practice for Domestic Wastewater Treatment Systems (PE ≤ 10)) involving soil percolation will not work.  The main focus will be on carrying out full-scale field trials using willow evapotranspiration (ET) systems in an Irish climate in areas with representative low permeability soils in order to provide data from which robust design guidelines can be developed for zero discharge wastewater treatment systems.

Existing full-scale willow ET systems constructed in Ireland in recent years have demonstrated that they can act as effective nature-based passive treatment systems that operate effectively, thereby significantly lowering risks of pollution to either groundwater or surface waters.  They also produce a renewable source of energy (wood biomass) which needs to be harvested every three years to ensure vigorous growth of the willows.  However, previous trials have shown that such open systems backfilled with native soil in an Irish climate will inevitably produce some runoff / overflow during the winter following periods of heavy rain.  One aim of this project is to assess and quantify the water pollution impact of these existing fully open willow ET systems (designed according to the current EPA guidelines) with respect to local receiving water quality in low permeability areas, especially after periods of heavy rainfall in the winter.

This project will use all the data collected from these existing willow ET treatment systems, as well as the continued monitoring of a number of them, in order to characterise the realistic evapotranspiration performance by the willows on Irish wastewater.  This knowledge will then be used to modify the process design to incorporate a surface mound which is partially covered with an impermeable liner (in order to limit the rainfall ingress) to determine if they can operate as zero-discharge systems.  Three of these new partially covered systems will be constructed in collaboration with Leitrim County Council and monitored over the period of the project, to assess whether they can indeed act as zero-discharge systems.  The data from the fieldwork studies, as well as simulations from numerical models that will be developed, will then be used to produce robust design guidelines for zero discharge solutions (if feasible) for different geographic locations across Ireland, based on the concept of a mounded impermeable cover diverting a fraction of the rainfall away from the ET system.

The project will also synthesis the current state of knowledge with respect to alternative low / zero effluent wastewater treatment, eco-sanitation and nature-based technologies via a combination of literature review and international contacts.  Finally, the research project will provide a quantitative comparison of the long-term environmental sustainability of these nature-based systems (compared to other more conventional on-site wastewater treatment systems) using Life Cycle Assessment methodologies.

PhD Supervisor:  Prof Laurence Gill